Edge curling effect on interface delamination of concrete overlays for bridge decks.

摘要

Deterioration of reinforced concrete bridge decks caused by various environmental conditions and traffic loads is a problem of significant concern. A protective thin overlay of about 2-inches is typically placed over newly constructed concrete bridge decks. Concrete overlay mixtures offer high resistance to traffic loads and environmental attacks, and often serve as leveling finished surfaces. A critical problem however has been the debonding and delamination propagation of the interface between overlay and substrate concrete, as observed in several bridges in West Virginia. Several studies indicated that during early-age of overlay curing, the debonding usually starts at corners or edges of the slab by curling stresses due to differential shrinkage between newly cast overlays and mature substrate. With time the crack front progressively grows within the interface leading to delamination of the overlay, and further distress cracking and deterioration.As a part of a large-scale study on bridge deck overlays for the WVDOH, the current investigation focuses on early age curling and delamination behavior of four types of overlays for three substrate surface conditions at two ambient temperatures. A number of bi-layer prismatic specimens were produced and conditioned and tested in an environmentally controlled chamber. A circular interface delamination notch was introduced at the specimen's corner. The corner and edge crack-openings due to possible delamination propagations were continuously monitored using, respectively, a clip-on gage and three displacement transducers (LVDT) connected to a data acquisition system. An ultrasonic pulse velocity method was used to predict the interface crack front propagation. The waveform was recorded with a digital oscilloscope, and through Fast Fourier Transform the amplitude-time history was converted into power spectrum for analysis. The temperature-time gradient across the overlay was measured with thermocouples to evaluate its role on edge curling and delamination.Results showed that the initial corner crack propagated along the interface edges and within the interface surface. The readings were consistent within and among the specimens. Higher temperature resulted in more delamination and the substrate surface with saturated condition and applied bonding slurry performed best, while the dry surface condition was the worst. Among overlays, the silica fume modified concrete cracked and debonded more than the latex modified concrete. However the use of shrinkage reducing admixtures significantly reduced the curling and delamination. The results of the ultrasonic pulse velocity test to map the crack-front propagation were consistent with the displacement-time data obtained with the transducers. The spectral analysis showed that the highly irregular overlay-substrate delaminated interface surface generated additional frequency components lower than the natural frequency of the transmitting transducer. The time-temperature profile did not appear to affect the curling and debonding as the temperature gradient was nearly stable within 24 hours and before delamination growth was detected.