Dowel bar retrofit
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A dowel bar retrofit (DBR) 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.
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[edit] History
As a vehicle travels on jointed concrete roads the weight of the vehicle passes from one concrete panel to the next. When the vehicle traverses the joints the full 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 need replacement. Instead, the cohesion between the panels and the strength of the road bed was relied upon to transfer the load. Over time the joint and road bed 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.
[edit] Dowel bar retrofit
Many states are retrofitting older highways with new 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 and 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 excess grout and remove 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 dowel bar retrofit will greatly increase this interval.
[edit] 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.
