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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]

Adding fibers is an effective way to enhance the printability and mechanical performance of 3D printable cementitious materials. Glass fibers are commonly used owing to their sound mechanical properties, high durability and affordable price. However, there is still a lack of systematic and in-depth research on the effects of adding glass fibers to cementitious materials. In this study, a series of 3D printable mortars with varying glass fiber content and water/cement (W/C) ratio were produced to evaluate their printability, flexural strength and compressive strength. The results showed that decreasing the W/C ratio generally has positive effects on printability and mechanical performance, whereas increasing the glass fiber content from 0% to 1% would substantially improve the extrudability, dimensional stability and buildability; increase the flexural strength by up to 82%; but decrease the compressive strength by up to 35%. Such large differences in the effects of glass fibers on the flexural and compressive strengths indicate significant material anisotropy. In fact, comparison of the strength results of printed specimens to those of un-printed specimens reveals that the printing process could increase the flexural strength by 98% but decrease the compressive strength by 47%. [ABSTRACT FROM AUTHOR]

In tandem with recent development of nanotechnology and reduction in cost of production, many nano-materials have been tried in concrete. In this study, the effect of nano silica (NS) on the carbonation resistance of concrete was investigated by producing a number of trial concrete mixes with varying water and NS contents for carbonation depth test. The results demonstrated that the addition of NS could significantly reduce the 28-day and 56-day carbonation depths of concrete, indicating that NS may be a promising supplementary cementitious material for producing high-performance concrete.

With rapid urban development, natural aggregate resources have become scarce and a large number of ageing buildings are being demolished, which has resulted in a significant reduction in natural resources and a large increase in construction waste. Therefore, the reuse of solid waste, including waste powder and recycled aggregate, has attracted more and more attention. Additionally, as a prominent way to alleviate the urban heat island effect and manage stormwater runoff, pervious concrete has been widely studied and applied. In this paper, the effects of waste powder (fly ash, volcanic powder and blast furnace slag) and recycled aggregate (recycled concrete aggregate and recycled brick aggregate) on the mechanical properties, water permeability, water filtration and durability of pervious concrete are summarized and introduced, and some prospects are put forward. From the literature review, it can be found that adding the appropriate amount of solid waste or applying proper treatment methods to solid waste will not bring negative effects; rather, it would even improve the performance attributes of pervious concrete. Therefore, the use of solid waste in pervious concrete has great potential for urban construction and environmental protection. [ABSTRACT FROM AUTHOR]