Section 2: Full-Depth Repair
In this chapter, Portland cement concrete (PCC) pavement distress types that require full depth repair are discussed, followed by the repair procedures.Anchor: #i1002829
Pavement Distress Types that Require Full Depth Repair (FDR)
In concrete pavement contraction design (CPCD), the following distresses require FDR:
- transverse cracks
- shattered slabs and corner breaks.
Transverse cracks occur due to temperature/moisture variations and/or wheel load stress that extend through the depth of a slab (but not plastic shrinkage cracks which occur when the rate of evaporation from the surface exceeds the rate at which the bleed water is available – the depth of plastic shrinkage cracks is limited to about 1 to 2 in. from the surface).
Transverse cracks in CPCD that extend through the depth of a slab are caused by inadequate slab lengths and deficient slab thickness (design issues), concrete with high coefficient of thermal expansion or modulus (materials issues), or non-uniform or insufficient base support (construction issues).
Shattered slabs and corner breaks result from insufficient slab thickness (design issue) and base support (design/construction issue).
In continuously reinforced concrete pavement (CRCP), the following distresses require FDR:
- deep spalling.
Punchouts in CRCP are caused by excessive wheel loading applications and insufficient structural capacity of the CRCP, such as deficient slab thickness (design issue) or subbase support (design/construction issue). It is manifest by block(s) of concrete connected by transverse and longitudinal cracks which are depressed and those block(s) are depressed. Normally, longitudinal steel at the transverse cracks of the punchouts eventually ruptures. Punchouts are the most serious distress type in CRCP. Better design and construction practices by TxDOT over the years significantly reduced the frequency of punchouts. Figure 10-1 shows typical punchouts. Note the asphalt patch was applied to restore the surface elevation, implying that the concrete block was pushed into the subbase.
Figure 10-1. Typical punchouts in CRCP.
Spalling is another distress type in CRCP. Spalling is the breaking, chipping, or fraying of concrete at the cracks. There are several causes for spalling. In Texas, spalling is more prevalent when certain types of coarse aggregates are used. The depth of spalling varies widely, from less than half an inch to as deep as half the slab thickness. Shallow spalling causes functional rather than structural problems in PCC pavement. However, deep spalling cause substantial structural damage to the pavement and requires FDR. Figure 10-2 shows deep spalling.
Unlike punchouts, it’s not easy to distinguish deep versus shallow spalling. The most efficient way to identify deep spalling is by coring or non-destructive testing such as ground penetrating radar (GPR) or portable seismic pavement analyzer (PSPA).
Figure 10-2. Deep spalling.Anchor: #i1002920
Full Depth Repair (FDR) Procedures
Once it has been identified that FDR is required, the procedures below need to be followed:
- Identify the repair limits
- Saw-cut the perimeters
- Remove the concrete slab
- If needed, repair damaged subbase
- Drill holes for longitudinal and transverse tie bars
- Provide longitudinal and transverse steel continuity
- Place concrete.
Each step is explained in more detail.
1. Identify the repair limits.
Item 361 requires the repair areas to be at least 6 ft. long and at least half a full lane width, unless otherwise shown on the plans.
It is important to properly identify the limits of the FDR needed. Repair area needs to include all areas that have developed voids under the concrete pavement. This area typically extends beyond the boundary of the failed areas. In TxDOT, it is a normal practice to determine the limits of the repair by evaluating the extent of the distress by visual observations only. Sometimes, this method does not include all the damaged area and results in pavement failures later on as shown in Figure 10-3. It illustrates that the repair limits should have been extended further to the left side.
Figure 10-3. Full depth repair of punchouts in continuously reinforced concrete pavement (CRCP).
For the FDR of extensive transverse cracking and shattered slabs in concrete pavement contraction design (CPCD), it is a normal practice to remove and replace the whole slab to ease sawing and removal operations. Use good engineering judgment to determine the repair limits for corner breaks.
For the punchouts repair limit determination, evidence such as the extent of pumping or the use of falling weight deflectometer (FWD) testing provides useful evaluation tools. For deep spalling, coring or non-destructive testing is the most efficient evaluation method. It is recommended that the limits of FDR are somewhat extended beyond the limits determined by the evaluations.
2. Saw-cut the perimeters.
Once the FDR limits are established, saw-cut the concrete using diamond-bladed saws through the full depth of the concrete slab. Saw-cutting with diamond-bladed saws will result in a smooth cut surface with little damage to the surrounding concrete. This operation will cut all the existing reinforcing bars along the perimeter. Carbide-tooth wheel saws can cause damage to the surrounding concrete and shouldn’t be used for saw-cutting for FDR.
During the summer, saw-cuts should be done in the morning while the concrete temperature is relatively low. When the temperature is high, the concrete is in compression and the diamond blade saw may bind.
Even though Item 361 allows up to seven days between saw-cutting and concrete placement, the saw-cut concrete blocks should be removed and subsequent repair operations should follow as quickly as possible. When this is not feasible, adjust saw-cut operations so that the subsequent repair operations can immediately follow. Since there is no load transfer in CRCP between saw-cut concrete blocks and the surrounding CRCP, any wheel loading applications will result in higher deflections in both saw-cut boundaries and the concrete block. These large deflections could cause damage not only in the subbase but also in the concrete, which will increase the potential for pavement distress for the full-depth repaired pavement.
3. Remove the concrete slab.
Once the repair limits are saw-cut, the concrete is removed in two ways. One is the lift-out of the slab, and the other is the breakup of the slab. At TxDOT, most of the slab removals are done using lift-out method, as it is faster and less labor intensive than the breakup method. While the breakup method could cause damage to the surrounding concrete, a properly conducted lift-out method will not damage the subbase and surrounding concrete.
To lift the slab, it is necessary to drill holes and insert pins as shown in Figure 10-4.
Figure 10-4. Slab ready for lift.
Once the lift pin arrangements are complete, cranes or front-end loaders lift the slabs vertically. The lifting should be done as vertically as possible with minimum sway, since any deviation from this can damage the surrounding concrete.
4. If needed, repair the subbase.
After the slab removal, the subbase condition must be evaluated to determine whether subbase repair work is needed. If distresses are extensive in the subbase, the subbase should be repaired with an approved subbase material to the original top of the subbase grade. TxDOT districts typically use the same subbase material type in the project for repair.
For punchouts, shatter slabs, or corner breaks, chances are that there will be some evidence of distress in the subbase. If the distress is not extensive, it is usually sufficient to remove the loose materials and compact the remaining subbase. However, if the distress is extensive, the subbase should be repaired with an approved base material to the original top of the subbase grade. Also, if the distress in the subbase extends beyond the perimeter of the FDR, a decision must be made whether further saw-cutting and additional concrete removal is needed. If the distressed subbase beyond the perimeter of the FDR is not repaired, the same type of pavement distress will take place.
For deep spalling, chances are that the subbases will be in good condition and no repairs will be needed except removal of loose materials and compacting the remaining subbase. However, if there is extensive distress in the subbase, the subbase needs to be repaired with an approved base material to the original top of the subbase grade.
5. Drill holes for longitudinal and transverse tie bars.
In concrete pavement contraction design (CPCD), dowels need to be provided in all transverse perimeters. In transverse patch joints, a minimum of four dowels are recommended for each wheel path. Holes are drilled in the existing concrete for dowels.
In continuously reinforced concrete pavement (CRCP), providing continuity of the reinforcing steel in the longitudinal direction is essential to maintaining good performance of CRCP. To provide tie bars in the existing concrete, holes are drilled in the concrete in the longitudinal direction. The length of the drill extended into the remaining concrete is important as the continuity of longitudinal steel is critical to the performance of CRCP. The spacing and depth of the holes are specified in the design standard “Full Depth Repair of Concrete Pavement: FDR(CP)-05.” For transverse tie bars, the bar spacing should be the same as that for tie bars or transverse steel in the adjacent lane. Figure 10-5 shows the drilling operation using a single drill. Sometimes, a multiple drill system, called a gang drill, is used for higher efficiency.
Figure 10-5. Drilling holes at longitudinal joint.
The design standard FDR(CP)-05 requires a minimum length of the drilled hole of 12 in. and Item 361 requires the contractor to demonstrate the bond strength is sufficient. Bond strength testing in the field rarely happens. Whether this length requirement is adequate or not should be evaluated, since failures have been observed due to pull-out of the embedded rebars. Failure of bars that have been drilled and epoxied to achieve proper strength may be more related to complete filling of drill holes with epoxy than the length embedded.
6. Provide longitudinal and transverse steel continuity.
If tie bars installed at the repair perimeters are not functioning well, the potential problem of lane separation at the longitudinal joint or distresses at the transverse repair perimeters will increase. For detailed steel designs for FDR, refer to TxDOT standard, FDR(CP)-05. For FDR of CRCP, this step of providing longitudinal continuity of tie bars is of utmost importance. To achieve the optimum bond between concrete and tie bars inside drilled holes, all dust inside the holes should be completely removed. The most commonly used method to achieve this is to apply compressed air to the holes. During this operation, it is important that the air is not contaminated with grease or oil, which will weaken the bond between the concrete and steel bars. Otherwise, the condensation of oil vapor inside the drilled holes will act as a bond breaker. To check whether there is oil vapor, compressed air is shot into white paper for a few seconds and the paper is checked for any oil residue.
Once the drilled holes are cleaned, they are completely filled with epoxy materials. The epoxy materials used in this operation have a low viscosity and it’s not easy to fill the holes completely. Item 361 requires the use of grout retention disks to keep the epoxy in the holes. However, this requirement is not always met in the field. More attention needs to be paid to this requirement.
Design standard FDR(CP)-05 indirectly requires a minimum lap length of 24 in. for #6 bars. This requirement comes from the fact that, to prevent pull-out failure, the steel embedded length should be a minimum of 33 times the diameter of the reinforcing steel. This will work as long as there are no cracks near the transverse perimeter of the repair area. If cracks develop near the transverse perimeter, this requirement will not be met. FDR(CP)-05 should be revised to increase the lap length requirement.
Figure 10-6 shows the full-depth repaired slab that was removed for additional repair due to distresses. The problem was that continuity of the longitudinal steel was not provided. The bars in the red circle show tie bars inserted into the drilled holes during the previous repair. The bar in the blue circle is the existing longitudinal steel. It is noted that tie bars are not fully bonded to the concrete. Epoxy is not fully filled in the holes either. If the pullout testing was done, it would have failed. The inability of the tie bars to provide continuity of the longitudinal steel caused wide joints on one side of the transverse perimeters in the repair area, which resulted in premature failure and another repair was required.
Figure 10-6. Previously repaired slab.
7. Place concrete.
Once the steel placement is complete, concrete is poured and finished. Since the concrete placing is done manually in FDR, the consolidation of concrete is achieved by hand vibration. In order to provide a good bond between the steel and concrete, a high quality consolidation operation is required. Surface texturing comparable to the surrounding concrete should be provided.
Normally, the full-depth repaired pavement is required to be opened as soon as possible. Prior to 2004 Specifications, Class K concrete was used for FDR. However, there were no statewide uniform strength requirements for Class K concrete. For 2004 Standard Specification Item 421 created a new class of concrete, high early strength (HES) concrete, when high early strength is required. The maximum water-cement ratio is set at 0.45 and strength requirements are specified in Item 360. Item 361 requires the use of class HES concrete if the pavement has to be opened within 72 hr. after concrete placement. Since high early strength is required for early opening to traffic, it’s normal practice to use high cement content as well as Type III cement, which is for high early strength. That practice could be beneficial if the repairs are done in winter when the ambient temperature condition is not conducive to achieving high early strength. However, if the repairs are done in the summer when the ambient temperature is high, the practice could result in high heat of hydration and premature pavement distress due to thermal cracking problems.
Another important item in concrete finishing is curing. Since the concrete used in FDR will often have requirements of high early strength as described above, higher cement content than normal Class P concrete is used. Also, Type III cement is sometimes used. These practices result in high heat of hydration as well as larger drying shrinkage of concrete. High heat of hydration creates large temperature gradient through the slab depth, especially near the surface. Also, high concrete temperature encourages the evaporation of the water from the surface. If the proper curing is not provided, the combination of large temperature gradients as well as high evaporation will cause cracking problems as shown in Figure 10-7. Proper curing is essential.
Figure 10-7. Map cracking in FDR slab.
For FDR project, the use of maturity in accordance with Tex-426-A, Estimating Concrete Strength by the Maturity Method” rather than conventional strength testing, is strongly encouraged. It is because the accurate estimate of the in situ strength can be best achieved by the maturity method. To use the maturity method, contact the Rigid Pavements and Concrete Materials Branch of the Construction Division.