Basic principles

CRC is a steel fibre reinforced high strength concrete and has spectacular properties - especially in bending. An example of this - admittedly a rather extreme example - is described below.

The cross section shown is taken from a beam with depth 100 mm and width 155 mm. The load-deflection curve from the beam test is also shown.

With the dimensions of the beam in mind it is easily discerned that conventional design rules can often be disregarded in connection with CRC.

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Cover to the reinforcement is only 10 mm, which is fully adequate even for heavily loaded beams in chloride contaminated environments. Other considerations - e.g. fire resistance - most often lead to reinforcement covers of 15 mm.

Also, the rebars are closely spaced and generally the recommended distance between rebars should be comparable to at least one fibre length - about 10-12 mm. In this case prestressing cables have been used as tensile reinforcement, but they were not prestressed, they were simply used because of their high tensile capacity - 1700 MPa. Nevertheless, no large cracks were observed during yielding of the steel. Compressive strength of the concrete in the beam was 300 MPa.

This an example of a heavily reinforced type of CRC. Reinforcement ratios for balcony slabs and staircases are typically around 2-3%, partly for economic reasons, partly because utilising the high bending strength would also lead to large deflections.