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Concrete In Australia : March 2008
TECHNICAL Table 1. Mechanical properties of steel reinforcement (Tested by Smorgon Steel). Mesh RL918 p( y Rm/Re Re (MPa) RL818 614.36 RL718 Rm (MPa) 612.56 646.53 Agt (%) 1.055 2.870 651.17 1.060 3.421 618.42 645.81 1.044 1.667 even number of point loads better simulate the distributed load . Near the ultimate load, the concrete blocks were, of necessity, spread out over the whole span rather than only at four points. During the testing, the data was recorded from strain gauges, transducers, load cells, photogrammetric survey and observations were made to determine the crack widths and pattern at various stages of the loading. Later, this data was used to calculate the moment redistribution and to plot the moment rotation and load defl ection curves for the critical sections of the test slab. Material properties The average compressive strength from fi ve standard concrete cylinder tests was found to be 32MPa. The mechanical properties of the steel reinforcement used in both test slabs, as tested and provided by Smorgon Steel Reinforcing, are provided in Table 1. More details of the test results are available in . Experimental results Load deflection curve A curve for total applied load on each span (excluding self- weight) versus the average mid-span defl ection is shown in Figure 3. The defl ection is measured with reference to the position of the slab after it had been displaced by the support settlement. It is noticeable that the curve near the maximum Figure 4a. Test slab at the failure load. reached the ultimate design load (1.2x Dead Load + 1.5 x Live Load). It was able to withstand this load despite the initial application of the support settlement. In fact, failure occurred g Slab behaviour under loading and crack pattern ) The test slab developed a crack over the intermediate support due to the self-weight moment reaching the cracking moment of the slab section. At the start of the test, the slab settled downward by an amount of 17mm at the intermediate support. A downward support reaction of 3.65kN was recorded at the intermediate support. This support settlement closed the initial crack developed over the intermediate support due to self-weight. Applying the four-point loads caused cracking within the spans initially. Due to this cracking, significant moment redistribution occurred from the mid-span regions to the intermediate support. After the serviceability load of 8kN was reached, cracks also appeared over the intermediate support. The slab exhibited well distributed cracking when the load Figure 4b. Failure section at the intermediate support. Figure 3. Load versus average mid-span deflection curve. load does not reach a plateau, indicating that the full plastic collapse mechanism is unable to form before localised failure occurs. The failure itself was very sudden and brittle resulting from the abrupt snapping of the top steel reinforcement close to the mid-span support. A defl ection of Span/97 was recorded at the penultimate load, indicating that the large reduction in stiffness after cracking led to the development of significant deflections. 40 Concrete in Australia Vol 34 No 1 at a much higher load as shown in Figure 3. The application of further loading after the ultimate design load of 14kN had been reached caused major cracking of the span regions with further cracks over the intermediate support region. At the failure load, both spans had developed widespread cracks and the cracks over the intermediate support had also widened considerably. The slab resisted a moment of 15.72kNm just before failure compared to a design moment of 8.35kNm. The main reason for this was the higher actual strength of the materials, in particular, the steel having an ultimate tensile strength of (kt)