Section 7: Dowel Bar Retrofit
This rehabilitation technique is applicable to only jointed concrete pavement (JCP or concrete pavement contraction design [CPCD]), not continuously reinforced concrete pavement (CRCP). Load transfer is defined as the ability of a joint or a crack to transfer wheel load from one slab to the next. In CPCD, the majority of load transfer is achieved by dowels. Aggregate interlock is supposed to provide some load transfer; however, the effectiveness of aggregate interlock diminishes with time as concrete contracts due to drying shrinkage. Also, the contribution of aggregate interlock to the load transfer is substantially diminished during winter when the temperature is low and joint opening becomes large. Good load transfer at the joints improves the performance of jointed concrete pavement, and dowels have been used to provide good load transfer. However, there are jointed concrete pavement sections that do not have dowels. The absence of dowels along with deficient subbase or subgrade support could result in faulting and cracking problems at joint areas. Figure 10-13 shows faulting in CPCD.
Figure 10-13. Severe faulting at the transverse joint.
This pavement shown in Figure 10-13 does not have dowel bars and the subbase support was not adequate. Faulting increases the stresses in concrete due to increased dynamic wheel loading, which further deteriorates the pavement. One of the most efficient ways to restore load transfer at the joints is dowel bar retrofit (DBR). In DBR, slots are cut, concrete removed, dowel bars inserted, backfill material filled, the surface finished, cured, and most often diamond ground. Many states have used DBR to restore old CPCD where dowels were not used and have had success. In TxDOT, DBR is a relatively new procedure and currently, TxDOT does not have guidelines for using DBR. However, TxDOT developed special specifications and design standards, even though they are not statewide (Special Specification 3012).
Anchor: #i1003943DBR Procedures
The following describes the steps needed to complete the DBR.
- Identify the need for DBR.
- Cut the slots.
- Prepare the slots.
- Place dowels.
- Backfill the slots.
1. Identify the need for DBR.
As described above, currently TxDOT does not have established faulting or load transfer efficiency criteria for DBR. However, ACPA and FHWA identified conditions that will benefit from DBR. Those conditions are:
- load transfer efficiency (LTE) of 60% or less
- faulting greater than 0.1 in.
- differential deflection of 10 mils or more.
Since the load transfer efficiency and deflections depend on the temperature conditions of concrete, the above first and third criteria need to have reference temperature. Also, the determination of LTE and deflections require the use of FWD, while the magnitude of faulting requires a simple ruler. Even though Georgia Fault Meter is much more sophisticated than a simple ruler and has been used in other states, a simple ruler will do the job for the determination of the need for DBR. Currently, TxDOT does not have a Georgia Fault Meter. If the faulting is due to deficient base support, DBR alone should not be used to restore LTE. In this case, subbase repair should be done along with DBR.
For young pavement, where the pavement condition is satisfactory but dowels were not used, DBR can be used to strengthen the pavement’s structural conditions. This is more of a preventive maintenance type operation.
2. Cut the slots.
Once the need for DBR is established, the next step is to cut the slots using diamond saw slot cutters. It is customary to provide three dowel bars in each wheel path. The diamond saw slot cutter shown in Figure 10-14 cuts multiple slots in a single pass. These cuts form the edges of the slots. When cutting slots, it is important that the slots are aligned parallel to centerlines. The slots need to be sufficiently wide to permit the largest aggregates in the concrete mix or proprietary mix to flow around the bars and consolidate properly.
Figure 10-14. Diamond Saw Slot Cutter.
Figure 10-15 shows the cuts made for slots. Slots are usually 12 in. apart center to center. Note that the cuts are clean and no spalling is observed on the edges of the slots.
Figure 10-15. Cuts made for slots.
3. Prepare the slots.
Slot preparation consists of:
- removing the concrete fins,
- flattening the bottom,
- cleaning the slots, and
- caulking the joint.
Small handheld jackhammers are used to remove the fins as shown in Figure 10-16. Once the fins are removed, the bottom of the slots is flattened using a small brush hammerhead. The flat slot bottom allows the dowels to sit level and be properly aligned. Normally, the slots are cleaned with sand blasting first as shown in Figure 10-17. Figure 10-18 shows the flat bottom after being water cleaned. Maintaining clean slots is important since the bond between backfilling material and slots is critical to providing good load transfer. Joint caulking is necessary to prevent patch material from entering the joint.
Figure 10-16. Removing concrete fins.
Figure 10-17. Sandblasting the slots.
Figure 10-18. Flat bottom after being water cleaned.
4. Place dowels.
Dowels used for DBR are similar to the ones used for new CPCD. Dowels are lubricated with some type of bond breaker. Figure 10-19 shows the dowel bar assembly used for DBR with a joint reformer, endcaps, and chairs. Joint reformer and endcaps allow movement for the slab to expand into without bearing on the patch or bar. Chairs are used to support dowels in the base of the slots and allow backfill material to surround the dowels, and should fit snugly in the slot to keep dowels properly aligned.
Figure 10-19. Dowel bar in place for backfill material application.
5. Backfill the slots.
Once the dowels are placed, backfill materials are applied. Backfill materials should have similar thermal properties to the concrete, provide strong bond to the existing concrete, be fast setting, have little shrinkage, and develop enough strength to allow traffic in a short time. Both high early strength concrete and proprietary mixes have been used successfully. High early strength concrete usually contains Type III cement, accelerators, and aluminum power. Accelerators and aluminum powder improve set times and reduce shrinkage. Aggregates in the mix should be small enough to allow the concrete to flow around the bar and consolidate properly. Consolidation of backfill material is done with a small spud vibrator. Care should be taken not to hit the dowels with the vibrator, since touching dowels with a vibrator will knock it out of alignment.
Once the backfill material is applied, the surface is finished flush with the surrounding surface. Curing compound should be applied as soon as practical. Sawing is done over reformer and the entire project is diamond ground, with resealing the joints as a final step. Figure 10-20 shows the section shown in Figure 10-13 after DBR, slab jacking, and diamond grinding (DG).
Figure 10-20. After DBR, slab jacking and diamond grinding of the section shown in Figure 10-13.