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Dowel bar retrofit (DBR)
Dowel bar retrofit (DBR) is a process of placing dowel bars into in-service pavements to restore load transfer either across a joint or a crack. Due to having a higher level of control than in new construction, DBR can use a limited amount of dowels (2-4 per wheelpath) to restore load transfer. This method has been applied since the 1980's with many pavements to help them reach or exceed their design life.
CHAPTER 1 Proper Procedure Checklists
Slot Cutting and Preparation
- Cut slots using a diamond-bladed slot cutting machine unless the pavement is to have a bonded concrete overlay, in which case a carbide milled slot is acceptable.
- Remove the concrete fins within the slots with lightweight chipping hammers (maximum size of 30 lb [14 kg]).
- Clean the slot walls and bottom by sandblasting.
- Ensure your process creates slots that can properly hold dowels parallel to the pavement surface and centerline, and at the mid-depth of the slab.
- Check that every dowel is lubricated or has a manufacturer- applied bond breaker.
- Ensure every dowel has support chairs and an expansion cap on each end.
- Use a filler board or expanded polystyrene foam material, placed at the mid-length of each dowel.
- Caulk the rim of the joint or crack inside of each dowel bar slot.
Patch Placement Procedures
NOTE: Patch material/placement is the most critical factor for successful dowel bar retrofits.
- Ensure all patch materials have been pre-evaluated in a laboratory to meet specifications for work time, rapid-early strength gain, shrinkage, and durability.
- Follow the manufacturer’s placement procedures for proprietary patching materials.
- Ensure the patch material is placed and consolidated properly, eliminating all air voids.
- Follow an appropriate (i.e. similar to previous laboratory testing) curing procedure.
CHAPTER 2 Dowel Bar Retrofit
- Use diamond-bladed slot-sawing machines to saw slots, unless a bonded concrete overlay is to follow.
- Use lightweight chipping hammers to remove the fins of concrete in the slots.
- Clean repair area thoroughly (typically by sandblasting, which is then followed by the use of clean, compressed air) before placing dowels and patch material.
- Ensure dowels are placed in the slots within alignment tolerances.
- To prevent point bearing, re-form the joint (or crack) by placing compressible material across the patch before placing repair material.
- Use a durable repair material, particularly for freeze-thaw resistance.
- Mix repair material in small quantities; follow manufacturer’s instructions, if applicable.
Dowel bar retrofit, or load transfer restoration, is primarily used on roadways that receive heavily channeled loadings (i.e. highways). Transverse joints or cracks that would benefit from improved load transfer can be identified by measuring the existing load-transfer efficiency with heavy-weight, nondestructive, deflection testing devices such as a Falling Weight Deflectometer (FWD). These tests must be conducted during periods of cooler temperatures (less than about 80°F [27°C]), when the slab joints and cracks are not tightly closed due to thermal expansion. Joints or cracks with a load-transfer efficiency (ratio of the deflection on the unloaded side of a joint or crack to the deflection of the loaded side) less than 60% should be considered for dowel bar retrofit. Dowel bar retrofits should also be performed on joints or cracks that have greater than 1/10 in. (2.5 mm) of faulting or differential deflection of 10 mils (250 µm) or more.
Different sizes of dowels should be specified for different pavement thicknesses (Table 2.1). A minimum length of 14 in. (350 mm) is recommended to allow for at least 6 in. (150 mm) of embedment on each side of the joint or crack, adequate room for an expansion cap on each end of the dowel bar, and reasonable placement tolerances. Three to four dowels, spaced 12 in. (300 mm) apart, should be used in each wheelpath, with the outermost dowel being 12 in. (300 mm) from the lane edge, except where tiebars from adjacent lanes or shoulders are encountered. A recommended layout for retrofitted dowels is depicted in Figure 2.1. The dowel bars, including the ends, should be epoxy coated or otherwise corrosion resistant.
Table 2.1. Dowel Size Requirements for Dowel Bar Retrofit.
|Pavement Thickness in. (mm)||Diameter in. (mm)||Min. Length in. (mm)||Spacing in. (mm)|
|< 8 (< 200)||1.0 (25)||14 (350)||12 (300)|
|8 to 9.5 (200 to 240)||1.25 (32)||14 (350)||12 (300)|
|10 + (250 +)||1.5 (38)||14 (350)||12 (300)|
Figure 2.1. Layout of retrofit dowel slots in relation to longitudinal joints.
2.1 Slot Cutting and Preparation
Using a diamond-bladed slot cutting machine (Figure 2.2), slots are cut across each joint (Figure 2.3) that is to receive a dowel bar retrofit (slots can alternatively be milled out if a bonded concrete overlay is to be applied to the pavement). Multiple-blade saws may be used to expedite slot production. The slots should be cut so the dowels are allowed to rest parallel to the pavement surface and centerline, and at mid-depth of the slab (Figure 2.4).
Lightweight chipping hammers (maximum size of 30 lb [14 kg]) are used to remove the concrete fins within the slots (Figure 2.5). The slot is then cleaned by sand- blasting to ensure removal of all sawing residue, dirt, or oil that may prevent complete bonding of the patch material to the slot faces. The rim of the joint or crack inside the slot is then caulked to prevent the patch material from locking up the joint or crack.
Figure 2.2. Slot cutting machine.
Figure 2.3. Dowel bar slots cut using a diamond-bladed slow cutting machine across a skewed joint (note that the slots are parallel to the center-line and not perpendicular to the joint).
Figure 2.4. Retrofit dowel installation details.
Figure 2.5. Lightweight chipping hammer being used for concrete fin removal in the slot.
Lubricate the dowels, or alternatively, use a manufacturer-applied bond breaker to allow the joint or crack to open and close with temperature changes. Each dowel is placed on a support chair to allow the patch material to surround and consolidate under it. Expansion caps are placed on both ends to provide space for lateral movement (Figure 2.4). A filler board or expanded polystyrene foam material must be placed at the mid-length of each dowel to prevent intrusion of the patch material into the joint or crack (causing point bearing), as well as to help form the joint in the slot (Figure 2.6).
Figure 2.6. Photo showing dowel retrofit assemblies, including joint re-formers and expansion caps, ready to be cast in place.
2.2 Patch Placement Procedures
The patch material is the most critical factor in the performance of retrofitted load- transfer devices. To date, high early-strength concrete mixtures have been used in most dowel retrofit installations. The patch material must be durable and should allow a sufficient bond to be established between the existing concrete and the patching material. For this reason, a thorough laboratory evaluation must be performed on any patch material used for load-transfer device retrofitting. The primary variables that must be evaluated are working time, rapid early-strength gain, shrinkage, and durability.
The manufacturer’s recommendations should be followed with all proprietary patching materials. If used, bonding agents should be those recommended by the manufacturer for the placement conditions. The patch material should be placed and properly consolidated (Figure 2.7) to eliminate all voids at the patch/existing concrete pavement interface and at the patch/load-transfer device interface. In general, the same requirements and procedures used for partial-depth repair mate- rials may be used as patch material in dowel-bar retrofit slots. It is also important to follow adequate curing procedures for proper hydration and strength gain of the repair material; doing so will prevent drying shrinkage and subsequent debonding (Figure 2.8). A properly performed dowel bar retrofit, with a mix comparable to the existing pavement, will result in repair areas that are hardly noticeable to drivers (Figure 2.9).
Figure 2.7. Vibrating backfill in dowel bar retrofit slots.
Figure 2.8. Dowel bar retrofit slots finished and covered with curing compound.
Figure 2.9. DBR – Final product.
Sawcuts (sides of slots) are not parallel to each other or the pavement centerline:
- Use a saw slot cutting machine.
- Check blade alignment.
Slots are cut too shallow:
- Re-saw the slots and remove concrete to the proper depth.
Slots are too deep:
- Use a lighter weight jackhammer, 30 lb (14 kg) max.
- Do not lean on the jackhammer.
- Do not orient the jackhammer vertically; use a 45-degree angle and push the tip of the hammer along the bottom of the slot.
- Stop chipping when within 2 in. (50 mm) of the bottom of the pavement.
- If jackhammers punch through the bottom of the pavement, make a full-depth repair across the entire lane width at the joint/crack.
Concrete fin is not easily removed:
- Check for mesh reinforcement and sever the steel at each end of the slot before attempting to remove the fin of concrete.
- Use a concrete saw to cut an additional kerf within the slot perimeter before chipping for removal.
Dowel cannot be centered over joint/crack because slot does not extend far enough:
- Chip out additional slot length with jackhammer so that at least 6 in. (150 mm) of dowel extends on each side.
Dowels are misaligned after vibration:
- Ensure that the slots are sized the exact width of the plastic dowel bar chairs.
- Do not allow the vibrator to touch the dowel assembly.
- Check for over-vibration; each slot should only require two to four short, vertical penetrations of a small-diameter spud vibrator.