Abstract
The corrosion of steel reinforcement is one of the main causes of chemical deterioration in reinforced concrete (RC). The replacement of conventional steel bar reinforcement by corrosion-resistant materials has been evaluated over the years. Fiber-reinforced polymer (FRP) composite bars have been under constant development to be used as internal reinforcement for concrete structures due to their high-strength, lightweight, and, most importantly, non-corrosive properties. Among pultruded FRP bars, glass fiber reinforced polymer (GFRP) bars are the most widely used. Using this type of non-corrosive material provides an alternative to replace fresh water with seawater in the production of concrete since the use of seawater in RC structures is prohibited due to its high presence of chloride that favors the corrosion of steel reinforcement. This dissertation focuses on the microstructural, flexural, and durability performance of GFRP bars in seawater-mixed concrete.
First, the constituent contents of four (4) commercially available pultruded GFRP bars were evaluated by quantitative analysis of scanning electron microscope (SEM) micrograph of cross-sectional samples through digital image processing (DIP). The fiber and resin matrix volume fractions acquired from the DIP method were converted to weight fraction by means of constituent relationship equations and were compared to the standardized resin burn-off technique (ASTM D2584). Comparable values were obtained from both methods; however, the DIP method has the ability to provide additional microstructural information. In addition, the correlation between void/defect content, moisture absorption capacity, and tensile properties was also evaluated. The void/defect content correlated with the moisture uptake at a substantially saturated condition. After the absorption/desorption protocols, the tensile strength and ultimate strain rupture were significantly affected, while the elastic modulus remained mostly unaltered.
Then, a total of forty-eight (48) GFRP-RC slabs reinforced with a 9.5 mm (.375 in.) diameter GFRP bar were cast using two different seawater-mixed concrete mixtures. The GFRP-RC slabs were exposed to accelerated aging [seawater at 60°C (140°F)] and field [25°C (77°F)/71.2% RH] conditioning. After 1, 6, 12, and 24 months of exposure, three-point bending tests were performed on all GFRP-RC slabs to investigate their flexural behavior. Strength capacities were calculated using an analytical and simplified approach (ACI 4401.R-15). The experimental test results were compared with the expected values in terms of flexural performance (first crack, ultimate and design capacity, and deflection). The type of concrete mix design, as well as the accelerated aging, seems to affect the ultimate capacity of GFRP-RC slabs. Analytical and ACI approaches reasonably predicted the experimental failure-moment capability of most of the seawater-mixed GFRP-RC slabs, specifically those exposed to field conditioning. The ACI 440 equations were in good agreement with the experimentally measured deflections, where the largest deviations were observed for accelerated-aged specimens.
Finally, the residual physico-mechanical properties of GFRP bars embedded in seawater-mixed concrete were evaluated over 24 months. The mechanical test results of GFRP bars are provided as residual capacities of tensile strength, longitudinal elastic modulus, transverse shear strength, and apparent horizontal shear strength. Physical evaluations are reported in terms of glass transition temperature (Tg) and microstructural integrity through scanning electron microscopy (SEM) images and energy-dispersive X-ray spectroscopy (EDS) analysis. Among all tested properties, tensile strength was the most affected by the environmental conditions. Based on an exponential degradation model, the long-term prediction of the tensile strength capacity was on average 92% under field exposure and 72% under the more aggressive conditioning [seawater at 60°C (140°F)].