Section 3: Partial Depth RepairAnchor: #i1003188
Pavement Distresses that Require Partial Depth Repair (PDR)
In TxDOT, the pavement distress types requiring partial depth repair (PDR) are limited to shallow spalling problems. For this document, shallow spalling is defined as the spalling with depths less than 4 in. It appears that concrete material properties and construction practices have a significant effect on the occurrence of spalling. Also, horizontal delaminations are frequently observed at spalled areas. Figure 10-8 shows typical spalling in CRCP.
Figure 10-8. Spalling in CRCP.
Spalling causes ride as well as noise problems for the traveling public. Research studies are underway to develop design/construction methods to prevent or minimize spalling. Spalling occurs more often when certain coarse aggregate types are used. Spalling can occur as early as within two years after construction. In some projects, it took more than 10 yr. before spalling occurred. Also, spalling appears to progress with time. Initially, spalling is confined to small areas, and then it extends to larger areas. To repair spalling problems, TxDOT has tried a number of different materials for PDR with varying results. Currently, there is a research study to evaluate the PDR procedures including patching materials. It is expected that the study findings will improve the effectiveness of PDR and this section will be revised as findings become available.Anchor: #i1003216
Partial Depth Repair (PDR) Procedures
The following procedures need to be followed for PDR.
- Identify the repair limits.
- Remove deteriorated concrete.
- Clean the repair surfaces.
- Place the repair material.
- Finish the surface.
Each step is explained in more detail.
1. Identify the repair limits.
When it is determined that PDR is required, it is important to properly evaluate the extent of the spalling and determine the limits of the repair. When spalling occurs, the damage is often extended beyond the visible spalled area. Since most of the spalls that require PDR appear to be caused by shallow delaminations, the extent of delaminations should be identified. One of the most efficient ways of evaluating the extent of delamination is the use of a sounding test. A hammer or steel rebar can be used for sounding testing. A hammer or rebar is dropped near the spalls. If there is no delamination, the sound will be solid. On the other hand, a dull or hollow sound indicates a high probability of delamination. Figure 10-9 shows coring in the spalled area that has been repaired.
Figure 10-9. Coring from spall repaired area.
The repair material applied (left side of the picture) has greenish color. A core was taken several inches away from the limits of the spall repair and several delaminations at various depths were observed. Note the top portion of the core came out disintegrated; this is an example of a PDR where the limits were not properly identified. If a sounding test had been conducted, areas with delamination would be identified and the area could be included in the repair. To assure removal of all delaminated concrete, it is good practice to extend the limits of the repair boundaries several inches beyond the limits determined by the sounding tests. As mentioned earlier, a research project is currently underway to identify the best method to identify delaminated areas. So far, researchers have evaluated ground penetrating radar (GPR) and portable seismic pavement analyzer (PSPA) for their potential to detect delaminations. Once the research is complete and more information becomes available, this section will be updated to provide information on which non-destructive testing (NDT) devices are the best at determining delaminated areas.
2. Remove deteriorated concrete
After the repair limits are determined, the delaminated concrete should be removed. A typical method for removing spalled concrete is chipping. A shallow vertical saw-cut, approximately the depth of the spall, made around the perimeter of the spalled area can be used to prevent a tapering of the repair around the perimeter. Tapering to a shallow depth can contribute to subsequent spalling. Sawing and chipping is sometimes used for large repair areas. As described in the previous section, it is important to remove all the delaminated concrete. Chipping is done with light pneumatic tools.
3. Clean the repair surfaces.
For PDR to succeed, good bonding between the exposed concrete surface and repair material is essential. Without good bonding, the repair material will be separated from the concrete due to environmental and wheel loading. When delaminated concrete is removed, it is important to expose a fresh concrete surface. Fresh concrete surfaces should have rough textures and be cleaned with water to remove any dust, which will improve bonding of the patching material.
4. Place repair materials.
As described above, a number of different repair materials have been used for PDR. There are different opinions as to which repair materials work best. However, the general characteristics of good repair materials should include:
- good bond strength,
- less volume change potential due to temperature and moisture variations, and
- strength and modulus of elasticity comparable to those of the existing concrete.
As mentioned before, a research study is underway to evaluate repair materials for spalling and it is expected that the findings will provide more complete information on the best repair materials.
When applying the repair materials, it is desirable to screed from the center of the patch out to the patch boundaries. This construction process will improve the bond of the repair materials to the concrete.
5. Finish the surface.
If cementitious materials are used for PDR, good curing is essential. Because the areas and depth of PDR are relatively small, this makes the surface to volume ratio of the repair material applied higher than that of normal concrete pavement. This high surface to volume ratio makes the repair material subject to larger volume changes due to drying shrinkage if moisture is not kept by good curing. The loss of moisture due to poor curing also results in evaporation heat loss.
The optimum time for the application of curing compounds is when the bleeding water no longer comes to the surface. However, repair materials for PDR have a low water-cement ratio; there may be no visible bleeding water . In that case, curing operations should begin as soon as possible.