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Concrete In Australia : September 2011
34 Concrete in Australia Vol 37 No 3 FEATURE: FLOOR CONSTRUCTION/SLABS ON GRADE e use of SFRC at high dosages has eliminated the need for induced contraction joints within slabs on grade, thereby simplifying design by reducing unsupported edge condition checks. e consequence of this technology is a modest increase in construction joint opening. Different metallic joint systems have been developed to maintain the load transfer even for joint opening up to 15 mm to 20 mm. ese armoured systems protect the ariss of concrete and are designed to prevent any differential vertical movement while allowing horizontal movements in both directions. 3.2 Ground floor slabs suspended on piles Following the successful application of SFRC for slabs on grade, the composite material has been further improved to allow its use in structural application such as industrial suspended slabs on piles and raft foundations. Noticeable advantages are the gain on program and the possibility of using latest technology equipment to lay the industrial concrete floor (less preparation, no steel fixing, larger panel sizes, fewer joints, faster casting, flatter floor slabs). e difference is that ground-supported slabs are assumed to be supported uniformly by the subsoil, whereas pile-supported slabs are assumed to be supported only temporarily by the platform but eventually only by piles at discrete points with no assistance from the soil. e preferred option is usually a fibre only, simply supported, two-way spanning flat slab as shown below. e slab bears on the piles but is not connected to them, and a polythene slip membrane is usually provided, allowing horizontal movement to take place. e steel fibres used in piled floor slabs are different to those used in standard "joint-free" slabs on grade. Broadly speaking they are at the higher end of the performance spectrum and they are typically drawn wire steel fibres with a higher tensile strength (around 1450 MPa), higher aspect ratio (length/ diameter ≥ 60) and are used at higher dosage rate (40-50 kg/m³). e consequences are better post crack toughness and residual strength, which are required in piled floors. As with ground bearing "joint-free" slabs, construction joints must guarantee the load transfer and their location needs to be carefully defined with regard to their proximity to piles. Typically, they will be positioned at contraflexion points, generally between 1/3 and 1/4 span. e total volume of slabs suspended on piles throughout Europe since 1992 designed in accordance with these design methods (SFRC without any form of steel bar) exceeds 10,000,000 m2. On the majority of these projects, the primary reinforcement was provided purely by structural steel fibres; the only other reinforcement was anti crack mesh incorporated at re-entrant corners and column isolation detailing. 4.0 CONSTRUCTION e process of successfully building large "joint-free" panels remains completely dependent on the skill and experience of the practitioner employed by the client or contractor. e practical aspects of fibre mixing, concrete mix design, site quality control, sub-base preparation, detailing, installation and correct programming are of paramount importance and if not mastered, could lead to a less than satisfactory result. A few of these critical elements are considered below. 4.1 Free movement e slab has to be free to move during the drying phase; any restraint to the concrete shrinkage can create tensile stresses in excess of the concrete strength, and therefore lead to cracking. Restraint can be caused by sub-base irregularities and friction, fixed obstructions (such as re-entrant corners and columns) and structural tie bars. Fine grading and controlled levelness of the sub-base are essential conditions for "joint-free" slabs. A polythene slip membrane without re-entrant folds is 50m 50m Traditional type of floor 2,500 m² with 800 m of saw-cut joints to maintain SFRC Jointless floor 2,500 m² with NO saw-cut joints Figure 1. Diagram showing slabs with and without saw cut joints.