Wen-Wei Wang

Prestress Losses and Flexural Behavior of Reinforced Concrete Beams Strengthened with Posttensioned CFRP Sheets

An experimental study was conducted to investigate the flexural behavior and long-term prestress losses of reinforced concrete (RC) beams strengthened with posttensioned carbon fiber-reinforced polymer (CFRP) sheets. The experimental program consisted of tensile tests of flat CFRP coupons under sustained loads and flexural tests of a total of eight RC beams: six strengthened with posttensioned CFRP sheets, one strengthened with nonprestressed CFRP sheets, and one control beam. The main objective of the tests was to gain a better understanding of the long-term prestress losses of CFRP sheets in the posttensioned system under different prestress levels and strengthening ratios. It is shown that the prestress losses of CFRP sheets in the posttension system are mainly attributable to anchorage set (approximately 12.6 to 18.2% of the initial prestress), whereas the time-dependent losses caused by creep and shrinkage of concrete and relaxation of CFRP sheets are relatively small (approximately 2.3 to 3.9% of the initial prestress). DOI: 10.1061/(ASCE)CC .1943-5614.0000255.

Long-Term Behavior of Prestressed Old-New Concrete Composite Beams

This paper presents both theoretical and experimental studies of the long-term behavior of prestressed old-new concrete composite beams under sustained loads. General differential equations governing the relationship between the incremental deflection and incremental internal forces of the composite beams were deduced in the theoretical study. Closed-form solutions for simply supported composite beams were obtained and validated using test results reported in previous literature on steel-concrete composite beams. The experimental investigation consisted of static long-term load tests carried out on four prestressed old-new concrete composite beams. The behavior of the old-to-new concrete interface, time-dependent deflections, concrete strains, and prestress losses was carefully observed over 260 days. The long-term test program showed that the midspan deflections and concrete strains increased with time because of creep and shrinkage of the new prestressed concrete. The slip strains at the old-to-new concrete interface were found to be relatively small, indicating that the interface bond was sound enough to prevent slip and that the prestressing loads were effectively transferred to the old concrete. The proposed theoretical models predicted the long-term behavior of the prestressed old-new concrete composite beams with an acceptable degree of accuracy. DOI: 10.1061/(ASCE)BE.1943-5592.0000152.

Intermediate crack-induced debonding in RC beams externally strengthened with prestressed FRP laminates

Intermediate crack-induced debonding is often a dominant failure mode in fiber-reinforced polymer (FRP)-strengthened reinforced concrete (RC) beams in flexure. It has been extensively studied for RC beams externally strengthened with unstressed FRP laminates. However, very little work has been done on FRP debonding for RC beams strengthened with prestressed FRP. This article presents a sectional analysis model for predicting the flexural capacity of RC beams strengthened with prestressed FRP laminates with due consideration of different failure modes. The focus is placed on the effective strain in the prestressed FRP at the ultimate states of intermediate crack-induced debonding or rupture of debonded FRP. Through back-calculation analysis of 51 RC beams strengthened with post-tensioned FRP laminates, a model for predicting the effective strain of prestressed FRP for ultimate strength prediction based upon sectional analyses was developed and validated through comparisons with test results.

Performance Evaluation of RC Beams Strengthened with an Externally Bonded FRP System under Simulated Vehicle Loads

Carbon fiber-reinforced polymer (CFRP) sheets can be used to strengthen existing RC bridge girders/decks. The objective of this study is to investigate whether transient vehicle loads present during the installation of CFRP influence the bond performance between the CFRP and the concrete substrate and, therefore, the consequent strengthening effect. A total of eight RC beams were tested. Two were unstrengthened reference beams, five were strengthened with CFRP sheets while subject to transient loads, and the final beam was strengthened with CFRP sheets while subject to only a sustained static load. The test parameters included the amplitude of the transient load, the anchorage length of the CFRP sheets, and the reinforcing/strengthening ratio. The transient loads were continued for 2 days (during CFRP cure) before all five CFRP-strengthened RC beams were tested to failure in four-point flexure. The test results were compared with those of the reference beams and the one strengthened under a sustained load. It was shown that a 1-Hz sinusoidal transient load varying between 30 and 50% of the ultimate capacity of the unstrengthened beam during the installation and curing of the CFRP sheets does not affect the structural performance of CFRP-strengthened RC beams. This result demonstrates the applicability of the fiber-reinforced polymer (FRP) strengthening technique for bridge girders that are subjected to continuous vehicle loads.

Self-Stressed Steel Fiber Reinforced Concrete as Negative Moment Connection for Strengthening of Multi-span Simply-Supported Girder Bridges

A self-stressed steel fiber reinforced concrete (SS-SFRC) moment connection is proposed to strengthen multi-span simply-supported T-section reinforced concrete (RC) girders. In the proposed moment connection technique, old concrete needs to be removed from adjacent girder ends and negative moment reinforcement is placed into cast-in-place SS-SFRC to develop continuity over the interior support. Two series of tests consisting of 15 girder specimens were conducted to investigate the reliability of the proposed SS-SFRC moment connection. An analytical model is presented to predict the load-carrying capacity of the resulting continuous composite girders. Test results show that SS-SFRC is excellent in preventing concrete cracking at the interior support. Both the load-carrying capacity and the serviceability of the strengthened girders are significantly improved. The analytical model is demonstrated to predict the load-carrying capacity of the resulting continuous composited girders with an acceptable degree of accuracy.

Prediction of prestress losses in RC beams externally strengthened with prestressed CFRP sheets/plates

This paper presents an analytical model for predicting the short and long-term prestress losses in reinforced concrete (RC) beams strengthened with prestressed carbon fiber reinforced polymer (CFRP) sheets/plates. The long-term prestress loss model is developed by treating the strengthened beam as two sub-elements: an RC beam and a prestressed CFRP sheet/plate, connected through an adhesive bonding layer. Through force equilibrium and displacement compatibility of the two sub-elements, the influences of concrete shrinkage, concrete creep as well the interlayer slip between the RC beam and the CFRP sheet/plate on the prestress losses of CFRP are modeled using an incremental model. For a post-tensioned system with strong end anchorages, closed-form solutions for the prestress losses due to creep and shrinkage of concrete are obtained. The reliability of the proposed analytical model is validated through comparisons with previous test results reported by the authors and others research groups. The maximum difference between the experimentally observed values and the predicted values for the long-term prestress losses was found to be only 2.2% of the initial prestress of the CFRP.