International Concrete Abstracts Portal

With their strength and their specific stiffness, composite materials present a significant interest in the conception of bearing structures. The influence of combined effects "time-temperature-loading" on composite reinforcement adhesive layer was studied to identify the long-term mechanical behavior of RC beam reinforced with FRP. A set of tests were conducted on reinforced concrete structures with carbon epoxy composites. The tests consist of applying a tensile shear stress during six months to obtain the long-term creep data and to carry out thermo-stimulated test to assess short-term creep data. The master curves set up with this method predicts with reasonable accuracy the long-term creep test data. The time-temperature superposition method is used to determine several master curves with several levels of shear stress. This method permits an evaluation of the long-term shear stress to apply in the adhesive layer to minimize the creep. The durability of repaired or reinforced structure depends on the adhesive behavior. We have assessed that the identification of the long-term creep can be done with a thermo-stimulated test. This test allows setting up the safety factor for any polymer to guaranty the structure durability.

Many pretensioning prestressed concrete (PC) beams using the grid glass fiber reinforced polymer (GFRP) reinforcements as prestressing tendons were manufactured. The initial prestressing forces were selected at various levels, namely from 0 to 52.5 % of the tensile capacity of the grid GFRP reinforcement. Then, the PC beams were left outdoors for a long time, namely from seven to eight years. Thereafter, they were demolished to take the grid GFRP reinforcements out. First, the tests on tensile properties of the grid GFRP reinforcements were carried out. Their residual tensile capacities decreased only a little, and moreover, their residual tensile rigidities did not change. Then, the cross sections of the glass fibers of the grid GFRP reinforcements were observed with a scanning electron microscope (SEM). The cross sections remained real circular and the glass fibers were not attacked by alkali of concrete.

This paper clarifies, experimentally, the degradation of aramid fiber, glass fiber and carbon fiber, used as reinforcement for concrete, in various solutions (alkaline solution, hydrochloric acid aqueous solution and pure water) at different temperatures. A calculation model is proposed to estimate the progress of the degradation by the solution. The accelerated degradation test, immersing fibers in several solutions, was carried out at the temperatures of 20, 40 and 60 degrees Celsius and the strength of the fiber after the immersion test was examined. Observation of the fibers was carried out by scanning electron microscope (SEM) in order to clarify the degradation of the fibers. As a result of this study, the strength changes of Kevlar 49 and Technora were quantitatively estimated using the weakest link theory of Weibull.

In Japan, continuous fiber bars are not allowed to replace reinforcing steel and prestressing steel for load-bearing members, mainly because: 1) Fibers and binders forming continuous fiber bars are made of combustible materials, for which the permissible temperature in case of fire is unknown. Also, these reinforcements are difficult to evaluate by the fire test methods currently practiced in Japan. 2) Safety factors according to members and durability, which are necessary for determining the design strength of continuous fiber bars, are not known, and no performance evaluation methods have been established. The Architectural Institute of Japan (AIJ), Japan Society of Civil Engineers (JSCE) and other institutions proposed about ten test methods for evaluating continuous fiber bars. However, the deteriorating external forces are not known yet in regard to durability, and few reports have been made regarding fire resistance. This paper reports on the commercial use of continuous fiber bars for load-bearing members related to performance evaluation methods and standards on the basis of the proposed standard test methods and state-of-the-art technology on their durability and fire resistance.

The external bonding of FRP to concrete beam is an effective method in order to increase the structural capacity of such beams. The test under fatigue loading of RC beam reinforced with FRP shows that the strengthening of beam with carbon epoxy composite increases fatigue durability [1]. To design composite reinforcement for a RC beam under fatigue solicitation, one has to determine the mechanical behavior of each material and above all, of the concrete composite interface. We have carried out a set of tests with fatigue solicitation on the concrete/composite interface and a set of tests on composite plate. Those tests set up the fatigue life for several levels of shear stress in the interface and in the composite plate and so allow to assess the maximum fatigue stress curve. The second part of the work is based on the information obtained from the first one to design the reinforcement. The design of the composite reinforcement is determined by a software developed by P. Hamelin and H. Nasseri [2,3] using a non-linear calculation method. The design method of those reinforcements is based on classical methods used for RC structures, taking into account the slip phenomena at the interface and considering the mechanical behavior of each material. Suitable range of stress for concrete, steel, composite and interface are determined to predict the fatigue life of the reinforced concrete beams. So, the load range for the test on the beam is set up. Safety factor are determined for the structure loading.