New polymer coat wears well
The Poplar Street bridge across the Mississippi River in St. Louis, Mo., is an orthotropic steel-plate deck bridge. It carries three major interstate highways, I-70, I-64 and I-55, across the Mississippi River at St. Louis. Approximately 130,000 vehicles including about 15,000 large trucks use the bridge each day. The five-span bridge is 2,165 ft long and consists of two independent bridges supported on a set of common piers. Each bridge carries four lanes of traffic and is supported by two box girder with girder depths ranging from 16 ft to 25 ft. The deck-plate thickness is typically 9?16 in. and is stiffened by closed trapezoidal stringers or ribs. The 5?16-in.-thick stiffeners on 13-in. centers are 11 in. deep and run along the length of the bridge. Load from the deck also is transferred to the box girders by transverse floor beams coinciding with every fourth floor beam location, providing the bridge additional torsional rigidity. There are approximately 30 such orthotropic steel-plate bridges in the world including six in the U.S.
When the bridge was constructed in 1967, the steel deck was covered with a wearing surface consisting of two layers of epoxy tack coat and 11?2 in. of rubberized asphalt concrete wearing surface. Stone chips were embedded in the second layer of epoxy as an anchor for the rubberized asphalt layer. This wearing surface system performed well until 1983. When the first wearing surface was no longer serviceable it was removed completely to expose bare metal of the deck and a replacement wearing surface was applied. This second wearing surface was intended to be identical to the original surface. It lasted less than three years. The eastbound lanes were replaced for a third time in 1986. This third wearing surface incorporated a proprietary system that included a fiberglass reinforcing mat in the asphalt wearing surface layer. Despite the reinforcing mat, unacceptable amounts of rutting and shoving necessitated the placement of a new wearing surface during the 1992 construction season.
Six wearing surface material systems were evaluated as potential replacements for the wearing surface on the Poplar Street Bridge by researchers and the University of Missouri-Columbia (MU) under a Missouri Highway and Transportation Department (MHTD) contract. Two were asphaltic concretes, three were epoxy-polymer concrete and one was a methyl-methacrylate polymer concrete. Laboratory tests included flexural fatigue tests on wearing surface-deck plate composite specimens at a constant temperature of 0°F, flexural fatigue tests under cyclic temperature variations from 0°F to 160°F and ancillary tests (resistivity and pull-out tests) to evaluate the condition of the surfaces before and after the fatigue tests. Concurrently with the laboratory tests, a test section of each material was placed on the bridge by its supplier and subjected to normal traffic up to two years. These field test sections were observed regularly for evidence of rutting, shoving, cracking, delamination or other signs of deterioration. Based on the performance of these materials in the laboratory tests and the test sections on the bridge, Transpo T-48 polymer concrete was observed to be the best of the materials tested and the only material that did not delaminate or fail during the field test period.
The MHTD selected and specified the polymer concrete for the new wearing surface. Placement of this wearing surface was started in July 1992.
A total of 20 working days was used by the contractor, Pace Construction Co. of St. Louis, to place the 226,000 sq ft of 1?2-in.-thick overlay. Transpo Industries worked with Eltech Inc. to design a specialized and highly accurate self-contained mobile mixer for the large volume mixing of the slurry.
No prior field experience was available on the use of polymer concrete wearing surface systems on large steel decks subjected to a combination of severe environmental and heavy traffic conditions. Also since there was potential for cracking and delamination, particularly at the colder temperatures, and conventional crack maintenance programs were considered unlikely to work for the new material, the MHTD was interested in closely monitoring the long-term performance of the new wearing surface system. MU was awarded a five-year contract for long-term inspection and testing of the wearing surface. The yearly inspection includes resistivity tests to qualitatively monitor cracking in the wearing surface, pull-out tests to establish the adhesion strength in tension between the wearing surface from the deck plate, observations to record wearing surface thickness, aggregate loss and other signs of deterioration.
Two visible cracks, one at each end of the southmost-lane (in the transition zone from 21?2 in. thick at the finger joint to 1?2 in. thickness of the overlay on the eastbound bridge) which appeared after one year of service were repaired using epoxy polymer. No further crack growth or deterioration of the wearing surface in the vicinity of the cracks were observed. While the cause for these cracks has not been conclusively established, differential thermal expansion significant to the only lane with the cracks whose beam was exposed to sunlight is being studied as a possible culprit.