Experimental and numerical evaluation of rapid post-tensioning of damaged reinforced concrete girders with unbonded near surface mounted shape-memory alloy wires
Date
2019
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Publisher
University of Delaware
Abstract
American Society of Civil Engineers (ASCE) classified 9.1% of the national bridges as structurally deficient in 2017. Several of the deficient concrete bridges have girder serviceability issues such as excessive cracking and deflection. Traditional external post-tensioning techniques involve high management of traffic costs and use of heavy construction equipment. To that end, a novel rapid post-tensioning technique using unbonded near-surface mounted shape-memory alloy (SMA) wires was herein evaluated. The suggested method relies on the intrinsic ability of the SMA to recover seemingly permanent deformation upon activation via heating. Mechanical characterization tests demonstrated that the SMA wires can generate 67 ksi of prestress. Within the range of SMA reinforcement ratios considered in this study (up to 0.17%), the structural-scale tests showed that post-tensioning of cracked reinforced concrete girders can reduce the crack width and residual midspan deflection by up to 370 µm (74%) and 0.06 in. (49%), respectively, and increase the ultimate moment capacity up to 83 kip-ft (45%). Finite element model of post-tensioned girders was developed in a damage mechanics framework and was validated against experimental data – the predicted yield and ultimate moment of the strengthened girders were 14% less than those observed in the experiments. The difference was due to the cumulative effect of errors embedded in the model assumptions. The model was used to evaluate the service and ultimate behavior of strengthened girders over a range of variables such as the extent of cracking in concrete, SMA reinforcement ratio, interaction between SMA and concrete (unbonded vs. bonded), and effective prestress in SMA. The partial loss of prestress due to elastic shortening of concrete is insignificant (<1 ksi) due to a relatively low Young’s modulus of the SMA. A modest increase (up to 3 times) in the ratio of SMA reinforcement ratio resulted in an increase in the yield moment by up to 99%. Reducing the effective prestress to 80% of the applied prestress decreases the yield moment by up to 7.8% in the strengthened girder. Overall, the study demonstrated the promise of the proposed post-tensioning technique in being used as a repair and strengthening strategy for reinforced concrete structures.