Showing papers by "Hugo Biscaia published in 2013"

Linear and nonlinear analysis of bond-slip models for interfaces between FRP composites and concrete

Abstract: The paper analyses different analytical and numerical solutions for the debonding process of the FRP-to-concrete interface on shear tests with the FRP plate submitted to a tensile load in one of its ends. From the point of view of the state of the art, two different ways of finding the bond-slip curve from experiments are discussed and analysed. Essentially, three different linear bond-slip models, one exponential model and another power based function are employed in the numerical process. The results are analysed and compared. The differences found in the stress field along the interface, maximum load, maximum slip, ultimate slip, fracture energy and effective bond length are reported. The load-slip behaviour is also presented for the linear and non-linear models herein studied and the influence of the local bond-slip model on the debonding process is discussed. The numerical integration process used on the present study proved to be coherent with the analytical expressions determined for the linear bond-slip models and allowed to verify that maximum load transmittable to the FRP plate is influenced by the square root of the FRP stiffness and fracture energy even when nonlinear bond-slip models are assumed.

Bond–slip on CFRP/GFRP-to-concrete joints subjected to moisture, salt fog and temperature cycles

Abstract: Research on bond between composite and concrete on beams externally reinforced with fiber reinforced polymers (ERBs) has generated many publications, but uncertainties remain. The issue of the long term behavior of those joints, especially the effect that severe and prolonged environmental actions may induce, justifies the search for additional data and recommendations to avoid premature debonding and failures. The present paper devotes attention to the effects of environmental aging on the constitutive bond-slip curves. Aging was imposed on an accelerated manner and several actions used to degrade the behavior of the joints evaluated by mechanical testing. Prismatic blocks of reinforced concrete linked on the upper side by a stainless steel hinge and externally bonded by a continuous strip of carbon or glass fiber reinforced polymer (CFRP or GFRP) adherent to the soffit were made so as to fit in commercially available laboratory climatic chambers. After aging, they were tested till failure under four point loading. Aging conditions imposed on the CFRP beams were (i) cycles of high-low relative humidity, (ii) salt fog cycles, and, on the GFRP beams, (iii) salt fog cycles, (iv) dry/wet cycles (water with 5.0% weight of NaCl), (v) total immersion in salt water, and (vi) freeze/thaw cycles. The results of the experimental program enabled the proposal of bond-slip laws that take into account the aging of the beams. They also showed that salt fog cycles were more severe in the case of CFRP, while freeze/thaw cycles were more degrading on bond of GFRP-to-concrete. The salt water effects on the GFRP beams appeared to be beneficial, most likely by improving the tensile strength of concrete. Numerical modeling certified by the obtained experimental data is presented that allows more general estimates of the environmental effects.

Nonlinear numerical analysis of the debonding failure process of FRP-to-concrete interfaces

Abstract: The paper analyses numerical solutions for the process leading to debonding failure of fibre reinforced polymers (FRP)-to-concrete interfaces in shear tests with the FRP plate subjected to a tensile load at one end. Any realistic local nonlinear bond–slip law can be used in the numerical analysis proposed in the present study. However, only a Popovics’ type expression is employed in the numerical process due to its use in different studies found in the literature. Effective bond length ( L eff ) is discussed and an expression depending on the Popovics’ constant ( n P ) is proposed to calculate it. Assuming a fracture in pure Mode II, the debonding process is analysed in detail and distributions of bond stresses and strains in the FRP plate along the interface are presented. The load–displacement behaviour is also presented and the influence of the local bond–slip law on the debonding process is discussed.

Influence of Temperature Cycles on Bond between Glass Fiber-Reinforced Polymer and Concrete

Abstract: Reinforced concrete (RC) beams externally strengthened with glass fiber-reinforced polymer (GFRP) strips bonded to the soffit may see their load-carrying capacity reduced due to environmental conditions-especially due to the deterioration of bond between the adhesively bonded laminates and concrete, causing premature failure. More research has been published on the detachment of the laminate progressing from the anchorage zone than on failure induced by the formation of flexural or shear-flexural cracks in the midspan followed by fiber-reinforced polymer (FRP) separation and failure designated as intermediate crack (IC) debonding. An experimental program to study degradation of the GFRP laminate beam specimens after accelerated temperature cycles, namely: 1) freezing-and-thawing type; and 2) cycles of the same amplitude (40°C [104°F]) and an upper limit approximately 70% of the glass vitreous transition temperature of the resin, Tg, is described. Effects on the bond stress and ultimate capacity are reported. Substantial differences between shear and bending-induced failure and a decrease of bond stresses and engagement of the laminates on the structural response are analyzed.