The purpose of partial-depth repairs is to correct localised areas of concrete pavement distress. Repair of this type restores ride ability, deters further deterioration, reduces foreign object damage potential, and provides proper edges so that joints can be effectively resealed.
In this article we study about need and in-depth step wise procedure of partial-depth repair of pavement.
Partial-depth repair is typically used to repair spalling either at pavement joints or at mid slab locations. Spalling can occur when unsealed joints or cracks are filled with in-compressible materials that prevent movement of the slab in hot weather and result in breakage of the concrete.
The spalling at joints is also caused due to poor construction, poor repairs, dowel bar lockup, improperly located dowels, and dowels in reamed-out sockets.
Fig 1: Spalling at Joints.
Spalling at mid slab is generally caused by reinforcement that is too close to the surface, or foreign matter, or poor surface finish, in the original PCC. Spalls create a rough ride and can accelerate deterioration.
Fig 2: Spalling at Mid span
Procedure of Partial-depth Repair in Pavements
1. Selection of Repair Boundaries
A preliminary survey is done to determine areas of unsound or delaminated concrete should be made to establish the repair boundaries. During the survey, all areas of unsound concrete or delamination should be determined by using a sounding technique. This technique is done by striking the existing concrete surface with a steel rod or carpenters hammer. Delaminated or unsound concrete will produce a dull or hollow thud, while sound concrete will produce a sharp metallic ring.
The repair boundaries should be extended beyond the detected delaminated or spalled area by 3 inches (75mm) to assure removal of all unsound concrete. The repair boundaries should be kept square or rectangular in line with the jointing pattern to avoid irregular shapes. Irregular shapes may cause cracks to develop in the repair material.
Fig 3: Marking of unsound concrete
If repair areas are closer than 24 inches (600 mm) apart, they should be combined. This will help reduce costs and eliminate numerous small patches
2. Removal of Existing Concrete
The boundary of the unsound concrete is fixed in the above step. In this step removal of existing concrete is accomplished by sawing and chipping or by a milling process. Additional saw cuts are made within the repair area to speed chipping. A saw cut 2 inches away from joints might reduce the possibility of damaging the opposite joint face.
A saw cut along the opposite joint face made by skimming the blade along the joint face will remove sealant residue and leave a clean vertical joint face. Concrete within the repair area should be removed to the bottom of the saw cuts or to 1/2 inch into visually sound and clean concrete, whichever is deeper, with light pneumatic tools. After removal of the concrete in the repair area, the pavement should be sounded again to ensure all unsound or delaminated concrete has been removed.
Fig 4: Removal of Unsound concrete
Occasionally, what appears to be spalling at the surface will actually extend through the full-slab depth or for more than one-half the slab thickness. Partial-depth repair should not be tried at such locations.The area should be marked and full-depth repair accomplished.
Under nocircumstances should partial-depth repair material rest upon dowel bars or reinforcement.
3. Cleaning of Concrete Surface
Prior to patching, the exposed faces, bottom of the patch area, and any exposed steel should be sandblasted to remove all loose particles, oil, dirt, dust, asphaltic concrete, rust, and other contaminants. As a minimum, air blow, wash with high-pressure water, and air blow again . The prepared surface must be checked prior to placing the new patch material. Any contamination of the surface will reduce the bond between the new material and the existing concrete.
Fig 5: Cleaning of concrete surface by sand blasting.
4. Joint Preparation
When placing a partial-depth patch adjacent to any joint, there must be no bond of the repair patch to the face of the adjacent concrete. Patches that abut working joints or cracks that penetrate the full-depth of the slab require a compressible insert or other bond-breaking medium to reform the joint or crack. This will form a uniform face against which the joint or crack can be properly sealed and will separate the patch from the adjacent slab.
The new joint should be not less than the same width as the existing joint or crack. When placing a partial-depth patch along a shoulder joint, place a piece of joint material along the slab edge even with the surface to prevent the patch material from penetrating the shoulder interface. Otherwise, the material may restrict longitudinal movement of the slab in response to thermal changes and result in damage to the repair or the shoulder.
5. Patch Materials
High early-strength PCC is used when early opening to traffic is required. When using this material, an epoxy bonding agent should be used. The concrete should not be placed until the epoxy becomes tacky. Normal set PCC can be used when the patch material can be protected from traffic for 24 hours. A light coat of bonding mortar is applied to the patch area. Patches using normal-set concrete should not be placed when the air temperature is below 10 degrees Celsius. At temperatures below 13 degrees Celsius, a longer curing period and/or insulation mats may be required.
6. Placement of Patch Materials
All sandblasting residue should be removed using oil-free airblowing equipment just prior to placing the bonding agent. The bonding agent should be applied with a stiff bristle and scrubbed into the patch area. It should be applied evenly and in a thin coat (approximately 1/16 inch) and should not be allowed to puddle . The volume of material required for a partial-depth repair is usually less, therefore patching material should be mixed on site in a small mobile drum or paddle mixer.
Fig 6: Placement of PCC
The repair area should be slightly overfilled to compensate for consolidation. Primary consolidation should be by tamping. The patch material also may be consolidated by small spud vibrators to eliminate voids at the interface of the patch and the existing concrete.
The finishing area should be finished to the cross section of the existing pavement without leaving excess material on the adjacent pavement surface. After finishing, the patch should be given a burlap drag or broom finish to approximately match the surface finish of the existing adjacent concrete pavement, unless a grinding operation is to follow.
Fig 7: Finishing of top PCC
8. Saw-cut Run outs
Epoxy mortar should be used to fill any saw-cut runouts that extend beyond the patch perimeter at patch corners. The mortar will help to prevent moisture penetration that may undermine the bond.
9. Sealing Patch/Slab Interface
An important procedure in placement of partial-depth repairs is sealing the patch/slab interface. The patch/slab interface is sealed with a one-to-one cement grout. This grout will form a moisture barrier over the interface and impede delamination of the patch. A joint sealant may also be used.
Proper curing of partial-depth repairs is very important due to the large surface of small patches compared to the volume of patch material, as well as the fact that concrete gains bond strength much slower than it gains compressive strength. This relationship is conducive to quick moisture loss and is different from most other concrete applications. Proper curing generally employs the application of curing compound at the time bleed water has evaporated from the surface.
Because curing is critical for partial-depth patches, the first 24 hours should be wet cured with burlap or a similar material. The balance of the 7-day curing period may be with liquid membrane compounds. In hot or dry climates, the patches should be cured for 14 days with a double mat of saturated burlap covered with polyethylene sheeting, over which plywood or lumber is placed.
11. Joint Resealing
Resealing the repair joint is extremely important, because it will help prevent moisture and in-compressible from causing further damage. It is important that the new transverse and longitudinal joints constructed within the patch area be formed or sawed to provide the proper joint seal reservoir, and match surrounding joints. The joint faces must be clean and dry for good sealant performance.