Prediction of shear strength of reinforced recycled aggregate concrete beams without stirrups

For decades, recycled coarse aggregate (RCA) has been used to make recycled aggregate concrete (RAC). Numerous studies have compared the mechanical properties and durability of recycled aggregate concrete (RAC) to those of natural aggregate concrete (NAC). However, test results on the shear strength of reinforced recycled aggregate concrete beams are still limited and sometimes contradictory. Shear failure is generally brittle and must be prevented. This article studies experimentally and analytically the shear strength of reinforced RAC beams without stirrups. Eight RAC beams and two controlled NAC beams were tested under the four-point flexural test with the shear span-to-effective depth ratio (a/d) of 3.10. The main parameters investigated were the replacement percentage of RCA (0%, 25%, 50%, 75%, and 100%) and longitudinal reinforcement ratio (ρw) of 1.16% and 1.81%. It was found that the normalized shear stresses of RAC beams with ρw = 1.81% at all levels of replacement percentage were quite similar to those of the NAC counterparts. Moreover, the normalized shear stress of the beam with 100% RCA and ρw = 1.16% was only 6% lower than that of the NAC beam. A database of 128 RAC beams without shear reinforcement from literature was analyzed to evaluate the accuracy of the ACI 318-19 shear provisions in predicting the shear strength of the beams. For an RCA replacement ratio of between 50% and 100%, it was proposed to apply a reduction factor of 0.75 to the current ACI code equation to account for the physical variations of RCA, such as replacement percentage, RCA source and quality, density, amount of residual mortar, and physical irregularity

Seismic strengthening of low strength concrete columns using high ductile metal strap confinement : a case study of Kindergarten school in Northern Thailand

The 2014 Chaing Rai earthquake (Thailand) caused extensive damage in many reinforced concrete (RC) buildings built before the introduction of modern seismic design guidelines. Much of the damage on these buildings was attributed to the inadequate capacity and/or ductility of columns. As a result, suitable and cost-effective strengthening techniques for such substandard elements are necessary. This article presents a case study on the seismic strengthening of a one-story RC kindergarten school located in Ampor Pan, Chaing Rai province. The building was partially damaged during the afore-mentioned earthquake, which led to cracking in walls, columns, and beam-column joints. As part of the initial assessment, innovative repair solutions were sought to minimize construction time, labor, and material cost. Accordingly, an innovative strengthening technique that uses Post-tension Metal Strapping (PTMS) was proposed to strengthen the damaged RC elements. This article presents details of the structural assessment performed on the building, as well as details of the PTMS strengthening strategy, which was applied for the first time in a real full-scale structure. This article contributes towards the validation and application of the PTMS strengthening on real structures, which had not been possible until now

Punching shear capacity of recycled aggregate concrete slabs

This article investigates the punching shear behavior of recycled aggregate concrete (RAC) two-way slabs. Ten 1500 mm × 1500 mm × 100 mm slabs were tested monotonically. Eight slabs were cast with RAC, whereas two control slabs were cast with natural aggregate concrete (NAC). The RAC incorporated coarse recycled concrete aggregate (RCA) at replacement levels of 25%, 50%, 75% and 100%. Two flexural reinforcement ratios (0.8% and 1.5%) were examined. The results show that the normalized punching shear strength of 100% RAC slabs decreased by 6.5% and 9% compared to NAC slabs for r = 1.5% and r = 0.8%, respectively. Doubling the amount of flexural reinforcement can increase the punching shear capacity of 100% RAC slabs by up to 45%. A punching shear database of 44 RAC slabs from literature and the 8 RAC slabs presented in this study revealed that the punching shear strength of RAC slabs predicted by ACI 318 was conservative, except for slabs with low reinforcement ratios (<0.6%). The punching shear strength predicted by Eurocode 2 gave more conservative results for all levels of RCA replacement and all flexural reinforcement ratios. A yield-line analysis also showed that the failure mode of the RAC slabs was controlled by punching shear

Torsional strengthening of low-strength RC beams with post-tensioned metal straps : an experimental investigation

This article investigates the behaviour of low-strength reinforced concrete beams under pure torsion with and without strengthening. Four beams were cast and tested in torsion: i) a control beam without vertical reinforcement, ii) two beams with internal stirrups designed for shear and torsion demands using different stirrup spacing (50 and 100 mm), and iii) a beam having steel stirrups with a spacing of 100 mm strengthened using high ductile post-tensioned metal straps (PTMS). The main objective of the PTMS strengthening solution was to investigate the enhancement of torsional strength confined along the beam. The failure modes, torsional capacities, rotation, and strengthening performance in torsion are discussed in in this study. The experimental results indicate that the PTMS improved the cracking torque capacity by up to 15 % compared to the control beam. Moreover, the PTMS also increased the ultimate torque by up to 19 % compared to the unstrengthened beam. Current code equations to predict the torsional capacity of RC beams are also compared with the experimental results. It is found that the predictions obtained by current ACI equation gives a good agreement and yield in general conservative values compared to the experimental ones