Buildings (Mar 2025)
Investigation on Mix Proportions of Ultra-High Performance Concrete with Recycled Powder and Recycled Sand
Abstract
The construction waste of brick and concrete can be used to produce recycled powder and recycled sand, which can replace cement and natural sand, respectively, in concrete. This can reduce the cost of concrete, reutilize construction waste and decrease environmental pollution. The idea of producing UHPC incorporating both RP and RS by standard curing instead of steam curing is proposed in this study. The optimal mixture design of ultra-high-performance concrete (UHPC) with both recycled powder and recycled sand is investigated. Based on the revised Dinger–Funk model, the optimal mix proportions of green UHPC (GUHPC) with recycled powder and recycled sand were calculated on this basis, and the effects of the superplasticizer content, water–binder ratio, recycled powder and recycled sand replacement ratio on the workability and mechanical properties of GUHPC at different ages were investigated through the designed experimental program. The test results show that when the superplasticizer to cementitious material ratio was 0.8%, the flowability and the 28 d compressive strength were highest. When the water–binder ratio was 0.16, the flexural strength and compressive strength of the GUHPC at different ages were the largest. As the replacement ratio of the recycled powder increased, the workability of the GUHPC decreased. However, even when replacement ratio of recycled powder was 30%, the flowability was still higher than 180 mm. The flexural strength and the 28 d compressive strength increased first and then decreased. Compared with mixtures without RP, the 28 d compressive strength increased by 6.4% and reached the maximum value when the replacement ratio of the RP was 30%. The comprehensive contribution of recycled powder to the strength was analyzed. Recycled powder can enhance the contribution of cement to GUHPC strength, and the enhancement effect increases with increases in the recycled powder content and age. The optimal replacement ratio of recycled powder is 30%. As the replacement ratio of the recycled sand increased, the flowability of the GUHPC first increased and then decreased, and the flexural and compressive strength decreased. The toughness was analyzed by the flexural strength to compressive strength ratio (f:c). With increases in the recycled sand, the f:c at 3 d of age increased, the f:c at 7 d of age showed no significant change, and the f:c at 28 d of age first increased and then decreased. The f:c at 28 d of age reached a maximum value of 0.316 when the replacement ratio of recycled sand was 50%. Therefore, the replacement ratio of the recycled sand was selected to be 50%. The optimum mix proportions of GUHPC were obtained by considering the workability, mechanical properties and amount of recycled material.
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