Use of Salvaged Utility Poles in Roadway Bridges: Time-Dependent Behavior of Composite Wood-Concrete Beams

This research study focused on the long-term and repeated load behavior of wood-concrete composite beams in which the wood layer is composed of recycled utility poles. Previous research studies have found this type of a beam to have high efficiency and to be a potentially viable replacement for short span bridges on low traffic roadways. Since both wood and concrete exhibit time-dependent behaviors, it is necessary to examine the effects of sustained and repeated loads on the composite beam. Two lab specimens were utilized to examine these behaviors. A long-term load, estimated to be 11% of the maximum capacity, was applied to the first specimen. Due to the rheological effects of the composite beam, the final deflection of the specimen at a time period of 256 days was 1.75 in., which was more than twice the elastic deflection due to the application of the dead and live loads. In order to predict the timedependent deflection, an existing finite element program was utilized as well as a simplified evaluation method. It was determined that, with modifications, both methods provide accurate predictions. The other specimen in the study was subjected to repeated loads up to the design service load for 35,000 cycles. It was found that the repeated loading does affect the effective stiffness of the beam as it dropped to 94.6% of the initial value as a result of the 35,000 load cycles. In addition, the time-dependent deflection of the specimens during the 28-day concrete curing period was monitored, and an empirical relationship previously developed for specimens with a different span was successfully modified to predict the deflection of specimens with various spans.