International Concrete Abstracts Portal

This study evaluates the potential to use shrinkage-reducing admixture (SRA) and pre-saturated lightweight sand (LWS) to shorten the external moist-curing requirement of ultra-high-performance concrete (UHPC), which is critical in some applications where continuous moist-curing is challenging. Key characteristics of UHPC prepared with and without SRA and LWS and under 3 days, 7 days, and continuous moist curing were investigated. Results indicate that the combined incorporation of 1% SRA and 17% LWS can shorten the required moist-curing duration because such a mixture under 3 days of moist curing exhibited low total shrinkage of 360 με and compressive strength of 135 MPa (19,580 psi) at 56 days, and flexural strength of 18 MPa (2610 psi) at 28 days. This mixture subjected to 3 days of moist curing had a similar hydration degree and 25% lower capillary porosity in paste compared to the Reference UHPC prepared without any SRA and LWS and under continuous moist curing. The incorporation of 17% LWS promoted cement hydration and silica fume pozzolanic reaction to a degree similar to extending the moist-curing duration from 3 to 28 days and offsetting the impact of SRA on reducing cement hydration. The lower capillary porosity in the paste compensated for the porosity induced by porous LWS to secure an acceptable level of total porosity of UHPC.

Engineered cementitious composites (ECCs) have excellent toughness and crack-control abilities compared to other cement-based materials, which can be used in underground and hydraulic engineering. Nevertheless, excellent impermeability and workability and low drying shrinkage are also required. Two groups of ECC mixture proportions with high fly ash-cement (FA/c) and watercement ratios (w/c) were chosen as baselines, and silica fume (SF) and a shrinkage-reducing agent (SRA) were introduced to improve the impermeability, workability, and mechanical behaviors. The workability laboratory evaluation indexes of ECC were also discussed. ECC mixture proportions with excellent workability (pumpability and sprayability), high toughness (ultimate tensile strain ɛtp over 3.5%), good impermeability (permeability coefficient K = 1.713 × 10–11 m/s), and low drying shrinkage (drying shrinkage strain ɛst = 603.6 × 10–6) were finally obtained. Then, flexural and shear tests were carried out for the material flexural/ shear strength and toughness evaluations, giving the characteristic material properties for the final ECC mixture proportions.

To improve the added application value of an industrial waste stone powder (SP), the optimizing mechanism of SP for the structure and composition of hydrothermal synthetic hardened cement stone was investigated in this paper. Cement was partially replaced by SP, silica fume (SF), or ground-granulated blast-furnace slag (GGBS), and then the microstructure with different SP content was tested through X-ray diffraction, thermogravimetric analysis (TG-DTG), mercury intrusion porosimetry (MIP), and scanning electronic microscopy. The findings indicate that the incorporation of SP in autoclaved products significantly enhanced compressive and flexural strengths. As the proportion of SP in cement was increased, a corresponding increase in the content of tobermorite within autoclaved cement mortar was observed. This increase in tobermorite concentration results in an initial rise followed by a subsequent decline in both compressive and flexural strengths. The maximum compressive and flexural strengths were achieved at an SP content of 15%. In addition, the mechanical strength was further improved by adding SP+GGBS or SP+SF. The strengthening mechanism of SP reveals that the change in the ratio of calcium and silicon ions (C/S) caused by SP in the sample was conducive to the formation of tobermorite and strength increase. Meanwhile, an increase in the quantity and a decrease in the crystal size of tobermorite were observed with an increase in the content of stone powder, resulting in a more compact microstructure of the sample. Moreover, the mechanical strength of cement composites doping SP+GGBS or SP+SF was further improved through superposition effects of SP and GGBS or SF with high activity. Currently, it is mainly applied to pipe pile products, and the strengthening effect of SP increases its use value. Meanwhile, the study of SP strengthening mechanism has laid a theoretical foundation for its application in high-strength autoclave and improved the relevant theory.

This paper is aimed at investigating the effects of different fiber inclusion on the mechanical properties of ultra-high-performance concrete (UHPC) by adding mineral admixtures as cement replacement materials to reduce production costs and CO2 emissions of UHPC. Throughout this research, 21 mixture designs containing four cement substitution materials (silica fume, slag cement, limestone powder, and quartz powder) and three fibers (steel, synthetic macrofibers, and polypropylene) under wet and combined (autoclave, oven, and water) curing were developed. To investigate the mechanical properties in this research, a total of 336 specimens were cast to evaluate compressive strength, the modulus of rupture (MOR), and the toughness index. The findings revealed that at the combined curing, regarded as a new procedure, all levels of cement replacement recorded a compressive strength higher than 150 MPa (21.76 ksi). Furthermore, the mechanical properties of the mixture design containing microsilica and slag (up to 15%) were found to be higher than other cement substitutes. Also, it was shown that all levels of the fiber presented the MOR significantly close together, and samples made of synthetic macrofibers and steel fibers exhibited deflection-hardening behavior after cracking. The mixture design containing microsilica, slag, limestone powder, and quartzpowder, despite the significant replacement of cement (approximately 50%) by substitution materials, experienced a slight drop in strength. Therefore, the development of this mixture is optimal both economically and environmentally.

Self-healing is a process by which concrete is able to recover its properties after the appearance of cracks, which can improve mechanical properties and durability and reduce the permeability of concrete. Self-healing materials can be incorporated into concrete to contribute to crack closure. This study aims to evaluate the influence of crystalline admixtures and silica fume on the self-healing of concrete cracks. The rapid chloride penetration test was performed on cracked and uncracked samples, from which it was possible to estimate the service life of concretes. The concretes were characterized by tests of compressive strength and water absorption by capillarity. The use of crystalline admixtures did not have a negative influence on concrete properties, but did not favor the chloride penetration resistance. The concrete with silica fume showed the lowest charge passed and highest values of estimated service life.