Chapter 12

Forms and Joints The riding quality of concrete pavement depends largely on the quality of support that the forms give to the form-riding equipment. Good form alignment alone does not ensure a good surface because bent or inadequately supported forms will contribute to surface irregularities. Steel paving forms contain concrete in a specific area and provide a track for the form-riding equipment. Joints are constructed in concrete pavements to— •

Allow concrete to contract and expand without breaking.

•

Relieve warping and curling stresses.

•

Separate sections of pavement that are placed at different times.

SECTION I - FORMS 12-1. Steel forms are made in 10-foot lengths and held in position by three wedged pins and end locks. Their weight varies from 204 pounds for 8-inch forms to 292 pounds for 12-inch forms. Figure 12-1 shows a standard steel form.

Locking wedge Receiving key

Locking plate

Anchoring pin

Figure 12-1. Steel Concrete Form

Forms and Joints 12-1

FM 5-436

PROCEDURES 12-2. To hold the forms in position, insert pins in each hole and drive them into the subgrade. The pin length depends on the height of the form and the type of subbase or subgrade. The pins should be at least 18 inches long for 8or 9-inch forms and 30 inches long for 12-inch forms. The pins should be longer for a very plastic, low-density subgrade but can be shorter for a wellcompacted, high-density subbase because it provides good lateral support to the portion of the pin below the form. 12-3. Forms have sliding lock plates at one end to fit under the flanges of adjacent forms and to ensure positive alignment at joints. The plates slide easily into the locking position; this prevents them from being battered out of shape when installed and removed. Failure to lock a joint may cause the formriding equipment to stop and the surface to align irregularly.

CONSTRUCTION 12-4. Standard forms are designed so that they can be used on curves with a radius as short as 150 feet. Shorter radii require some type of flexible form, such as sheet metal or 1-inch wood. Heavy construction of standard forms is not required because form-riding equipment cannot turn on sharp curves that have a radius of less than 150 feet (hand-finishing methods are normally used under these condition). Secure flexible metal or wooden forms into position by driving stakes at frequent intervals to hold them in alignment. 12-5. Form height should be equal to the specified thickness of the concrete, and the base should be at least 80 percent the thickness of the height. When necessary, increase the height of the forms by placing them on wooden risers. Ensure that the thickness of the wooden base does not exceed 25 percent of the original form height and that it brings the forms to the required height. 12-6. Ensure that the surface of the subgrade or subbase is the same height or slightly higher than the elevation of the form base so that the form rests on compacted material. Trim the form base to the proper elevation before placing the form in position.

PLACEMENT AND ALIGNMENT 12-7. After trimming the form base, place the form, set a pin in each hole, and align the form with the string line. Set the string line at the proper height for grading the form base. If grading by hand, set the stakes so that the string is on line with the form face and is at the elevation of the form top. The string serves as a guide for setting forms. If grading by machine, set the string line twice—first on an offset and at a height to meet the requirements of the grader, and second, as for hand grading. Set the stakes at 25- to 50-foot intervals on vertical and horizontal curves. Drive the form pins with a sledgehammer or a mechanical pin driver (compressed air). 12-8. A pin driver (Figure 12-2) consists of a clay spade (digger or 35-pound pavement breaker) with a modified moil-point tool. The sharp end is removed from the moil point, and a collar is welded on so that it rests over the head of the pin. When air is supplied, the clay spade exerts enough force to drive the

12-2 Forms and Joints

FM 5-436

pin down. The collar prevents the tool from vibrating off the head of the pin. The pneumatic pin-driver method saves man-hours and decreases the mushrooming effect on the pinhead. When the forms are at the proper elevation, drive wedges and end plates to hold them firmly in position.

Figure 12-2. Pin Driver 12-9. Ensure that the forms are rigid and the support is firm. The forms should not deflect more than 1/4 inch when tested as a simple beam carrying a load that is equal to the heaviest equipment being used. The forms should rest on a firm base, not on pillars or columns of material. To ensure this, pack the material under the forms by rolling it with a form tamper or tamping bars. 12-10. Minor variations in elevation may exist, and they can be recognized by sighting along the top of the forms. To correct a low spot, raise the form by loosening the locking wedges and end plates, place a bar under the base, and tamp additional material beneath the form. Correct high spots by removing the form and trimming off excess material.

HANDLING 12-11. Spray or brush forms with form oil immediately before each use. This prevents concrete from sticking to the forms and facilitates removal, cleanup, and reuse. Oil also prevents concrete damage when keyways are removed. 12-12. Remove forms as soon as the concrete slab has hardened sufficiently. To avoid chipping the new concrete when removing the forms, use pin pullers with adequate bearing on the forms. To prevent bending the forms when lifting them, loosen them thoroughly from any spilled concrete, soil, or other material. Do not use a form if the top is out of line by more than 1/8 inch in 10 feet. With proper handling, forms retain the straightness that is essential to an even surface finish. 12-13. Clean forms before reusing or storing them. When storing forms, stack them about five high for easier handling. 12-14. Unless a special bridge truck with a 22-foot extendible boom is available, use a 25-ton, low-bed trailer to transport forms from one area to another. When dropping forms off a low-bed trailer, ensure that they do not hit one another and that the ground is not rocky enough to damage them. If the area is level, use a forklift or a small crane to move the forms short distances.

Forms and Joints 12-3

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SECTION II - JOINTS

TYPES OF JOINTS 12-15. Construction, expansion, and contraction joints are used in concrete pavements. Joints that are parallel to the centerline of the pavement are longitudinal, and joints that are at right angles to the centerline are transverse. CONSTRUCTION 12-16. Install construction joints as follows: •

•

Place longitudinal construction joints along the interior edges of each paving lane according to the design criteria. Joints can be doweled or keyed. Place transverse construction joints at the end of a day’s placement or at other points on a paving lane where placement has stopped long enough for the concrete to start setting. When practical, these joints take the place of a planned transverse or expansion joint. When impractical, install a transverse construction joint not less than 10 feet from the closest, regularly spaced transverse joint. Use dowels on transverse construction joints that are at a planned joint location, and use tie bars when transverse construction joints are along the center of a slab.

EXPANSION 12-17. Expansion joints allow the concrete to move without disturbing the adjacent structure or intersecting pavement. They also decrease compressive stress, which may cause sections of the concrete pavement to blow up. 12-18. Use expansion joints (Figure 12-3) in pavement that is