Dowel bar retrofit for lasting road repairs
Dowel Bar Retrofit (DBR), used in conjunction with other Concrete Pavement Restoration (CPR) practices such as diamond grinding, can return a concrete roadway to a structurally sound, smooth condition that can exceed the smoothness and noise values attained at the time of construction.
DBR restores load transfer across joints and cracks by installing dowel bars linking the adjoining slabs. By linking slabs, the traffic load is shared, preventing differential vertical movement of the slabs at the joints and cracks, thereby eliminating the formation of faults or step-offs. It is these faults that cause the rough ride and wheel slap that is sensed when traveling on a concrete roadway that has lost its ability to transfer load from one panel to the next.
Load transfer across transverse joints of jointed plain concrete pavements is essential for long-term performance, especially when there are heavy truck traffic loadings. If there is sufficient load transfer, tensile stress and deflections will be reduced, lowering the potential for joint spalling, base and/or subgrade pumping, transverse joint faulting, and cracking. The methods to obtain load transfer include aggregate interlock, treated bases, and/or dowel bars placed at transverse joints. Aggregate interlock alone may not provide sufficient load transfer to minimize tensile stress and deflections if there are heavy truck transfer loadings. In general, load transfer efficiencies between 70 to 100 percent are considered to be adequate, while load transfer below 50 percent can lead to joint faulting, panel cracking, and poor ride quality.
DBR is a good pavement restoration method in select pavement conditions. DBR should be considered when:
• Pavements exhibit load transfer below 60 percent.
• Joint and crack faulting is between 1/16 to 3/4 in.
• Transverse cracks are reasonably tight with minimal spalling. If DBR is applied early, the amount of diamond grinding may be greatly reduced.
• Pavements that were constructed as non-doweled jointed pavements can have DBR applied to prevent future faulting as an effective pavement preservation treatment.
The general steps for DBR construction are fairly straight forward. First, cut DBR slots and remove existing concrete from slots. Then clean the slots of all debris followed by sand or water blasting and place caulking compound at all joints/cracks. Next, place dowel bar assemblies in the slots. Place the patching material, then consolidate and finish patching material. Finally, diamond grind the pavement surface.
Early Beginnings in Georgia
In the early 1970’s, lack of load transfer was identified as one of the interrelated causes of pavement faulting. The Georgia Department of Transportation (GDOT) added dowel bars as a standard design detail for new PCC pavements in the mid 1970’s. Although joint faulting on their older PCCP was an issue, none of the existing CPR activities studied in the 70s addressed the lack of effective load transfer in the existing pavements except through stabilization of the slab using undersealing techniques.
GDOT became interested in improving the load transfer capabilities of their existing PCC pavements after observing the re-faulting of pavements following CPR and diamond grinding. It was felt that the effectiveness of CPR could be extended if load transfer could be added at the joints. Around the same time, the Federal Highway Administration (FHWA) published a report in 1977 with conceptual proposals for two load transfer restoration devices. GDOT initiated a research project in 1980 to evaluate these and other devices. The goal was to develop construction procedures for adding load transfer to existing pavements and to evaluate the effectiveness of the methods. Various load transfer devices were placed by maintenance forces in a total of 461 joints on I-75 between Atlanta and Macon in 1981 with some additional installations done in 1982. Some of the devices placed included the Georgia Split Pipe, the Figure Eight device used in experiments in France, the Vee device researched in the 1977 FHWA report, the Double Vee device developed at the University of Illinois and sold under the trade name of LTD plus, and smooth steel dowel bars. Patching materials used on the project included three types of proprietary fast setting grouts, polymer concrete, and plain fast setting concrete. The number and spacing of the various devices and dowel bars were also variables in the project.
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