PUNCHING SHEAR BEHAVIOUR OF RECYCLED AGGREGATE CONCRETE FLAT-PLATES

Abstract

Punching shear behaviour of Recycled Aggregate Concrete (RAC) balanced slab-column connections was experimentally investigated and analytically modelled for the following variables: Recycled Concrete Aggregate (RCA) volumetric replacement level (0%, 50% and 100%), RAC grade (normal-strength, medium-strength and high-strength), amount of tension reinforcement (1.16% and 2.0%), steel fibre volume fraction (0%, 0.4% and 0.8%), steel fibre type (hooked-end and crimped fibres) and aspect ratio of the steel fibres (50 and 80). The RCAs were obtained by crushing laboratory produced waste concrete of unknown provenance and only the coarser fraction of the RCAs were used in this investigation. The nominal maximum size of the RCA particles was kept at 12.5 mm which was the same as that of the Natural Coarse Aggregate (NCA). Mechanical testing of the non-fibrous RAC and the Steel Fibre Recycled Aggregate Concrete (SFRAC) indicates that compressive and splitting tensile strength of these concretes were practically insensitive to the RCA replacement level. However, steel fibre type and dosage in the RAC significantly affected the splitting tensile strength and depending on the fibre characteristics this property for the SFRAC increased in the range of 29% - 59% when compared to that of the control non-fibrous RAC. Flexural performance evaluation of the SFRAC was conducted using prism specimens subjected to four-point bending test. Measured residual strengths and toughness of the SFRACs indicate that these properties were more influenced by the steel fibre characteristics than the RCA replacement level. In general, the hooked-end steel fibres were more efficient than the crimped fibres in enhancing mechanical performance of the RACs. Punching shear test results of the non-fibrous and the SFRAC slabs indicate that the measured punching strengths were practically unaffected by the RCA replacement level, though RAC grade had a more significant effect on this property. In comparison to the normal-strength RAC slabs, the increase in the punching strength of the medium- and the high-strength RAC slabs were in the range of 15% - 23%, and 19% - 25% respectively. The punching capacities of the RAC slabs were enhanced by 23% - 30% upon inclusion of the hooked-end steel fibres in the RAC. A fibre volume fraction of 0.4% was indicated as the optimum dose for enhancing punching strength of RAC slabs. Compared to the RCA replacement level, steel fibre inclusion was more effective in improving serviceability behaviour of the RAC slab specimens. Service-load deflections and crack widths of the SFRAC slabs reduced in the range of 10 % - 18% and 27% - 41% respectively when compared to those of the non-fibrous slabs. Increasing fibre dosages lead to marginal increases in inclination of the critical punching shear crack with these inclinations for the slabs containing 0%, 0.4% and 0.8% volume fractions of the hooked-end steel fibres being in the range 280-290, 290-320 and 330-350 respectively, regardless of the RCA content. Energy absorption of the slabs was noted to be more sensitive to the RAC grade and fibre dosage than the RCA replacement level. At 0.4% and 0.8% volume fraction of the hooked-end fibres, the energy absorption of the slabs increased by 73% and 159% respectively when compared to the non-fibrous slabs. When compared to the normal-strength non-fibrous RAC slab, energy absorption of the medium- and the high-strength RAC slabs increased by 65% and 95% respectively. The punching shear performance of the RAC slabs reinforced with the hooked-end steel fibres was in general superior to the slabs reinforced with the crimped steel fibres. On the basis of the measured steel strains and load-deflection curves of the slab specimens, broadly, three types of failure modes were identified in the slab specimens viz. pure punching failure, flexural-punching failure and flexural failure. The use of steel fibres in the RAC transformed the failure mode of the slabs from pure punching to the more desirable flexural-punching or even a flexural failure. The selected current design codes under appraisal gave highly conservative punching capacities for the slab specimens of this investigations with relatively the highest conservative punching load prediction being obtained from the IS 456 and the least from the Eurocode 2. A punching shear strength predictive model based on the strut-and-tie analogy for the RAC and the SFRAC slabs has been developed from a conceptual model reported in the literature. The proposed analytical model predicted conservatively and accurately the punching strengths of all the slab specimens of the present investigation as well as the slab specimens of similar tests available in the literature.