International Concrete Abstracts Portal

In this research, the interaction behavior between fiber and concrete was investigated by biaxial plain loading experiments on mortar panels strengthened with various fiber sheets (carbon, aramid and glass) and analyses based on the Modified Compression-Field Theory. As result of the experiment and analysis, it was confirmed that (1) pure shear and pure tensile strengths of the strengthened panels are in proportion to the tensile strength of the fiber, (2) analysis based on the Modified Compression-Field Theory can express the experimental results excellently, and (3) in the case of fibers with small elastic modulus, there is a vast difference between local shear strains and average strains. Further analysis was conducted with the elastic modulus and weight per unit area set as the variable factors.

This paper presents the results of laboratory testing of concrete beams and field testing of two concrete T-Beam bridges that were externally strengthened with CFRP. As a result of the lab testing all the systems tested showed a considerable increase of 68% for the ultimate strength and 45% decrease in deflection for the beams with 2-#4 reinforcing bars . The beams with 2-#7 reinforcing bars showed a 60% increase in ultimate strength and a 26 % decrease in deflection when comparing the loads that produced yielding of the steel in the baseline and the 5 layer test. Both structures were load tested and monitored before and after application of the CFRP with strain gages and LVDTs. The results showed an increase up to 8% in stiffness and up to 15% decrease in strain. In addition to the load test a quality assurance NDT evaluation was performed on the bond using infrared thermography. The results showed typically less than 2% voids between the CFRP and the concrete.

The uniaxial tensile tests of Reinforced Concrete elements with Carbon fiber sheet (RCC) are conducted to clarify the basic mechanical characteristics which affect the tension stiffness of RCC. This paper mainly presents the difference between RCC and ordinary Reinforced Concrete member (RC) in the load carrying capacity, the average crack spacing, stress and/or strain distributions of steel, and the average stress – strain relationship of concrete. Crack spacing of RCC becomes smaller between 25% and 60% of that in RC as the amount of Carbon Fiber Sheet (CFS) increases. The strain distributions of steel in RCC before and after the yielding of steel differ from those in RC. The tension stiffness developed by the bond action of CFS increases as the amount of CFS increases but that of steel becomes less. With the steel and CFS contributions combined, the tension stiffness of concrete as a whole generally becomes greater than that in RC. There is, however, a case in which tension stiffness of concrete in RCC decreases more than that in RC, because the average bond stress of steel rapidly decreases after the yielding of steel.

The simple-supported concrete beams reinforced with aramid FRP (AFRP) rods and carbon FRP (CFRP) sheets were tested to failure using a symmetric two-point concentrated static loading system. AFRP rods were used instead of steel reinforcing bars, and CFRP sheets were epoxy bonded to the tension face of the concrete beams to enhance their flexural strength. Moreover, a 5-cm wide strip of CFRP sheet in some places were wrapped around the web (hereafter, called “U-jacket”) after the CFRP sheets were bonded. The strain distributions on AFRP rods and CFRP sheets, and flexural behavior of the beams with AFRP rods and CFRP sheets were examined experimentally. The results showed that; 1) peeling of CFRP sheets occurred near the maximum flexural moment region, 2) ultimate load and deflection of the beam with U-jackets were higher, and 3) the U-jacket was a significant factor affecting the ductile behavior of beams with CFRP sheets.

In addition to the conventional modes of failure observed in RC beams, new ones can be detected in RC members strengthened by means of externally bonded FRP reinforcement. Concrete cover delamination is a mode of failure caused by shear transfer and local regions of tension stress fields. A series of tests were carried out in order to study the concrete cover delamination failure, wherein the variables were length of beam span, bonded area, number of plies, and U-jacketing schemes. Two mechanisms within the concrete cover delamination failure were observed: one starting at the cutoff point of the FRP, which is originated by a high concentration of normal (out-of-plane) and shear stresses, and second one starting at an intermediate crack. The latter mode of failure is caused by normal and shear stresses at the level of the steel reinforcement. From the point of view of design, it is important to recognize this premature type of failure, and determine algorithms for its prediction.