Experimental Investigation of Combined Recycled Fine and Coarse Aggregate Replacement in Concrete

Abstract

This study experimentally investigates the feasibility of replacing natural fine and coarse aggregates with recycled concrete aggregates (RCA) in M25 concrete. A control mixture and four recycled aggregate mixtures containing 10%, 20%, 30% and 40% combined replacement were produced using a design procedure based on IS 10262:2019. Fresh-state performance was evaluated by slump, while hardened-concrete performance was assessed using 7- and 28-day compressive strength, 28-day split tensile strength, and 28-day flexural strength. The control mixture exhibited a slump of 75 mm and a 28-day compressive strength of 32.8 MPa. At 10% and 20% RCA replacement, the compressive strengths remained close to the control value, at 32.5 and 31.8 MPa, respectively. Beyond 20%, the reduction became pronounced, reaching 28.1 MPa at 30% and 24.4 MPa at 40% replacement. Similar trends were obtained for split tensile and flexural strengths. The 20% mixture maintained a slump of 75 mm and achieved split tensile strength of 3.05 MPa and flexural strength of 3.70 MPa at 28 days. Although the 10% mixture produced marginally higher mechanical properties, 20% was identified as the optimum replacement level investigated because it enabled greater utilization of recycled aggregate without a loss of control-level slump and with satisfactory mechanical performance. The findings indicate that controlled combined replacement of natural fine and coarse aggregates can provide a technically viable route for incorporating processed C&D concrete waste into M25 concrete.

Introduction

The text investigates the use of recycled fine aggregate (RFA) and recycled coarse aggregate (RCA) from construction and demolition waste as partial replacements for natural aggregates in M25 concrete. The main motivation is to reduce natural-resource depletion, landfill disposal of construction waste, and the environmental burden associated with conventional concrete production.

The study recognizes that recycled aggregates generally contain adhered old mortar, making them more porous, less dense, and more water-absorptive than natural aggregates. These characteristics can reduce concrete workability and mechanical strength, particularly at high replacement levels.

Experimental methodology

M25 concrete was produced with combined recycled fine and coarse aggregate replacement levels of:

• 0% – control mix (M0)
• 10% – M1
• 20% – M2
• 30% – M3
• 40% – M4

The study evaluated:

• Fresh concrete slump/workability
• 7- and 28-day compressive strength
• 28-day split tensile strength
• 28-day flexural strength

The concrete was designed according to relevant Indian Standards, using OPC 43-grade cement, 20-mm coarse aggregate, Zone-II natural sand, and processed recycled aggregates.

Major findings

1. Workability

Workability remained close to the control level up to 20% replacement:

The significant reduction beyond 20% is attributed mainly to the higher water absorption and rougher surface texture of recycled aggregates.

2. Compressive strength

28-day compressive strength decreased progressively:

• 0%: 32.8 MPa
• 10%: 32.5 MPa
• 20%: 31.8 MPa
• 30%: 28.1 MPa
• 40%: 24.4 MPa

The reduction was minor up to 20%, but became substantial at 30–40%.

3. Split tensile strength

The 28-day split tensile strength decreased from 3.25 MPa for the control to:

• 3.18 MPa at 10%
• 3.05 MPa at 20%
• 2.61 MPa at 30%
• 2.52 MPa at 40%

4. Flexural strength

Flexural strength followed a similar downward trend:

• 0%: 4.20 MPa
• 10%: 3.90 MPa
• 20%: 3.70 MPa
• 30%: 3.20 MPa
• 40%: 2.90 MPa

The deterioration is attributed to weaker interfacial regions, increased porosity, and the presence of adhered mortar in recycled aggregates.

Optimum replacement level

The study identifies 20% combined recycled aggregate replacement (M2) as the optimum performance-balanced level. Although the 10% mixture produced slightly higher mechanical strengths, the 20% mixture provides substantially greater waste utilization while maintaining:

• 75 mm slump, equal to the control
• 31.8 MPa 28-day compressive strength
• Acceptable tensile and flexural performance
• Greater conservation of natural aggregates

The authors emphasize that 20% should not be treated as a universal optimum; it applies specifically to the materials, proportions, recycled-aggregate source, and curing conditions investigated.

Practical implications and limitations

The results support controlled use of processed recycled aggregates in M25 concrete, particularly when aggregate moisture conditioning, quality control, contaminant removal, and potentially water-reducing admixtures are properly managed.

However, the study does not quantify life-cycle carbon emissions, economic savings, or long-term durability. It also uses one recycled-aggregate source and does not separately investigate the effects of RFA and RCA.

Future research should therefore examine finer replacement intervals around 20%, different RFA/RCA proportions, aggregate pre-treatment and moisture conditioning, admixtures, durability properties, life-cycle assessment, and economic feasibility.

Conclusion

1) Combined replacement of natural fine and coarse aggregates with processed RCA influenced both workability and hardened concrete properties. 2) Slump was 75, 78, 75, 65, and 50 mm at 0%, 10%, 20%, 30%, and 40% replacement, respectively; the principal workability loss occurred beyond 20%. 3) 28-day compressive strength was 32.8, 32.5, 31.8, 28.1, and 24.4 MPa for 0-40% RCA, respectively. The reduction was limited to 20% but became pronounced at higher concentrations. 4) 28-day split tensile strength reduced from 3.25 MPa for the control to 3.05 MPa at 20%, 2.61 MPa at 30%, and 2.52 MPa at 40%. 5) 28-day flexural strength reduced from 4.20 MPa for the control to 3.70 MPa at 20%, 3.20 MPa at 30%, and 2.90 MPa at 40%. 6) Although the 10% mixture showed slightly higher mechanical properties than the 20% mixture, the 20% mixture was selected as optimum because it enabled greater RCA utilization while retaining a 75 mm slump and satisfactory mechanical performance. 7) Under the tested materials, mix proportions, and curing conditions, 20% combined RCA replacement is the recommended performance-balanced level for the investigated M25 concrete.

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Copyright

Copyright © 2026 Ranveer Singh, Himmi Gupta, Hemant Sood. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.