Structural Performance and Mechanical Evaluation of Two-Layer Reinforced Concrete Beams Incorporating Clam Shell Aggregates

Abstract

Concrete remains a primary building material in civil engineering, but its overall cost is heavily dictated by its constituent materials. In region-specific contexts like the Niger Delta and coastal Nigeria, traditional coarse aggregates such as crushed granite incur substantial costs due to heavy transport logistics and inflation, while discarded clam shells create severe waste disposal and environmental challenges. This study investigates the structural performance, cost-benefit feasibility and failure modes of two-layer reinforced concrete (RC) beams utilizing recycled clam shells as a partial alternative aggregate. By strategically allocating premium, high-strength aggregates to high-stress compressive zones and lower-performing, environmentally friendly recycled clam shell aggregates to lower-stress zones, this study investigates the structural performance, failure modes of a two-layered composite beam under flexural loading and can it be maintained alongside cost reduction? Laboratory testing was conducted on a scaled beam specimen composed of an upper compressive layer and a lower tensile layer joined at a defined shear interface. Load-deflection curves, interfacial shear slip, and ultimate failure mechanisms were systematically monitored up to structural collapse. Various mix configurations on rectangular beam measuring (1200X100X150) of Types MC (control), M1, M2, M3 and concrete cube specimens MP were subjected to comprehensive laboratory testing, including sieve analysis, compressive strength testing, tensile stress analysis, shear strength measurement, and flexural load-deflection evaluation. The beam specimens (M1 and M2) clam shell concrete was formed with 1:2:3 mix with depth of 75mm at the compression layer and the crushed stone aggregate concrete was formed with 1:2:4 mix with a depth of 75mm and beam specimens (M3) clam shell concrete was formed with 1:2:3 mix with depth of 50mm at the tension layer and the crushed stone aggregate concrete was formed with 1:2:4 mix with a depth of 100mm at the compression layer . Experimental findings indicate that while 28-day cube compressive strength decreases with clam shell integration (28.60 Mpa for control down to 12.30 -17.33 Mpa for modified mixes) the two- layered beam exhibits a maximum load-bearing capacity of [35.06Kn] with an ultimate mid-span deflection of [5.96mm]. Initial linear-elastic behavior was observed up to [60.0% - 80.0% depending on the sample] of the ultimate load, after which composite action began to degrade due to progressive interface shear failure. Full-depth flexural cracking originated in the lower tension layer and propagated upward into the compression zone, triggering complete delamination at the layer interface prior to ultimate failure, the two-layer beam structural configurations demonstrate comparable or superior ultimate load capacities. Specifically, Beam Type-M3 (50 mm clam shell concrete tension layer / 100 mm conventional concrete compression layer) achieved an ultimate IJEMT load capacity of 35.06 Kn, outperforming the control beam Type-MC (34.82 Kn). Modified beams exhibited enhanced ductility and deflection capacity prior to failure.

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