Punching behaviour of concrete slabs incorporating coarse recycled concrete aggregates

Abstract This paper presents experimental, numerical and analytical investigations about the effects of incorporating coarse recycled concrete aggregates (CRCA) on the punching behaviour of reinforced concrete (RC) slabs. For this purpose, four concrete mixes were designed with various substitution ratios of natural coarse aggregates (NCA) by CRCA: 0% (reference mix), 20%, 50% and 100%. Subsequently, eight 1100 × 1100 × 90 mm reinforced concrete slabs (two per concrete type) were cast and subjected to punching tests. The experimental results indicate that the incorporation of CRCA affects the mechanical behaviour of RC slabs according to the following trends: (i) the punching resistance of slabs made of concrete with CRCA (CRCAC) is similar to that of the reference mix; (ii) the stiffness decreases, particularly in the uncracked state; and (iii) the cracking load slightly decreases. The numerical study comprised the development of three-dimensional non-linear finite element models of the slabs. The numerical models were able to trace the experimental responses of the slabs tested, highlighting the influence of the concrete fracture energy on the numerical results. The best predictions of punching strength were obtained when similar fracture energy was adopted for all concrete mixes. Finally, the accuracy of some of the most relevant design codes in predicting the punching strength of CRCAC slabs is assessed. The most accurate estimates were obtained using Model Code 2010 (MC 2010) with levels of approximation II to IV. Eurocode 2, ACI 318 and MC 2010 with level of approximation I provided less accurate and relatively conservative punching strength predictions.