Assessment and prediction of damage in aged concrete bridge decks using the impact-echo method

A non-destructive testing program has been performed to evaluate the integrity of damaged concrete bridge decks on three pre-cast, reinforced concrete slabs that were removed from decommissioned bridges built in the 1950's in South Carolina, USA. Impact-echo tests were first performed to non-destructively assess the initial condition and distribution of damage throughout the slabs by analyzing the variation in propagation wave velocity. Two deterioration zones were characterized in Slab 1 that had an average Cp of 3700 m/s and fc' of 2600 psi for rough surface and 4100 m/s and fc' of 3600 psi for smooth surface. It was found that the P-wave velocity varied by as much as 1000 m/s throughout the slabs. Two slabs were non-destructively evaluated in the laboratory using the impact-echo method. Impact-echo tests were performed concurrently with full-scale monotonic and fatigue load tests. Impact-echo tests were performed before and after the loading sequence for each slab and between each application of fatigue loading for the second slab. Results from tests on the monotonically loaded slab detected a significant reduction in P-wave velocity after failure indicating a reduction in the slab stiffness. Loading history of the aged slabs was deducted from the observed asymmetry in the P-wave velocity distribution, indicating certain loading concentrations on the bridge while in service. For the monotonically loaded slab, it was observed that the impact-echo method differentiated between points of stiffness loss, crack propagation, localized damage and no damage. Results were compared to cores and were in good agreement.; Impact-echo tests on the fatigue-loaded slab quantified the degradation of the slab during fatigue testing by tracking the change in P-wave velocity and growth of cracks. Significant damage such as cracking was detected earlier than visually observed and before the slab reached service failure. Loading behavior obtained by IE varied for points equidistant from the loading plate due to the aged condition of the slab prior to load tests. Theoretically they should behave the same. In addition, the crack distances were determined with an absolute error of 0 to 7%. For the fatigue loaded slab it was observed that localized failure and crack propagation within the slabs were a function of initial concrete quality, cracks, and proximity to loading plate.; Finally, the results obtained from impact-echo tests were used to predict the remaining service life of the aged slabs. The prediction was performed using the modulus of elasticity of concrete estimated from impact-echo tests, the applied service moment on the slab and input in three different fatigue models.