Behaviour of thin bonded plain concrete overlays on bridge decks under fatigue loading

Abstract

Fatigue is one of the main causes of failure in structures subjected to cyclic loading. Since bridges are constantly subjected to cyclic loading due to the high traffic loading, deterioration in stiffness is often the result. A new remediation system or strengthening method was developed and tested for its behavior under fatigue. The newly developed approach at Vienna University of Technology of testing large structural elements by benefiting from the resonance effect which effectively reduces the experimental time and cost was implied. The testing setup at the Institute for Structural Engineering at Vi-enna University of Technology provided the possibility of fatigue loading for full-scaled specimens under higher forces and with higher frequencies.<br />A thin bonded plain concrete overlay is added to the substrate or existing bridge deck to enhance or restore its strength. As the overlay is also the new concrete carriage way, it will reduce the weights on the bridge as no asphaltic surface or overlay is needed anymore. Due to the thin bonded overlay system the overlay substrate interaction would be stronger.<br />To exactly model the behavior of the overlay on a box girder bridge in the field region and in the flange region, two types of specimens were tested. Specimens with the overlay working as a compression zone supplement which idealize the field region and specimens with the over-lay working as a tension zone supplement representing the flange region. For the course of the experimental work two specimens with compression zone supplement and five with a tension zone supplement were constructed. Three of the specimens with a tension zone supplements were doweled for comparison reasons.<br />For high cycle fatigue testing, the specimens were all subjected to four million cycles and two of them underwent a static bending test after the fatigue loading test. Specimens with the plain concrete overlay in the compression zone displayed positive response as the overlay fully contributed in the loading capacity of the section. The bonding layer showed no signs of deterioration during the fatigue loading test as the recorded deflections were very small. The behavior of the bonding layer could be detected through the static bending test afterwords. The bonding layer turned out to be very strong making the capacity of the composite section nearly as big as the capacity of a homogeneous section. Providing a strong bond the horizontal movement between overlay and substrate at the loading capacity of the section was nearly only 1 mm.<br />The specimens with the overlay in the tension zone showed lower efficiency since the plain concrete overlay cracked at some stage or the other. Due to the cracking the plain concrete overlay did not contribute to the loading capacity. Accordingly large deflections and excessive crack growth and propagation in the section were observed which led to high stiffness deterioration during the fatigue loading test. Also the bonding layer experienced only small relative horizontal movement despite the crack propagation through overlay and substrate. In spite of some local weaknesses observed in the bonding layer the new remediation system was efficient. Similarly the static bending test showed no excessive relative horizontal movement between overlay and substrate.