Abstract
In order to improve the sustainability of the construction industry, which is responsible for 12 % of all fresh-water consumption, seawater could be an advantageous replacement for fresh water in mixing concrete especially in coastal regions where fresh water may be scarce. Seawater could potentially be used in unreinforced concrete and mortar (i.e. bricklaying, renders, etc.) or in combination with non-corrosive reinforcement (i.e. Glass Fiber Reinforced Polymer (GFRP) bars). In order to achieve the widespread usage of such technology, the fundamental behavior of seawater-mixed concrete and embedded GFRP bars need to be studied. This dissertation consists of three studies which cover the durability of GFRP bars in seawater-mixed concrete (Study 1), compressive strength of seawater-mixed concrete under different curing regimes (Study 2), and shrinkage behavior of seawater-mixed concrete (Study 3). Study 1 investigates the effect of seawater used as mixing water in concrete on the long-term properties of GFRP bars. The durability of GFRP bars embedded in seawater-mixed concrete was studied in terms of residual mechanical properties (i.e. tensile strength, horizontal and transverse shear strength, and GFRP-concrete bond strength) after immersion in seawater at 60 °C for a period of 24 months. Benchmark specimens were also cast using conventional concrete. Results showed comparable performance between the two sets of bars. Some degradation of the mechanical properties was observed in both cases, with the most degradation being observed in the bond strength. Tensile strength decreased by 21 – 26%, tensile modulus by 6 – 12%, horizontal shear strength by 21 – 26%, and transverse shear strength by 25 – 28%. The bond strength showed the highest degradation, with 47 and 55% reductions for bars extracted from conventional and seawater-mixed concrete, respectively. Scanning electron microscopy was used to identify degradation mechanisms. Areas with large concentrations of voids near the bar edge, formed during manufacturing, may provide a pathway for moisture and alkalis into the bar which could lead to fiber disintegration and debonding between fibers and the resin. Over time, a greater number of fibers are affected, which leads to the formation of significant cracking near the edge. This could explain the greater degradation in bond strength. Study 2 reports the results of an investigation on the effect of different environments (curing regimes) on the compressive strength development of seawater-mixed concrete. Fresh properties of concretes prepared with seawater and concrete mixed with tap water were comparable, except for set times, which were accelerated in seawater-mixed concretes. Concrete cylinders were cast and exposed to subtropical environment (outdoor exposure), tidal zone (wet-dry cycles), moist curing (in a fog room), and seawater at 60 °C (140 °F) (submerged in a tank). Under these conditions, seawater-mixed concrete showed similar or better performance when compared to conventional concrete. In order to further understand strength development of such mixtures, Thermogravimetric Analysis (TGA), Energy-dispersive X-ray spectroscopy (EDX), and electrical resistivity measurements were performed at the end of 24 months on specimens exposed to seawater at 60 °C (140 °F). In this curing regime, concrete mixed with seawater constantly performed better than conventional concrete by 10 – 18% over the 24 months. The reason for the better performance is lower leaching of the calcium hydroxide from the concrete mixed with seawater, due to a reduction in ionic gradients between the pore solution and curing solution in concrete mixed with seawater. These results suggest that concrete mixed with seawater can potentially show better performance when compared to conventional concrete for marine and submerged applications due to lower leaching. The shrinkage behavior of cementitious materials mixed with seawater is investigated in Study 3. Cement mortar mixtures were prepared with two water-to-cementitious materials ratios (w/cm = 0.36 and 0.45), two binder compositions (namely, ordinary Portland cement (OPC) and OPC with 20 % fly ash replacement), and two types of water (tap water and seawater). The autogenous and drying shrinkage behavior of these mixtures are examined using ASTM standard test methods for 65 days. The use of seawater as mixing water increased the autogenous shrinkage. At w/cm 0.36, the ultimate autogenous shrinkage increased from 213 μs in the mixture with tap water to 387 μs in the mixture with seawater; corresponding values were 149 μs and 314 μs for mixtures with w/cm 0.45. An acceleration of the cement hydration at early ages due to the seawater is identified as the cause of the increase in autogenous shrinkage in mixtures with seawater. At w/cm 0.36, seawater did not have a strong effect on the drying shrinkage and tested mixtures had ultimate drying shrinkage values between 543 μs and 663 μs. At w/cm 0.45, in mixtures without fly ash, ultimate drying shrinkage increased from 838 μs in the mixture with tap water to 1027 μs in the mixture with seawater. In mixtures with fly ash, the ultimate drying shrinkage increased from 738 μs in the mixture with tap water to 1370 μs in the mixture with seawater. The drastic increase in the drying shrinkage in mixtures containing fly ash and seawater at w/cm 0.45 seems to be due to the development of a finer pore size distribution and internal water movement. In applications where drying shrinkage may be a concern, the use of fly ash in seawater-mixed concrete could be problematic.