This is the first of three articles on the role of reinforcement in the design and construction of concrete industrial floors.
The articles primarily consider the most common application of industrial floors which is to be found in warehouses and distribution centres. The three pieces will broadly relate to Chapters 6, 7 and 8 of Technical Report 34 Fourth Edition(1) as follows: Design-structural properties; Structural design of ground-supported slabs; Structural design of pile-supported slabs. Tony Hulett of Face Consultants reports.
It is self-evident that the most important aspect of floor design is that of the reinforcement and yet up until around ten years ago, there was little independent guidance available based on modern design techniques such as yield line theory.
Design methods had moved ahead significantly during the 1990s largely as a result of initiatives by steel-fibre suppliers in Europe. These initiatives had the positive effect of reducing slab thicknesses for ground-supported floors by about 15–20% and were a valuable step towards better economy.
However, design methods did not become independently codified until 2003 when The Concrete Society published the Third Edition of TR34(2). Generally across Europe, floor design had become reliant on the proprietary data and design methods of fibre suppliers, although in the UK a number of specialist designers began using similar design methods for both conventional and steel-fibre reinforcement around 2000.
The publication of the Third Edition for the first time provided integrated and comprehensive design guidance for ground-supported floors using either conventional reinforcement or fibre reinforcement and TR34 remains the only independently produced guidance of this type to be found in the world. It is the case that there are other generic guidelines on steel-fibre concrete but TR34 is distinctly different in that the guidance is specific to floors.
In the Fourth Edition in 2013, the guidance was updated in respect of fibre reinforcement based on wide international experience gained in the intervening ten years. This latest edition now also includes comprehensive design guidance for pile-supported floors. Although the guidance in TR34 is now widely used around the world, proprietary designs provided by fibre suppliers are still common and are particularly prevalent across mainland Europe.
Basis of design in TR34
The principles of design in TR34 are in limit state format; they are based on yield line analysis and are in accordance with Eurocode 2(3). Material properties are primarily based on European Standards.
Material properties and testing
Steel-fabric reinforcement is generally regulated by national Standards. At present, there is no European Standard. In the UK, steel fabric should be in accordance with BS 4483(4) and should preferably be supplied under a product certification scheme such as CARES, which provides independent verification of material performance.
Fibre-reinforced concrete is a more complex problem in terms of testing and performance reporting. The performance of steel-fibre concrete has traditionally been assessed based on beam tests. Square and round plate tests have been promoted in the past but have not gained acceptance in the floor construction sector, or European Standards.
In practice, the system of performance testing and reporting has not been subject to independent verification. Manufacturers have not systematically tested for the range of fibre types and contents used in floors. Where tests have been performed, some suppliers have appeared reluctant to publish their data and there is concern that claimed performance could be unreliable.
In the Fourth Edition of TR34, the notched beam test has been adopted in accordance with BS EN 14651(5). A beam under test can be seen in Figure 1. TR34 requires that 12 beams are tested and that the mean value is used for design purposes, subject of course to a materials partial safety factor as discussed later. Beam tests should use concrete that conforms to a standard. That ‘standard’ concrete should be, in so far as this is possible, compatible with the concrete that is being used in real floors.
BS EN 14651 requires that a standard concrete is used as prescribed in BS EN 14845-1(6). In that Standard, the use of concrete with a maximum size of 20mm is mandatory. This is obviously sensible as most concrete in the UK has aggregate of this size. In parts of Europe the use of larger aggregates such as 32mm is quite common.
Despite this, it has been noted that some tests have been based on aggregate as small as 14mm. The point here is that generally, fibre distribution and orientation is less uniform where larger aggregates are used and this has the effect of reducing the effective strength of the steel-fibre concrete. The corollary is that the use of smaller aggregates can artificially enhance test results.
The key issue here is that designers need to have confidence in the performance data provided.
Reinforcement as a commodity
It would generally be agreed that steel for reinforcement is a basic commodity in construction. Steel bar and wire are used in fabric and fibre production. It follows that for steel fabric, a standard mesh of say A193 is the same, irrespective of the manufacturer. The material will comply with the Standard and an independent quality scheme such as CARES is there to ensure that conformity is achieved from all suppliers.
Similarly, TR34 itself sets out to bring uniformity and conformity to floor design and construction irrespective of the designer or constructor.
There is however a missing link and that is the performance of the steel-fibre-reinforced concrete and it is suggested that this is an area that needs to be developed. Clearly there are limits on how far this can be taken because concrete is an inherently variable material, using as it must, local materials.
However, this constraint does not apply to the steel fibres themselves. Steel fibres are no more than shaped pieces of wire and the wire itself is a commodity that is produced in a relatively limited number of plants around the world. Furthermore, it is self-evident that a fibre of given geometry and steel strength is the same irrespective of supplier. In the UK there are probably only four fibre types in common use and it is also the case that the number of fibre contents used in concrete are relatively limited, with perhaps only three in common use in the UK.
Therefore in order to provide designers with standardised data it is possible that less than 20 combinations of steel fibre type and content need to be tested. It is suggested that this is work that should be considered by an independent body if funding could be found for such a project.
Material safety factors in design
The partial safety factor for conventional reinforcement is 1.1 in accordance with normal practice.
In TR34, the partial safety factor for steel-fibre-reinforced concrete is 1.5 and is the same as for normal concrete. By observation, steel-fibre concrete is a composite, the components of which have to be measured and mixed in much the same way as for normal plain concrete. Therefore the variability incurred cannot be less than that of normal concrete.
A strong case
Reinforcement is critical to the successful outcome of the design and construction of industrial floors. Reinforcement can generally be considered as a commodity.
In the UK, steel fabric reinforcement is supplied under an independent conformity scheme to established Standards. European Standards have been available for testing steel-fibre-reinforced concrete but have not been widely or consistently implemented. As a result, designers cannot have the confidence that they could reasonably expert.
There is a strong case for setting up an independent testing process for fibre reinforcement.
[References]:
1. CONCRETE SOCIETY. Concrete Industrial Ground Floors. A guide to design and construction. Technical Report 34, Fourth Edition, The Concrete Society, Camberley, 2013.
2. CONCRETE SOCIETY. Concrete Industrial Ground Floors. A guide to design and construction. Technical Report 34, Third Edition, The Concrete Society, Camberley, 2003.
3. BRITISH STANDARDS INSTITUTION, BS EN 1992-1-1. Eurocode 2 – Design of concrete structures. General rules and rules for buildings. BSI, London, 2004.
4. BRITISH STANDARDS INSTITUTION, BS 4483. Steel fabric for the reinforcement of concrete. BSI, London, 1998.
5. BRITISH STANDARDS INSTITUTION, BS EN 14651. Test method for metallic fibre in concrete. BSI, London, 2005.
6. BRITISH STANDARDS INSTITUTION, BS EN 14845. Test method for fibres in concrete. BSI, London, 2006.[/fusion_builder_column][/fusion_builder_row][/fusion_builder_container]
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