Effect of Coarse Recycled Aggregate Size on Flowability and Compressive Strength of Self-Compacting Concrete

Abstract

Background and Objectives: Concrete is the most widely used construction material globally. The production of cement, the primary component of concrete, accounts for approximately 8% of global carbon dioxide (CO₂ ) emissions. One popular method for reducing such gas emissions involve the use of recycled concrete aggregate. The present research then aimed to determine an optimum size ratio for recycled aggregate in self-compacting concrete, taking into account both the flowability and compressive strength. The aim is to provide convincing information to concrete users in Thailand toincrease the use of coarse recycled aggregate. The approach can reduce the reliance on natural gravel, mitigate environmental degradation and decrease the greenhouse effect by lowering CO₂ emissions, which are primary contributors to global warming and the recent phenomenon of global boiling.

Methodology: The present study considered coarse aggregate size ratios of 1/2 inch and 3/8 inch at 0:100, 20:80, 40:60, 60:40, 80:20, and 100:0. Mixtures with water-to-cement (W/C) ratios of 35%, 40%, and 45% were tested. Flowability was determined by considering the slump flow and the flow time through a standard funnel. Compressive strength was determined at 3, 7, and 28 days.

Main Results: The results indicate that higher amounts of superplasticizer, at 1.4–1.5%, were required when using only the 1/2-inch coarse aggregate size; this would clearly adversely affects the cost of concrete. The flow time through the standard funnel ranged from 3.8 to 28 seconds. A flow time of shorter than 5 seconds posed a risk of segregation. Concrete mixtures with flow times shorter than 5 seconds were observed in samples with water-to-cement (W/C) ratios of 35% and 40%. However, no segregation was found in any mixtures. The average compressive strength increased by approximately 29.5% at 28 days when compared to that at 3 days. The ratio of coarse aggregate sizes (1/2 inch to 3/8 inch) that yielded the highest compressive strengths in the mixtures with W/C ratios of 35%, 40%, and 45% were 20:80, 40:60, and 80:20, respectively, with compressive strengths of 592.8, 489.3, and 424.8 ksc.

Conclusions: Utilizing coarse recycled aggregate for new concrete production should take into account the size ratio of the aggregate to achieve self-compacting concrete with optimal properties. It is not recommended to use a single aggregate size as doing so requires a higher amount of superplasticizer and may not yield a specimen with the highest compressive strength. A suitable size ratio depends on the water-to-cement (W/C) ratio. The recommended size ratios when using aggregates of 1/2 inch to 3/8 inch are 20:80, 40:60, and 80:20 for concrete with W/C ratios of 35%, 40%, and 45%, respectively.

Practical Application: Engineers and self-compacting concrete users can apply the findings from the present research by utilizing the optimal coarse aggregate size ratios of 1/2 inch to 3/8 inch in a mixture with water-to-cement (W/C) ratio of either 35%, 40%, and 45%. These mixtures would result in concretes achieving high compressive strengths, even when using 100% coarse recycled aggregate. These data can be effectively applied to various projects to enhance the performance and sustainability of concrete production.