The shear design of reinforced concrete beams that contain stirrups and are made with recycled coarse aggregate (RCA) still rests on code equations calibrated for natural aggregate concrete. Rahal and Alrefaei (2018) tested 18 such beams at a single practical longitudinal reinforcement ratio (1.38%), found average shear-strength reductions of 5% and 9% at 20% and 100% RCA replacement, while the mechanisms of shear transfer in cracked RCA remained qualitative. Subsequent work enlarged the test database (Ju et al., 2021), quantified the model-uncertainty penalty of RCA and traced it to overestimated aggregate interlock (Pacheco et al., 2020), and advanced mechanism-based analysis (Trindade et al., 2023; Chakraborty and Subramaniam, 2026), yet a 2026 review of 123 studies concludes that codes are conservative only up to about 50% replacement, become unconservative beyond (the unconservative regime being driven mainly by members without stirrups), and are not calibrated for RCA source variability. The open problem is a representative, mechanism-informed evaluation of code shear provisions for RCA beams with stirrups, a quantitative attribution of the shear transfer mechanisms in cracked RAC, and the derivation of design-specific reduction factors for recycled aggregate concrete.

Background

Shear in reinforced concrete. A reinforced concrete (RC) beam resists shear through several coupled actions: the uncracked concrete compression zone, aggregate interlock (the shear transferred across an inclined crack by mechanical interlocking of its rough faces), dowel action (the transverse shear resistance of the longitudinal tension bars), and the stirrups, i.e. closed transverse bars used as shear reinforcement, which carry tension across inclined cracks and restrain their opening. Design codes such as ACI 318, Eurocode 2 (EC2), CSA A23.3 and the fib Model Code give shear equations for members with stirrups of the form V=Vc+VsV = V_c + V_s, where VsV_s is the steel contribution computed from stirrup geometry and yield strength and VcV_c is a concrete contribution calibrated on databases of tests on natural aggregate concrete (NAC). A prediction is called conservative when it falls below the measured strength (safe side) and unconservative when it exceeds it. In several code formats VcV_c grows with the longitudinal reinforcement ratio ρ\rho (tension-steel area divided by the effective concrete area), so the population of beams behind a code evaluation matters.

Recycled aggregate concrete. Recycled coarse aggregate (RCA) is crushed demolition concrete; each particle carries adhered old mortar, so concrete made with RCA (RAC) contains weak, porous inclusions and weak old-to-new mortar interfaces (interfacial transition zones, ITZ). Because RCA particles are softer than natural rock, inclined cracks in RAC tend to cut through them rather than routing around them, giving smoother crack faces and reduced aggregate interlock, as measured directly on cracked RAC planes by Xiao, Xie and Yang (2012). What fraction of a beam's shear resistance this actually removes in the presence of stirrups, and how it depends on RCA source quality, aggregate size and concrete strength, has never been quantified; the stirrup contribution, which is RCA-independent, partly masks the loss.

Origin of the question. Rahal and Alrefaei (2018) tested 18 beams with stirrups at a single practical ρ=1.38%\rho = 1.38\% with RCA replacing 0%, 20% and 100% of the coarse aggregate: shear strength fell on average 5% (20% replacement) and 9% (100%), while cracking patterns, failure modes, longitudinal steel strains were essentially unchanged and midspan deflections grew at full replacement. Comparing their results with code calculations, the fib Model Code 2010 (at level of approximation II — the second, more refined tier of that code's hierarchy of shear formulations of increasing accuracy) was conservative while ACI, CSA and EC2 were slightly unconservative, especially with RCA. Because the campaign tested a single reinforcement ratio and VcV_c grows with ρ\rho in several code formats, however, its verdicts cannot settle how the codes behave across the reinforcement ratios encountered in practice, and the mechanism behind the RCA penalty was only hypothesized (reduced interlock offset by RCA-independent stirrup action). A rational evaluation of current code shear equations therefore requires further testing at practical and varied levels of longitudinal reinforcement, and detailed investigation of shear transfer in cracked RCA, including the effects of RCA source quality, aggregate size and concrete strength and the relative contributions of aggregate interlock, dowel action and stirrup restraint, as the basis for design-specific reduction factors for recycled aggregate concrete.

State after 2018. Test databases have grown (a 2021 compilation holds 96 RCA beams with shear reinforcement spanning ρ\rho from 0.75% to 4.09%), the model-uncertainty penalty of RCA has been quantified for EC2 and MC2010 and traced to overestimated aggregate interlock, and pure-shear panel tests and mechanism-based analyses have sharpened the physical picture. Yet the codification background for the fib Model Code 2020 and the revised Eurocode 2 proposed RAC adjustments for shear only in members without shear reinforcement, and a 2026 review of 123 studies concludes that existing codes are conservative only up to about 50% RCA replacement and unconservative beyond it (an unconservative regime driven mainly by members without stirrups), and blind to RCA source; no calibrated, source-aware shear provision for members with stirrups exists.

Problem Statement

The open problem is to place the shear design of recycled aggregate concrete (RAC) beams with stirrups on a rational, mechanism-informed basis. Concretely: (1) evaluate, on representative experimental evidence spanning practical and varied longitudinal reinforcement ratios (including the low ratios, at and below roughly 2%, typical of construction practice), the full range of RCA replacement ratios, and the variability of RCA sources and qualities, whether the shear equations of current codes — ACI 318, Eurocode 2, the fib Model Code, CSA A23.3 — are conservative, accurate or unconservative for RAC beams with stirrups; (2) quantify the shear transfer mechanisms in cracked RAC — the relative contributions of aggregate interlock, dowel action, the uncracked compression zone and stirrup restraint — and their dependence on RCA source quality, aggregate size and concrete strength, replacing the current qualitative picture; and (3) derive design-specific reduction factors (or partial factors) for the shear resistance of recycled aggregate concrete members with transverse reinforcement, with a demonstrated safety level. These three results are jointly required and serve one overarching objective: safe, calibrated shear design provisions for RAC beams with stirrups.

References

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