EFFECT OF ADHESIVE STIFFNESS AND CFRP GEOMETRY ON THE BEHAVIOR OF EXTERNALLY BONDED CFRP RETROFIT MEASURES SUBJECT TO FATIGUE LOADS

摘要

Nine 10" (254 mm) deep, 6" (152 mm) wide and 186" (4730 mm) long concrete beams having three #4 longitudinal steel reinforcing bars as primary flexural reinforcement, were tested under midpoint cyclic loading until fatigue-induced failure or to 2,000,000 cycles. Eight beams were strengthened with four different soffit-mounted externally bonded carbon fiber reinforced polymer (CFRP) arrangements. A commercially available 4" (102 mm) wide, 0.055" (1.4 mm) thick preformed unidirectional carbon fiber reinforced strip system was used along with two commercially available adhesive systems. The adhesive systems consisted of a high-modulus and a low-modulus adhesive with their respective stiffness's varying by a factor of two. The effects of CFRP geometry, the width of the strip to the width of the soffit of the beam ratio (bf/b), and adhesive shear stiffness on the behavior of fatigue loaded specimens were investigated.Four of the retrofit specimens failed due to fatigue-induced internal reinforcement rupture prior to achieving 2,000,000 cycles. Observations of these specimens include stress range drift from cycle N=2 and N=Nf, degradation of secant stiffness, and the effects of retrofit geometry on the stress carried by the steel reinforcement. Stresses at cycle N=Nf were noted to increase significantly from cycle N=2. The CFRP was noted to increase the secant stiffness of the retrofit specimens and slow the rate of decay of stiffness when compared to the control fatigue specimen. As CFRP area increased, the stress in the steel reinforcing was noted to decrease. The reduction in steel reinforcement stress for the load range in this research is proportional to the amount of CFRP attached to the soffits of the beam.The other four retrofit specimens were cycled to 2,000,000 cycles and then tested monotonically to failure. Observations of these specimens indicate that the cyclic loading had significant effect on the bond using the low-modulus adhesive. In the companion thesis, Reeve (2005), Reeve states that the low-modulus adhesive consistently exhibited superior debonding behavior to the high-modulus adhesive. This effect is apparently negated by the effect of cyclic loading for the low-modulus adhesive.