Field monitoring of integral abutment bridge approach slabs

Integral abutment bridges (IABs) eliminate the joints between the deck and abutments and hence have become more prevalent in the U.S. due to lower maintenance costs and longer service life. Cracking of approach slabs immediately adjacent to IABs, which requires maintenance, repair, and even premature replacement, has been observed recently at a number of IABs in Illinois. Agency survey across the U.S. indicates that other states have also observed similar cracking in IAB approach slabs. Field inspections of existing approach slabs for bridges in Illinois suggest there are common crack patterns among mainline interstate highway bridges and also in cross-road bridges. To study the long-term behavior of IAB approach slabs, a four-lane cast-in-place approach slab and a three-lane precast approach slab were instrumented. Changes in concrete strain and temperature at distributed plan locations and at several locations through the slab thickness were measured. Global displacements of the slabs due to thermal deformations were also measured. Results indicate that slab deformations are in many cases linearly related to temperature change. Nonlinear behavior is observed at some locations of the approach slabs close to the abutment, especially during the summer of 2018. Static truck testing was conducted to study the effects of live load on the approach slabs. Although live load effects were distinctly observed, the thermal strain change even during daily cycles is more substantial compared to live load induced strains. Finite element analyses of the IAB approach slabs under live load and temperature change were also conducted. These numerical results were used to estimate the elastic modulus of subgrade reaction for the soil beneath the approach slabs. To date, no significant cracks have been observed in the instrumented approach slabs, and the modest measured field strains are consistent so far with physical observations