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Steel-fibre-reinforced concrete

Design of steel-fibre-reinforced concrete is not covered by design standards such as BS 8110 (now withdrawn) or Eurocode 2, which assume the use of bar reinforcement.

Various design approaches are currently adopted.

The first may be described as Design on the basis of material properties. In this approach, material properties such as residual tensile strength are determined from standard small beam or statically determinate slab tests. These properties are then inserted into equations determined mathematically or empirically that define the performance of the concrete element to determine the load capacity. In general, the design equations will be linked to properties determined from a specific test.

This approach is discussed in detail in Concrete Society Technical Report 63, Guidance for the design of steel-fibre-reinforced concrete.

Other approaches are commonly used, such as Design assisted by testing. This approach has been used widely for the design of pile-supported slabs and more recently has been used for fully suspended slabs. The basic premise is that the performance of the concrete in small beam tests, while satisfactory for linear elements, is not representative of the performance of elements such as slabs. Thus, the process consists of testing round indeterminate slabs to determine the design flexural resistance. The slab can then be designed by yield line theory. This design procedure is justified by the results of load tests on large-scale sub-assemblies of suspended slabs.

A partial application of this approach is the design of composite slabs on steel decking where tests to determine the local performance of fibre-reinforced concrete, e.g. around stud shear connectors, have been used to demonstrate that the performance is adequate. Slabs with fibre-reinforced concrete can thus be designed in accordance with the Standards for composite slabs with fabric reinforcement. The approach is approved by SCI (Steel Construction Institute) for specified combinations of steel fibres and decking.

Design by performance testing, or proof testing, is applicable to precast units where a large number of items are required for a particular purpose. The element’s dimensions, fibre content, etc. will be determined on the basis of judgement or experience. Representative completed units will be tested to demonstrate their ability to carry specified loads.

fib Model Code for concrete structures (2010) contains some clauses for design.

The 4th edition of the Concrete Society report TR34 Concrete industrial ground floors (2013) provideds a design approach for warehouse floors.

Swedish Standard SS 812310 Design of fibre concrete strcutures (in English), which is complementary to Eurocode 2, is due for publication in 2014

Acknowledgement: The Concrete Society

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TR63 Guidance for the design of steel-fibre-reinforced concrete

TR34 Concrete industrial ground floors
a guide to design and construction - 4th Edition

Concrete industrial ground floors – user expectations