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  Strengthening bridgesCCE graduate student John Rhodes hopes to give bridges a new life by strengthening pre-stressed concrete structures that have sustained damage during vehicle impact. Repairing the beams is significantly less expensive than replacing them. In his Iowa DOT-funded research, Rhodes simulates the damage caused by over-height vehicles to pre-stressed concrete girders and then repairs the damaged girder by attaching carbon fiber-reinforced polymer sheets. The 20-inch-wide CFRP sheets are systematically applied to the concrete using a bonding epoxy. Unidirectional fibers of each sheet provide a tensile strength approximately ten times that of standard structural steel. Rhodes’ goal is to determine the behavior of the CFRP system for strengthening impact-damaged pre-stressed concrete girders. In the testing of the system, two hydraulic cylinders are positioned on top of a repaired pre-stressed CFRP-reinforced concrete girder; each cylinder can apply up to 200,000 lbs of force. As the applied load is increased, strain gauges and displacement transducers monitor the girder’s behavior. A computer-controlled data acquisition system plots the reading and stores data relating to changes in concrete strains, CFRP strains, and deflections. Rhodes and CCE professors Terry Wipf and Wayne Klaiber then analyze the test data to determine the strength of the repaired pre-stressed beam. The CFRP system is being tested for strength considerations as well as weather-related durability and endurance factors. Carbon-bonded concrete elements have been exposed to the natural environment for over a year now and will be periodically tested for reductions in bond strength from freeze/thaw cycles and humidity. In September, this repair technique was used on six beams of a pre-stressed concrete bridge that was struck by an over-height vehicle on an Iowa highway. The bridge, which was tested prior to the repairs, will be re-tested in the next few weeks to determine the effectiveness of the repairs. Rhodes’ study of the CFRP system on bridges holds promise for other pre-stressed or reinforced concrete elements. |
