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Concrete In Australia : March 2008
TECHNICAL Behaviour of one-way continuous reinforced concrete slabs – constructed with grade 500 Class L mesh steel, under support settlement U Siddique, H Goldsworthy and R J Gravina U Siddique and H Goldsworthy are from the Department of Civil and Environmental Engineering at the University of Melbourne, Victoria. R J Gravina is from the School of Civil, Environmental & Chemical Engineering at RMIT University, Victoria. The impact of constantly changing properties of construction materials on the behaviour of the structures and the relevant code provisions needs to be reassessed on a regular basis. The introduction of higher strength steel, Grade 500MPa, in Australia has led research to study the behaviour of concrete structures constructed with the new steel. A number of analytical and some limited experimental studies have been carried out and have shown concerns about its ductility and/ or the related provisions of AS3600-2001. In particular, the ductility of class L mesh has surfaced as a controversial issue. Some studies have concluded that the use of class L mesh in suspended slabs is undesirable, in particular under accidental loadings like relative support settlements. However, AS3600-2001  permits the use of class L steel with certain restrictions. Further detail about these issues is available in another publication  of the authors. The present study consists of full-scale experimental testing of one-way continuous slabs under imposed settlement and aims to examine their ductility, moment redistribution, moment- rotation relation and crack pattern. This paper is intended to provide some results and details of the second experiment conducted, while the results from the fi rst experiment are available in . The experimental program Two full-scale continuous one-way slabs were constructed and tested. The design was carried out following the provisions of AS3600-2001, incorporating Amendment No1. The simplifi ed method for continuous one-way slabs (clause 7.2) was followed for the design purposes without considering foundation movement (clause 126.96.36.199). This approach was adopted to consider an adverse scenario where a structure, designed without considering such effects, undergoes support 17mm a) 5000 17mm Figure 1. Details of support settlement a) Test Slab 1 b) Test Slab 2. 4000 b) 4000 settlement. This experimental program has been devised to study the behaviour of suspended slabs constructed with class L mesh under such an eventuality. In the first test slab, settlement was imposed in the upward direction while 5000 Figure 2. Plan and longitudinal section showing the dimensions of the test slab. (All dimensions in mm) to impose the support settlement. Two hydraulic jacks were used to lower the support by a pre-determined amount and the support reaction was recorded using a load cell at each jack. Also, various other instruments and techniques were used to observe/record the experimental data consisting of conventional instruments like strain gauges and transducers, and modern techniques like digital photogrammetry surveying. During the test, the slab was loaded until failure occurred. The following loading sequence included: • self-weight • intermediate support settled by a predetermined amount • dead and live loads on each span. After the support settlement, dead and live loads were imposed by using concrete blocks of various weights at four points on each span. The four-point loads were used since the Concrete in Australia Vol 34 No 1 39 the second test slab was settled downward (see Figure 1). However, only details of the second experiment are given in this paper. The material properties and loads considered for the design of the slab for the second experiment are as follows: • dead load (G) • live load (Q) = 1kPa (excluding self weight) = 3 kPa • load combination = 1.2 G + 1.5 Q • concrete ƒc ´ = 32MPa • steel ƒsy = 500MPa (class L type mesh). Test slab set up and instrumentation The test slab was cast in position and tested after 28 days. All the supports were pin-ended to allow free rotation of the test slab and built by an assembly of steel plates and a roller. A special arrangement was made at the intermediate support