States don't always want to take chances when it comes to paving highways, but South Carolina had a choice to make. It could use dowel bar inserters (DBIs) on the $64 million I-95 reconstruction project through Darlington and Florence counties, or stick with more conventional methods, which, though “tried-and-true,” could cause excessive delays for motorists. The state's Department of Transportation had been burned by new technologies before. Back in the 1980s, it experienced a less than adequate performance of a highly touted continuously reinforced concrete pavement.
Not wanting to repeat the blunder, the state was reluctant to accept a new technology that could possibly compromise long-term pavement performance for motorists' convenience. However, SCDOT risked using a second new technology to employ the first.
This second risk involved a new technique to conduct quality assurance and quality control, making sure the dowels were where they were supposed to be. The MIT Scan-2—made by Magnetic Imaging Tools GmbH, Dresden, Germany and distributed in the United States and Canada by ARA Inc.—was developed specifically for measuring dowel and tie bar locations.
The unit uses an array of detectors and data analysis algorithms to locate a dowel within ±5 mm. The device emits a weak, pulsating magnetic signal and detects the transient magnetic response induced in the metal bars. Magnetic tomography then determines the position of the bars in the pavement.
The unit, which can be used on fresh or hardened concrete, is set on rails, straddling the joint. It can detect steel at a depth of 10 inches or more. Testing takes less than 2 minutes per joint. Field data analysis is fully automated and the results can be printed from the onboard computer.
The Scan-2 works well for locating inserted dowel bars, but it can also be used to locate epoxy-coated dowels in baskets. However, it isn't so good with non-epoxy-coated varieties because the dowel becomes magnetically indistinguishable from the basket.
Why location is important
Dowel bars are fairly inexpensive items, though the role they play in multi-million-dollar concrete pavement projects is critical. Improperly placed, they can compromise the integrity of the pavement.
Dowel bars are used in jointed plain concrete pavements to provide load transfer, which reduces faulting and improves performance. They can be placed manually before concrete placement (dowel baskets) or during the paving process (DBIs). Dowel baskets, which are fabricated or manufactured items, are placed ahead of the paving train, while dowel bar inserters are part of the train.
Unfortunately, research has shown that regardless of how they are placed, the dowels can sometimes get misaligned. Dowel baskets can be pushed over by the wet concrete, or they can become misaligned during the paving process. Misalignment can result in improper load transfer.
Uncertainty about dowel bar placement is one of the main reasons dowel bar inserters have not been widely adopted in the United States. The University of Minnesota, in association with the National Cooperative Highway Research Program (NCHRP), is investigating the problem of dowel misalignment. The hope is to one day offer guidelines and tolerances for dowel placement in concrete placement.