Degradation Kinetics of Pultruded E-Glass/Vinylester in Alkaline Media

Results are presented from a 75-week investigation aimed at characterization of deterioration mechanisms and performance of E-glass/vinylester in an alkaline environment. The material is characterized using dynamic mechanical thermal analysis, fouriertransform infrared spectroscopy, gravimetric moisture uptake, tensile and short-beam-shear tests, and microscopy. Results show a range of deterioration mechanisms initiating with reversible plasticization of the resin and transitioning to irreversible fibermatrix debonding, hydrolysis, and chain scission of the resin, and pitting and material loss of the fiber. Reconditioning, following periods of immersion, results in some regain in tensile strength over at least half the exposure period, whereas after 15 weeks there is almost no regain in interlaminar shear strength. Predictions of longterm response match fairly closely with experimental results at the 75-week level and show that, under self-similar continuation of deterioration, 26.89% of the original tensile strength and 58.24% of the short-beam-shear strength can be expected to be retained after 50 years.

Keywords: alkali; deterioration; fiber-reinforced polymer; glass fiber.

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INTRODUCTION

Due to their noncorrosive nature, high specific strength, and light weight, fiber-reinforced polymer (FRP) composites are increasingly being considered for use in civil infrastructure. The use of these materials in the form of externally bonded reinforcement for rehabilitation or in the form of reinforcing bars and tendons for new reinforcement provides a potential means of retarding, or even completely mitigating, the deterioration seen in conventional materials such as steel and concrete. The relatively low cost and high level of ultimate strain achievable, impact resistance, damage tolerance, and general ease of processing makes the use of E-glass fiber reinforcement very attractive, even when compared with the vastly superior stiffness and inertness of carbon fiber.

There is, however, a concern related to the long-term durability of E-glass reinforced FRP with concrete due to the well-established vulnerability of these fibers to deteriorate when in contact with alkalis.1-3 Bare glass fibers, in contact with alkalis of chemistry and pH similar to pore water of concrete, are degraded rapidly through a process of pitting, etching, leaching, and embrittlement. The presence of high pH with hydroxyl ions breaks siloxane bonds in the glass network2 with the calcium hydroxide (which is a critical component of concrete) and calcium carbonate, inducing the formation of a thin film around the fiber and causing hydroxylic attack resulting in transport of Ca ions to the surface and the gradual extraction/leaching of silica from the fiber.4 The chemical attack due to hydration and dissolution reactions results in the formation of a hydroxide, or of a calcium-silicate hydrate, culminating in the existence of a gel-coat of H2SiO3 that causes rapid degradation of the glass structure itself.5 Besides the effect of chemical attack resulting in fiber surface pitting, cracking, and degradation, it should be noted that these forms of damage further act as flaws reducing fiber properties in the presence of moisture.

Although the use of a polymeric resin as a matrix in composites affords a level of protection to the fibers from direct contact with such environments and precludes the immediate formation of alkali products at the fiber surface, it does not restrict the eventual migration of moisturecarrying alkaline salts and ions towards fiber surfaces by diffusion through the bulk resin, or by wicking along fibermatrix interfaces. Results of previous studies have indicated substantial degradation with premature failure in some cases6,7 and rapid loss in tensile strength8,9 in others, with alkaline salts penetrating through the resin towards and onto the fiber surfaces.10,11

The potential for rapid deterioration of properties when combined with the propensity of glass fiber-reinforced composites for creep and stress-rupture has resulted in recommendations of fairly high, and often very conservative, factors of safety related to its use.12-14 In a recent publication, Sen, Mullins, and Salem15 reported that specific E-glass/vinylester reinforcement used by the U.S. Navy had a predicted service life of between 1.6 and 4.6 years in an unstressed condition and only 0.5 and 1.7 years when stressed to 10% of ultimate. This is, however, not universally accepted with diverse claims on durability being made by various researchers and manufacturers. A primary reason for this is the fact that, even today, degradation kinetics of E-glass composites subjected to alkaline exposure are not well understood. In addition, vinylesters, which are often used as resins in reinforcing bar and structural applications, have a complex progression of cure due to simultaneous but ratedifferentiated reactions of homopolymerization and copolymerization leading to incomplete polymerization of the bulk polymer.16,17 This can lead to changes in properties with time due to slow progression in degree of cure, a greater degree of susceptibility in some cases of swelling in solvents, and a lower initial resistance to hydrolysis.18 Thus, although vinylesters have been shown to have good attributes for low-cost processing and appear to have enhanced durability characteristics, there are still a large number of fundamental questions related to the level of long-term durability of E-glass/vinylester composites when placed in direct contact with, and even encapsulated in, fresh concrete.