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Bridge Maintenance Skills Training
Program of Instruction
SECTION : V. Carpentry
Lesson: E5 Form Building, Installation & Removal
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Exhibit V-1 Job Built Form
TRAINING OBJECTIVES:
The employee will learn how to measure and cut lumber, build and remove forms and identify forms for re-use or destruction.
MATERIALS, RESOURCES, TRAINING AIDS:
SB-96-6 Hand and Portable Power Tools
SB-97-5 Eye Protection
SB-00-4 Work Colthing Guidelines
SB-04-04 Electrical Safety
SB-08-4 Safety Footwear Policy
ANSI – A10.9-2004 Safety Requirements for Masonry and Concrete Work
LESSON:
A form is a temporary structure or mold for the support of concrete while it is setting and gaining sufficient strength to be self-supporting. Formwork is the total system of support for freshly placed concrete including the mold or sheathing which contacts the concrete as well as all supporting members, hardware and necessary bracing. One of the most common methods of surface repair of vertical and, in some cases, overhead locations is the placement of formwork and pouring of concrete into the prepared cavity. Rodding or internal vibration is necessary to remove air and provide intimate contact with the existing concrete substrate. Forms are made with necessary shoots to provide access for the concrete into the formed cavity.
Forms are essential to concrete construction. Formwork holds concrete until it sets, produces the desired shapes and, sometimes, surface finishes. It must support its own weight, the freshly placed concrete, plus construction loads such as materials, equipment and workers.
Because building and setting up forms can represent up to one-third of a concrete structure’s repair cost, this phase of a project is very important. To design forms, you must know the strength of the forming materials and the loads they must support. The proper selection of materials and equipment, careful planning of fabrication and erection procedures, and efficient reuse of forms can expedite the job, assure best use of labor and save money.
TRAINING OBJECTIVE 1
The employee will explain and demonstrate how to measure the size of form needed and how to cut the lumber.
KEY POINTS:
▪ TWO GOLDEN RULES FOR CUTTING LUMBER –
➢ RULE ONE - Measure TWICE and cut ONCE!
➢ RULE TWO – See rule one!
▪ When cutting lumber, allow for corner overlaps to insure proper inside dimensions.
▪ Use chamfer strips for corners, edges and drip stops.
TRAINING OBJECTIVE 2
The employee will explain and demonstrate proper form characteristics to include:
▪ The forms must be correctly designed to handle expected loads.
▪ Forms must be tight, rigid, and strong. Loose forms permit loss of cement and/or water and result in honeycomb.
▪ The formwork must stay in place until the concrete is strong enough to carry it’s own weight. Brace forms enough to align them and make them strong. Forms for walls and columns need to be braced and/or tied down to prevent up-lifting of the forms.
▪ The quality of the surface finish of the concrete is directly affected by the forms and form material.
▪ Poor workmanship and lack of attention to detail while installing formwork will lead to form leakage and rough finishes.
▪ Forms should be built to permit easy removal without damage to the concrete.
▪ Before concrete is placed, forms are treated with oil or other coating material to prevent the concrete from sticking. Cover the form surfaces evenly, but do not allow the oil or coating to contact construction joint surfaces or any reinforcing steel in the formwork. Wetting with water may be substituted to prevent sticking but only in an emergency.
TRAINING OBJECTIVE 3
Employee will explain the about the different types of forms and form materials to include:
BASIC TYPES:
• Job Built Forms – Typically built of wood with sheathing nailed to studs. Spreaders are placed between the forms, ties are inserted through predrilled holes in the sheathing and the walers, and the wall forms are aligned and braced.
• Prefabricated Forms – Manufactured forms that are generally purchased or leased. Made of plywood, metal, fiberglass or plastic. There are removable or stay in place forms available. The panels are erected basically the same way as built in place forms.
BASIC MATERIALS:
Forms are generally constructed from plywood, steel, fiberglass, aluminum alloys, earth, particle board, hard board, cardboard, rubber, and plastic.
• Wood – Most common, economical, easy to work with and adaptable to many shapes. Form lumber must be straight, structurally sound and strong. Douglas Fir, western hemlock and kiln-dried pine are generally available and economical for formwork. ¾” plywood is commonly used for wall and floor forms. Exterior type plywood is necessary. Grade B-B or A-C should be used. It is warp resistant, has a smooth surface and can be re-used more often.
• Metal – These are used for added strength. They are initially more expensive, but are more economical than wood forms if reused often enough.
• Fiber / plastic – are ideal for round concrete columns and other applications where preformed shapes are needed.
TRAINING OBJECTIVE 4
The employee will explain and demonstrate the components of a wood form and forming hardware needed.
• Sheathing – the form wall.
• Studs – Reinforce the sheathing.
• Braces – stabilize the forms.
• Spreaders – placed between the panels to maintain proper wall thickness.
• Form anchors – fasten the forms to previously placed concrete.
• Form hangers – fasten the forms to a structural frame of steel.
• Spacers or chairs – hold the rebars the specified distance from the inside surface of the form.
• Duplex nails – nails tht are easily removed without damaging the lumber.
• Form ties – keep the form from spreading under the fluid pressure of the concrete.
• Tie rods – used with form ties.
• Walers – reinforce the studs when they extend upward more than 4 or 5 feet. They should be made of doubled 2 x 4’s and are lapped at the form corners to add rigidity. They also tie form panels together and keep them aligned.
TRAINING OBJECTIVE 5
The employee will explain and demonstrate design considerations of forms to include:
KEY POINTS:
• Forms are the molds that hold concrete and support it until it hardens. They must keep deflections within acceptable limits, meet dimensional tolerances, prevent paste leakage, and produce a final product that meets appearance needs.
• Designing formwork is a job concerned with details, such as joint spacing, chamfer strips at corners or joints, working scaffolding, keyways, screeds, stripping forms and form materials. The design of the formwork is critically important. Concrete weighs about 150 lb. per cu ft. Forms are unevenly loaded while concrete is being placed. The form must be designed to withstand the fluid pressure placed against them.
• Strength, rigidity and watertightness are the most important form design considerations.
• Even with adequately designed forms, forms may fail due to failure to control rate of placement of concrete, insufficient nailing of members, inadequately tightened or secured form ties, or use of old or damaged form materials.
• Forms must support all weights and stress to which they are subject, including the dead load of the forms; the weight of people, equipment and materials that transfers to the forms; and any impact due to vibration.
• Design for ease of erection and removal.
• When concrete is placed in forms, it is in a plastic state and exerts hydrostatic pressure on the form. The pressure depends on the rate of placing and the ambient temperature. The pressure continues until the concrete takes its initial set, usually in about 90 minutes, but longer in lower temperatures.
• The pressures on a one-side wall form or double-side wall form are the same. Pressure is, however, transmitted through the one-side form differently. Blowouts, bulges, and out-of-tolerance walls often occur due to improper bracing or omitting ties.
DESIGN CALCULATION
• The amount of pressure, rate of placing and ambient temperature and form sheathing will determine the maximum spacing for studs, walers and ties. Knowing these factors, you calculate a tentative design. The Bridge Engineer, using details on loads and stresses for the formwork, the construction method, concrete placement rates and temperatures, form materials equipment to be used and other pertinent information, will approve form designs.
• A typical form building scenario might be:
Materials available =
2x4 studs,
double 2x4 walers,
¾-in plywood sheathing,
heavy coil ties (3,000 lb.) and
maximum concrete pressure of 400 lb./sf.
The calculation shown here is for a “typical” repair job and for training purposes. Consult the Bridge Engineer for guidance with other form layouts.
STEP:
1. Determine materials and maximum concrete pressure.
2. Use Table 2 of handout to find maximum stud spacing. Refer to the column headed Maximum Concrete Pressure and find the value you have for the maximum concrete pressure. If the value you have falls between two values in the column, round it up to the nearest given value. Move right to the column identified by the sheathing thickness. This number is the maximum stud spacing in inches.
(400 lb/sf = 16”)
3. Determine the uniform load on a stud by multiplying the maximum concrete pressure by the stud spacing. Convert the answer to pounds per linear foot by dividing the result by 12.
Max concrete pressure (lb/sf) x max stud spacing (in) ÷ 12 (in/ft)
400 lb/sf x 16 in ÷ 12 in/ft = 533 lb/ft (round up to 600)
4. Use Table 3 to determine the maximum waler spacing. Refer to the column headed Uniform Load and find your value. If the value you have falls between two values in the column, round it up to the nearest given value. Move right to the column identified by the size waler you are using. This number is the maximum wale spacing in inches.
Uniform Load = 600,
2x4 walers = 24”
5. Determine the uniform load on a waler by multiplying the maximum concrete pressure by the maximum waler spacing:
400 lb/sf x 24” ÷ 12 in/ft = 800 lb/ft
6. Use Table 4 to determine the tie spacing based upon the Uniform Load of Waler. Refer to the column headed Uniform Load and find the value you have for the Uniform Load of Waler. Again round up if needed. Move right to the column identified by the size of the waler you are using. This number is the maximum tie spacing in inches based on waler size.
800 lb/ft = 30”
This would require forms to be built with the following maximum spacing:
▪ 2x4 studs on 16” spacing,
▪ Walers on 24’’ spacing
▪ Ties at 30” spacing.
• Place the first tie at one-half the maximum tie spacing from the end of the waler.
• Determine the number of studs on one side of a form by dividing the form length by the maximum stud spacing and add one. The first and last studs must be placed at the ends of the form.
• Determine the number of wales for one side of a form by dividing the form height by the maximum waler spacing. Place the first waler one-half of the maximum space up from the bottom and the remainder at the maximum wale spacing.
TRAINING OBJECTIVE 6
The employee will explain and demonstrate when to remove and how to remove, inspect, and store forms to include:
KEY POINTS:
• Forms should not be removed until the concrete has thoroughly hardened. Ties may be removed as early as 24 hours after casting to loosen forms slightly and permit entry of curing water between form and concrete. Curing water should not be much cooler than the concrete. Otherwise it may cause cracking.
• As a general rule forms should not be removed until the concrete has reached 70 percent of it strength or is strong enough to support its weight and the weight of any loads that will be imposed on it. In cool weather it is good practice to leave the forms in place as long as possible.
• Careless workers can cancel out the value of good detailing and planning by indiscriminate use of a wrecking bar. Care should be taken when removing forms from a fresh pour.
• Good care of forms between uses, including careful handing without dropping, not only prolongs their life but also may improve stripping, because damaged forms are harder to strip.
• Remove residual concrete from forms, being careful not to mar face.
• At the end of each job nails should be tightened, the forms repaired, cleaned and oiled.
• Stack forms flat with faces together. Keep out of rain and sun.
Exhibit V – 2 After Form Removal
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