Dowel Bar Retrofit
Encyclopedia
A dowel bar retrofit is a method of reinforcing cracks in highway
pavement
by inserting steel dowel
bars in slots cut across the cracks. It is a technique several states have successfully used to address faulting in older jointed plain concrete pavements. The typical approach is to saw cut and jackhammer out the slots for the dowels. Following dowel placement the slots are then typically backfilled with a non-shrink concrete mixture (grout
) and the pavement is diamond-ground to restore smoothness.
In modern highways, dowel bars are once again placed across joints and at intervals along the pavement. Modern dowel bars are coated with epoxy to prevent corrosion.
The final step often involves diamond-grinding of the entire road surface to remove any bumps or dips. Without the dowel bar retrofit this grinding would have to be repeated every six to eight years, but it is predicted that the retrofit will greatly increase this interval. Dowel Bar Retrofit has been proven to outlast Asphalt Overlay by at least ten years, and significantly improves ride quality.
First, make two saw cuts in the pavement to outline the longitudinal sides of each dowel bar slot. Saw to a depth and length that allows the center of the dowel to be placed at mid-depth in the pavement slab. Saw slots parallel to each other and to the centerline of the roadway with a maximum tolerance of 1⁄4-inch per 12 inches of dowel bar length. Employ saws equipped with gang mounted diamond blades capable of cutting a minimum of three or four slots simultaneously in each wheel path. Skewed joints or cracks may require slots longer than the length specified in the plans. Pick up and remove water and paste residue from the pavement surface immediately by means of a vacuum attachment on the sawing machine. For smaller projects, a slot sawing machine might not be feasible. Consider allowing the use of walk-behind saws for slot cutting when the project contains less than 100 retrofit bars.
Remove existing concrete from the slots followed by the cleaning out of all debris. Use maximum weight 30 pound jackhammers to remove the concrete remaining between the saw cuts. If the concrete removal operations cause damage to the pavement that is to remain, discontinue concrete removal operations and only resume after taking corrective measures. During concrete removal operations, use a small bush hammer as necessary to produce a flat, level surface within the slot so the backfill flows and consolidates under the dowel bars.
Complete installation of retrofit dowels, with dowel bars and backfill placed prior to reopening the lane to public traffic. Place silicone at all joints and cracks followed by the dowel bar assemblies in each slot. Following that, place patching material and then consolidate and finish patching material. Finally, diamond grind the pavement surface within 30 working days of dowel bar retrofit placement.
Highway
A highway is any public road. In American English, the term is common and almost always designates major roads. In British English, the term designates any road open to the public. Any interconnected set of highways can be variously referred to as a "highway system", a "highway network", or a...
pavement
Pavement (material)
Road surface or pavement is the durable surface material laid down on an area intended to sustain vehicular or foot traffic, such as a road or walkway. In the past cobblestones and granite setts were extensively used, but these surfaces have mostly been replaced by asphalt or concrete. Such...
by inserting steel dowel
Dowel
A dowel is a solid cylindrical rod, usually made of wood, plastic or metal. In its original manufactured form, dowel is called dowel rod.Dowel rod is employed in numerous, diverse applications. It is used to form axles in toys, as detents on gymnastics grips, as knitting needles, as structural...
bars in slots cut across the cracks. It is a technique several states have successfully used to address faulting in older jointed plain concrete pavements. The typical approach is to saw cut and jackhammer out the slots for the dowels. Following dowel placement the slots are then typically backfilled with a non-shrink concrete mixture (grout
Grout
Grout is a construction material used to embed rebars in masonry walls, connect sections of pre-cast concrete, fill voids, and seal joints . Grout is generally composed of a mixture of water, cement, sand, often color tint, and sometimes fine gravel...
) and the pavement is diamond-ground to restore smoothness.
History
As a vehicle travels on jointed concrete roads the weight of the vehicle passes from one concrete panel to the next. As the vehicle traverses the joints its weight is placed on the edge of the panel, where the panel is least able to withstand the deflection force. This can cause cracks as pavement shears off the edge of the panel. On older highways dowel bars (steel rods) were placed across the joints to help transfer the load from one panel to the next. This was discontinued because the dowel bars tended to corrode and require replacement. Instead, the cohesion between the panels and the strength of the roadbed was relied upon to transfer the load. Over time the joint and roadbed tends to break down, resulting in cracks and displacement of the panels.In modern highways, dowel bars are once again placed across joints and at intervals along the pavement. Modern dowel bars are coated with epoxy to prevent corrosion.
Dowel bar retrofit overview
Many states are retrofitting older highways with epoxy-coated dowel bars. The retrofit begins with cutting of six slots (three in each wheel path) across all transverse joints or cracks. The slots are cut with ganged diamond saws that make six cuts in each wheel path. The concrete between the saw cuts is then removed with lightweight jackhammers (heavy-weight jackhammers tend to damage the concrete around the cuts). The epoxy-coated dowel bars are placed in the slots, then the slots are filled with grout and the joints or cracks are filled with waterproof caulk. The final step is to diamond-grind the joint to remove both excess grout and any displacement of the panels.The final step often involves diamond-grinding of the entire road surface to remove any bumps or dips. Without the dowel bar retrofit this grinding would have to be repeated every six to eight years, but it is predicted that the retrofit will greatly increase this interval. Dowel Bar Retrofit has been proven to outlast Asphalt Overlay by at least ten years, and significantly improves ride quality.
Dowel bar retrofit process
For dowel bar retrofit, dowels, bond breaker, expansion caps, dowel bar support chairs, foam core insert, caulking filler, non-shrink concrete backfill material and submittals are needed. The dowels need to be smooth, round, epoxy and made of bond breaker coated steel conforming to requirements. The bond breaker is to be applied on all surfaces of the dowel bar. The expansion caps belong at each end of the dowel bar and the dowel bar support chairs are used to firmly hold the dowels centered in the slots during backfill operations. The foam core insert is used to re-establish the joint or crack, which allows the bar within each side of the slot to expand and contract. The caulking filler is used to prevent backfill material from flowing into the joint or crack. Non-shrink concrete backfill material that is tested as rapid set concrete patching materials is recommended for use on DBR projects.First, make two saw cuts in the pavement to outline the longitudinal sides of each dowel bar slot. Saw to a depth and length that allows the center of the dowel to be placed at mid-depth in the pavement slab. Saw slots parallel to each other and to the centerline of the roadway with a maximum tolerance of 1⁄4-inch per 12 inches of dowel bar length. Employ saws equipped with gang mounted diamond blades capable of cutting a minimum of three or four slots simultaneously in each wheel path. Skewed joints or cracks may require slots longer than the length specified in the plans. Pick up and remove water and paste residue from the pavement surface immediately by means of a vacuum attachment on the sawing machine. For smaller projects, a slot sawing machine might not be feasible. Consider allowing the use of walk-behind saws for slot cutting when the project contains less than 100 retrofit bars.
Remove existing concrete from the slots followed by the cleaning out of all debris. Use maximum weight 30 pound jackhammers to remove the concrete remaining between the saw cuts. If the concrete removal operations cause damage to the pavement that is to remain, discontinue concrete removal operations and only resume after taking corrective measures. During concrete removal operations, use a small bush hammer as necessary to produce a flat, level surface within the slot so the backfill flows and consolidates under the dowel bars.
Complete installation of retrofit dowels, with dowel bars and backfill placed prior to reopening the lane to public traffic. Place silicone at all joints and cracks followed by the dowel bar assemblies in each slot. Following that, place patching material and then consolidate and finish patching material. Finally, diamond grind the pavement surface within 30 working days of dowel bar retrofit placement.
Problems
Dowel bar retrofits have occasionally been plagued with problems. These problems are usually due to poor workmanship and can be avoided by careful following of specifications and close supervision of workers. Use of heavy jackhammers (heaver than 14 kg class) tends to cause cracking of the concrete all around the slots. Lack of caulk or voids in the caulk put in the joints or cracks leads to cracks around the joints. Voids in the grout (especially below the dowel bar) can cause failure of the grout.External links
See also
- Diamond grindingDiamond grindingDiamond grinding is a pavement preservation technique that corrects a variety of surface imperfections on both concrete and asphalt pavements. Most often utilized on concrete pavement, diamond grinding is typically performed in conjunction with other concrete pavement preservation techniques such...
- Diamond BladeDiamond BladeA diamond blade is a saw blade which has diamonds fixed on the blade's base to use the diamonds to cut hard or abrasive materials. There are many types of diamond blades, and they have been applied in many industries, for example, construction industry to cut stone, concrete, asphalt, bricks, coal...
- Diamond segmentDiamond segmentDiamond segments are the function parts of a metal-bonded diamond tool. The metal-bonded diamond tool can be a metal-bonded diamond blade, a diamond grinding cup wheel, a diamond core drill bit, a diamond gang saw blade, etc...
- Diamond tool
- Concrete grinderConcrete grinderA concrete grinder can come in many configurations, the most common being a hand-held Angle grinder, but it may be a specialized tool for countertops or worktops. There are also purpose-built floor grinders that are used for grinding and polishing marble, granite and concrete...
- Diamond grinding of pavement