Strength Degradation of Laminated Composites under Hygrothermal Loading Conditions

Abstract

Fiber-reinforced polymer (FRP) composites are becoming increasingly popular materials for reinforcing and/or strengthening concrete members. FRPs have high strength, light weight, excellent resistance to aggressive environments, and represent a very effective solution to the deterioration problem caused by the corrosion of steel reinforcement in structural concrete.

FRPs materials, as is known, are influenced by environmental conditions such as freezing and thawing, moisture, temperature, solar radiation, and aggressive chemical agents that occur during their service life. The degrading effect of environmental actions on the mechanical properties of FRPs reduces the durability of concrete structures. As a consequence, for an efficient use of FRPs as substitutes for traditional steel reinforcements in reinforced concrete structures, an analysis of the durability of FRP materials considering all possible deteriorating conditions is necessary.

One of the most important environmental factors affecting the durability of FRPs is the temperature variation. In hot-climate regions, the concrete structures reinforced with FRP rebars, in fact, are continuously subjected to thermal actions corresponding to daily temperature variations. So, the natural environmental conditions represent a source of degrading effect that needs accurate investigations in evaluating the durability of FRPs.

This thesis presents some results of an ongoing experimental program undertaken to investigate the suitability of FRP laminated composites working in natural environment conditions. The experimental investigations performed include thermal, with temperature value of 450C, water wetting and drying cycles, tensile and bending tests of GFRP (Glass FRP) and CFRP (Carbon FRP) after their conditioning.

The mechanical and physical properties tensile strength, Flexural strength, capacitance, micro hardness, diffusivity and area of fraction of FRP laminates, determined from hygrothermal exposure, are compared with those of the unexposed laminated composites. The mechanical degradation (strength reduction and decrease in the elastic modulus) is then analyzed and discussed. The degradation is also analysed at macroscopic and microscopic levels by using SEM analyses of the exposed and unexposed specimen. An attempt has been made to relate macroscopic and microscopic behaviors.