Author: Cheng Chen

Publisher:

Published: 2016

Total Pages:

ISBN-13: 9781339825847

Near-surface mounted (NSM) method using Fiber Reinforced Polymer (FRP) composites proves to be promising in structural strengthening and rehabilitation of deficient reinforced concrete (RC) and prestressed concrete members. However, the fatigue behavior of strengthened members is still not well understood and needs extensive research before practical design guidelines are established. The objective of this research is to experimentally and analytically assess the fatigue performance and predict the fatigue life of NSM CFRP strengthened RC beams. In particular, it aims to: (1) experimentally evaluate the bond performance of the NSM CFRP reinforcement-epoxy interface and its degradation; (2) propose an analytical model to predict the bond performance; (3) evaluate the test results of the flexural performance of the NSM CFRP strengthened RC beams subject to fatigue loadings; and (4) develop models to predict the corresponding flexural performance and fatigue life of the strengthened concrete members. Thirty-six NSM strengthened concrete block specimens were fabricated and then subject to various fatigue loadings. Direct pull-out test was conducted on each specimen to obtain the bond characteristics and performance. Variables considered in this study include: cycles of fatigue loadings and types of NSM reinforcements. The result shows that specimens using NSM rods had higher bond strength at the epoxy-NSM interface than that at the concrete-epoxy interface, resulting in the governing failure due to breakage of epoxy and concrete. Specimens using NSM strips, on the other hand, failed mainly due to debonding (at the epoxy-NSM interface). Moreover, fatigue loading tended to shift the FRP strain and the local bond stress distribution from the loaded end towards the free end in NSM-strengthened specimens. The local bond stress-slip relationship under fatigue followed a hardening-softening behavior for both types of NSM reinforcement. Fatigue loading generally decreased the bond strength while the corresponding slip remained almost unchanged. A theoretical solution to this bond characteristics under fatigue loading is derived to predict the debonded length progression and fatigue behavior of the bond related variables (e.g. distribution of bond stress, slip, and tensile stress of the NSM reinforcement) for concrete blocks strengthened by NSM CFRP reinforcement. Twenty RC beams strengthened with NSM FRP were constructed in 2 batches, one strengthened by NSM rods and the other by NSM strips. Sixteen NSM strengthened specimens were subject to different levels of fatigue loading until failure, while four strengthened specimens were tested under monotonic loadings. Typical elastic-plastic behavior was observed in monotonic test, yielding of specimen occurred when the tensile strain of steel rebar reached around 0.2%. Most fatigue specimens failed by rebar rupture and small group of specimens failed by debonding between NSM and concrete. However, unstable crack propagation was also observed to occur with rebar rupture in certain specimens, especially specimens with lower fatigue load and longer fatigue life. The crack and deflection progression could be generally divided into three distinct stages: (1) initial stage with decelerating increase; (2) steady stage with increase of constant and small rate; (3) final unstable stage of accelerating increase, and the second stage occupied most of fatigue life. An analytical solution to the flexural response of the RC beams strengthened with NSM CFRP reinforcement is derived, using a trilinear bond-slip relation for the CFRP-epoxy interface and the previously derived theoretical solution to bond characteristics. As the analytical model provides solution to flexural response, an independent fatigue life prediction model is also developed for reinforced concrete beams strengthened with NSM FRP rods under flexural fatigue loading. The model is based on the fracture-mechanics approach using cohesive model and capable of predicting fatigue life due to rebar rupture, where the effect of cohesive stresses due to aggregate, steel rebar, NSM FRP reinforcement is approximated by specific cohesion laws.In conclusion, this dissertation presents thorough and detailed research to experimentally and analytically investigate the flexural behavior of NSM FRP beams, including the strain of FRP, rebar and concrete, deflection, and crack progression, etc. Moreover, approaches to address the fatigue life prediction is also proposed and verified by experimental results.