International Concrete Abstracts Portal

This study comprehensively investigates the development of ambient-cured self-consolidating geopolymer concrete (SCGC) based on the chemical composition of binders and alkaline activators. Five factors of the chemical composition of binders and alkaline activators, each with four levels, are used to evaluate and optimize the workability and compressive strength of the high-strength SCGC. The designed SCGC mixtures provided sufficient workability properties and compressive strength between 28 and 70.3 MPa (4061 and 10,196 psi). It was found that the SCGC mixture with a binder content of 600 kg/m3 (37.4 lb/ft3), a CaO/(SiO2 + Al2O3) mass ratio of 0.55, an Na2O/binder mass ratio of 0.11, an SiO2/Na2O mass ratio of 1.2, and an Na2O/H2O mass ratio of 0.35 was the optimum mixture, which achieved a slump flow of 770 mm (30.3 in.), 28-day compressive strength of 70.3 MPa (10,196 psi), and final setting time of 80 minutes. The CaO/(SiO2 + Al2O3) ratio in binders, binder content, and Na2O/binder mass ratio have been found to be the most influential factors on the workability and compressive strength of ambient-cured SCGC. Microstructural analysis of SCGC mixtures showed that the increase in the CaO/(SiO2 + Al2O3) ratio promoted the formation of calcium- aluminate-silicate-hydrate (C-A-S-H) gels and enhanced the compressive strength by filling voids and creating a compact and dense microstructure.

At low temperatures, the development of concrete strength is slow and can seriously hinder construction progress. The traditional early-strength components cannot meet the requirements of green and high-performance concrete. In addition, research on the early-strength accelerators at low temperature is paltry, and the effect of early strength was limited, but the mechanism of early strength at low temperature remains unclear. Calcium bromide (CaBr2) was used as a new kind of early-strength component, and its effects on mortar strength, cement paste setting time, and early hydration characteristics were evaluated. The incorporation of CaBr2 shortened the setting time of cement pastes and accelerated the strength development of specimens of all ages (the 28-day strength continued to increase dramatically). The compressive strength of mixed mortars increased between 18 and 376%, and the mortar strengths after 3 days met or even exceeded that of the contrast sample, cured at 20°C (293.15 K). The presence of CaBr2 decreased the solubility of Ca(OH)2, so it reached saturation and precipitated more easily. CaBr2 also significantly increased the dissolution and hydration rates of C3S by shortening hydration induction and advancing acceleration. Furthermore, the maximum heat-release rate and cumulative heat release could be increased by CaBr2, which shortens the nucleation and crystal growth (NG) stage and the phase-boundary reaction (I) stage. Large amounts of Ca(OH)2 formed after only 12 hours, and new products, such as bromine-containing calcium-silicate-hydrate (C-S-H) gels and hydrated calcium bromoaluminate (Ca4Al2O6Br2∙10H2O), were also generated. These products piled up and bonded, resulting in a denser microstructure.

In this paper, variable temperature (VT) curing was used on ultra-high-performance concrete (UHPC) specimens to determine the effects of early-age temperature histories on UHPC mechanical property development. A systematic test plan evaluated 16 programmed VT profiles with different target temperatures as well as VT start and end times. VT curing profiles were intentionally exaggerated from typical time-temperature curves of hydrating concrete to expose the effects of timing on mechanical property development. Specimens that were placed into VT curing immediately after molding produced higher maximum temperatures compared with specimens that were exposed to VT curing 1 day after molding. However, specimens that were cured at room temperature for 1 day prior to VT curing had significantly higher compressive strength and elastic modulus. These findings show the importance of predicting in-place temperatures that develop in UHPC structures as mechanical properties can be drastically altered based on when high temperatures occur.

Relatively limited work has been performed to quantify how internal curing influences curing specifications. This paper examines the performance of internally cured mixtures (made using fine lightweight aggregates) compared to conventional concrete cured with wet burlap and curing compounds. Mortar mixtures were prepared using ordinary portland cement (OPC), fly ash, and silica fume (SF) with water-cementitious materials ratios (w/c) of 0.35 and 0.45. Neutron radiography (NR) was used to determine the nonevaporable water content as a function of curing time and distance from the exposed surface. The curing-affected zone (CAZ) was determined using the nonevaporable water profiles. The CAZ was used to develop equivalent curing durations for conventionally cured and internally cured samples. Internally cured mixtures reduced the depth of the CAZ, especially in the samples with limited external curing durations (reduction up to 15 mm [0.6 in.]). The application of internal curing in all mixtures reduced the duration of external curing by 50 to 60%, except for the internally cured SF samples, which showed a slight reduction. This dramatically impacts the construction schedule.

In this study, an optimal curing method was established for very early-strength latex-modified concrete (VES-LMC), which is frequently used in partial-depth repair (PDR) of deteriorated concrete pavements. The appropriate starting time of curing, when the surface of the VES-LMC was not damaged, was found for various curing conditions such as ambient air, polyethylene (PE) sheet, blanket, curing membrane, PE sheet on curing membrane, and blanket on curing membrane. The hydration characteristics of the VES-LMC and ordinary portland cement concrete (OPCC) were then compared by evaluating their respective properties such as water loss, bleeding, autogenous shrinkage, and compressive strength. In addition, the optimal curing method was investigated by determining the water loss, water absorption, drying shrinkage, and compressive strength of the VES-LMC specimens cured under the aforementioned conditions. The test results revealed that VES-LMC performed better than OPCC as a PDR material. In addition, covering the VES-LMC with a PE sheet 3 minutes after placement was observed to be the most effective curing method in PDR.