Sustainable Concrete Paver Blocks with Unprocessed C&D Waste: Flexural Strength as the Critical Compliance Parameter

A 2×3 factorial investigation on M30 paver blocks with Class F fly ash (0%, 10% of OPC) and unprocessed C&D waste (0%, 15%, 30% of NCA), benchmarked against IS 15658:2021, yields the following conclusions: (i) Flexural strength — elevated to mandatory status in the 2021 revision of IS 15658 — is the sole discriminating compliance criterion. Three mixes (M30-FA0-RCA30, M30-FA10-RCA15, M30-FA10-RCA30) fail the 3.30 MPa threshold despite passing compressive, water absorption, and abrasion requirements. (ii) M30-FA10-RCA0 is the mechanical optimum (40.8 MPa CS, 4.68 MPa FS, 3.5% WA, 12,600 mm³ abrasion), outperforming the pure OPC control across every parameter while displacing 10% of OPC. M30-FA0-RCA15 is the waste-diversion optimum, retaining full compliance while incorporating 154 kg of unprocessed C&D waste per m³. (iii) Unprocessed C&D waste degrades flexural strength disproportionately (29–36% reductions at 30% replacement) versus compressive (12–18%), consistent with multi-zone ITZ fracture under tensile loading. Class F fly ash at 10% benefits all properties at zero and 15% C&D replacement, but the combination with unprocessed C&D waste at these levels amplifies the flexural deficit beyond IS limits. (iv) For non-traffic pedestrian paving under Indian conditions, 15% unprocessed C&D waste represents a practical upper bound for single-variable substitution; 10% fly ash can be independently adopted for mechanical and durability enhancement. Combined substitution at these levels is not recommended without aggregate pre-conditioning.

1.         Mehta, P.K., Monteiro, P.J.M.: Concrete: Microstructure, Properties, and Materials. 4th edn. McGraw-Hill Education, New York (2014) 2.         Jain, S., Singhal, S., Jain, N.K.: Construction and demolition waste generation in cities in India: an integrated approach. Int. J. Sustain. Eng. 12(5), 333–340 (2019). https://doi.org/10.1080/19397038.2019.1612967 3.         Siddique, R.: Performance characteristics of high-volume Class F fly ash concrete. Cem. Concr. Res. 34(3), 487–493 (2004). https://doi.org/10.1016/j.cemconres.2003.09.002 4.         Etxeberria, M., Vázquez, E., Marí, A., Barra, M.: Influence of amount of recycled coarse aggregates and production process on properties of recycled aggregate concrete. Cem. Concr. Res. 37(5), 735–742 (2007). https://doi.org/10.1016/j.cemconres.2007.02.002 5.         Malhotra, V.M., Mehta, P.K.: High-Performance, High-Volume Fly Ash Concrete. CANMET, Ottawa (2002) 6.         Lothenbach, B., Scrivener, K., Hooton, R.D.: Supplementary cementitious materials. Cem. Concr. Res. 41(12), 1244–1256 (2011). https://doi.org/10.1016/j.cemconres.2010.12.001 7.         Kou, S.C., Poon, C.S., Chan, D.: Influence of fly ash as cement replacement on the properties of recycled aggregate concrete. J. Mater. Civ. Eng. 19(9), 709–717 (2007). https://doi.org/10.1061/(ASCE)0899-1561(2007)19:9(709) 8.         Attri, G.K., Gupta, R.C., Shrivastava, S.: Impact of recycled concrete aggregate on mechanical and durability properties of concrete paver blocks. Mater. Today Proc. 42(3), 975–981 (2021). https://doi.org/10.1016/j.matpr.2020.11.977 9.         Singh, P., Sharma, A., Roy, A.B.D.: Utilization of Linz–Donawitz slag and brick bat for the production of sustainable paver blocks. Mater. Today Proc. (2024). https://doi.org/10.1016/j.matpr.2023.07.183 10.       Tam, V.W.Y., Tam, C.M.: Assessment of durability of recycled aggregate concrete produced by two-stage mixing approach. Constr. Build. Mater. 22(10), 2003–2010 (2008). https://doi.org/10.1016/j.conbuildmat.2007.07.012