Search

Underwater three-dimensional (3D) concrete printing has a great potential to be applied in submarine construction, but there tends to be significant dispersion and strength reduction of the printed concrete due to washout by surrounding water. This research tried to overcome such problems by enhancing the washout resistance and strength of the printable concrete. A series of seawater coral sand mortar mixes containing silica fume (SF), nano silica (NS), glass fibers (GF) and/or hydroxypropyl methyl cellulose (HPMC) were produced, printed under seawater, and tested for their underwater printability and mechanical properties. The test results showed that both SF and NS can enhance the washout resistance, extrudability and buildability, and improve the flexural, compressive, and bond strengths. GF has also positive but relatively smaller effects. On the other hand, HPMC can improve the underwater printability and bond strength, but may have adverse effects on the flexural and compressive strengths. GF and HPMC, when added together, could cause difficulties in underwater printing, and thus are incompatible. Nevertheless, the combined addition of SF, NS, and HPMC greatly enhances the underwater printability and mechanical properties. Based on these results, some anti-washout 3D printable mortars for submarine construction were developed. [ABSTRACT FROM AUTHOR]

Waste concrete recycled powder (WCRP) occupies up to 30% of crushed old concrete and is hard to be reutilized due to its high porosity and low cementitious reactivity. Traditionally, the WCRP is reused as cement substitution in mortar/concrete production, but this substitution method has certain disadvantages. In this research, a novel substitution method, called the paste substitution method, was applied by replacing part of the cement paste with WCRP; a total of 28 trial mortar mixes containing WCRP added as either paste substitution or cement substitution were made for evaluation and analysis. By comparing the test results, it could be found that the incorporation of WCRP using the paste substitution method can remarkably enhance the compressive strength by at most 38.3%, densify the microstructure, and meanwhile greatly reduce the cement consumption by at most 26.7%. The cementing efficiency factor of WCRP in compressive strength was calculated for the first time, and its value is 0.289. Regression analysis showed that there exists good correlation between the compressive strength and effective water/cementitious materials (W/CM) ratio. [ABSTRACT FROM AUTHOR]