Roland Mayr explains how selecting the right structural synthetic fibres for sprayed concrete depends on when and how the concrete will be loaded
Fibre-reinforced sprayed concrete is widely used for rock support, with fibres replacing conventional mesh reinforcement. It can be applied safely, fast and in a highly mechanised way.
The 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. They 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 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.
The performance of PP fibres depends on the fibre strength and the properties of the interface between the fibre and the concrete matrix. This is influenced by both the surface structure of the fibre (flat, embossed) and the strength (age) of the matrix.
The age at which energy absorption panels are tested varies from country to country, from 7 to 28 days. The 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 words, the fibres should only start to be pulled out as the maximum breaking load of the concrete lining is approached.
Flat versus embossed sprayed concrete
To compare the performance of flat and embossed fibres in different strengths and ages of concrete, tests were carried out on square panels at 7, 28 and 90 days. (Round test panels are often used in mining but results from square panels can be converted to those obtained from round panels by dividing the EA obtained by 2.5).
The performance of the flat fibres can be seen in Graph 1. With increasing age and strength of the concrete, the performance of the system improves.
The embossed fibres show a different behaviour (see Graph 2). The performance in the lower deformation range is very similar. In the higher deformation range and with increasing concrete strength and age the performance decreases.
After the tests, the fibres in the cracks were counted and classified into snapped and pulled out fibres. The percentage of snapped fibres increases with strength/age for both fibre types, but the increase of broken embossed fibres is significantly higher.
What do the sprayed concrete test results tell us?
A number of conclusions can be drawn from these results. Firstly, that modern sprayed concrete tends to develop significant strength with age. Secondly, it is important to consider at what age the sprayed concrete will be loaded and how big the deformations will be. The results also show that flat fibres can take higher loads and deformations, as long as the loads are applied when the concrete has reached a higher age/strength. Meanwhile, embossed fibres can be better for lower age/strength concrete (up to 14 days). Also, they provide greater stiffness with lower overall levels of deformation.
Master Builders Solutions provides various types of structural PP fibres in its MasterFiber range. The firm’s knowledge of concrete technology and chemistry allows it to design the concrete mix and select the fibres for a maximum structural response, depending on the geology of the mine.
For reinforced sprayed concrete, the main requirement is to maintain a certain level of ductility under high deformation conditions. The strength and age of the concrete matrix, as well as the fibre type, must be considered to achieve the desired performance.
Roland Mayr is global technical manager sprayed concrete at Master Builders Solutions