Water, saltwater, and concrete leachate solution effects on durability of ambient‐temperature cure carbon‐epoxy composites

Abstract

Wet layup carbon epoxy composites are extensively used for rehabilitation of civil infrastructure. This study focuses on foundation building for understanding durability of such materials through long‐term (up to 5 years) hygrothermal aging in water, saltwater, and concrete leachate, solutions. Moisture uptake is described using a two‐stage model incorporating diffusion and relaxation/deterioration parameters with the concrete leachate solution showing further transitions and the highest value of the relaxation/deteriorative coefficient. Glass transition temperatures decreased with an increase in moisture content reaching an initial minimum followed by fluctuations and small increases with an increase in the time of exposure. Immersion in concrete leachate results in the largest decrease. Tan δ height shows an initial decrease followed by an increase and then a continued decrease indicative of an initial plasticization and post‐cure followed by deterioration at the resin and fiber–matrix levels. Although short‐term response in mechanical properties shows differences with immersion in water having the least effect, solution type had very little difference on residual mechanical characteristics at the end of the 60‐month period of immersion. The results emphasize the need for further understanding of competing effects in ambient‐cured composites and of the need for longer term testing for determination of design factors.

Seawater Effects on Thermally Aged Ambient Cured Carbon/Epoxy Composites: Moisture Kinetics and Uptake Characteristics

“…Interface degradation can result in the formation of annular microchannels, resulting in more rapid diffusion [39], which is the driver for non-Fickian diffusion phenomena [40]. The extent and type of change at the level of the polymer network and fiber-matrix interphase is not only dependent on the time of exposure but also on the type of aqueous solution [41][42][43][44]. For composites cured under ambient conditions, initial immersion in aqueous solutions can lead to the progression of curing and acceleration in the initial increase of glass transition temperature, followed by significant decreases [44][45][46].…”

“…The extent and type of change at the level of the polymer network and fiber-matrix interphase is not only dependent on the time of exposure but also on the type of aqueous solution [41][42][43][44]. For composites cured under ambient conditions, initial immersion in aqueous solutions can lead to the progression of curing and acceleration in the initial increase of glass transition temperature, followed by significant decreases [44][45][46]. Of special interest to the longterm durability of these materials are the phenomena of hydrolysis and chain scission, which decrease density and can eventually result in the leaching of low-molecular weight species from the bulk polymer and interfacial regions.…”

“…The increase in diffusivity can similarly be explained by the enhanced initial formation of cracks and microcavities in the polymer and composite [51]. Though there is a body of work on the effect of seawater exposure, there is still a significant need to gain a comprehensive understanding of these phenomena and the resulting effects [41,44], especially as related to multiple and sequential exposure conditions.…”