Dowel bar retrofit for lasting road repairs
| November 20, 2012 |
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.
The area where the load transfer retrofit tests were to be constructed on I-75 was rehabilitated in 1976 by GDOT maintenance forces but significant increases in joint faulting had redeveloped by 1980. The original plan to cut the slots for the dowels utilized a specially built carbide tipped Rotomill mandrel, but an early field trial showed that this method of concrete removal created too much damage to the joints. The GDOT then decide to cut the slot perimeters using diamond saw blades removing the remaining concrete with lightweight jack hammers as is done today. Core holes were drilled for the placement of the other devices.
Performance evaluations were made in January 1982, September 1982, and March 1983. Static weight and Dynaflect load transfer measurements were made along with horizontal joint movement measurements, faulting measurements, slab cracking, and visual observations of the load transfer devices for bond failures and spalling, cracking, etc., of the patching materials.
The results from the test installations showed that retrofitted dowel bars were the best means of reestablishing load transfer to existing joints determining that three dowels should be placed in the outside wheel path and two dowels in the inside wheel path. It also stated that retrofit dowel placement in the inside wheel path could possibly be eliminated once long term performance data became available. It was also recommended that joints with large slab movements should be stabilized though undersealing prior to placing dowels.
DBR in Washington State
In Washington State, plain jointed concrete pavements constructed prior to the 1990’s did not contain dowel bars across the transverse joints. After being in-service for 30 or more years, a significant number of the Washington State concrete pavements had developed transverse joint faulting, many with average faulting greater than ½ in. Since sufficient funding was not available to reconstruct the faulted and rough concrete pavements, in 1992 the Washington State Department of Transportation (WSDOT) initiated a study to investigate the cost effectiveness of load transfer restoration techniques. Since then, WSDOT has dowel bar retrofitted more than 626 lane miles or approximately 1,322,000 bars, of faulted concrete pavements.
Since its inception in Washington State, DBR projects have also included diamond grinding of the entire project length, and to the extent necessary, full-depth replacement of concrete panels with two or more cracks, partial-depth spall repair, crack sealing, and for all but one project, resealing transverse and longitudinal joints. It must also be noted, that when WSDOT initiated DBR there existed a sizeable (approximately 600 to 800 lane miles) backlog of concrete pavements in need of rehabilitation. Due to this backlog, the majority of which were on the heavily traveled interstate system, WSDOT conducted DBR in a worst-first manner, implying that projects that received DBR first, were in the worst condition (primarily heavily faulted).
According to WSDOT research, the average construction costs for DBR was approximately 16 percent less (2006 dollars) than the typical cost of a four-inch asphalt overlay, which is the minimum recommended overlay depth for rehabilitating a faulted concrete pavement. The success realized by WSDOT has promoted widespread acceptance of the process and as a result, 20 states and one Canadian province have completed numerous successful projects.
DBR application, in conjunction with panel replacements and diamond grinding, has proven to be an effective rehabilitation treatment for faulted concrete pavements. Based on the review of approximately 380,000 DBR slots in Washington State, the presence of cracking, spalling and debonding of the patching material was nearly non-existent, indicating that superior construction and inspection practices have led to long-term performance. It was determined that Washington State has experienced very little DBR slot-related distress, with less than 3 percent of all DBR slot distress combined on any given project and typically less than 1 percent on all projects. Further, after reviewing DBR performance, it was found that 5 of the 21 projects examined showed superior longer-term performance as compared to all other DBR projects. This was due in part to applying the DBR process to pavements earlier in the rehabilitation cycle and therefore with much less faulting.
Based on the findings in Washington, DBR is a highly effective solution for long-term pavement repair. Critical to the success of DBR projects are appropriate specifications and construction inspection processes, as well as contractors firmly establishing themselves in DBR construction techniques, bringing a high level of experience and quality consciousness to the project. As all states seek ways to repair aging concrete pavements, DBR can be the ideal solution when long-lasting, cost-effective repairs are desired.
DBR has proved to be a long lasting repair, lasting 15 to 20 years, and is a sustainable pavement rehabilitation process as existing concrete pavements are rehabilitated rather than reconstructed. It is traffic friendly since projects can be completed during off-peak hours with short lane closures. The process is flexible, in that it only has to be applied to the lanes that show distress, whereas other treatments require the entire roadway to be treated. Other repair options, such as asphalt overlays, can result in raising guard rails, overhead signs, and bridges, increasing the overall project costs. Additionally, the simple design bid process allows projects to be designed and advertised in a fraction of the time required for most overlay processes. DBR is the right repair process for faulted PCCP at a competitive price.