Behavior of High-Strength Lightweight Concrete Slabs under Punching Loads

Abstract

During the last decade eight bridges and 18 concrete platforms for oil exploration have been constructed of high-strength lightweight (HSLW) aggregate concrete in many European countries. However, the use of HSLW aggregate concrete is increasing faster than the development of appropriate design recommendations. Punching shear failure at an interior column can readily initiate a progressive collapse in a flat plate structure. In this study, six slabs were tested under central loads. Four slabs were constructed of HSLW concrete of compressive strength higher than 70 MPa, with steel ratios ranging from 0.5% to 2.0%. The two reference specimens were constructed with normal-strength (NS) concrete and lightweight aggregates and had steel reinforcement ratios of 1% and 0.5%. The results included the ultimate loads, deflections, modes of failure, crack patterns, ductilities, concrete strains, and steel strains. The test results were compared with other test data on high-strength (HS) and NS normal weight concrete slabs and code predictions for slab strength. Normally, a reduction factor of 0.85 is recommended for lightweight aggregates by the American Concrete Institute code. A similar reduction of 0.80 is suggested by the BS 8110 code. These reduction factors are conservative when applied to HS concrete. A reduction factor of 0.95 is more appropriate for HSLW concrete, and of 0.85 for NS lightweight concrete. The results revealed that the change of reinforcement has a more serious effect on the general behavior of HSLW concrete slabs compared with NS and HS concrete slabs.