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Steel or macro synthetic fibres may be added to the concrete either at the ready-mixed plant or to the mixer truck on site. The addition of fibres to the concrete should be in accordance with an agreed method statement, to ensure that the fibres are adequately distributed throughout the concrete, there are no fibre balls that may cause pump blockages and the quality of the final concrete is not unduly affected.
Fibres can be supplied in degradable paper bags or containers, plastic bags or in bulk. The type of fibre and/or packaging has a direct impact on how the fibres are dosed into the concrete. Care should be taken to ensure proper dispersal and complete mixing. Degradable paper bags or containers can be put directly into the concrete truck or mixer, or they can be opened and the loose fibre dispensed. The plastic bags are not degradable and so cannot be put directly into the concrete. Fibres can also be blown with compressed air into the concrete to aid dispersion.
Where the fibres are added directly to the truck on site, care should be taken to ensure that the fibres are completely distributed throughout the load, with no loss by sticking to the hopper, mixer blades etc. Mixing of the concrete after the addition of the fibres depends greatly on the truck’s efficiency; it is recommended that the fibres should be mixed for approximately five minutes at optimum speed, or a minimum of 100 revolutions of the drum. Where fibres are added at the ready-mixed plant, it is suggested that they should be the first item in the mix. The subsequent addition of aggregate helps to distribute the fibres.
Concretes containing fibres can be placed, compacted and finished using the same methods as for concrete without fibres. Vibration should be applied, except in the case of self-compacting concrete, to ensure full compaction and elimination of voids. Finishing by float or trowel, should not be excessive but sufficient to produce the required finish without the production of excessive surface laitance or surface exposed fibres. It is expected that small numbers of fibres may be visible on the worked surface.
Further information on the use of fibres may be found in Concrete Society Technical Report 63, Guidance for the design of steel-fibre-reinforced concrete and Technical Report 65, Guidance on the use of macro-synthetic-fibre-reinforced concrete.
Acknowledgement: The Concrete Society
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