This research focuses on developing a mixture design for highstrength geopolymer concrete (HSGPC) complying with the highstrength concrete criteria mentioned in Indian standards. This study focuses on optimizing the content of alkaline activators and binders proportionately. The compressive strength of different proportions of geopolymer mortar was carried out meticulously to determine the optimal proportions of solution-binder (S/B) and sodium silicatesodium hydroxide (SS/SH) ratios. The aforementioned ratios were optimized using the Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) analysis for further calculation. The mixture proportions for Grades M70, M80, M90, and M100 were determined and verified through experimental validation. To assess the suggested mixture design, a slump test was conducted to quantify the workability, subsequently followed by the evaluation of compressive strength after 24 hours, 7 days, and 28 days. After achieving the desired workability, promising compressive strength was observed as 76, 89, 93, and 104 MPa at 28 days. Finally, the mechanism of strength increment was investigated using various characterization techniques, such as X-ray diffraction (XRD) and scanning electron microscopy (SEM) equipped with energydispersive spectroscopy (EDS). The SEM/EDS analysis of the HSGPC proves the dense microstructures of different gel formations. The proposed mixture design procedure falls under the target strength-based method category. It has successfully yielded a strength of 104 MPa for ground-granulated blast-furnace slag (GGBS)-based geopolymer concrete incorporating coarse and fine aggregates.

This work aims to decrease the damage in concrete caused by freezing-and-thawing. For this, concrete was healed using the polymer containing phosphazene after the freezing and thawing. The Taguchi method was used to decrease the experimental numbers. The experimental variables were determined as cement dosage, the phosphazene percentage, and curing time. 100 × 100 × 100 mm (3.94 × 3.94 × 3.94 in.) cubes were prepared for experiments. After demolding, the samples were cured in a water tank at 20°C ± 2°C (68°F ± 3.6°F) until the test ages (28, 60, 90, 180, and 365 days) were reached. These samples were then subjected to the freezing-and-thawing cycles. The healing process was conducted to the samples by impregnation with the polymer containing phosphazene after freezing-and-thawing cycles. Lastly, the compressive strength, ultrasonic pulse velocity, and weight change of concretes were determined. Scanning electron microscope, energydispersive X-ray spectroscopy, and X-ray powder diffraction analyses were performed to examine the microstructures of the samples. The results showed that the impregnation of polymer containing phosphazene after the freezing-and-thawing increased the strength and durability of the concrete.

The use of seawater as mixing water in reinforced concrete (RC) is currently prohibited by most building codes due to potential corrosion of conventional steel reinforcement. The issue of corrosion can be addressed by using noncorrosive reinforcement, such as glass fiber-reinforced polymer (GFRP). However, the long-term strength development of seawater-mixed concrete in different environments is not clear and needs to be addressed. This study 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 seawater-mixed concrete and concrete mixed with potable water were comparable, except for set times, which were accelerated in seawater-mixed concrete. 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 reference concrete. Specifically, when exposed to seawater at 60°C (140°F), seawater-mixed concrete shows higher compressive strength development than reference concrete, with values at 24 months being 14% higher. To explain strength development of such mixtures, further detailed testing was done. In this curing regime, the seawater-mixed concrete had 33% higher electrical resistivity than the reference concrete. In addition, the reference concrete showed calcium hydroxide leaching, with 30% difference in calcium hydroxide values between bulk and surface. Reference concrete absorbed more fluid and had a lower dry density, presumably due to greater seawater absorption. Seawater-mixed concrete performed better than reference concrete due to lower leaching because of a reduction in ionic gradients between the pore solution and curing solution. These results suggest that seawater-mixed concrete can potentially show better performance when compared to reference concrete for marine and submerged applications.

In this project, the effect of using lightweight aggregate (expanded slate) on the early-age cracking tendency of mass concrete mixtures was evaluated. Concretes representative of mass concrete mixtures—namely, normal-weight concrete, internally cured concrete, sand-lightweight concrete, and all-lightweight concrete—at two different water-cementitious materials ratios (0.38 and 0.45) were tested in cracking frames from the time of setting until the onset of cracking. The development of early-age concrete stresses caused by autogenous and thermal shrinkage effects were measured from setting to cracking. The behavior of concretes containing lightweight aggregates was compared with normal-weight concrete placed under temperature conditions simulating fall placement in mass concrete applications. Increasing the amount of pre-wetted lightweight aggregates in concrete results in systematic decrease in density, reduced modulus of elasticity, and reduced coefficient of thermal expansion. All these factors effectively improve the concrete’s early-age cracking resistance in mass concrete applications.

This research work was on the evaluation of the flexural strength and compressive strength relationship of spent foundry sand (SFS) concrete. The relationship was established using a concrete mixture of 1:1.71:2.56, a cement content of 404 kg/m2, and a water-cement ratio (w/c) of 0.52. This was used to cast beams of dimensions 150 x 150 x 500 mm (6 x 6 x 20 in.) cured for 90 days in a water curing tank under laboratory conditions. The SFS was used to replace fine aggregate (FA) 0 to 40% by wt. The evaluations on the statistical characteristics of the flexural strength data results showed that the addition of SFS to concrete improved the hydration process. This was reflected in the strength development of the concrete and the strong correlation and level of significance observed with the variables (mixture and age). The values of the modulus of rupture (MOR) obtained are in the range of 4.6 to 6.6 MPa; this was at the optimum replacement of 10%. At this level, the value of the flexural strength was approximately 29% of the compressive strength. The two models chosen that represented the flexural strength and compressive strength relations are the square root and 2/3 models. The relative predictive error (RPE) for each is 0.1 and 0.2, respectively.