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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.

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.

    Figure 12-1. Steel Concrete Form

 

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 8- or 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 well-compacted, 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 form-riding 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 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.

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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 <10 inches thick, at pavement intersections, and around structures that are in contact with the pavement. If placing concrete when temperatures exceed 95°F, omit expansion joints in slabs that are <10 inches thick. If the concrete contains aggregate with a high coefficient of thermal expansion, also use expansion joints in pavement that is >10 inches.

    Figure 12-3. Expansion Joint

 

12-19. The expansion-joint filler may be a wooden board or a preformed bituminous-treated fiber that conforms to specification requirements and dimensions. Use dowels for all transverse expansion joints to maintain alignment of the adjoining slabs and to provide load transfer between the slabs. Thick edge slabs should be free, undoweled, longitudinal expansion joints. These free joints are usually installed in large paved areas and at ridge lines where a change of grade is required.

Contraction

12-20. Contraction joints are installed to—

  • Control pavement cracking when the concrete contracts.
  • Limit curling or warping stresses created by differences of temperature and moisture between the top and bottom of the slab.

12-21. Longitudinal and transverse contraction joints are weakened-plane joints that are formed by cutting grooves in the top portion of the pavement to ensure contraction cracking at the joint. Form contraction joints in plastic concrete during pavement construction, or saw them in the pavement after it has hardened.

12-22. Construct the weakened-plane joints (dummy joints) according to the design criteria. For pavement ³10 inches, the spacing of the transverse contraction joints should be equal to the width of the paving lanes, usually 20 to 25 feet. However, concrete made with certain types of aggregate, such as slag, may shrink more and require closer joint spacing. The depth of contraction joints is usually equal to one-fourth of the pavement thickness.

12-23. The cost of sawing joints is directly related to the depth of the cut, and the least depth possible should be determined by a trial in the field. If a concrete saw is unavailable, construct contraction or dummy joints using a steel form that is 1/4 by 4 by 19 3/4 inches. Weld hooks to the top edge of the form (the hooks will be used to pull the form out after the concrete has set). Coat the form with heavy grease and apply bond paper. Use a hammer or a vibrator to insert the steel form in the concrete. Pull out the form after the concrete sets to leave a clean vertical joint.

Dowels

12-24. Dowels are mechanical load-transfer devices built as an integral part of certain transverse joints. Install dowels across a joint so that the joint can open and close and the slab ends on each side of the joint will be held at the same elevation. The deflection of one slab under load is resisted (through the dowel) by the other slab, which is deflected and carries a portion of the load imposed on the first slab.

12-25. Use dowels for transverse expansion joints and transverse construction joints installed at planned joints. Also use dowels in longitudinal construction joints, at intersections, at frequent stopping places, and at outside lanes or slabs of the pavement. The diameter, length, and spacing of dowels vary with the pavement's thickness. All dowels should be straight, smooth, and free from burrs at the ends. Paint and grease half the length of the dowel so that it will bond with the concrete.

12-26. Cap the ends of dowels used in expansion joints; do not cap the ends of dowels used in construction joints. Ensure that the dowels in construction joints or other locations where a filler is not installed are the same size, type, and spacing as transverse expansion joints. Table 12-1 lists approved sizes of dowels.

Installation of Joints

12-27. Joints are necessary in concrete pavement. However, even the best joints are weak under load stresses and deteriorate because of the weather. Most concrete pavement failures start at the joints. Inspect the area before, during, and after placing the concrete to ensure good joints. Construct all joints perpendicular to the finished grade of the pavement. Place transverse joints at right angles to the centerline of the pavement, and ensure that joints continue along a straight line through all lanes.

12-28. Place longitudinal joints parallel to the centerline of the pavement at the required spacing. Install all expansion and construction joints before placing the concrete. Inspect tie bars and dowels in contraction joints early in the placing process. Do not extend steel, reinforcement, or unrequired dowels or tie bars through any joint.

Longitudinal Construction Joints

12-29. Longitudinal construction joints are defined by the direction of the forms. Ensure that dowels are the required size and are accurately installed in the forms at the proper location and required spacing. The dowels should be held securely and accurately in position so that they will not be displaced when the concrete is placed. Forms usually have an outside channel or other holding device for keeping the dowels in position. Check the embedment depth while placing the concrete, and embed the fixed portion of the dowel during the first placing operation. Paint and grease the exposed portions of the dowels before placing the adjoining lane.

12-30. When using key longitudinal joints, key forms should be made of steel. If steel forms are unavailable, use wood forms. Fasten key longitudinal joints securely to pavement forms at required locations.

Transverse Construction Joints

12-31. Install transverse construction joints after the paving operation is complete. Transverse construction joints are generally formed by installing a temporary bulkhead between the forms or at the end of them. Ensure that the bulkhead is securely fastened in place. Install dowels when the transverse construction joint is located at a scheduled joint. When concrete placement stops at the interior of a slab, place tie bars in the joint to form a continuous slab unit when additional concrete is placed. Construct the next regular joint in a planned location.

Expansion Joints

12-32. Expansion joints are the most difficult to set and maintain in the correct position. Check them carefully throughout the paving operation. Ensure that the joint filler and dowels or other load-transfer devices are held securely in place with available devices, such as wires, stakes, or chains. Ensure that the joint filler is in a vertical position and supported so that it will not be disturbed or damaged during concrete operations. Carefully prepare the subgrade and accurately cut the joint material so that it will extend continuously from the required position at the top of the slab and from edge to edge on the pavement.

Concrete Placement

12-33. While placing concrete, protect the top edge of the filler with a metal channel cap that forms a space for the poured sealer when the cap is removed. Hold the dowel bars in place, with metal baskets or other installing devices, parallel to the surface and the inside line of the form on the slab. Check the bars with a template, coat the free end of the dowel with heavy oil or grease, and place a close-fitting expansion cap on the dowel. Immediately correct dowel displacement that is in excess of the allowable tolerance.

Final Positions

12-34. Check the horizontal position of the dowels with a steel tape that has the dowel-bar spacings marked, and then check the vertical position with a dowel-bar checker. The checker is basically a rectangular U-frame with two legs about 10 inches long and an adjustable level, which is mounted on the crossbar. When checking 16-inch dowels, the legs should be spaced 12 inches apart; when checking 20-inch dowels, the legs should be spaced 16 inches apart.

12-35. Adjust the level to compensate for the grade of the pavement by placing the checker on top of the forms at each expansion joint and centering the bubble. Set the legs of the checker on the dowel bar, straddling the joint an equidistant between the legs. The position of the level's bubble should be within the limits for the specified tolerance. If a dowel-bar checker is unavailable, allow for the grade of the pavement and use a level and a rod to check the vertical position of the dowels.

Joint Surfaces

12-36. Carefully finish the surface of the concrete at the joint, and check it with a straightedge to ensure that it provides a smooth riding surface. Use an edging tool to round the top edges of the joint. Remove concrete that has entered the space above the joint filler during the finishing process. When removing the side forms, remove wedges of concrete that are across the ends of the joints. Concrete left spanning the joint prevents free expansion and causes spalling.

Contraction Joints

12-37. Form weakened-plane (dummy) contraction joints by grooving the top portion of the freshly placed concrete with a suitable tool or by sawing the grooves in the pavement after the concrete has hardened. In general, weakened-plane joints are constructed without any steel. The load transfer depends on the interlock of aggregate and mortar in the fractured plane below the joint opening. Install dowels or tie bars, when required, in contraction joints before placing the concrete. Use baskets or other suitable supports to accurately hold the steel in place during paving operations. Check dowel alignment.

Hand-Formed Joints

12-38. Form weakened-plane joints in plastic concrete using a metal bar or another tool that produces a groove of the required dimensions in the top of the slab. Form the joints when the concrete has properly hardened. If the joints are formed too soon, the concrete will sag under the pressure of the forming tool and leave a depressed area in the region of the joint. Also, the concrete will flow together when the bar is removed. If the concrete is too hard, the forming tool will disturb the concrete when it is inserted or removed. Forming a joint too late produces a high joint, and even slight irregularities in the joint area produce a rough riding surface. Carefully inspect joint forming to avoid faulty joints.

Sawed Joints

12-39. The following are advantages of sawed joints:

  • The number of required finishes is reduced because hand finishing is unnecessary at the joints.
  • A smoother, better riding surface is provided by machine finishing.
  • The deterioration at joints is reduced because the concrete is not manipulated after it is placed and partially set.
  • The curing compound can be applied earlier because the joints are sawed afterward. The compound is not removed before the joint is sealed.

12-40. Saw joints before cracking occurs and after the concrete has hardened so that it will not be torn or disturbed. Various factors (concrete consistency, air temperature, humidity, wind) influence how long it takes concrete to harden, which determines when sawing can be started. For example, during hot, dry weather, sawing can probably be started after 6 hours; in cold weather, it may be delayed 24 hours. In the field, examine the concrete or make a trial cut to determine when sawing can be started.

12-41. Saw or hand-form control joints in the concrete at intervals of 80 to 100 feet (intermediate joints are sawed later). With this method, cracking between the joints can occur because of the delay in sawing intermediate joints.

12-42. Use a guide to saw the joint in a straight line. Using a mechanical guide can delay the sawing operation because it takes time to install the guide and move it between joints. Saw carefully if using a chalk line marked on the pavement as a guide. Thoroughly flush the groove with water when finished sawing. Saw the joint to about 1/8 inch in width, and enlarge the top to a width of 3/8 to 5/8 inch. The depth should be 5/8 to 7/8 inch to provide suitable space for sealing the joint.

Sealing Joints

12-43. Seal joint openings with a poured joint sealer to prevent moisture movement between the pavement surface and the subgrade. The sealer also prevents solid material from entering and spalling concrete joints under heavy traffic. Seal joints as soon as possible after the curing period, but do not seal them when the air temperature is below 50°F or during wet weather. Before sealing joints, thoroughly clean them with a power-driven saw blade, a sand blaster, compressed air, or a suitable joint-cleaning machine. Sawed joints usually retain some cuttings from sawing; therefore, clean them carefully. Loosen the material in the sawed joints by running a worn-out saw blade in the joint.

12-44. Use hot-poured or cold-applied sealers for airfield pavements. Heat the material in containers that will maintain a constant temperature by keeping the material thoroughly agitated. Control the material's temperature according to the manufacturer's recommended temperature range. Do not overheat the sealer because it could affect its quality. Do not reheat sealers or any material that is left in the heating pot after a scaling operation; this material is considered waste. Do not hold sealing material at the pouring temperature for a long period because it may affect its quality; melt the sealer and use it as required. Fill the joints flush with the pavement surface, and remove excess material that spills over the pavement. Use equipment for installing cold-applied sealers according to the manufacturer's recommendations. Use a jet-fuel-resistant material on airfields that handle jet aircraft.



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