Recycled aggregate can be used as a substitute for natural aggregate to produce good regenerated concrete, so the text use recycled coarse aggregate and recycled fine aggregate to substitute for natural aggregate with different replacement rates preparing double recycled aggregate concrete, and researching the effect of the quality and replacement rate of recycled aggregate on the mechanical properties of recycled aggregate systematically. The results show that the compressive strength of double recycled aggregate concrete is increased gradually with the improvement of the quality of recycled aggregate, and the effect from weak to strong is class I, class II and class III. On the other hand, the compressive strength of double recycled aggregate concrete is decreased gradually with the increase of the replacement rate of recycled aggregate, and the maximum reduction is up to 57.2%. Therefore, the simultaneous use of recycled coarse aggregate and recycled fine aggregate have an effect of "double weakening" on the compressive strength of recycled concrete, and it can be applied to the concrete products of low strength grade. © 2020 IOP Publishing Ltd. All rights reserved.

In metro engineering, due to the large amount of lining concrete pouring, it is easy to produce large shrinkage deformation in the maintenance stage, which leads to cracks in lining concrete and seriously affects the operation and safety of metro. Therefore, this paper adopts the test scheme of large dosage of mineral admixtures to compare and analyze the influence of the amount of cementing material, the dosage of mineral admixtures and the binder/water ratio on the dry shrinkage performance of metro lining concrete. The results show that the shrinkage ratio of lining concrete increases with the increase of cementitious material content or binder/water ratio, but decreases with the increase of the dosage of mineral admixtures. When the cement variety is P.I 42.5 and the dosage of mineral admixture is 40%, or the cement variety is P.I 52.5 and the dosage of mineral admixture is 50%, the shrinkage deformation of lining concrete is small, which can meet the needs of subway construction and will have a broad application prospect in practical engineering. © 2020 IOP Publishing Ltd. All rights reserved.

Recycled aggregate can be used as a substitute for natural aggregate to produce good regenerated concrete, so the text use recycled coarse aggregate and recycled fine aggregate to substitute for natural aggregate with different replacement rates preparing double recycled aggregate concrete, and researching the effect of the quality and replacement rate of recycled aggregate on the mechanical properties of recycled aggregate systematically. The results show that the compressive strength of double recycled aggregate concrete is increased gradually with the improvement of the quality of recycled aggregate, and the effect from weak to strong is class I, class II and class III. On the other hand, the compressive strength of double recycled aggregate concrete is decreased gradually with the increase of the replacement rate of recycled aggregate, and the maximum reduction is up to 57.2%. Therefore, the simultaneous use of recycled coarse aggregate and recycled fine aggregate have an effect of "double weakening" on the compressive strength of recycled concrete, and it can be applied to the concrete products of low strength grade. © 2020 IOP Publishing Ltd. All rights reserved.

The effective mineral admixture in mortar can consume the calcium hydroxide produced by hydration reaction because of pozzolanic effect. For high volume mineral admixture mortar, when the replacement ratio exceeds a certain 'threshold' value, the supply of Ca(OH)2 amount is insufficient, the superfluous and ineffective mineral admixture will no longer react in mortar but as fine aggregate. This study presents an experiment study on the threshold value of effective replacement ratio of fly ash mortar by the comprehensive analysis of Ca(OH)2 amount and consumption curves. Under the conditions of different curing temperature, the effective replacement ratios of fly ash mortar have been determined ultimately. The results showed that Ca(OH)2 amount and consumption in mortar both decreased with replacement ratio. For fly ash mortar, at 20°C curing temperature, when the replacement ratio was less than 40%, the Ca(OH)2 amount decreased obviously with replacement ratio. However, when replacement ratio was more than 40%, Ca(OH)2 amount at 91d changed slightly with replacement ratio, the Ca(OH)2 amount was consumed to a limit value at this time. Under the condition of 30°C curing temperature, when the replacement ratio exceeded 30%, the change of Ca(OH)2 amount with replacement ratio was close to a straight line, the superfluous fly ash no longer consumed Ca(OH)2 basically. Moreover, the consumption of Ca(OH)2 was not much difference to the Ca(OH)2 amount at the same replacement ratio under the condition of 20°C curing temperature. This result has a great significance on effective utilization of mineral admixtures in engineering application. © 2020 IOP Publishing Ltd. All rights reserved.

In metro engineering, due to the large amount of lining concrete pouring, it is easy to produce large shrinkage deformation in the maintenance stage, which leads to cracks in lining concrete and seriously affects the operation and safety of metro. Therefore, this paper adopts the test scheme of large dosage of mineral admixtures to compare and analyze the influence of the amount of cementing material, the dosage of mineral admixtures and the binder/water ratio on the dry shrinkage performance of metro lining concrete. The results show that the shrinkage ratio of lining concrete increases with the increase of cementitious material content or binder/water ratio, but decreases with the increase of the dosage of mineral admixtures. When the cement variety is P.I 42.5 and the dosage of mineral admixture is 40%, or the cement variety is P.I 52.5 and the dosage of mineral admixture is 50%, the shrinkage deformation of lining concrete is small, which can meet the needs of subway construction and will have a broad application prospect in practical engineering.
© 2020 IOP Publishing Ltd. All rights reserved.