Roland Mayr explains how selecting the right structural synthetic fibres for sprayed concrete depends on when and how the concrete will be loaded

safely, fast and in a highly mechanised way. Te use of synthetic fibres, generally


polypropylene (PP), allows the concrete structure to behave in a ductile way, enabling significant deformation before failure. Structural synthetic fibres form an internal network and add superior tensile properties to the sprayed concrete. Tey have advantages over steel fibres in that they do not corrode and have a far smaller carbon footprint. Effective surface support and protection against the weathering of rock and strata are critical to the safety, efficiency and longevity of any mining operation. Different types of synthetic fibre cause the reinforced sprayed concrete to behave differently. Making the right choice of fibre depends on how much loading the


ibre-reinforced sprayed concrete is widely used for rock support, with fibres replacing conventional mesh reinforcement. It can be applied

concrete must take and how quickly that loading will be applied – and that depends on the ground and on planned mining operations. As shown in Table 1, there are three main shapes of structural PP fibres available: flat, embossed and crimped. To compare the performance of different fibers, energy absorption (EA) tests on panels of sprayed concrete containing the fibres can be used. EA is a measure of the toughness of sprayed concrete, with a load applied to the centre of a panel to mimic deformations. Typically, fibre testing panels are prepared

by spraying the concrete in the same way it would be applied underground, but with less accelerator. Sometimes the panels are also cast under lab conditions, but this can give misleading results, depending on the types of fibres used. Embossed fibres typically show a better performance under cast conditions because the liquid cement paste penetrates all the fine indentations in

the fibre to form a good bond. However, under spraying conditions, the cement paste stiffens immediately due to the addition of the accelerator and hence does not penetrate the indentations as well. Te performance of PP fibres depends on the fibre strength and the properties of the interface between the fibre and the concrete matrix. Tis is influenced by both the surface structure of the fibre (flat, embossed) and the strength (age) of the matrix.

Te age at which energy absorption

panels are tested varies from country to country, from 7 to 28 days. Te testing can be done either when a certain strength is reached or at a certain age. It should be noted that, if a shotcrete shell is loaded a long time after it is applied, the strength and performance can be quite different than if it is loaded immediately Ideally, a fibre would not be pulled out easily or break as a load is applied. In other

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