CRACKING OF PRE-STRESSED CONCRETE GIRDERS ON BL CREEK BRIDGE

THIS PAPER REVIEWS the cracking of the pre-stressed concrete girders on BL Creek bridge in the Racecourse Mill area. The bridge is a curved, four-span (each 12 m long) bridge built in 1982 to service an area that now delivers 30 000 to 35 000 tonnes of cane from six cane farms. Cracking in the girders was first noted during bridge inspections in 2005/06 non crush season and following load testing, load and speed restrictions were put into place for the 2006 crushing season. Deflection monitoring was introduced and cracking continued to grow with a large longitudinal crack forming along the horizontal centre-line of a girder in span 4. Longitudinal cracks became evident on all visible girder surfaces and increased in number and length as time progressed. The cause of this cracking was attributed to alkali-silica reaction (ASR). Examination of the girder drawings showed that they were constructed with no shear reinforcement with the exception of the ligatures near the end supports. Structural analysis of the girders shows that they are do not comply with the Bridge Code requirements for longitudinal shear for the loadings induced by the passage of locomotives. In 2014 a risk assessment determined that the girders needed to be replaced given the amount of deterioration that had occurred coupled with the ingress of water. The assessment determined that the method of failure for the bridge would likely be very sudden with little to no warning when it occurred. The girders were replaced for the 2014 crushing season. This experience shows a need for drawings of bridges with pre-stressed concrete deck units to be checked for the presence of voids and shear reinforcement due to the susceptibility of ASR and for regular monitoring of bridges for cracking where voids exist and shear reinforcement is not present.