New York State Department of Transportation



Bridge Maintenance Skills Training

Program of Instruction

SECTION : IV. Concrete

Lesson: D4: Concrete Mixing

TRAINING OBJECTIVES:

The employee will demonstrate the knowledge and understanding of the different types of cement and mortar mixes, field mixing concrete, and slump testing.

MATERIALS, RESOURCES, TRAINING AIDS:

FHWA – Bridge Maintenance Training Manual (VII-11 – VII-16)

SB-97-5 – Eye Protection

SB-00-4 – Work Related Clothing

SB-06-01 – Safety Footware Policy

SB-07-01 – High-Visibility Apparel & Hard Hat Policy

LESSON:

Although ready mixed concrete is generally used for large construction jobs, it is not always practical for small jobs. For many such uses the amount may be less than 1 cu yd, which is less than many ready mix concrete producers care to deliver. Therefore, it is necessary to mix small batches of concrete on the site. Ingredients must be measured accurately to ensure uniform quality concrete from batch to batch. For weight proportioning, a bathroom scale can be used. Place the empty container that will be used for weighting the material on the scale and then set the scale at zero. After weighting each ingredient, mark the level of the correct weight of material inside the container. Subsequent batches can be measured by filling the container to these marks. When volume proportioning is used, buckets can be marked in a similar manner to obtain correct volumes of cement, sand and gravel.

Concrete is made of fine and coarse aggregates, cement and water. The quality of concrete is directly related to the quality of the cement paste. The quality of the paste, in turn is directly related to amount of water mixed with the cement. For a given amount of cement, as the amount of mixing water is reduced, the strength of the paste increases, making the concrete stronger and more durable.

The hardening of concrete mix is the result of chemical reactions between the water and cement. That reaction is called “hydration”. Hydration of cement requires a certain amount of water in the mix, which depends on the amount of the cement; as the amount of cement in the mix is increased, the amount on water required for hydration increases. More water is used in mixing concrete than is required for hydration. This is because the mix should be plastic and workable so that it can readily be placed and

consolidated. This additional water should be kept to a minimum, since it will evaporate after the concrete has gained strength and will leave voids in the concrete.

These voids will reduce the strength and watertightness of the concrete.

TRAINING OBJECTIVE 1

The employee will explain concrete composition and desirable properties.

KEY POINTS:

• See Lesson D1 – Concrete Theory

TRAINING OBJECTIVE 2

The employee will explain the various types of cement and mortar mixes, to include:

• Portland cement is the most commonly used cement.

▪ Generally there are five types of cement to meet certain physical and chemical requirements for specific purposes:

← Type I – General purpose

← Type II – resist moderate sulfate attack. Used in structures of considerable mass, such as large piers, abutments.

← Type III – High-Early strength

← Type IV – used in massive concrete structures

← Type V – sulfate-resistant

• Portland cement concrete has the following classes:

▪ Class A – general purpose structural.

▪ Class C – pavement: slipform paving, form paving.

▪ Class D – thin structural applications.

▪ Class E – structural slabs.

▪ Class F – high early strength.

▪ Class G – tremie.

▪ Class H – pumping applications.

▪ Class I – slip form highway median barriers.

▪ Class J – slip forming structural median barriers.

▪ Specialty Concrete – high performance concretes are used in modern bridge decks. These mixes have a much higher than normal cement content and produce a very high strength concrete.

▪ Mortar and concrete typically use the same types of portland cement; the most common is a Type I general-purpose cement. Lime is added to mortar mixes to make them retain water longer, to improve workability or handling, and to make the hardened mortar less brittle and less prone to shrinkage. There are two typical mortar mixes. For interior work and outdoor work that is above grade, use a Type N mix. This mix is composed of 1 part portland cement, 1 part hydrated mason's lime, and 6 parts mason's sand. The sand should be a well-graded masonry sand that has a range of grain sizes from fine to coarse. For below-grade construction and for paving projects, use a Type S mix in a 1:1/2:4-1/2 proportion.

Some manufacturers produce factory-blended masonry cements that are a combination either of portland cement and lime or of portland cement and natural or chemical agents. They are available in Types N and S--these will produce a Type N or S mortar when mixed with sand in a one to three proportion (one part masonry cement to three parts sand).

▪ Type N Mortar

Type N mortar is suitable for general use in exposed masonry above grade. It is recommended for use in parapet walls, chimneys and exterior walls when subject to severe exposure.

▪ Type S Mortar

Type S mortar is recommended for use in reinforced and unreinforced masonry where higher flexural strengths than Type N are required.

▪ Type M Mortar

Type M mortar is recommended for use in masonry in contact with earth such as foundations, retaining walls, paving, sewers and manholes, and in reinforced masonry. 

▪ Type O Mortar

Type O mortar is suitable for interior use in non-loadbearing applications.

TRAINING OBJECTIVE 3

The employee will explain the procedures to use when mixing concrete on the job to include:

• Determining type of mix needed for the repair.

• Selecting proper mixer.

• Measuring the correct proportions of cement, sand and aggregates.

• Dry mixing until a uniform color is obtained and then adding the water.

• Mixing to desired slump.

TRAINING OBJECTIVE 4

The employee will explain slump testing and the procedures used to perform, to include:

▪ Slump testing is a means of measuring the consistency of concrete using a "slump cone." The cone is made of galvanized metal with an 8-inch-diameter base, a 4-inch-diameter top, and a 12-inch height. The base and the top are open and parallel to each other and at right angles to the axis of the cone. A tamping rod 5/8 inch in diameter and 24 inches long is also needed. The tamping rod should be smooth and bullet-pointed. Do not use a piece of reinforcing bar (rebar).

▪ When making a slump test, dampen the cone and place it on a flat, moist, nonabsorbent surface. From the sample of concrete obtained, immediately fill the cone in three layers, each approximately one-third the volume of the cone. In placing each scoop full of concrete in the cone, move the scoop around the edge of the cone as the concrete slides from the scoop. This ensures symmetrical distribution of concrete within the cone. Each layer is then "rodded in" with 25 strokes. The strokes should be distributed uniformly over the cross section of the cone and penetrate into the underlying layer. The bottom layer should be rodded throughout its depth.

▪ If the cone becomes overfilled, use a straightedge to strike off the excess concrete flush with the top. The cone should be immediately removed from the concrete by raising it carefully in a vertical direction. The slump should be measured immediately

after removing the cone. You determine the slump by measuring the difference between the height of the cone and the height of the specimen. The slump should be recorded in terms of inches of settlement of the specimen during the test.

▪ After completing the slump measurement, gently tap the side of the mix with the tamping rod. The behavior of the concrete under this treatment is a valuable indication of the cohesiveness, workability, and placability of the mix. In a well-proportioned mix, tapping only causes it to slump lower. It doesn't crumble apart or segregate by the dropping of larger aggregate particles to a lower level in the mix. If the concrete crumbles apart, it is oversanded. If it segregates, it is undersanded.

TRAINING OBJECTIVE 5

The employee will explain the procedures used to finished and cure concrete, to include:

• After the concrete is placed, struck off with a straightedge and immediately darbied or bull floated to level the surface.

• It should stand until the water sheen on the surface has disappeared, and then it is floated and given the desired finished.

• Curing should begin immediately after the surface is finished and the concrete hardens sufficiently, so it will not be damaged by the method of curing.

• Curing should continue for seven (7) days. Cure by covering the surface with a polyethylene sheet, wet burlap (kept continuously moist), or a membrane forming curing compound.

• In cold weather the fresh concrete should be protected against freezing with some type of insulating material.

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