Guidelines



Guide on

IS 3370 - 2020,

(2nd Revision)

Code of Practice -

Concrete Structures for

Retaining Aqueous Liquids :

Part 1- General Requirements,

Part 2- Reinforced Concrete Structures.

November 2020.

by

Lalit Kumar Jain

Consulting Structural Engineer

Nagpur

lkjain.ngp@

Published on Web Jointly by

Indian Concrete Institute

Nagpur Centre

and

Indian Water Works Association

Nagpur Centre

November 2020

CONTENTS

PREFACE p 3

Guide to IS 3370 Part 1 – 2020

R 0 INTRODUCTION p 4

R 1 SCOPE p 4

R 2 REFERENCES p 5

R 3 TERMINOLOGIES (& definitions) p 5

R 4 MATERIALS p 8

R 5 EXPOSURE CONDITION p 9

R 6 CONCRETE p 12

R 7 DURABILITY p 16

R 8 SITE CONDITIONS p 18

R 9 CAUSES AND CONTROL OF CRACKING p 19

R 10 STABILITY p 25

R 11 DESIGN, DETAILING & WORKMANSHIP AT JOINTS p 25

R 12 JOINTING MATERIALS p 35

R 13 CONSTRUCTION p 37

R 14 TEST OF STRUCTURE p 38

R 15 LIGHTNING PROTECTION p 39

R 16 VENTILATION p 39

R 17 DESIGN REPORT AND DRAWINGS p 39

APPENDIX 1 p 40

Guide to IS 3370 Part 2 - 2020

R 0 GENERAL p 41

R 1 SCOPE p 41

R 2 REFERENCES p 41

R 3 GENERAL REQUIREMENTS p 41

R 4 DESIGN p 41

R.4.2 Loads p 42

R 4.3 Method of Design p 44

R 4.4 Limit State Design p 45

R 5 FLOOR p 50

R 6 WALLS p 51

R7 ROOFS p 51

R 8 DETAILING p 51

R ANNEX A p 60

R ANNEX B p 60

ANNEX C: Concrete Finishes p 61

Guide to IS 3370 Part 1 & 2

- 2020 (2nd revision)

Code of practice -

Concrete Structures for

Retaining Aqueous Liquids

PreFaCe

“Retaining aqueous liquid” should be taken synonymous to ‘storage of, or containing aqueous liquids or its exclusion on one side’. In this guide use of terms ‘aqueous liquid’ and ‘water’ are synonymous. In the title word ‘storage’ is changed to ‘retaining’, and clarified that only ‘aqueous liquids’ are dealt and liquids not in general. Here after ‘Liquid Retaining Concrete’ is abbreviated to ‘LRC’. The code does not differentiate between “water contact” and “water retaining” members. All “water contact” members may not be “water retaining” members.

These standards are also applicable to the units of structure conveying e.g. channels, handling e.g. sump and pump-houses, and treating water and waste water (sewage), i.e. for environmental engineering structures, and water resource engineering structures, though not mentioned specifically. Code is mainly for aqueous retaining, and other concrete structures where water-tightness and durability are of prime importance. For structures dealing with waste water and sewage or storing liquids which may attack concrete, additional requirements may also be needed, and some guidelines are given at appropriate places. If likely chemical attack is slow (in relation to design life of structure in years), higher concrete grade is needed. With increase in potential of chemical attack, surface finishes, and protective coatings are needed. Linings are to be provided where chemical attack may be very severe or rapid.

For water conveying, or cross drainage structures in water resource engineering (e.g. aqueducts, canal syphon, sump and pump-house etc.), IS 3370 is being traditionally referred for liquid retaining members, till a separate code would be available for such structures. All the requirements for these types of structures are not covered in this code. For such structures limiting crackwidth of 0.2 mm is enough and tighter limits may not be required.

Those interacting with code revision are normally dealing with bigger size works. Large number of works are for small water supply schemes; and for these few common requirements have become unnecessarily little heavy.

Working stress method is to be applied for LRC designed as plain cement concrete (PCC). For reinforced concrete liquid retaining structures, the working stress design (WSD) method is deleted. These are to be designed by limit state design method only. Design approach is made more rationalized in present revision, while keeping issues simple as far as possible. Design by LSD (compared to WSD) gives economy. With LSD, present revisions have very little effect on the cost economy of the liquid retaining structures.

All four parts of IS 3370 are revised. A new part to deal with construction practices, quality management and maintenance is required. Part 3 for prestressed LRC, is revised specifying limit state design, working stress design deleted and it is in line with IS 1343. The prestressing in one direction only or partial prestress is also considered.

This guide is dealing with the subject in wider perspective, and some opinion may not be from the standards. In few situations code is silent, keeping subject brief, or not explicitly clear, and these are discussed. Views are not necessarily ‘word to word’ interpretation of code, but a guide for understanding for designer to take decisions. The provisions are explained to understand background information.

Reader is assumed to be well conversant with concrete technology and reinforced concrete design and that dealt in basic code (IS 456:2000)# and text books*. The aim is to give guidance to an average engineer for small and usual projects, and may not cover all the requirements for large projects.

In this guide water-cement (indicated as w:c or w/c) ratio, and water-cementitious or water-binder (w:b) ratio (as used in IS 456) are used as synonymous.

For further understanding of the design of LRC structures reference can be made to EN 1992-3: 2006 - Part 3; ACI 350:2019; New Zealand NZS 3106; British code BS 8007:1987. For more details refer to specialist literature. Details can be added as per reader’s demand or suggestions. A handbook or design aids may also be prepared if demand is indicated. Reader may communicate disagreement on a specific issue, or suggestions for giving more explanation, thus help to revise or improve the guide.

Clause number of the IS code is preceded by letter R, and subsequent text is guide, remark or commentary on the concerned clause. Remarks are not given on every clause. Additional remarks are also given in clause numbers which do not exist in the code. The information given is as per the opinion of the author.

As a sample, clauses from the standard (in blue) are added with prefix S, in the section 1 of part 1.

For any contract, the recommendations given in this guide if in variance with IS code, shall not be applicable, unless the contract also specifies this reference.

For supporting structure for elevated tanks, refer “Guide for Design & Construction of RCC Elevated Water Tanks”. For more details on concrete and for guide on construction aspects of LRC, refer “Guide on Construction of Concrete Structures for Retaining Aqueous Liquid”. These guides are by same author.

# IS 456 -2000, Indian Standard Code of Practice for Plain and Reinforced Concrete, with 6 amendments, (standard under revision).

$ IS 3370 part 1, 2, 3 & 4 -2020, Indian Standard Code of Practice – Concrete Structures for Storage of Liquid, Part 1 General requirements & part 2 Reinforced Concrete Structures.

*Suggested books : 1. Properties of Concrete, A. M. Neville; 2. Concrete microstructures, properties and materials, P. K. Mehta & P.J.M. Monteiro, Indian edition by Indian Concrete Institute ; 3. Concrete Technology Prof. M. S. Shetty, S. Chand publishers 2005.;

Guide to IS 3370 Part 1- 2020 (2nd Revision),

Code of Practice - Concrete Structures

for Retaining Aqueous Liquids :

Part 1, General Requirements

R 0 INTRODUCTION

‘Terminologies’ are added (refer R3). ‘Exposer condition’ dealt in more details (refer R5). A concept of H/t (hydraulic gradient a ratio) at the construction joint has been introduced (refer R3.17, R11.2.b(i)). Factor of safety against uplift is deal in little more details (refer R8.c). Information on ‘joints’ has been expanded (refer R11). More details about construction joint are added (refer R11.2.b). IS 456 is still the mother code, though in some of the areas, the provisions in that are not made applicable.

For LRC members’ minimum exposure taken is ‘severe’. For members in liquid contact i.e. surrounded on all sides by liquid, wherein liquid travel under hydraulic gradient does not take place through the member thickness over the major part of life; situation is not as severe as for a liquid retaining. For members in contact with liquid and not retaining, the provisions in IS 3370 part 2 can be bit relaxed except the clear cover which will be as per IS 456, the minimum concrete grade can be bit lower (M25 in place of M30) and the crackwidth requirement can be 0.2 mm and need not be lower. Water-tightness class consideration is not required. Column inside tank is a ‘water contact’ member, similar is a baffle wall in a treatment unit always having water on both faces.

Code now deals with the weakness at construction joint, leading to design action. Designer should check strength capacities in direct shear, and crackwidth at the construction joints. Location of construction joint is to be specified and checks for adequacy of strength and satisfactory performance, are to be applied. Detailed specifications for construction joints are given. Autogenous healing of cracks is mentioned.

Coated steel and stainless steel have been permitted for reinforcement. Bond strength reduction for coated bars, is recommended. Fibres are permitted for improving concrete performance.

For PCC design, details are not there, and designer has to develop understanding and design strategy. It can be designed by working stress method for very small components. For PCC, the permissible tension in concrete is reduced.

Requirement and desirability of concrete surface finish, plaster, lining, coating etc. on concrete surfaces is not dealt. Guidance on surface finishes and smoothness is not given, which in sewage treatment plants may become important.

Design and execution of works are to be done under of a qualified and experienced engineers.

Importance of low concrete permeability is emphasized, however requirement of tests and limiting values of permeability of concrete are not given. Prescriptive specifications and deemed-to-satisfy rules are given.

IS 456 and IS 1343 to the extent applicable, are to be treated as part of IS 3370. Few provisions of IS 456 are over-ruled by these codes, and few others are not applicable as specified in IS 3370.

1 Scope :

R 1. Pollutants or water transportation through the concrete thickness may increase over the design life, and affect the durability and functional requirement of the member. With guidance given, the design life of 50 years can be considered for non-replaceable main structural components, and average life may be >80 years with maintenance and interventions. This life would be approximate in view of action level of environment remaining undefined. Planned maintenance should be envisages for items other than main structural components. Ideally structural components should perform over design life without any intervention or maintenance. Maintenance may be required for cleaning, colouring, movement joints, secondary items, finishes, non-structural items, and for defects in concrete. Service life of structure may reduce due to inadequate quality of construction, especially the variation in size and quality of clear concrete.

S 1.1 This standard (Part 1) lays down general requirements for the design and construction of plain, reinforced or prestressed concrete structures, intended for storage or retaining of aqueous liquids. A concrete structure or member may function as liquid retaining, when the amount of liquid permeating through its thickness, under hydraulic gradient, is practically negligible.

The recommendations are generally applicable to the storage/retaining of aqueous liquids having temperature not exceeding 50° C and no detrimental action on concrete and steel or where sufficient precautions have been taken to ensure protection of concrete and steel from damage due to action of such liquids.

The requirements applicable specifically to plain and reinforced concrete and prestressed concrete liquid retaining structures are covered in IS 3370 (Part 2), and IS 3370 (Part 3) respectively.

R 1.1 Aqueous liquids in temperature range 1(C to about 40(C are normal. Reactivity increases above 40(C, requiring additional precautions. The code gives a limit of 50(C, internationally it is 40(C. Generally water temperature is lower by about 15(C from the maximum ambient. The daily temperature variation of water is less than that of ambient air. The code is applicable to the retaining the aqueous liquids and solutions having no detrimental action on concrete and steel, or where sufficient precautions are taken to ensure protection of concrete and steel from damage due to actions such as in the case of sewage. Outside the above range of temperature, design will have additional considerations and provisions like lining or coatings etc. For ambient temperature below 1°C i.e. freezing condition, designer may require more design actions and precautions for durability, serviceability. Design for temperature gradient (of any range) across the thickness if persistent over long time, needs a design action.

S 1.2 This standard does not cover the requirements for concrete structures for storage/retaining of hot liquids, hazardous materials and liquids of low viscosity and high penetrating power such as petrol, diesel and oil. This standard also does not cover dams, pipes, pipelines, tunnels and damp-proofing of basements. This standard does not cover all the requirements of pressurised tanks, floating structures and tanks having the additional requirement of gas tightness. The selection and design of coatings and linings are not covered in this standard.

R 1.2 The code applies to all components of LRC and roof members enclosing the space above the aqueous liquid, excluding well ventilated (i.e. ventilation area >4% of the free liquid surface), and free height above liquid is > 1.5 m.

Parts of IS 3370 apply to the units of structure conveying (channels), handling (pump-houses), treating water and waste water (sewage) for environmental engineering structures, and may be applied to water resource engineering structures till separate standards are formulated for these.

Hot, cryogenic, low viscosity liquids (high penetrating power like petrol, diesel, oil, etc.), hazardous, or those susceptible for explosions are excluded from the code; and those would need additional requirements. Liquids at high temperature or pressure are not considered. For liquids detrimental to concrete, precautions and protections to ensure durability of concrete, are required. Special problems of shrinkage arising in the storage of non-aqueous liquids and the measures necessary where chemical attack is possible are also not dealt with.

This standard does not cover all the requirements of pressurised tanks, floating structures, and gas tightness. Requirements regarding coatings, linings, and retaining of chemically active or hazardous materials are also not dealt. The code also does not cover dams, pipes, pipelines, tunnels, lined structures and damp-proofing of basements.

For all types of liquid containments excluded in above, the guidelines from the code can be used, however additional criteria may also be needed. For all LRC waterproofing or damp-proofing treatment is not necessary, if required for a members, refer IS 6494. Water-tightness is necessary for LRC.

Tank to store potable water, shall be provided with roof and screens to prevent contamination and to avoid entry of vermin, birds, insects and mosquitos.

Junctions and joints between members shall be treated as element requiring design, detailing and proper construction for achieving reliable performance of the structure. Enough details are not covered in this document for connections of precast concrete for liquid retaining components.

S 1.3 The criteria for design of RCC staging for overhead water tanks are given in IS 11682.

R 1.3 IS 11682 is under revision. “Guide for Design & Construction of RCC Elevated Water Tanks” can be referred.

R 1.4 To ensure compatibility of the design assumptions (Ec, shrinkage etc.) as per the standard, the actual nominal maximum size of the aggregate (MSA) being used should be 16 mm or above, and normally 20 mm. Concrete with lower MSA might not support the design assumptions, e.g. aggregate interlock at construction joint, shear capacity, fracture energy, stiffness (Ec value), shrinkage catered for, etc. A small thickness (25 N/mm² is the target. Blended cement can reduce thermal cracking, improve durability of concrete, and are also improve environmental sustainability. With use of blended cement or SCM’s, the threshold chloride concentration reduces, hence addition of corrosion inhibitor in concrete can be recommended. For roof of tank retaining chlorinated water, if blended cement is used or flyash or GGBS is added, corrosion inhibitors should be used in the concrete, or only OPC should be used without blending.

4.2 Aggregates :

R 4.2 AGGREGATES:

Some engineers feel that water absorption of aggregates should not be more than 3% which appears to be very stringent limit. Porous aggregate increases the permeability of concrete. If satisfactory low level of concrete permeability can be achieved, absorption of aggregate will not affect the performance. However still there is no recommendation and specification of the permeability value permissible. To offset the possible effect of higher absorption of aggregate (>5%) one may adopt a little lower limit of water-binder ratio (or concrete grade higher) than that recommended.

Porous aggregates are normally not permitted for the components of structure retaining aqueous liquid or enclosing the space above liquid. Limits of porosity or absorption are not specified in the code. However for roofs of tanks, if higher grade concrete is used (≥ M40) some types of light weight aggregate may be used. For components enclosing the space above liquid, the percolation of liquid through concrete is not important, but the permeability influencing the deterioration mechanism of concrete is of importance. Aggregate of higher absorption (9%) can be used. Galvanized bars can also be used. If galvanized bars are used, ensure that the zinc coating shall be sufficiently passive to avoid chemical reactions with the cement or concrete shall be made with cement that has no detrimental effect on the bond to the galvanised reinforcement. Natural passivation of zinc coating can be achieved by storing the galvanised bars outdoors for more than a month. Instant passivation can be achieved by dipping the zinc coated product in passivation solution. Epoxy coated galvanised bars are also being used in other countries where environment is very severe for corrosiveness.

The tie wire or any corrodible item shall not transgress the concrete cover. The type of binding wire shall not cause bi-metallic (galvanic) reaction with reinforcement. If feasible, coated / insulated binding wire should be used. Different grades of uncoated steel and different types of steel should not be permitted in a reinforced cement concrete (RCC) component, without electrically insulating from each other.

Use of protective coatings should normally not permit reduction in concrete cover. For stainless steel bars or dual coated (zinc & epoxy) nominal cover can be reduced by 10 mm.

Compared to un-coated reinforcement, for coated reinforcement the bond strength (at limited slip) will reduce, and crackwidth can be higher. Hence, their use shall be accounted in design.

As reinforcement, fiber (continuous) reinforcement products (FRP rods or mats) can be used. Such composites are of carbon, glass or aramid fibres in matrix resin. Refer ISO 14484:2019. These bars have low or negligible ductility, hence cannot be substituted on design force basis. These bars do not corrode, and hence can be used with small cover (say reduce by 15mm) and at lower stress limits suited for small members.

R 4.4 ADMIXTURES

S 4.4.1 Mineral Admixtures :

R 4.4.1 Mineral admixtures, i.e. pozzolanic materials like flyash, GGBS, Metakaolin, silica-fume or micro-silica, etc. as supplementary cementitious materials (SCM’s) or additives, are used to improve micro structure and thus reduces the permeability of concrete. Use of flyash and GGBS also reduce early age cracking due to less heat of hydration in initial period. There may also be a small saving in cost. It is preferable to use mineral admixtures, being advantageous for many chemical exposures. While SCMs are used, addition (3to 5% of cementitious) of lime stone (>80% CaCO2) powder (0.45). Hence with their use, choose a concrete ≥M40.

S 4.5 Jointing Materials :

R 4.5 JOINTING MATERIALS

Jointing materials are required at construction joint, and movement (contraction & expansion) joints. All materials used at present at the joints in LRC, are not covered by Indian Standards. For such materials specifications should be obtained from the manufacturer or the other standards (like BS or ASTM) can be referred. Use of bituminous preparations are not desirable for structures retaining potable water, and similarly some other materials may not also be compatible. Compatibility with liquid in contact needs to be checked for the relevant LRC. See 12 and also 11.5.

The life of most jointing material is much shorter than the design life of LRC. Hence for the design and selection of materials, consider maintainability and restorability of joints.

Some Indian Standards related to joints are given in R-Appendix 1, at the end of this part 1.

S 5 EXPOSURE CONDITIONS

R 5 EXPOSURE CONDITION

Classification of exposure conditions is given in Table 3 of IS 456. Components of LRC should be assumed to be exposed to not less than ‘severe’ condition on both faces for design. Outer face of roof may be taken as ‘medium’ exposure, or higher. Roof top and outer surface of a tank may have higher exposure condition in polluted industrial area, coastal area or sea-face. If conditions demand or chlorine attack could be significant, inner face of roof enclosing space above chlorinated liquid, is to be assumed to be exposed to ‘very severe’ exposure, else it could be ‘severe’.

A face of a component may be subjected to higher exposure like ‘very severe’ or ‘extreme’ if liquid in contact or environment demands so. Consequently the two faces of a component may have for different exposures for design, e.g. one severe and other very severe. The grade of concrete has to be chosen for higher class of exposure. From a concrete surface, clear cover over the bar and limiting crackwidth are functions of the design exposure condition on that face. Map indicating climatic zoning, and susceptibility to corrosion of reinforcement can also be considered.

Components which for most of the time during design life will be surrounded on all its side by non-injurious liquid can be treated as exposed to moderate condition, e.g. column inside tank. These are ‘liquid contact members’. (See R0 2nd para). In most cases such members may be small and it may not be worthwhile to reduce the grade of concrete for small quantity. Many members in structures of water resource engineering require this consideration.

Higher exposure conditions e.g. ‘very severe’ or ‘extreme’, calls for protective surface treatment. The code does not specify lower crackwidths for higher exposures. Crackwidths below 0.2 mm, do not have significant effect on corrosion of reinforcement. However, estimation of crackwidth has many approximations, hence at important locations for better reliability crackwidth may be specified less than 0.2 mm. Also for smaller crackwidths, the water-tightness improves which in turn affect the long term durability.

Take an example of filter house in a water treatment plant. There are three locations of concrete components to be distinguished for design.

a) Floor slab and wall of filter boxes, troughs (launders/channels) are LRC. Adjoining to filter box is pipe gallery, where water due to leakages from joints and valves come. If pipe gallery floor is suspended (not directly supported on ground), it is also designed as liquid retaining member. At top of filter boxes cantilever walkways are provided, which are always above liquid surface, however are designed as LRC.

b) Operating platform above (>2m) pipe gallery is provided. Space between pipe gallery & operating platform is well ventilated like typical building. Operating platform is designed for clear cover required for moderate exposure, and crackwidth limiting to 0.2 mm under serviceability limit state. These types of members are not dealt in the code, and designer has to take decisions. Usually grade of concrete is same as provided in other components at that level.

c) Roof of filter house is usually ≥ 3 m above the top of filter box i.e. walkway & operating platform level. The space below roof is well ventilated like typical building. Though roof can said to be enclosing space above liquid in filter box, the space is large and well ventilated due to doors & windows of filter house. Roof of filter house is designed like any other building for mild or moderate exposure condition as the case may be.

Similarly situation occurs in chemical solution room, wherein solution tanks are treated as LRC and other parts as normal building work. Also consider an example of sump and pump house. Wall, floor & roof of sump are designed as LRC. Floor of pump house is LRC. Floor of the pump house has some openings for access to sump and for installation of pumps etc. Space in the pump house is well ventilated and treated like industrial building. Above floor of pump house all RCC is treated like a building only and not LRC.

The modern approach is to recognize the possible combination of mechanisms of deterioration of concrete component, and design aim should be to achieve an expected durability for the design life.

On the surface of steel embedded in concrete, a protective oxide film tightly held on the bars, by hydration product of cement, is formed by the highly alkaline (pH greater than 12.5) chemical environment present in concrete. This thin passive film protect the steel from further corrosion reaction. As the concentration of chloride ion (acid soluble other than chloride combined in cement reaction, free in pore water) increases, to a threshold (critical), it brakes the protective film and initiates the corrosion of steel. With further continuing penetration of chloride ions, corrosion rate increases.

Apart from chloride ions (radical of salt or acid), chlorine gas or nascent chlorine also reacts with the concrete (like acid attack), reducing hydrated cement to powder, loosing capacity to bind. For roof enclosing liquid with high chlorine, refer R 7.2. Chlorine reaction is less severe in saturated concrete. Underside of roof not saturated, may be affected much more. Whereas wall in freeboard zone is mostly saturated, does not experience the damage by chlorine.

Tanks having chlorine dissolved in water i.e. chlorine solution tank, chorine contact tanks, or tanks holding water having break-point (high dissolved concentration) chlorination, will have nascent chlorine temporarily in air above water, which is highly corrosive to concrete. The underside of roof shall be assumed to be exposed to very severe condition. Such roofs shall be in concrete minimum M40 grade and water-binder ratio ≤0.40. Anti-chlorine surface coating (e.g. epoxy) should also be applied. Note that the life of treatment like coating if applied will be much less than the design life of structure, and this coating will remain a maintenance item.

All tanks of water supply scheme contain water which is normally chlorinated. The dissolved chlorine may be less than break-point chlorination, after few hours of adding chlorine at treatment plant. In such cases, the quantity of chlorine evolved will be less, and corrosive action of chlorine could be slow. However, similar treatment should be given to the underside of roof, considering long life,

The grade of concrete has to be chosen for higher level of exposure condition on any one of its surfaces.

The surface treatment, its smoothness and applications of coatings also depend upon the exposure condition. Concrete in contact with the sewage, requires smooth surfaces.

R 5.1 Detailed exposure classification related to environmental actions causing loss of durability, needs consideration. Select exposure classes based on the environmental conditions of LRC in service and its place. Considerations may include, coatings or lining, and other special treatments.

Different surfaces of a component at different times, may be subjected to different environmental actions. It may be subjected to more than one actions. Based on actions affecting durability, exposure classes are given in Table A. One may also refer to ICI TC/08-01 handbook on durability.

Table A - Exposure classes (based on ISO 22965-1:2006, EN 206-1 & ICI TC/08-01 duly modified)

|Designation |Description of environment related to concrete |For information, examples of the exposure class, concrete would be |

|/ Class | |subjected to - |

|1. Penetration resistance or resistance against permeability of water |

|P0 |No risk of water contact |Resistance against water permeability is not required e.g. interior |

| | |building elements remaining mostly dry & no condensation |

|P1 |Exposer to water |Requiring low permeability e.g. water retaining concrete |

| | |or that exposed directly to very heavy rainfall |

|2. No risk of corrosion or attack on reinforcement or embedded metal |

|X0 |(a) PCC (no reinforcement or embedded metal): | |

| |Exposures except freeze & thaw cycles, | |

| |abrasion or chemical attack. | |

| |(b) For concrete with reinforcement |Inside buildings with very low humidity in air say |

| |or embedded metal: Almost dry |relative humidity RH 2.0% (in soil), or SO3 > 5000 ppm in water |

|7. Freezing and thawing attack on concrete |

|Exposed to significant attack by freeze/thaw cycles whilst wet, the exposure classified as follows: |

|XF1 |Moderate water saturation, without de-icing agent |Vertical concrete surfaces exposed to rain and freezing |

|XF2 |Moderate water saturation, with de-icing agent |Vertical concrete surfaces of structures |

| | |exposed to freezing and airborne de-icing agents |

|XF3 |High water saturation, without de-icing agent |Horizontal concrete surfaces exposed to rain and freezing |

|XF4 |High water saturation, with de-icing agent or sea |Road and bridge decks exposed to de-icing agents, Concrete surfaces |

| |water |exposed to direct spray containing de-icing agents and freezing, Splash |

| | |zone of marine structures exposed to freezing |

|8. Chemical attack on concrete |

|Exposed to chemical attack from natural soils & ground water as given in Table A1, the exposure classified as below. |

|Classification of sea water depends on the geographical location, therefore the classification valid in the place of use of the concrete |

|applies. |

|Note: Special study needed to establish relevant exposure condition where there is - limits outside of Table A1; other aggressive chemicals; |

|chemically polluted ground or water; high water velocity in combination with the chemicals in Table A1. |

|XA1 |Slightly aggressive chemical | |

| |environment according to Table A1 | |

|XA2 |Moderately aggressive chemical environment | |

| |according to Table A1 | |

|XA3 |Highly aggressive chemical | |

| |environment according to Table A1 | |

Table A1 - Limiting values for exposure classes due to

chemical attack from natural soil & ground water

|Aggressive chemical environments class based on natural soil and ground water at water/soil temperature between|

|5°C to.30°C and a water velocity sufficiently slow to approximate to static conditions. |

|The most onerous value for any single chemical characteristic determines the class. |

|Where two or more aggressive characteristics lead to the same class, the environment should be classified into |

|the next higher class, unless a special study for this specific case proves that it is not necessary. |

|Chemical |Reference test |XA1 |XA2 |XA3 |

|characteristic |method | | | |

|Ground Water |

|SO42- mg/l |EN 196-2 |≥ 200 and ≤ 600 |> 600 and ≤ 3000 |> 300 and ≤ 6000 |

|pH |ISO 4316 |≤ 6.5 and ≥ 5.5 |< 5.5 and ≥ 4.5 |< 4.5 and ≥ 4.0 |

|CO2 mg/l aggressive |EN 13577 |≥ 15 and ≤ 40 |> 40 and ≤ 100 |saturated |

|NH4+ mg/l |ISO 7450-1/2 |≥ 15 and ≤ 30 |> 30 and ≤ 60 |> 60 and ≤ 100 |

|Mg2+ mg/l |ISO 7980 |≥ 300 and ≤ 1000 |> 1000 and ≤ 3000 |> 3000 to saturation |

|Natural Soil |

|SO42- mg/kg a total|EN 196-2 |≥ 2000 to ≤ 3000c |> 3000c to ≤ 12000 |>12000 to ≤ 24000 |

|Acidity ml/kg |DIN 4030-2 |>200 Beaumann Gully |Not encountered in practice |

|a. Clayey soils with a coefficient of permeability below 10-5 m/s may be moved into a lower class. |

|b. The test method should prescribe the extraction of SO42 by hydrochloric acid; alternatively, water |

|extraction may be used, if experience is available in the place of use of the concrete. |

|c The 3000 mg/kg limit should be reduced to 2000 mg/kg, where there is a risk of accumulation of sulphate |

|ions in the concrete due to drying and wetting cycles or capillary suction. |

5.1.1 For exposure classes given in Table A, the concrete parameters are recommended in Table B.

Table B – Recommended Concrete Parameters for exposure class as per Table A.

|Exposure Class |Minimum cement content |Maximum water-cement ratio |Minimum concrete grade |

|No risk X0 |260 |0.60 |M20 |

|Penetration resistance or resistance against permeability of water |

|P1 PCC |300 |0.55 |M20 |

|P1 RCC |350 |0.50 |M25 |

| | | | |

|Carbonation induced corrosion in RCC |

|XC1 |300 |0.55 |M25 |

|XC2 |320 |0.50 |M30 |

|XC3 |330 |0.48 |M35 |

|XC4 |340 |0.45 |M40 |

|Chloride induced corrosion : chloride other than from sea water |

|XCl 1 |320 |0.48 |M35 |

|XCl 2 |340 |0.45 |M40 |

|XCl 3 |360 |0.42 |M45 |

|Chloride induced corrosion : sea water action |

|XCs1.0 |330 |0.45 |M25 |

|XCs1.1 |350 |0.45 |M35 |

|XCs1.2 |360 |0.40 |M40 |

|XCs2 |360 |0.42 |M40 |

|XCs3 |380 |0.40 |M45 |

|Aggressive chemical environment |

|XA1 |330 |0.48 |M35 |

|XA2* |360 |0.45 |M40 |

|XA3* |400 |0.41 |M45 |

|* When SO4 leads to exposure class XA2 or XA3, it is essential to use sulphate-resisting cement. |

|If classified, high sulphate-resisting cement should be used for exposure class XA3 |

|Freeze-thaw attack |

|XF1 |300 |0.50 |M30 |

|XF2 |320 |0.48 |M35 |

|XF3 |350 |0.45 |M40 |

|XF4 |380 |0.44 |M40 |

|Minimum entrain air content should be 4% for XF2 to XF4 |

Note : Recommendations in Table B are not same as per ISO 22965-1 or EN 206-1.

R 5.2 In construction the minimum cement content and the minimum grade of concrete shall be higher of the values as recommended from Table 1 of IS 3370 part 1, Table 2 of IS 456 and Table B above. Similarly maximum water-cement ratio should be lower of the values as recommended in these tables. The concrete characteristics shall be enveloping the requirement from different considerations.

S 6 CONCRETE

Provisions given in IS 456 and IS 1343 for concrete shall apply for reinforced concrete and prestressed members respectively subject to the following further requirements:

a) The concrete shall conform to Table 1.

b) The cementitious content excluding mineral admixtures, such as flyash and ground granulated blast furnace slag, should not be used in excess of 400 kg/m3, unless special consideration has been given in design to the increased risk of cracking due to drying shrinkage in thin sections, or to early thermal cracking and to increased risk of damage due to alkali silica reactions.

c) Cement plaster if applied to internal surfaces of concrete, should not be treated as an alternative to impermeable concrete.

Table 1 – Minimum Cementitious Content, Maximum free Water-cementitious Ratio

and Minimum Grade of Concrete

| Concrete |Minimum Cementitious |Maximum free |Minimum Grade |

| |content |Water-cementitious ratio |of Concrete |

| Plain Concrete |250 Kg/m³ |0.50 |M 20 |

| Reinforced Concrete |350 Kg/m³ |0.45 |M 30 |

| Prestressed Concrete |380 Kg/m³ |0.40 |M 40 |

d)

NOTES : 1 Cementitious content mentioned in this table is inclusive of mineral admixtures mentioned in IS 456 and is irrespective of the grades of cement.

2 For small tanks having gross capacity up to 50 m³ at locations where there is difficulty in providing M30 grade concrete, the minimum grade of concrete may be taken as M25 (with minimum cementitious content as 350 kg/m³). However, this exception shall not apply in coastal area, or the area where air pollution is high or liquid retained is aggressive like sewage.

R 6 CONCRETE

PCC base (or called mud-mat concrete, lean concrete, foundation PCC or blinding layer) is a non-structural concrete and not govern by the requirements specified in Table 1, and is excluded from the following discussion. PCC in foundation is discussed in R 3.3, R 3.30, R 9.2.8b, R 11.2a, R 11.4, R 13.1.1, R 13.1.2,

The concrete by itself should be watertight (i.e. low permeability), and plaster should not be relied for reducing leakages, but concrete should be grouted to reduce permeation, if required.

R 6.1 Table 1 specifies minimum cementitious or binder (i.e. cement + pozzolanas) content, maximum free water to cementitious /binder ratio, and minimum grade of concrete.

Cementitious content given in Table 1 is irrespective of the grades of cement and it includes mineral admixtures such as flyash or GGBS and are taken into account with respect to the binder content and water-binder ratio. Do not exceed the limit of pozzolana and slag specified in IS 1489 Part 1, IS 455 and IS 16714 respectively. With maximum size of aggregate less than 20 mm, the concrete may require higher binder (cement + mineral admixture) content, and for higher MSA minimum binder content can be less (refer Table 6 of IS 456). As the clinker (OPC) content in binder reduces, the w/b ratio should also reduce.

For higher exposure conditions (very severe or extreme), the requirements of Table 5 of IS 456 will also govern the specification of concrete.

R 6.1.1 If during construction there is a good control (small variation within narrow range) on aggregate grading, and standard deviation in compressive strength of concrete is less than 6% of the characteristic strength, i.e. quality control is very good, the minimum binder content in RCC can be taken as 320 kg/m³.

For RCC work, total binder content in concrete can be lower than the limit given in the Table 1, where aggregates and powder content are well grades and proportion arrived at by particle packing theory, wherein main role of cementitious (binder) material is to coat other particles and its action as filler (filling finer space) is very small. This approach will also require particles graded below 200 to about a micron size. OPC content of concrete can be much lower in these cases, such as 200 to 250 kg/m³. SCM’s, additives and filler materials can be used in addition to OPC. However, water-binder (w:b) ratio should be ≤ 0.4 for such concrete. It is also advisable that in a cubic metre of concrete total water content should not be more than 140 litres including free water on aggregates and the water in admixtures. Note that, thus total water and total paste in concrete will be very low and need of superplasticizer will be higher.

R 6.2 Concrete should satisfy all the requirements of IS 456, and specifically those in Table 5 of IS 456. Grade of concrete is a main criterion for specifying concrete. Though permeability is an important parameter for LRC, specific recommendation is not given. To control permeability, in addition to minimum grade (strength), maximum water-binder ratio is also specified. For water-binder ratio, the equivalent weight of SCM’s should be accounted. For flyash 0.2 to 0.4, for GGBS 0.5, Metakaolin 0.7 etc. and for ultrafine SCM’s the factor is higher.

R 6.2.1 Concrete as proportioned (mix designed) should have enough of workability for ease of working, in relation to the method of handling and compaction of concrete. For increasing the workability the dose of plasticizer (or superplasticizer) can be enhanced. Limit on water-binder ratio should always be maintained.

R 6.2.2 For concrete mix production, the specified water-binder ratio should be taken 0.01 less than the limiting value specified in Table 1 or Table B or the value taken for mix trial in laboratory. (Ref. ISO 22965-1). This is to account for the field variations. Water from all sources including that in admixture and the surface water with aggregate shall be accounted for calculating the total water in the mix, and also for water-binder ratio.

Most works of liquid-tank are small, and may not conform to note 1 of Table 8 in IS 456 (amendment 4), hence target mean strength shall be fck +1.65×6 MPa, i.e. for M30 grade the target mean strength should be 40 MPa. This margin of strength is required to cover variations in quality of materials, grading, batching, mixing and transportation etc., till a lower standard deviation is obtained from the record of strength test on the concerned work. For mix design, after obtaining standard deviation from the actual work (field) test the value can be used for arriving at the target strength, which shall not be less than fck +1.65×s MPa. Here “s” is standard deviation, taken not less than 4.

For RMC supplies the variations in concrete productions are small. However, variations in transportation (involving more than one hour time and temperature variation), placing, compaction and curing can take place, and the standard deviation can be taken as minimum 4.2 MPa or more as confirmed by the records test record from site of work (and not at RMC plant). Thus for RMC supplies the target mean strength would be 37 MPa for M30 grade. RMC supplier does not take this in to account and the average target strength of concrete as supplied is lower than that required. RMC supplier should leave a margin for variations in strength due to field operations. Hence at the time of placing order to a RMC, for the acceptance average strength shall be fck + 5 MPa for tests on samples taken on delivery.

R 6.2.3 With the modern cement as available (strength >53 N/mm² and as high as 70 N/mm²), for conformance of limiting maximum water-binder ratio (related to exposure condition), the achievable grade of concrete may be significantly higher than that being specified, and by mix design trials, it can be determined in laboratory. Also for good grading of aggregates, the strength can be higher at the specified w/c ratio. In such cases, to conform to the requirement of maximum water-binder ratio, the grade of concrete to be adopted in construction shall be related to developable strength at the limiting water-binder ratio conformed by test in laboratory. The field strength of concrete shall be not less than the average developable strength minus 1.65× standard deviation adopted for mix proportioning. The specified compressive strength should be reasonably consistent with the w:b ratio required for durability, which should be low enough, and the specified strength high enough, to satisfy both the strength criteria and the durability requirements.

In other words this means that for cements of much higher strength and optimised better graded aggregates, the concrete grade in construction should be higher than that given in Table 1. And designer has the option of designing and specifying higher grade concrete. When high strength OPC with ≤10% flyash and GGBS are used, the grade of concrete in construction should be M35 or more.

R 6.2.4 In the modern concrete practice, for enhancing the grade of concrete, cement content need not increase. It can be enhanced by lowering the w/c ratio and marginally increasing the plasticizer dose. Hence for enhancing the grade from M30 to M40, increase in cost is very marginal (say 2 to 4% only as cost of more plasticiser dose) provided the cement content (kg/m³) does not change. This can be easily verified by difference in quotations for the two grades of concrete from a RMC supplier. In general higher grade concretes are more durable and also economical in designs. Concrete grade as higher as practicable should be adopted, and still it can be economical.

R 6.3 Minimum concrete grade for LRC work in RCC is M30. Because of history of constructing tanks in M20 & M25 grade and satisfactory performance of many the tanks already constructed; small tanks up to 50 m3 in the environment of medium exposure and with H/t within 25, can be designed and constructed in M25 grade concrete, except those in coastal areas, or where air-pollution is high, or liquid retained is aggressive like sewage. However, minimum cement content will remain 350 kg/m³.

R 6.3.1 For LRC designed as PCC (see 9.2.1), M25 grade is permitted; however H/t is ≤20 and minimum reinforcement is as per IS 3370. Very small (70°C) in early life (1 to 3 days) of concrete, delayed ettringite formation (DEF) can occur (in later life) in certain mixes, with wetness for most part of its life. To avoid adverse effect of DEF on the performance of LRC in service, the peak temperature in early life should not be >70°C. To control this use recommendations given in R 6.3.5.

R 6.3.7 In LRC, chemical admixtures (plasticizers) enhance workability, reduce water-binder ratio and permeability, therefor advantageous. Avoided those containing chlorides. A particular admixture shall be permitted only after the compatibility test with the cement sample from the specific source. The source of admixture or cement, if any one changes, the compatibility test shall be carried out again.

Total acid soluble chloride content shall be ≤0.6 kg/m³ in concrete. For very sever environment like roof on chlorinated water, the chlorides shall be ≤ 0.4 kg/m³, and also 96%), by spraying water or covering concrete to avoid evaporation of water from it, i.e. applying curing compound. This phase continues till desirable properties are developed in concrete.

iii. Till liquid is filled in tank, it should not be allowed to dry-up i.e. RH ≥55% should be maintained.

R 6.6.1 Concrete members should be initially cured continuously (without intermittent drying time) for at least 14 days, and also till 80% of the specified strength is achieved. During summer the main phase of curing period should extent to 28 days and in other dry season to 21 days, during which concrete it kept moist (RH >70%) and may not be 100% saturated continuously after 14 days. Thereafter, LRC should be sprayed with water at least once a day for not allowing the concrete to dry out i.e. keeping RH>55%. Curing activity in the last phase may not be required if ambient temperature is below 10(C or humidity in air is high (>70%). [ RH = relative humidity ]

Proper curing of concrete is vital for controlling temperature-shrinkage cracks, and gaining durability. The method shall ensure that the surfaces of all concrete remain continuously moist in the curing period. With curing, the temperature of concrete shall also be kept in control. Temperature shock i.e. sudden cooling of concrete surface say by continuous cold water spray immediately after de-shuttering should be avoided.

R 6.7 For pneumatically applied concrete, the designer should approve the specifications, material requirements, mix proportions, effective water-binder ratio, mixing, placing, equipment to be used, and curing before the construction starts.

7 DURABILITY

S 7.1

R 7.1 Durability is the ability of a structure or structural element to withstand cumulative deterioration, which may otherwise is harmful to the required performance in the relevant environment, and deterioration should be small as well as tolerable till the design life. Durability should be satisfactory in all situations of limit states of serviceability; and also for few parameters in ultimate limit state for LRC. On reaching an ultimate limit state, LRC may get damaged and excessive leakage may take place, but otherwise after loads get reduced the performance can continue for short duration, till intervention if required will takes place.

Durability is also a limit state. The structure shall be designed such that deterioration will not reach to a limit which can affect the serviceability during the design life with the determined plan of maintenance (conservation). Repairs as additional maintenance would be required for short falls in quality of structure. Before reaching its limit state, structure should give enough warning.

It is a prime consideration for LRC. Refer 8 in IS 456 for its requirements. For it, exposure of concrete structure to environmental and service conditions during design life is to be considered. Each mechanism causing loss of durability, need to be taken in to account and enough resistance for that has to be designed in the structure.

R 7.1.1 Important agencies causing loss of durability are following.

Chloride ion penetration, and increase its concentration to a threshold value near the steel surface, which reduces the passivation for corrosion of steel.

Carbon dioxide permeation convert calcium hydroxide to carbonate, thus pH reduces in the protective concrete cover over steel bar. While concrete is saturated, carbonation does not take place. Normally concrete has pH >12 (in new concrete up to 14). With reduction in pH the rate of corrosion of steel increases.

With increase in permeability, calcium hydroxide comes out of concrete with water in dissolved form and deposits on the external surface of concrete. This results in more porosity, more permeability, and reduction of pH etc., all helping in loss of durability.

Loss of durability is caused by the permeation of agencies like water, oxygen, carbon dioxide, other gases, chloride ions, etc. through concrete. The permeation of each of these are not directly controlled, but indirectly by water-binder ratio and grade of concrete. Water to binder ratio (usually indicated as w:c or w:b) should be as low as possible (between 0.45 to 0.40), while keeping the concrete workable using plasticisers (chemical admixtures).

Other actions like sulphate attack, acid attack, freeze-thaw effect, and reactive aggregates can also contribute to loss of durability and need considerations, case-wise.

Defects like honeycomb and segregation, are havoc for durability, and these must be eliminated. Next is the very low seepage through cracks and joints. Better control over microstructure of concrete and low permeability, are desirable durability over the design life of LRC.

While mineral admixtures (e.g. flyash, GGBS) are added the threshold limit for chloride ion concentration may reduce, which can be compensated by using suitable corrosion inhibitor.

S 7.2

R 7.2 From experience of observing roof of tanks storing chlorinated water, the durability of bottom of roof is highly important. ‘Very severe’ exposure should be considered for underside of roof, hence concrete grade minimum M40, w:c ratio 0.4, and anti-chlorine surface treatment like epoxy coating are needed. See also R5 last 5 para’s.

Presence of other gases, like hydrogen sulphide in sewage treatment, may also require suitable coatings.

R 7.2.1 In environment prone to chemical attack, all jointing materials should be chosen such that they are resistant to the chemical exposure as conformed by tests performed by the manufacturer or the supplier.

R 7.2.2 Concrete should be protected against damage by abrasion, erosion and cavitation. Following actions may be taken- reduce velocity of flow to 0.42fy in limit state of serviceability. [See also Part 2, R 6.4.2 & R 8.3.1.]

It should be noted that for keeping H/t >20, the limiting crackwidth shall reduce requiring high amount of steel provision and sealing of joint at liquid face.

For a particular H/t value, stress (compression or tension) on the construction joint affects the seepage through joint (progressively increasing).

1. Whole section in compression, no tension.

2. Tension on one face & compression block ≥ 50 mm.

3. Tension on one face & compression block < 50 mm.

4. Tension on whole section (i.e. on both faces).

For better understanding the performance affected by H/t, this ratio can be modified for framing better criteria. Leakage through crack is also related to width of crack. Stress (compression or tensile) perpendicular to the construction joint (or a crack) has an influence of leakage. Part or full section (interface) may be subjected to compression, in presence of which the leakage will reduce. Hence an equivalent thickness can be accounted which is enhanced for part of section in compression.

If N is the size of compression block (depth of neutral axis), tension part = t – N ,

Equivalent thickness = te = t + N /4 = t + 0.25 N {constant 4 can reduce up to 2}.

The limiting hydraulic gradient (H/t) through monolithic member (away from any joint) can be much higher (say 3 times) where there is no structural crack.

Above the limiting H/t, water-bar is required to reduce the leakage through construction joints. It is preferable to provide higher thickness, compared to providing water-bar. Refer 11.5.1 for details at construction joint.

R 11.2 c) Temporary Open Joint (Gap Joint) :

Along a member like wall or slab, a temporary gap can be kept during construction, which will be filled in by structural concrete lately, and before the scheduled time for putting the structure in service. The width of gap could be of the order of 0.8 to 1 m. This gap facilitates to lap the reinforcement extending in to the joint from two sides. Usually there is no advantage of keeping it much wider or narrower gap. This temporary gap accommodates the contraction of adjoining concrete lengths on each side, due to temperature effect in young concrete and also partially shrinkage taking place till the time of filling the joint. Through this gap the reinforcement can act as continuous by lapping. If the length of concrete member on both side of gap is more than 14 m, it may be necessary to provide laps for all bars within the gap, to reduce the restrain on contraction of concrete. As all bars are lapped within this small region, lap length should be suitably enhanced, and distribution bars (perpendicular to lap) should about 1.3 times the minimum reinforcement. To allow the contraction of adjoining concrete to the maximum possible extent, the gap can be filled in as late as possible, and after few days of drying of the concrete on each side. The interfaces at the gap should also be treated as a set of two partial contraction joints.

R 11.3 DESIGN AND DETAILING OF JOINTS :

This is with reference to movement joints for ground supported tanks.

R 11.4 SPACING OF MOVEMENT JOINTS :

The movement joints are provided to divide a structure in parts, allowing movement of concrete panel in each part independently, by substantially reducing the friction between the concerned RCC panel and its sub-base. For permitting expansion or contraction of panel between movement joints, the top of PCC sub-base must be perfectly in a plane, flat and smooth. Flatness and smoothness of the surface must be specified, controlled and achieved in the construction.

For ground tanks, the spacing of joints should form a pattern, taking in to account the positions of columns supporting the roof. Preferably column should be at centre of a panel, i.e. equidistant from movement joint. Load of column can mobilise enough friction preventing horizontal displacement of slab at column location.

Even if the surface which may appears to be smooth, has very small roughness (seen under a magnifying lens) of sub-base top, the concrete above will set in to ups and down of roughness and friction will develop, thus resistance will be offered to free movement, and the purpose of providing movement joint may be partially defeated. Hence a separation (or bond breaking) sheet is put in between, which will facilitate sliding. The thickness of the separation sheet should allow friction-less in-plane shear deformation i.e. relative displacement between bottom and top layers. Normally 1 mm thick polyethylene (LDPE) sheet is specified with smooth top sub-base. Thickness of sheet is directly related to the roughness (on finer scale) of the top surface of sub-base.

It should be noted that projections of structural concrete below the plane interface shall lock the movement, and induce unwanted cracks. The sub-base PCC below the RCC floor slab should have specification such that it can be finished smooth. For good finishing-ability of base PCC it should have enough paste and finer aggregates.

The options for movement joint are for the cases where restrain to expansion and contraction movements are present, which is mainly in ground supported tanks. Following are the design options.

a) Option 1- Within the area of continuity, intermediate movement joints are not planned. Design assumes restraint. Cracking behaviour is controlled by provision of enough reinforcement (normally high amount) which has smaller spacing with minimum possible size of bar. Construction joints will induce crack pattern, and are to be sealed. This option is preferred for tank up to 22 m size. For higher size tanks reinforcement has to be designed more cautiously.

b) Option 2- Movement joints are introduced, therefore the amount of strain to be controlled by reinforcement reduces, and thus the requirement of reinforcement can reduce. Crack pattern is induced by spacing of movement joints.

c) Option 3- Cracking can be controlled by close interval of movement joint, thus significantly reducing the requirement of reinforcement. Cracks in between movement joints if develop will be significantly small. Total freedom for movement is very difficult to be achieved and hence some reinforcement should also be provided.

In Table 2 Column 4, option 1, the steel ratio can be significantly higher say >1.4 times ρcrit .

Calculation for ρcrit will be as per Annex A Part 2.

R 11.3.1 The steel ratio ρcrit as calculated (for 415 grade) will be as below. This refers to ground tanks only.

|fck |M20 |M25 |

|II |0.015 |1.5 % |

|III |0.020 |2.0 % |

|IV |0.027 |2.7 % |

|V |0.036 |3.6 % |

[ e.g. for zone III, base shear not less than 0.020 (i.e. 2.0%) of the total seismic weight. ]

R 4.2.4.13 If the tank or staging do not have main members as overhanging or cantilever, other than cantilever walkway, the effect of vertical seismic can be neglected for tanks in zone II and III. Otherwise, in all other cases, horizontal and vertical seismic effects should be combined.

All overhanging or cantilever members even in zone II or III, shall be designed for vertical seismic also. For the purpose of this clause cantilever walkway (≤ 1.2 m cantilever span) will not classify the tank as having cantilever members. However, the cantilever walkway or gallery shall be designed considering vertical seismic also.

R 4.2.4.14 Mass of pipeline, stair, ladder etc. should also be estimated for arriving at total horizontal seismic action for elevated tank.

R 4.2.4.15 For Industrial water tower of high importance or tower in highly populated urban area (where collapse of tower may damage many other buildings or structures), the importance factor may be taken higher than 1.5 (can be 1.75 to 2.0), if specified by the owner. Also refer IS 1893 part 4.

R 4.2.4.16 For tank having slab supported on ground, it (slab) acts as a diaphragm for transferring the seismic forces from tank to ground. Hence connection of the ‘slab on grade’ to vertical members shall be detailed for the seismic load path. Movement joints may interfere with the adequacy of action as diaphragm.

R 4.2.4.17 Members of tank participating in lateral load resisting frame (i.e. floor beams) shall be designed conforming to ductility requirements of IS 13920-2016 (under revision). If flat slab is provided for floor of an elevated tank it should be designed for high degree of ductility (norms are not available in the standards).

R 4.2.5 WL : Wind load. As per section 5 to 7 of IS 875-3, wind should be accounted as pseudo-static force.

[ Note that wind speed map of India is revised in 2020 as per amendment 2, or refer NBC.]

R 4.2.6 Earth Pressure (EP): The lateral pressure from earth backfill, may be symmetrical or asymmetrical. Including those caused by unequal backfill or surcharge, net lateral loads shall be determined by rational methods of soil mechanics based on soil and foundation investigations.

The wall does not deflect enough as required for active state of earth pressure. Hence effective earth-pressure coefficient could be higher (say 1.3 to 1.5 times active), and in between at-rest and active state. The coefficient shall not be less than 0.50 where soft soil is present. Where excavation is in hard strata like weathered rock or soft rock, the refilled trench is small and active pressure can be very small, and this active pressure can be enhanced by 50%.

If EP is definitely to remain always, it may counteract the liquid pressure from opposite direction. Such relieving earth pressure should not be more than half of the active state. Also refer R 8 a (i) in part 1.

Earth cover on roof may be taken as saturated dead load. Also take in to account construction loads of machinery and heaped earth which may exceed the force action at some sections compared to intended design load.

Allowance should be made for the effects of any adverse soil pressures (EP) on the walls, due to the surcharge and process of compaction of the soil and the condition of the structure during construction and in service. The lateral pressure from earth backfill, may be symmetrical or asymmetrical. Including those caused by unequal backfill, net lateral loads shall be determined by rational methods of soil mechanics based on foundation and soils investigations.

R 4.2.8 Liquid load (FL) :

FL shall take in to account the actual density of the contained liquid. Density of plain water can be taken as 9810 N/m³. Aqueous solutions (alum solution), suspensions or sludge can have higher densities. Accumulated sludge, deposited silt, grit, lime etc. if any, will add to the load. Wherever exists, liquid load includes dead storage which may have higher density due to sludge and silt etc.

FL may be accounted at zero or partial or full liquid load as may make a load combination more critical. It should be such as to cause the most critical effects at a position of a member. Liquid load also includes, both static and dynamic effect of the liquid as pressure. Each tank shall be designed and checked also for tank-empty condition.

For serviceability condition FL is to be taken up to normal working top liquid level (WTL) or the overflow level. This level is usually referred to as full supply level (FSL) in tanks. For overflow to match the rate of incoming liquid, usually heading of liquid above WTL is of the order of 20 to 50 mm. Such a small heading of liquid can be neglected in design. To keep a control over maximum FL, overflow arrangement should be designed for a discharge rate not less than the maximum filling rate of tank; an overflow pipe or weir arrangement of adequate size shall be provided to prevent overfilling of the tank.

If over flow is chocked, or for any other reason liquid level rises above WTL (/FSL), liquid load will be higher under such an unusual condition. Provision of such rare and temporary rise may be estimated, which may be termed as maximum top liquid level (MTL). For design, above FSL liquid may be considered to rise by amount as specified (need not be up to full freeboard), and level may be taken as MTL. In many cases height of MTL above WTL (/FSL) is specified between 150 (for smaller tanks 1000 m³). With this rise (i.e. MTL), FL is accounted only in ultimate load combination with DL & IL (limit state of collapse). However for all other combinations with WL/EL, FL up to FSL/WTL only is considered. Where freeboard available is very high, a suitable MTL shall be decided from hydrostatic design considerations.

Where MTL above WTL is higher by more than 300 mm, the load factor for FL can be taken as 1.40 (in ultimate load combination), the condition being very rare.

Except for pressurised tanks, vent(s) shall be provided in roof to regulate the internal pressure in space above liquid, while tank is being filled or emptied. See R 16 in part 1.

R 4.2.8.1 Internal pressure of liquid shall be assumed to act at the centre of thickness of the liquid retaining circular wall. Same criterion can be applied to rectangular wall panels. If lining or impermeable treatment is applied to the inner surface, or the pressure is due to granular material or soil, the inner surface of impermeable lining or wall may be assumed to be the point of application of force. The external pressure of liquid will be assumed to act at outer surface of structure, which may be in combination with earth-pressure. Weight of liquid on a member (e.g. slabs) shall be up to the internal surface (i.e. top face) of member.

For calculation of hoop force as a simplification, radius should be taken as clear radius plus half the thickness of wall i.e. centre-line radius of wall. Increasing the radius by half the thickness is the maximum possible. As per first level approximation, for actual hoop force the factor to thickness could be one third (1/3rd). Where wall thickness is higher (>200mm) the factor to thickness will be less than 1/3rd. In most cases, it may is appropriate to account increase of radius by 1/3rd of wall thickness, though code has introduced the factor as half as a simplification.

R 4.2.9 Other load actions which are significant for serviceability, strength and stability of the structure or its members as applicable, including the following shall be taken into account.

a) Construction load, (b) ground movement, (c) thermal effect on roof.

The clause is mainly for ground tanks.

R 4.2.10 An underground (or partly) structure subject to groundwater pressures should be designed for floatation (uplift). Design of each member shall account the pressure due to groundwater in suitable critical load combinations. See IS 3370 part 1, R8(c).

R 4.2.11 If concrete is allowed to dry, the moisture dependent drying shrinkage will take place. This can happen where tank is provided with impermeable lining. Normal recommendation in this standard (for temperature-shrinkage reinforcement) does not account this moisture related drying shrinkage. Hence this extra shrinkage if restrained will need more reinforcement to control crackwidth. See also R 9.1.B.1 & R 9.1.1 of part 1.

R 4.2.12 Loads should be grouped as ‘permanent loads’, ‘provisional loads’, ‘variable loads’ and ‘construction loads’. Environmental actions (physical, chemical or biological) are also types of loads.

R 4.2.13 Junctions (say connections) of members should be designed and detailed for giving rigidity and satisfactory crack control throughout the design life of structure.

R 4.3 METHOD OF DESIGN

The level of accuracy of various physical parameters of design should be refined by devoting more efforts for analyses, so as to arrive at improved accuracy in the behaviour and strength provided by design. In regard to a specific criterion, inadequate performance is considered to be a failure in that regard. In addition to serviceability and strength, degree of water tightness, absence of leakages and avoidance corrosion of reinforcement are prominent performance requirements amongst others.

R 4.3.1 Components of structure adjoining to LRC may not be designed as per IS 3370. However, continuity of these with LRC may need considerations of restrains imposed on LRC.

R 4.3.2 Significant plastic redistribution of moments (as in IS 456 - 22.7 & 37.1.1) is not permitted directly. Limited redistribution can be done, keeping a limit on deformations or maximum strain. [ACI 350 permits up to 20% of BM to be reduced on account of plastic redistribution with some limitations.]

Also simplified estimate by coefficients (as in 22.5 of IS 456) cannot be permitted.

R 4.3.2.1 For two way spanning slabs and wall panels, moment coefficients from Table 26 of IS 456 for uniformly distributed load, which are based on modified (multiplied by 4/3 times) yield line theory, with this readjustments are normally acceptable for design of liquid retaining concrete including crack control, as is proven by experience and also use in other countries. In such plates permitting the elasto-plastic behaviour of concrete and reduction of peak moments, in serviceability state for -ve BM tensile stress in steel should be ≤190 N/mm² or crackwidth be ≤ 0.15 mm (in place of 0.2 mm).

Use of elasto-plastic behaviour of structure can be made, for members with ductility and limiting plastic strain considered. Use of simplifying assumptions or elasto-plastic behaviour of structure can be made, with limit on plastic strain considered and by imparting enough ductility, having proved by long experience or technical literature or tests that it can give satisfactory performance at or near junctions (connections) of members, and in estimation of crackwidth. Because the control on possible crackwidth during service is required, adjustments on account of plasticity (say redistribution of moment) can be done only if crackwidth can be predicted for such situations within acceptable accuracy. Alternately, strain can be enhanced by plastic strain, for crackwidth estimation.

However it should be noted that tables for bending moment coefficients for triangular liquid pressure (say for walls) are generally based on elastic analysis or linear finite element analysis. For a loading configuration, uniform pressure and hydrostatic pressure (triangular) can be added. But using the coefficient tables, difference of the two cannot be permitted. In other words BM worked out by Table 26 of IS 456, for uniformly distributed load and BM for triangular load, cannot be deducted from each other, but can be superimposed as additive.

For rectangular wall plate subjected liquid load, elasto-plastic analysis can be done wherein peak moment is reduced up to ≤20% only. For moments in other plate shapes and loading, results of elastic analysis can be used. For flat slab design analysis for suspended floor of tank can be done by finite element stiffness method, and high ductility is to be imparted. Norms for achieve high ductility for column width of flat slab are not available at present in standards. Under combination of seismic action, the flat slab participates in the frame action, demanding very high ductility. Use literature for imparting high ductility.

Where flexural moments are calculated by a method of linear elastic analysis of frame, the redistribution of maximum -ve BM in a continuous span can be done to a limited extent. Follow procedure as per 37.1.1 of IS 456, except that limit of reduction shall be 20% in place of 30%. A reduction (on account of plasticity) of critical elastic moment (maximum –ve) up to 20% can be done (as permitted by ACI 350) if strain is substantial (i.e. moment is a peak or maximum value in the member) but limited, at the section where redistribution is permitted. Before readjusting the BM, the -ve BM section should under-reinforced. For under-reinforced sections the limiting strain limits are satisfactory. Such reduced BM shall be used for calculating redistributed moments at other sections of the member by principles of static equilibrium.

The sections designed in flexure shall be under-reinforced (i.e. less than 75% of balanced wherein compression governing failure is avoided), and should not be over-reinforced, where plastic redistribution moment is done.

For sway frames no redistribution on account of plasticity will be permitted.

R 4.3.2.2 Flat slab design as per 31.4 of IS 456 is based on plastic redistribution of moments, and cannot be permitted for suspended floor slab of elevated tanks, because of inadequate control on ductility and possible under estimate of crackwidth. Flat slab design as per 31.4 of IS 456 may be permitted for roof slab of tanks and also for floor slab on grade of ground tanks. Flat slab analysis done by finite element method will be acceptable for design in all cases. For floor of elevated tank it will be acceptable with provision of high degree of ductility imparted to it.

[ACI 350 permits the flat slab design based on direct coefficient method. However minimum reinforcement requirement is also quite high.]

R 4.3.2.3 The floor member of a tank, which is part of lateral action resisting frame has to be conforming to IS 13920. For other LRC members, the ductility requirement is small, and need not conform to IS 13920. For flat slab LRC floor, the ductility demand may be higher than that considered in IS 13920.

R 4.4 LIMIT STATE DESIGN (LSD)

Structural safety i.e. ultimate resistance to actions (force) bearing, serviceability (deflection & crackwidth) are required. In modern design considerations are also required to other limits like water-tightness, durability, robustness, sustainability, constructability, maintainability, restorability, etc.

R 4.4.1.1 Limit State of Collapse (Ultimate limit state) : Load combinations are given in Table 1.

R 4.4.1.2 Limit State of Serviceability :

R 4.4.1.2 a) Deflection check is required as per 23.2a of IS 456, and b is not applicable. For deflection check only 70% of FL can be treated as long term load (accounting creep coefficient) and remaining 30% as short term (no creep). For tanks which may remain filled up for a long time (say the provisional storage for firefighting or units of treatment plants remaining full most of time) 100% FL should be treated as long term. Earth load shall also be a long term load.

For deflection of long term loads, the short-term modulus of elasticity (Ec) divided by 1+θ will be the effective modulus. Creep coefficient θ be taken 2.2 for DL & FL, and zero for short term loads. (As per IS 456).

R 4.4.1.2 b) At the concrete surface, estimated crackwidth (by procedure specified) due to the restraining effects on temperature and shrinkage (length change) in young concrete should not exceed 0.2 mm. Also at the concrete surface crack-width is estimated (by procedure specified) for the serviceability loads (1 DL + 1 FL + 1 IL only). Here the effects of temperature & shrinkage is not taken additive to the loads. The temperature-shrinkage effect or load effect should be taken independently for crackwidth check, and will not be combined.

It should be noted that in IS 3370, while ‘severe’ exposure condition shall be considered, the limiting crackwidth is 0.2 mm, and not 0.1 mm as specified in IS 456 clause 35.3.2, and is over-ruled by the provision of IS 3370. For ‘very severe’ and ‘extreme’ exposure, a lower limiting crackwidth say 0.15 mm may be taken relevant.

However author recommends that, for the bottom face of roof of tanks storing chlorinated water, or for very severe or extreme exposure, or members in contact with sewage (in STP) crackwidth limit 0.10 mm may be adopted. For more guidance on reducing the limiting crackwidth, refer the requirement given in R 4.4.3.2, R 4.4.3.3, and R 4.4.3.4.

The international trend is to assume that there is no significant relationship between crackwidth below 0.2 mm and the durability of concrete member. However, in more severe environment, for better control on permeability, lower crackwidth can be preferred for higher reliability. Method of estimating crackwidth is an approximate one hence for reliability, in more important cases lower crackwidth limit is to be adopted.

Longitudinal cracks are more prone to durability problems, and shall be avoided. See R 9 last but one para.

R 4.4.1.2 c) In addition to crackwidth check, the general recommendation (internationally) is also to limit the tensile stress in steel and compressive stress in concrete for members requiring crackwidth ≤0.2 mm. Higher compressive stress can produce cracks transverse to direction of compression. For these limiting values refer the clauses and discussion at RB-1 for section in flexure, and RB-4 for member in direct tension. Also see R 6.4.2 and R 8.3.1

R 4.4.1.3 Partial Safety Factors : Recommended partial material factors (γm) are as per IS 456, 1.5 for concrete and 1.15 for steel. Designer may take higher γm for LRC for small works where quality systems are not complete and workman ship may not be at good level.

Increase in these factors by 15 to 20% for concrete and 9 to 15% for steel, will enhance the reliability without any significant increase in cost, as crackwidth check governs the design. Partial material factors (γm) can be 1.65 to 1.80 for concrete and 1.2 to 1.33 for steel.

High compression in concrete can also induce tensile strain in direction perpendicular to compressions (Poisson’s ratio effect). Due to micro-cracks and the effect of temperature and shrinkage superimposed, secondary cracking may result. As per EN code, excessive compressive stress (>0.45fck) in the concrete under the service load may lead to higher than predicted level of creep, and promote the formation of longitudinal cracks, and also result in micro-cracking in concrete. To control such secondary cracking, the design compressive stress in concrete could be reduced by about 15%. This amounts to the reduction of the utilised strength of concrete by 15% for purpose of design. Such reduction is not specified in the standard. If applied, it normally will have very little effect on sizes of members of LRC.

R 4.4.1.4 Load Combinations :

Load combinations with partial load factors are given in Table 1 (reproduced below).

Table 1 Load Combination and Load Factors

|Case |Ultimate Limit State |Limit State of Serviceability |

| |DL |

|Limiting crackwidth |Plain round bars |Deformed bars |

|0.10 mm |85 N/mm² |100 N/mm² |

|0.15 mm |95 N/mm² |110 N/mm² |

|0.20 mm |115 N/mm² |130 N/mm² |

Tensile stress higher than that in Table 2 (deemed to limits) is acceptable, if detailed check for crackwidth is carried out and found to be within limit.

Similarly Table 3 gives limiting stress in deformed bars and the spacing for ‘deemed to’ criteria for 0.2 mm crackwidth. If the criterion is fulfilled, no further detailed check for crackwidth is required.

If a member in combined bending and compression, has compression on the two extreme fibres (i.e. neutral axis is outside the section or eccentricity of compression is small), it can be said to be case of compression predominant. In such a case no tension develops, hence no cracking and crackwidth check is required.

For a member in combined bending and axial force (tension or compression), if tension develops on one face and compression on another (i.e. neutral axis is inside the section) it can be said to be case of bending predominant. In such a case crackwidth check is to be applied as a flexural member.

For a member in axial tension with or without combined bending, such that depth of neutral axis is less than 50 mm or it is outside the section (i.e. tension on both faces), the section will be termed as predominantly in direct tension, and calculations will be done as a member in direct tension.

Section size and reinforcement on each face as arrived at, is analysed for stresses under service load, which is same as working stress method. For this modular ratio shall be as per IS 456, annex B. The crackwidth formulae are adopted from British code, wherein higher value of modular ratio (m) is specified. Hence (recommended by author though not in code), that in place of ‘m’, a modified value of ‘1.5×m’ may be taken in to account. Thus the calculation involves grade of concrete, however its effect on crackwidth is very small. For cracking long term modular ratio should be considered which will be much higher, and can be enhanced by 100%, in place of increasing by 50%.

Section analysis for depth of neutral axis requires solution of cubic equation, which can be done by a computer programme. For a depth of neutral axis, the maximum compressive stress in extreme fibre and tensile stress on tension steel will be calculated, and further calculate crackwidth.

In serviceability state, it is prudent to limit both the compressive stress in concrete to 0.36 fck and tension in steel to 0.55 fy (228 N/mm² for 415 grade) for long term crack control at important locations. In British practice (refer Design of Liquid Retaining Concrete structures by R.D. Anchor) the stress in steel is also limited to a lower amount. Also recommends that in serviceability state, for in direct tension the steel stress should be limited to 0.50 fy (207 N/mm² for 415 grade steel). In highly critical locations, steel stress should preferably be reduced further. Also see R 6.42 & R 8.3.1.

From stress, strain in steel can be calculated, and reducing it for concrete stiffening, crackwidth can be calculated using equations given. The estimated tension stiffening is the maximum capacity possible, however this maximum value may not be mobilised in all the cases. The tension stiffening of concrete can further reduce at the section of construction joint, or at the section of curtailment of bars.

At a construction (or partial contraction) joint, the tension stiffening reduces (may be by 1/4th of its value), hence crackwidth will be higher. At any section, the correction to strain in the bar due to tension stiffening (duly reduced at construction joint or due to curtailment in tensile steel) cannot be more than 2/3rd of the strain in steel.

It should be noted that as per the procedure estimated crackwidth value is almost unaffected by the grade of concrete, its modulus of elasticity and its tensile strength. While the grade of concrete increases, the modular ratio (m) decreases, which has a very small effect on the calculated crackwidth. Hence the equations will need corrections when high strength concrete (say grade >M35) or concrete with higher flexural strength (>4 MPa) is used. Also the procedure cannot be applied to polymer concrete, fibre concrete or ferrocement.

Tension stiffening can be related to flexural strength of the concrete. Up to M30 concrete, flexural strength can be taken as fcr = 0.7√fck . For grades above M30 the value of fcr may be taken as 4 N/mm² or a characteristic value determined by tests. For fibre or polymer concrete the flexural strength shall be determined by test.

If average residual strength (fem,150) is ≥ 1.0 N/mm², the tension stiffening can be assumed to be enhanced by 20% (factor 0.25 becoming 0.30), for calculation of crackwidth. For FRC having average residual strength (fem,150) ≥ 1.50 N/mm², the tension stiffening can be assumed to be enhanced by 60% (factor 0.25 becoming 0.40), for calculation of crackwidth.

Equations (8, 9, 11 & 12 in Annex B) for tension stiffening (ε2) could be multiplied by 0.25fcr (in numerator), thus making the equation non-dimensional. For fibre concrete having average residual strength ≥1.50 N/mm², equations be multiplied by 0.4fcr ; and if ≥ 1.0 N/mm², multiplied by 0.3fcr .

Actual benefit of fibres are much higher, which can be accounted if detailed procedure as per accepted methodology can be applied. At construction joints, the contributions of fibres shall be neglected for strength, and for crackwidth the tension stiffening shall reduce. Refer R 4.4.3.5.

It should also be noted that in some cases, if steel on compression side of section is accounted the estimated crackwidth will be slightly higher, but it may be permissible to neglect steel in compression in such case, if the section (neglecting compression steel) remains under-reinforced.

R 4.4.3.2 A liquid retaining member may be classified in to water tightness class, as susceptible to the possible leakage related to crackwidth recommended. Note that concrete always permits the passage of small quantities of aqueous liquids by permeation and diffusion.

In long run, water tightness is reduced due to washout of particles by flowing water, leaching of calcium compound, and degradation of hydrate by ion exchange. Autogenous healing takes place only in initial life (say about a year) of LRC if crackwidth are very small, beyond that period the autogenous healing may be insignificant. At an age of 15 to 30 years, the seepages through cracks may enhance a bit, for which initially crackwidth may be controlled conservatively.

Classification of Water-Tightness - Giving limiting crackwidth in mm

|Tightness |Requirement for leakage |Cracks through, no |Compression block |Compression block |

|Cass |{ H/t is between 20 to 30 } |compression block |< 0.2 t or 50mm # |≥ 0.2 t or 50mm # |

| |[ # whichever is more ] |(direct tension) | | |

|1 |Leakage to be limited to a small amount. Some surface |0.15 mm |0.20 mm |0.20 mm |

| |staining or damp patches acceptable | | | |

|2 |Leakage to be minimal, Appearance not to be impaired by |0.10 mm |0.15 mm |0.20 mm |

| |staining. | | | |

|3 |No leakage permitted. |Crackwidth nil with prestressed. OR |

| |Special measures such as Prestressing or impermeable liner |Leakage prevented by lining; and |

| |required. |limiting crackwidth as for class 1, in concrete. |

Notes : 1. No compression block means member section with neutral axis outside section

(or eccentricity of tension small) or member in direct tension or hoop.

2. The tightness classes of the wall and floor of a tank can be different.

R 4.4.3.3 Recommendation about crackwidth related to water tightness class is given in table above. Most LRC can be assumed to be of class 1. Where aesthetics is important or the passage of pollution through concrete is important, the Tightness class 2 can be applied. Tightness class 3 is where no permeation of liquid through concrete or wetness is permitted. Where appearance of wet patches are not acceptable in addition lining is necessary.

R 4.4.3.4 The limiting crackwidth values as recommended in table above (related to water tightness class) may exceed by 0.05 mm, if H/t is ≤ 20. In no case crackwidth shall exceed 0.2 mm. If H/t is > 30, the limiting crackwidth shall reduce by 0.05 mm.

R 4.4.3.5 The crackwidth at construction joint can be calculated by reducing the tension stiffening by concrete by 25%. However, crackwidth shall be assumed to increase not more than 0.05 mm, nor more than 50% of the crackwidth without reducing tension stiffening i.e. the crackwidth estimate for monolithic section.

R 4.4.3.6 For crackwidth enhancement due to shear, at that section the flexural moment in slab shall be enhanced by equivalent moment due to shear force (Mes) for crackwidth check only. Mes = SF × (D/3),

D is overall depth of slab at the section considered.

If shear reinforcement is provided, enhancement by the equivalent moment need not be considered.

R 4.4.3.7 After accounting different reductions on limiting crackwidth if value is less than 0.05 mm, the requirement for limiting value can be taken as 0.05 mm.

R 4.4.3.8 The estimated crackwidth has a probability of not being exceeded. An occasional wider crack in a structure should not necessarily be regarded as evidence of local damage unless leakage is unacceptable. Reduce leakage by rectification say grouting. Even if crackwidth appear to be within permissible limit, but unacceptable leakage takes place which does not appear to reduce, it should be controlled by grouting.

R 4.4.3.9 Top or outer surface of roof member may be subjected to moderate exposure condition for which crackwidth limit need not be checked (0.3 mm is deemed to be safe), and for higher exposer crackwidth limit will be 0.2 mm.

R 4.5. STRESSES DUE TO MOISTURE OR TEMPERATURE CHANGES

R 4.5.1 The clause is applicable if the tank is to be used only for the storage of water or aqueous liquids at or near ambient temperature and the concrete never dries out; and adequate precautions are taken to avoid drying and hence cracking of the concrete during the construction period and until the tank is put to use.

In cases of calculating stresses due to shrinkage, assume shrinkage coefficient as 300×10-6 for concrete having cement (OPC) ≤ 400 kg/m³ or total cementitious content not more than 450 kg/m³. For higher cement content the shrinkage coefficient will be higher.

R 4.5.2 If tank can remain empty for more than a month, or where impermeable coating or lining is applied, the concrete can dry out and total shrinkage can be much higher, and the requirement of temperature-shrinkage (i.e. minimum) reinforcement would be higher (by 25% to 40%).

R 4.6 Between various members (e.g. between wall & floor, or wall & wall) junctions are intended to be rigid. The junctions (connection) should be designed accordingly and effect of continuity should be analysed and accounted in design and detailing of junction and each member. For LRC members the capacity of a junction to resist force actions (moment, shear etc.) should not be less than the maximum estimated actions within the junction.

R 4.7 TEMPERATURE AND SHRINKAGE EFFECTS (TS)

Experience has shown that minimum of 0.22% (434 mm² for 200 mm thickness) has given satisfactory performance without any movement joint for ground tanks up to 15 m size. For ground tanks of size about 22 m, above minimum steel is found inadequate. Hence for small tanks minimum steel should not be increased. Minimum steel should not be increased for vertical direction, unless tank height is more than 15 m or it is restrained vertically to other source.

Though not clarified in the code, it can be stated that horizontal minimum reinforcement may be higher depending up on the horizontal size of structure (continuous construction), or spacing of movement joints. For elevated tanks which are restrained by structural features or by other structure, minimum reinforcement will be similar to ground tanks. Otherwise elevated tanks normally have very little restrains against linear temperature, moisture & shrinkage movements, hence requirement of minimum reinforcement does not increase substantially with the size of structure. Alternately minimum steel shall be calculated as per A-1.2 in code. For a tank if roof is free to slide and is not rigid with the walls, % minimum steel in roof can be reduced to that in one lower range of size.

Continuous construction without movement joints (as per option 1) can be done for ground tanks normally up to 30 m size. Above 30 m size provision of expansion joint is a normal solution, though tanks of more length (say 50 m) can be designed without movement joints.

Designer has an option to calculate the minimum steel as per the provisions of the code, depending upon the spacing of movement joints. Where concrete grade is higher than M30, designer must calculate the minimum steel required as critical steel ratio.

To economize on the minimum steel for ground tanks, top of foundation PCC should be a flat and have smooth surface with bond breaking sheet (as per R 9.2.8.b IS 3370 part 1) to facilitate sliding and thus reducing the restrain. The thickness of such bond breaking sheet will depend upon the roughness of the top of PCC base. For PCC having flat in-plane and fairly smooth surface, about 1mm thick LDPE (polyethylene) sheet is recommended by British practice. Also see R 13.1.2.

Options (Table 2 part 1) may be used to design movement joints at closer interval and also design the temperature shrinkage steel. For the continuous construction (Option 1), PCC top may have slight slope and gradual thickening, and also bond braking layer is not required. Where tanks are small, the horizontal tank size can be treated as spacing of movement joint and minimum steel can be smaller (as per option in Table 2 in part 1).

Temperature-shrinkage effects are in two groups, one the linear effect (in the plane of member), and second the gradient across the thickness of section of the member. All these effects are relaxed by cracking. The effect can also be subdivided as long term and short term. Long term effect will also have further relaxation by creep of concrete.

R 4.7.1 Shrinkage Coefficient : In reinforced concrete (different from prestressed), the effect of temperature & shrinkage get relaxed (reduced) due to creep and cracking of concrete. While accounting temperature fall (from peak due to heat of hydration at about 1 to 3 days), shrinkage can be assumed to be negligible in the immature concrete. In the calculation method given in annex B, a reduction of strain (100×10–6) is suggested on account of creep.

Shrinkage has two components, one the irreversible (physio-chemical), other the reversible (i.e. moisture dependent), total shrinkage if not known can be assumed to be 300×10–6, in which relaxation due to cracking can be assumed to be included. For M30 grade concrete typical total free shrinkage is much higher compared to 300×10–6 (may be 800×100-6). It is reduced due to absence of moisture dependant component and relaxation due to creep and cracking.

While a component is in contact with aqueous liquid, only chemical shrinkage i.e. irreversible part (33 to 40 % of total) will take place, and this gets almost compensated by creep. Hence in combination with liquid load, shrinkage may be neglected.

It should be noted that if cementitious content increases (> 450 kg/m³) the shrinkage will be higher and higher minimum steel will be needed.

R 4.7.2 In tanks protected by internal impermeable lining, the design strain will be higher due to drying of concrete. Hence design has to consider higher strain by about 150×10–6 (say total 450×10–6), and permit higher crackwidth if crack bridging property of the lining can be assured.

R 4.8 Sustainability should be given consideration in the design and construction. Aim should be to minimise the use of OPC, and maximise use of flyash and GGBS, and optimise the material use. Consideration should be given to long life and minimise the maintenance requirements, and construction quality should avoid repairs during design life.

5 FLOOR

R 5.1 Provision of movement joint is linked to the basis of minimum steel. This is explained in Part 1.

R 5.2 Floor can be assumed to rest on ground if proper foundation conditions are met with. If subjected to uplift, it should be designed for bending due to net upward pressure,

R 5.3 Floors not supported on grade are also called suspended slabs, as are required for elevated tanks. For floor slabs up to 200 mm thick, in the region of +ve bending moment, reinforcement can be provided at the bottom face of the slab only (no steel at top face); and in such a case the bottom reinforcement shall conform to the minimum steel requirement for the total slab thickness. For slab thickness more than 200 mm, at least one third of minimum steel should be placed at top face, and total steel of top and bottom face together should conform to minimum steel requirement. In the region of -ve bending moment, the top reinforcement should not be less than two third of the minimum for gross thickness.

R 5.4 For suspended floor slab supported on beam or wall, the critical section for design of slab near support shall be 45 mm inside from the face of support.

R 5.5 In most cases for floor beams of tank, the depth to span ratio is >0.40, which requires reduction of lever-arm for calculations of tensile steel.

R 6 WALLS

For the monolithic construction of RCC floor and wall, sympathetic vertical cracks can develop at the locations of movement joints in the floor if provided. Hence walls should also be provided with movement joints similar to floor.

Where a pipe passes through a wall or floor, the pipe should be embedded while member is cast. Alternately a box-out may be kept with additional precautions. In all cases pipe should not be very near to a joint being provided in concrete or a joint be provided on all side and a little way from pipe (say about 2 to 4 ×thickness of member). The joint between member concrete and box-out should be treated as a construction joint and should be grouted adequately. At the position of pipe the thickness of concrete should be increased and extra reinforcement provided to take care of stress concentration and extra stresses due to force/ restrain due to movement and pressure on pipe. As well to give more hydraulic creep length for water seepage.

R 6.4.1 For cylindrical wall of elevated tank if assumed to have no radial displacement in simplified analysis, the design hoop tension in bottom portion of wall can be increased by an amount equal to one third (0.33%) of the amount it is short from membrane hoop tension.

R 6.4.2 Wall junction with floor of tanks shall be designed and detailed for rigidity and crackwidth. Note that crackwidth limit is governed by class of water tightness (R 4.4.3.2), size of compression block, adjustment due to H/t (R 4.4.3.4), and with crackwidth check moment enhancement due to shear (R 4.4.3.6). Tension in steel should be limited to < 0.50 fy (207 MPa in 415 grade) in limit state of serviceability. Also refer R 8.3.1.

The junction shall also be checked for direct shear on the interface of construction joint (for reduced shear capacity)

R 7 Roofs

Roofs should be designed with sufficient slope or camber to ensure adequate drainage accounting for any long-term deflection of the roof due to the dead loads, or the loads should be increased to account for all likely accumulations of water due to long term deflection by accumulated water itself. If deflection of roof members may result in ponding of water accompanied by increased deflection (in long term) and additional ponding, the design must ensure that this process is self-limiting.

For roof slabs up to 200 mm thick, in the region of +ve bending moment, reinforcement can be provided at the bottom face of the slab only (no steel at top face); and in such a case the bottom reinforcement shall conform to the minimum steel requirement for the total slab thickness. For slab thickness more than 200 mm, at least one third of minimum steel should be placed at top face, and total steel of top and bottom face together should conform to minimum steel requirement. In the region of -ve bending moment, the top reinforcement should not be less than half the minimum for gross thickness.

For large (>20 m) roof slab having multiple panels, exposed directly to external environment (i.e. without and soil cover or insulating cover), At the support where slab is continuous, the top reinforcement near the support (minimum for 0.1× clear span) shall not be less than 0.2% of gross section of the slab; unless slab is specifically designed for temperature and shrinkage requirement (duly relaxed by creep & cracking) superimposed with DL.

For roof slab supported on beam or wall, the critical section for design of tension reinforcement of slab near support shall be 45 mm inside from the face of support.

R 8 DETAILING

The dimensions of structural components in local areas of the structure, and specifying the structural details and reinforcement are the parts of structural detailing. Detailing is to be done on drawings.

R 8.1 Minimum Reinforcement

Minimum reinforcement is also called as temperature-shrinkage reinforcement which can take care of normal cracking due to same and avoid repeated calculations for normal effects of temperature and shrinkage. Minimum reinforcement is based on gross concrete area, and where steel is on both faces it shall be based on thickness of surface zone for each of the faces.

With the revision of codes this minimum reinforcement is being increased. In general the requirement of temperature-shrinkage steel increases with the size of structure and also with the degree of restrain. For large size tanks there are always some cases where need of steel may be more than the minimum specified.

However, the minimum steel recommended in standard is higher than the need for small works. In India there are very many small water tanks for rural water supply scheme. For the small tanks which have behaved satisfactorily for 6 decades, cannot be overburdened with higher minimum steel, due to higher need felt for larger work or bad experience of poor construction.

[ As per Table 2 in IS 3370 part 1, where joint are at smaller spacing (option 3) recommended steel is 2/3rd.

Thus in small tanks ≤ 7 m minimum reinforcement recommended can be reduced. ]

It should also be noted that, container of elevated tank has much smaller linear restrain and will need smaller temperature-shrinkage steel. Floor slab of elevated tank need more steel for BM due to load, compared to a floor slab of ground tank. That cannot be an argument for higher minimum steel for elevated tanks.

Hence the author recommends the minimum steel as per table below. {ACI requirement is >1.3× than below.}

|Type of reinforcement |Elevated Tank |Ground supported Tank |

| |≤10 m |14m |22 m |

| Top zone in mm |D/2 |D/2 |250 only |

| Bottom zone in mm |Nil |100 only |100 only |

Minimum % steel specified will apply to all members. For the floor slab on grade (i.e. continuously supported by ground), for which it will be in % of the surface zones thickness specified.

At bottom portion (0.6m or one eighth of height) of wall, the minimum % steel will be similar to that for slab below wall. For remaining height of wall, the minimum steel will be as per the vertical height of tank.

R 8.2.1 Size (i.e. diameter) of bar and spacing of bars (in slabs and walls) could be small as practically possible without causing congestion of steel or difficulty in placing and vibrating concrete. In walls and slab minimum preferable c/c spacing can be nearly equal to 5× diameter of bar or 75 mm whichever is higher; reducing spacing below such a limit has no significant advantage. Maximum permissible spacing is 300c/c. This limit on spacing is also applicable for minimum steel or distribution steel. For beams the minimum clear distance between bars can be smaller. However in beams one of the clear space between horizontal bars should not be less than 75 mm for pouring the concrete and for insertion of needle vibrator.

R 8.2.3 Condition of ‘spacing of bars not more than thickness of member’ should not be applicable to lightly loaded members provided with thickness in excess of the requirement. This spacing limit can be treated as a requirement for highly stressed members requiring much higher area of steel, and do not apply to roof slabs and members lightly loaded (service stress in steel 10φ, or reinforcement required is >500 mm²/m on a face of a member. It is preferable to keep spacing small and taking 8φ bars. Where reinforcement on a face of member required is >500 mm²/m, choose the bar size less than thickness of slab or wall.

Spacing of bar more than thickness can be permitted, where calculated crackwidth is 20φ K = 5, >28φ K = 6;

grade up to 500, bar up to 20φ K = 5 , >20φ K = 7, >28φ K = 8;

grade up to 550, bar up to 20φ K = 6 , >20φ K = 8, >28φ K = 10;

Where possible the radius of curvature should be bit more than the minimum recommended.

Bearing stress need not be checked in a hook or a bend near the end of a bar, as stress in bar will be small at end.

However at hook or bend in stirrup which has to take shear force, stress in bar at bend will be high, hence a cross bar (same or higher diameter) is necessary to give effective anchorage and to develop stress in stirrup.

High strength deformed bar may have slightly reduced strength at the bend or re-bend (i.e. bending and straightening or bending and reverse bending) position in bars and ductility may also reduce, more specifically under dynamic (/repeated reversal) loading. Bars should be checked before use under a re-bent test.

R 8.2.8 Hooks :

In the hooks, curvature inside bend can be as sharp as permitted. Diameter of mandrill (i.e. diameter clear inside of bend) permitted is- the bar diameter multiplied by ‘b’ as given below.

|Steel Type → |MS |Medium TS |High strength deformed bars | |

|Bar size↓ |250 |300 |415 |500 |550 |

|Standard hook |4 |6 |8 |8 |Values |

| | | | | |of |

| | | | | |‘a’ |

|Hook in rings |6 (4) |8 |12 |10 | |

|Rings for seismic detailing |8 (6) |10 (8) |16 (NP) |12 | |

Value in bracket are as per IS. NP = not permitted.

R 8.2.9 Congestion of Steel :

Reinforcement may be regarded as congested, if it poses difficulty in placing and compacting concrete. While designing the details of reinforcement, it is to be assured that there is space with margin for inserting, placing in position and tying the bar in final position with ease and simplicity; or else the sequence of tying of reinforcement should evolved and specified. During designing, as far as possible congested steel should be avoided by providing increased concrete section or less number of bars of higher size. Normally the clear gap between horizontal bars should be ≥ 2× maximum size of aggregate.

If bars are bundled, the permissible bond stress should be reduced by 33 to 50%. Bars if congested, need not be provided at uniformly spacing. Bars may be arranged having minimum spacing or grouped, leaving few wider spaces. One gap should be ≥75 mm wide nearly vertical, for pouring of concrete without getting screened, and also for insertion of needle vibrator. Only in exceptional cases, it can be 60 mm assuring vibrator needle of smaller size to be used.

While estimating the space between bars, allowance should be made for permissible tolerance (for bar placing and cover), and for laps required in some bars.

R 8.2.10 Development Length, Laps and Anchorage :

To develop stress in the bar at a section, minimum anchor length or an end anchorage or a combination of the two is required on each side of the section. Length of bar required to develop the desired stress level without end anchorage is the ‘Development Length’. The basic development length (Ld ) is given by: Ld = φ (s / (4 (bd) , where φ = diameter of bar, (s = stress in bar, and (bd = average bond strength between the bar and the concrete.

Bars in compression have higher bond.

Development length should be increased by about 15% for two bars in contact, 33% for three bars in contact, and 50% for four bars in a group.

Depending upon the value of end anchorage provided, the development length may be reduced up to 0.5 Ld.

Following aspects affect the lap or development length.

a) Type, grade and quality of concrete affects the bond strength.

b) Type and deformations on bar affect the ultimate bond strength.

c) Surface finish and surface condition of bars.

d) Stress gradient i.e. variation of stress (tension or compression) along the length of bar favourably reduces the development length. For members in pure tension the double lap length or anchor length is recommended; and in zone of constant tension due to bending increase by up to 40%.

e) For reliability of lapping, percentage of steel being lapped simultaneously (i.e. not staggered) affect the lap length. See table below.

|Between adjacent laps clear |From concrete surface clear |% of lapped bars relative to total |

|distance |distance of bar | |

|a |b |20% |25% |33% |50% |

|fctk0.05 |1.99 |2.31 |2.61 |2.89 |3.16 |

Annex C: Concrete Finishes [Not a part of the standard]

C.1 Formed Surfaces

Surface finish Type F1

The main requirement is that of dense well compacted concrete. No treatment is required except repair of defective areas, filling all form tie holes and cleaning up of loose or adhering debris. For surfaces below grade which will receive waterproofing treatment the concrete shall be free of surface irregularities which would interfere with proper and effective application of waterproofing material specified for use.

Surface Finish Type F2

The appearance shall be that of a smooth, dense, well- compacted concrete, may show the slight marks of well fitted shuttering joints. The Contractor shall make good any blemishes.

Surface Finish Type F3

This finish shall give an appearance of smooth, in plane, dense, well-compacted concrete, with shutter marks ordinarily not visible, stain free and with no discoloration, blemishes, arises, air holes etc. Only lined or coated plywood with very tight joints shall be used to achieve this finish. The panel size shall be uniform and as large as practicable. Any minor blemishes that might occur shall be made good by the Contractor.

C.2 Unformed Surfaces

Finishes to unformed surfaces of concrete shall be classified as U1, U2, and U3, ‘spaded or bonded concrete’. Where the class of finish is not specified the concrete shall be finished to Class U1.

Class U1 finish

A spaded finish shall be a surface free from voids and brought to a reasonably uniform appearance by the use of shovels as it is placed in the Works.

It is the first stage for Class U2 and U3 finishes and for a bonded concrete surface. Class U1 finish shall be a screeded and levelled, uniform plain or ridged finish which (unless it is being converted to Class U2, U3, or bonded concrete) shall not be disturbed in any way after the initial set and during the period of curing, surplus concrete being struck off immediately after compaction.

Where a bonded concrete is specified over its surface, the laitance shall be removed from the Class U1 finished surface and the aggregate exposed while the concrete is still green.

Class U2 finish

It shall be a wood float finish. Floating shall be done after the initial set of the concrete has taken place and the surface has hardened sufficiently to allow the floating operation. The concrete shall be worked no more than is necessary to produce a uniform surface free from screed marks.

Class U3 finish

It shall be a hard smooth steel-trowel led finish. Trowelling shall not commence until the moisture film has disappeared and the concrete has hardened sufficiently to prevent excess laitance from being worked into the surface. The surfaces shall be trowelled under firm pressure and left free from trowel marks.

The addition of dry cement, mortar or water shall not be permitted during any of the above operations.

C.3 TOLERANCES IN CONCRETE SURFACES

Concrete surfaces for the various classes of unformed and formed finishes specified in various clauses shall comply with the tolerances shown in Table 1 hereunder, except where different tolerances are expressly required by the Specification or shown on the Drawings.

In Table 1 ‘line and level’ and ‘dimension’ shall mean the lines, levels and cross-sectional dimensions shown on the Drawings.

Surface irregularities shall be classified as ‘abrupt ‘or ‘gradual’. Abrupt irregularities include, but shall not be limited to; off-sets and fins caused by displaced or misplaced formwork, loose knots and other defects in formwork materials, and shall be tested by direct measurement. Gradual irregularities shall be tested by means of a straight template for plane surfaces or its suitable equivalent for curved surfaces, the template being 3 m long for unformed surfaces and 1.5 m long for formed surfaces.

Table 1 Maximum tolerance (mm) in:

|Class of Finish |Line and level |Abrupt irregularity |Gradual irregularity |Dimension |

|U1 |± 12 |± 6 |± 6 |- |

|U2 |± 6 |± 3 |± 3 |- |

|U3 |± 5 |± 2 |± 2 |- |

|F1 |± 12 |± 5 |± 6 |+12, -6 |

|F2 |± 6 |± 3 |± 4 |+12, -6 |

|F3 |± 3 |± 1 |± 2 |+6, -2 |

C.4 RIGIDITY OF FORM WORK : (Rigid formwork are called moulds.)

RF1 : Highly rigid mould (or forms) are of very high rigidity as required for casting concrete samples for testing, say mould for cube or beam. Under the pressure of concrete while it is vibrated in mould the surface deflection should not be more than 0.2 mm. Tolerance for dimensions of mould should be within ± 0.2 mm.

RF2 : Very rigid form as are used in precast factory. Under the pressure of concrete while it is vibrated in mould the surface deflection should not be more than 0.5 mm. Tolerance for dimensions of mould should be within ± 0.5 mm or smaller if in the specification.

RF3 : Rigid form as are used for precast products cast on site but sensitive to tolerance. Under the pressure of concrete while it is vibrated in mould the surface deflection should not be more than 1 mm. Tolerance for dimensions of formwork should be within ± 1 mm or smaller if in the specification.

RF4 : Normal formworks as are used for in-situ works. Under the pressure of concrete while it is vibrated in mould the surface deflection should not be more than 3 mm. Tolerance for dimensions of mould should be within ± 3 mm or smaller if in the specification.

It is proposed that this document will be upgraded from time to time.

Hence send your comments to -

Er. L. K. JAIN, Consulting Engineer,

36 Old Sneh Nagar, Wardha Road,

NAGPUR 440 015, India

Email : lkjain.ngp@

Phone +91 712 228 4037 , M +91 9423101453

Other documents by the same author :

1. Guide on Construction of Concrete Structures for Retaining Aqueous Liquid.

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2. Guide on Design & Construction of RCC Elevated Water Tanks

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