Load transfer is usually good in new pavement joints, especially if they are doweled. Repeated strong loads, on the other hand, might cause the dowel sockets to elongate, resulting in dowel looseness and a loss in load-transfer efficiency. As load-transfer efficiency declines, several types of distress, such as pumping, spalling, faulting, and slab cracking, become more common. Restoration of load transmission is used to prevent future deterioration of the concrete pavement by lowering the risk of distress and the mechanisms outlined earlier.
Need for Load-Transfer Restoration
Measure the existing load-transfer efficiency with heavy-weight, non-destructive deflection testing instruments like the Falling Weight Deflectometer to identify transverse joints or cracks that would benefit from increased load transfer. When the slab joints and cracks are not tightly closed, these tests must be performed at colder temperatures (less than roughly 80 degrees Fahrenheit (27 degrees Celsius)). Load-transfer restoration should be considered for joints or cracks with a measured load-transfer efficiency of less than 50% (the ratio of the deflection on the unloaded side of a joint or crack divided by the deflection on the loaded side). Deflection measurements should be taken as close to the joint or crack as possible. Measurements should be obtained in the centre of the load plate and 12 inches (300 millimetres) across the joint if a sensor is employed. The measurements taken in the slab’s centre should be adjusted to account for normal slab bending.
Correction of Deficiencies
It’s important to figure out what’s causing the joint or fracture pain before installing load-transfer devices. Prior to load-transfer restoration, efforts should be made to remedy these inadequacies. Heavy distressed slabs may necessitate replacement of parts or the complete slab. Sound concrete near to the joint or crack is required for successful installation of load-transfer devices. If the concrete near the joint or crack has deteriorated sufficiently, a full-depth repair (with facilities for load transfer) should be installed instead of load-transfer devices. Prior to load-transfer restoration, additional work may be required, such as:
1. Subsealing (required if there is a loss of support) to fill voids in the pavement structure and restore the slabs.
2. Spall and full-depth repairs to replace highly distressed joints and slabs with corner breaks, “D” cracking, and other issues.
Methods of Load-Transfer Restoration
Dowels and proprietary shear devices have been utilised to restore load transfer of existing joints or cracks. Both strategies appear to be effective in transferring loads across joints and cracks based on short-term experience. The main purposes of load-transfer devices in concrete pavements are to keep contiguous slabs aligned and to restrict or reduce stress caused by loads on the pavement.
Dowels. Different dowel sizes should be given for different pavement thicknesses. Table shows the size and spacing of dowels for construction, contraction, and expansion joints.
When using extra-strength pipe for dowels, the ends of the pipe should be plugged or filled with a stiff mixture of sand-asphalt or portland cement mortar. If the pipe ends are filled, the plug must fit inside the pipe and be flush with the pipe’s end so that no projecting material bonds to the concrete and prevents the dowel from moving freely. When using smooth steel dowels, diamond-blade saws are used to cut slots for the dowels. To speed up the process, multiple blade saws might be employed. The slots should be carved so that the dowels can rest horizontally and perpendicular to the fracture or joint at the slab’s mid-depth. To remove the concrete from the slots, light-weight chipping hammers are utilised. The slot is next cleaned with sandblasting or another procedure that removes all sawing residue, dirt, or oil that could hinder the repair material from attaching to the slot faces. To allow the patch material to encircle the dowel, each dowel is placed on a support chair. An expansion cap is inserted on one end of the dowel after it has been oiled and painted. To prevent the patch material from intruding into the joint or crack and to form the joint in the slot, the dowels must be fitted with a filler board or Styrofoam material at mid-length.
Proprietary Shear Device
Load transfer across joints and cracks has been reported to be effective using proprietary shear devices such as the Double Vee Device and the Plate Stud Connector. The manufacturer’s guidelines for the placement of proprietary shear devices must be followed. The following are some general guidelines for using the Double Vee Device: Core a 6-inch (152-millimeter) diameter hole centred above the joint or crack, extending totally through the slab depth, in the coolest weather possible. To help create a mechanical interlock, the core sidewalls should be grooved.
The sidewalls of the core hole are next sandblasted and scraped clean to prevent dust from interfering with the patch material’s connection to the existing slab. To prevent the liquid element of the polymer concrete from escaping, the seams or cracks, as well as the bottom of the core hole, must be entirely sealed. At a depth of 1 inch (25 mm) below the slab surface, the precompressed Double Vee Device is introduced and properly orientated with the joint or fracture. A joint sealant reservoir must be installed above the shear device at the top of the slab.
Most installations to date have used polymer concretes and high early-strength PCC. The patch material employed in load-transfer devices, especially shear devices, is the most important component in performance. Between the device and the patching material, as well as between the existing concrete and the patching material, a strong bond must be produced. As a result, any patch material used for load-transfer devices must undergo a rigorous laboratory test. Working duration, quick early-strength increase, and shrinking are all important things to consider.
A bonding agent should be applied after the patch area has been thoroughly cleaned. The type of bonding agent utilised will be determined by the bond development criteria for traffic opening and the patching material used. All patching materials should be used according to the manufacturer’s instructions. The manufacturer’s specified bonding agents for the placement conditions should be used. The patch material should be put and solidified to eliminate virtually all voids at the patch-to-existing-concrete interface, as well as at the load-transfer device-to-patch interface.
To reestablish load transfer, mechanical devices (dowel bars) are put in an existing stiff (Portland Cement Concrete) pavement. Cutting six slots (three in each wheel path) across all transverse joints or cracks is the first step in the retrofit. Diamond saws are used to cut the slots, which make six cuts in each wheel path. Lightweight jackhammers are used to remove the concrete between the saw incisions (heavy-weight jackhammers tend to damage the concrete around the cuts). The epoxy-coated dowel bars are then inserted into the slots, which are subsequently filled with a quick-setting concrete (grout) and the joints or fissures with waterproof sealant. Diamond-grinding the repair spot or the entire road surface to remove any bumps or dips is a common finishing step.
Where: The retrofit is appropriate for transverse joints or cracks on PCC pavements in good structural condition (especially JPCP–jointed plain concrete pavements) with poor load transfer (50-60 percent), differential deflection greater than 0.01″, faulting of 0.12 to 0.5″, and fewer than 10% of slabs with multiple cracks. It’s also suitable for situations where dowel bars have failed or have been installed incorrectly.
Why (advantages): Load transfer decreases strains in the slab as well as joint deflections. By decreasing slab deflections, effective load transfer slows or stops the onset of pumping and faulting. It reduces tensile strains within the slab, which reduces cracking and corner breaks. The procedure enhances long-term rideability while also extending the life of the pavement.
Why not (disadvantages): The method is costly and time-consuming, but ignoring the issue will lead to poor rideability and premature pavement deterioration. Rehabilitation may be a better alternative if the pavement and its structure are in poor overall condition. When the presence of alkali-silica reaction (ASR), alkali-carbonate reaction (ACR), D-cracking, or considerable full or partial depth repair is required, the procedure is not acceptable.
When: After a distress assessment and deflection testing that shows the indicative criteria indicated above, the process should be completed. Aggregate interlock is used to distribute load across joints in joined plain concrete pavers. Aggregate interlock is lost as these joints erode due to age and trafficking, and load transfer is reduced.
The phrase “load transfer” refers to the movement (or distribution) of load across discontinuities such as joints or cracks (AASHTO, 1993). Both the loaded and adjacent unloaded slabs deflect when a wheel weight is applied at a joint or crack. The amount of deflection of the unloaded slab is proportional to joint performance. Both the loaded and unloaded slabs deflect equally when a joint is working perfectly. The following equation defines load transfer efficiency.