ALIGNING LONG STEEL FIBRES IN FRESH CONCRETE

摘要

The random nature of steel fibres in concrete slabs on grade means that they are inherently inefficient because many of the fibres do not provide much resistance to shrinkage or tensile stress in flexure in the direction of principal stress. The alignment of short steel fibres onto a chosen horizontal plane can be achieved using a magnetic device which is placed in a fin which is dragged through the concrete during screeding operations. The use of longer steel fibres to improve post-cracking ductility is desirable, but the existing alignment technique does not extend to these longer fibres because the magnets do not release them. This paper will describe how the re-design of the magnetic system enables the longer fibres to be aligned. A graphic illustration of the effects of the main parameters of magnet rotation speed and screed speed will be given, showing how the continuity of fibres on the chosen layer is preserved and under what circumstances the fibres are staggered, leading to potential cracking problems. The technique has been used in fresh concrete showing how, for the first time, the longer fibres are aligned on one plane at a chosen depth below the surface of the concrete slab.