Ralejs Tepfers

Bond of FRP Reinforcement in Concrete - a Challenge

The bond of ordinary steel reinforcement in concrete depends on many factors, such as the pullout resistance, the geometry of a concrete member, the placement of a bar in the member cross section, the cover splitting, the confinement caused by concrete and the surrounding reinforcement, the order of bond-crack appearance, and the bond-stress distribution along the bond length. The bond of FRP reinforcement depends on even a greater number of factors. Moreover, the types of FRP bars are numerous. Their surface is weaker than that of steel bars and may fracture by bond forces. The surface of FRP bars is softer and does not create as high local stress concentrations in bond contact points to concrete as the harder steel bars do. This fact often delays the appearance of cover splitting cracks along the bars. However, the load necessary for developing the crack pattern of ultimate splitting failure in concrete is then very dependent on whether the bar surface is glossy or rough. The FRP reinforcement can also be used for external shear and/or flexural strengthening of existing members. For this application, FRP bars are placed in grooves cut on the surface of the member to be strengthened and are fixed there with a cement mortar or epoxy paste. In such an application, the performance of bond between the FRP rod and the mortar or resin and then between the mortar or resin and concrete is critical for the effectiveness of the technique. The presence of two interfaces increases the number of parameters needed to characterize the global “joint” behavior and introduces new possible failure modes. The fundament for the bond resistance estimation should be an accepted bond philosophy linked to appropriate models. A system of bond tests should provide necessary coefficients for the models.

Ductility of nonmetallic hybrid fiber composite reinforcement for concrete

Reinforcing units, FRP, of unidirectional fiber composites for concrete have elastic behavior up to tensile failure. For safety reasons an elongation of 3% at maximum load is usually required for the reinforcement. Ductile behavior with the necessary elongation and stress hardening could be obtained with braided fiber strands around a core of foam plastic, thin glass fiber cylindrical shell, or unidirectional carbon fibers. Braids around a porous core reveal the ductility when epoxy resin breaks up and collapse of core enables the braids to rotate. The same seems to happen at that cross section, where carbon fiber core breaks in tension. The best result is obtained using a cylindrical glass fiber reinforced core shell surrounded with aramid fiber braid.

CONCRETE CYLINDERS CONFINED BY CFRP SHEETS SUBJECTED TO CYCLIC AXIAL COMPRESSIVE LOAD

T. ROUSAKIS , C. S. YOU , L. DE LORENZIS , V. TAMUŽS , R. TEPFERS 5 1 Dept. of Civil Eng., Democritus University of Thrace, Xanthi, Greece 2 Dept. of Mechanical Eng., Pohang Univ. of Sci. and Tech., Pohang, South Korea 3 Dept. of Innovation Eng., University of Lecce, I-73100 Lecce, Italy 4 Institute of Polymer Mechanics, Univ. of Latvia, Aizkraukles 23, Riga, Latvia 5 Dept. of Building Materials, Chalmers University of Tech., Goteborg, Sweden

The influence of sustained stress on the durability of GFRP bars embedded in concrete

A new test equipment has been created to study the combined effect of environmental exposure and external stress on the durability of a GFRP (E-Glass/Vinyl ester) rebar. GFRP specimens embedded in moisture-saturated concrete have been conditioned at two temperatures and at two levels of sustained tensile stress (5% and 25% of the ultimate tensile strength). According to the preliminary results, stress levels up to 25% of the original tensile strength do not appear to have a negative influence on the durability of GFRP bars embedded in concrete.