Concrete Proof

In Ontario, concrete pavement design for heavily trafficked highways consists of doweled jointed plain concrete pavement (JPCP) over an open-graded drainage layer (OGDL) and granular base and sub-base. Epoxy coated, smooth, 456 x 32 mm steel dowel bars are placed at 300 mm centers across the transverse joints to provide load transfer. Longitudinal joints are tied with 15 x 760 mm tie bars spaced at 600 mm intervals.

In the past, the position and alignment of dowel bars within a concrete pavement was difficult to verify. MTO required that cut-outs 2m x full paver width be made to ensure that the placement and alignment of the dowel bars met contract requirements. This type of destructive verification was typically carried out only at the beginning of concrete placement and not repeated.

The Ministry of Transportation's Materials Engineering and Research Office has been performing an ongoing evaluation of the Magnetic Imaging Tomography (MIT) Scan-2 as a innovative non-destructive test for evaluating dowel bar alignment on concrete pavement contracts. This was facilitated through an equipment evaluation program established by the Federal Highways Association (FHWA), which loaned the MTO an MIT scan device in September – October 2006. The scan was used by MTO to train staff and to measure selected joints throughout southern Ontario on three concrete pavement highways.

The three highways evaluated were: Highway 417, Highway 404 and Highway 401. The pavement structure on Highway 417 consisted of 200 mm thick jointed plain concrete pavement (JPCP) with 456 x 32 mm dowel bars as load transfer devices. The dowel bars were inserted by a four-track slip form paver with a rear-mounted automatic dowel bar inserter (DBI). On Highway 404 the pavement consisted of 250 mm JPCP with 456 x 32 mm dowel bars. The dowel bars were placed using metal baskets secured to the granular base. Finally on Highway 401 the pavement structure consisted of 260 mm JPCP with 456 x 32 mm dowel bars. The dowel bars were inserted with a four-track paver with a centrally located dowel bar inserter.

A total of 8388 dowel bars were scanned over the three highways. After analyzing the recorded data with the post processing software MagnaProoftm many issues with poor dowel bar alignment such as: uncut transport ties, dowel bars intersecting tie bars and major side shift became evident, helping prove the usefulness and need for the MIT Scan-2 on all concrete pavement contracts. Actual use of the MIT scan and its post processing software has allowed the Ministry to conclude that the MIT Scan-2 can be used as a successful real time, non-destructive inspection tool. When used as Quality Control (QC) tool by the contractor, the MIT Scan-2 can help identify dowel bar alignment problems early in the paving operation and identifying reject able dowel bars for repair.

The next step in implementing the MIT Scan-2 as a routine inspection tool on concrete paving contracts will be to convince industry of its usefulness and the validity of this technology as a QC/Quality Assurance (QA) tool.

When concrete pavements are designed correctly and constructed to specification, 30 years or more of service can be expected. Using the MIT Scan-2 to insure that all dowel bars are aligned properly provides another inspection tool to insure the MTO obtains the maximum life expectancy from this asset. Ultimately this technology will help reduce future costly repairs and allow the Ministry to allocate funding to other areas of need. The Ministry is committed to this technology and will continue to evaluate the MIT Scan-2 to insure that it meets the ministry's needs.