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New pavement joints typically exhibit good load transfer, particularly if the joints are doweled. However, repeated heavy loads can cause an elongation of the dowel sockets and result in dowel looseness and a reduction of load-transfer efficiency. As load-transfer efficiency decreases, many types of distress rapidly increase, including pumping, spalling, faulting, and slab cracking. Restoration of load transfer is used to retard further deterioration of the concrete pavement by reducing the potential for the distress and the mechanisms previously described.
Need for Load-Transfer Restoration
Transverse joints or cracks that would benefit improved load transfer can be identified by measuring the existing load-transfer efficiency with heavy-weight, non-destructive, deflection testing devices such as the Falling Weight Deflectometer. These tests must be conducted during periods of cooler temperatures (less than about 80 degrees Fahrenheit (27 degrees Celsius)) when the slab joints and cracks are not tightly closed. Joints or cracks having a measured load-transfer efficiency (ratio of the deflection on the unloaded side of a joint or crack divided by the deflection of the loaded side) of less than 50 percent should be considered for load-transfer restoration. The deflection measurements should be taken as near as possible to the joint or crack. If a sensor is used, the measurements should be taken in the centre of the load plate and 12 inches (300 millimetres) across the joint. The measurements taken in the centre of the slab should be corrected for normal slab bending.
Falling Weight Deflectometer
Correction of Deficiencies
Before load-transfer devices are installed, it is necessary to determine the cause of the joint or crack distress. Attempts should be made to correct these deficiencies prior to load-transfer restoration. Heavy distressed slabs may require portions of or the entire slab to be replaced. Successful installation of load-transfer devices requires sound concrete adjacent to the joint or crack. If the concrete near the joint or crack is significantly deteriorated, full-depth repair should be placed (with provisions for load transfer) in lieu of load-transfer devices. Additional work that must be performed prior to load-transfer restoration may include:
1. Subsealing (essential if loss of support exists) to fill voids in the pavement structure and to restore to the pavement slabs.
2. Full-depth and spall repairs to replace highly distressed joints and slabs with corner breaks, “D” cracking, etc.
Methods of Load-Transfer Restoration
Two methods of restoring load transfer of existing joint or cracks have been used: dowels and proprietary shear devices. Short-term experience indicates that both methods can be effective in transferring loads across joints and cracks. The important functions of load-transfer devices in concrete pavements are to help maintain alignment of adjoining slabs and to limit or reduce stresses resulting from loads on the pavement.
Dowels. Different sizes of dowels should be specified for different thicknesses of pavements. Dowel size and spacing for construction, contraction, and expansion joints are shown in Table
When extra-strength pipe is used for dowels, the pipe should be filled with a stiff mixture of sand-asphalt or portland cement mortar or the ends of the pipe should be plugged. If the ends of the pipe are plugged, the plug must fit inside the pipe and be flush with the end of the pipe so that there will be no protruding material to bond with the concrete and prevent free movement of the dowel. When using smooth steel dowels, slots for the dowels are cut using diamond-blade saws. Multiple blade saws may be used to speed operations. The slots should be cut so that the dowels are allowed to rest horizontally and perpendicular to the crack or joint and at mid depth of the slab . Light-weight chipping hammers are used to remove the concrete within the slots. The slot is then cleaned by sandblasting or any method that will ensure removal of all sawing residue, dirt, or oil that may prevent bonding of the patch material to the slot faces. Each dowel is placed upon a support chair to allow the patch material to surround the dowel. One end of the dowel is oiled and painted and an expansion cap placed on the end . The dowels must be provided with a filler board or Styrofoam material at mid-length to prevent intrusion of the patch material into the joint or crack and to form the joint in the slot.
Dowel Size Requirements
| Pavement Thickness inches (millimeters) | Minimum Dowel Length inches (millimeters) | Maximum Dowel Spacing inches (millimeters) | Dowel Diameter and Type |
| 8 (200) | 16 (400) | 12 (300) | 3/4-inch (19-millimeter) bar |
| 8 to 11.5 (200 - 290) | 16 (400) | 12 (300) | 1-inch (25-millimeter) bar |
| 12 to 15.5 (300 -395) | 20 (500) | 15 (380) | 1- to 1 1/4-inch (25- to 32-millimeter) bar or 1-inch (25-millimeter) extra-strength pipe |
| 16 to 20.5 (400 - 520) | 20 (500) | 18 (450) | 1- to 1 1/2-inch (25- to 38-millimeter) bar or 1- to 2 1/2-inch (25- to 64-millimeter) extra-strength pipe |
| 21 to 21.5 (530 - 550) | 24 (600) | 18 (450) | 2-inch (50-millimeter) bar or 2-inch (50-millimeter) extra-strength pipe |
| >26 (660) | 30 (762) | 18 (450) | 3-inch (75-millimeter) bar or 3-inch (75-millimeter) extra-strength pipe |
Dowel installation
Forming joint in slot
Proprietary Shear Device
Proprietary shear devices such as the Double Vee Device and the Plate Stud Connector have been used and are reported to be effective for load transfer across joints and cracks. The placement of proprietary shear devices must be in accordance with the manufacturer’s recommendations. The following are general recommendations for the Double Vee Device: In the coolest weather possible, core a 6-inch- (152-millimeter-) diameter hole centred over the joint or crack, with the core hole extending entirely through the slab depth. The core sidewalls should be grooved to assist in creating a mechanical interlock. The core hole sidewalls are then roughened by sandblasting and brushed clean to keep dust from interfering with bond of the patch material with the existing slab. The joints or cracks and the bottom of the core hole must be completely sealed to prevent loss of the liquid portion of the polymer concrete. The precompressed Double Vee Device is inserted and properly oriented with the joint or crack at a depth of 1 inch (25 millimetres) below the slab surface. A joint sealant reservoir must be provided at the top of the slab above the shear device.
Core hole centered over crack
Grooving sidewalls
Double Vee Device
Patching Material
Polymer concretes and high early-strength PCC have been used in most installations to date. The patch material used with load-transfer devices is the most critical factor in performance, particularly with shear devices. Sufficient bond must be established between the device and the patching material as well as between the existing concrete and the patching material. For this reason, a thorough laboratory evaluation must be made of any patch material utilized for the load-transfer devices. Prime factors which must be evaluated are: working time, rapid early-strength gain, and shrinkage.
Placing
After the patch area has been properly cleaned, a bonding agent should be applied . The type of bonding agent will depend on the bond development requirements for opening to traffic and type of patching material used. The manufacturer’s recommendations should be followed with all patching materials. Bonding agents should be those recommended by the manufacturer for the placement conditions. The patch material should be placed and consolidated to eliminate essentially all voids at the interface of the patch and the existing concrete and at the load-transfer device and the patch
Cleaned patch area
