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Are you 'covered'?

Standards and regulations for spacers in reinforced concrete construction.

When designing and building reinforced concrete structures, the main emphasis is on two major components: the reinforcement, and the concrete mix. There are national and international standards that provide guidance with regards to these materials and their usage. Nevertheless, there are further components, which play a minor role in volume terms, but are equally important from a quality and durability perspective. Spacers, which keep the reinforcement in the correct position, are such a component. Gerhard Bumes, Managing Director at Max Frank Ltd., gives an overview regarding standards to be aware of when specifying and using spacers.

Why is cover important?

Eurocode 2, officially known as BS EN 1992-1-1 and now widely used for the “Design of concrete structures” (as the title states), emphasises the importance of 'cover': It’s mentioned in the heading of chapter 4 on the same level as 'Durability'. Designers and Builders obviously aim to provide a 'durable structure' that does not have 'significant loss of utility or excessive unforeseen maintenance' (4.1). To achieve this, the concrete cover needs to have the right 'density, quality and thickness'. As section 4.4.1.2 states, this will ensure 'safe transmission of bond forces', provide 'protection of the steel against corrosion', and provide 'an adequate fire resistance'. All these points are supported by an effective 'Crack control', elaborated in more detail in Section 7 of EC2. In summary, to ensure the structural integrity of the structure, concrete cover plays an essential role.

Specifying the right cover.

A major influence on the above stated density and quality of the cover is the 'water/cement ratio and minimum cement content' (4.1) and, therefore, the specified concrete type and strength. Equally important is the 'thickness' of the cover, defined as the 'distance between the surface of the reinforcement closest to the nearest concrete surface' (4.4.1.1). This is called 'concrete cover'. The designer’s duty is to specify the nominal cover cnom on the drawings, where cnom = cmin + Δcdev. The minimum cover cmin is determined by bond or durability requirements, depending on environmental conditions. The requirement for Δcdev depends on, 'the type of construction and the quality control measures', as clarified by BS 8500-1 (A.3). According to the UK National Annex to EC2, a typical example using in-situ construction requires 10mm as the value for Δcdev.

Spacers must achieve the specified cover.

Once the correct nominal cover cnom has been specified, it must be achieved during the construction process, hence the requirement for spacers. BS EN 13670, a standard to be used in conjunction with EC2, advises spacers need to be 'suitable for achieving the specified cover', and – in the case of cementitious spacers – 'should have at least the same strength and should at least give the same corrosion protection as the concrete in the structure' (6.2). In addition to this, The National Structural Concrete Specification for Building Construction (NSCS 4) provides 'non-contradictory complimentary information'. It clearly states, that spacers 'shall be in accordance with BS 7973' (6.1.3.1).

Part 1 of BS 7973 refers to some essential characteristics of spacers:

  • Not to cause any deterioration to reinforcement (6.1).
  • Not to allow free passage of water (6.1).
  • Not to cause spalling (6.1).
  • Cementitious spacers: compressive strength at least 50N/mm² (6.3).
  • For normal duty spacers: point test load min 0.5kN, heavy duty point test load min. 3kN (Table 2).
  • To be factory made, according to BS 7973-1, and not formed on site (6.3).
  • Dimensional tolerance: +/- 1mm up to 75mm cover (7.2).
  • Maximum deflection: 1mm (9).

Part 2 of the standard advises on the application of spacers. Most significantly, spacers 'shall be staggered' (8.1.1.1), and heavy duty spacers need to be used where reinforcement exceeds 20mm (7). BS 7973 defines, in detail, the physical properties of spacers. However, required durability properties for acid, sulphate, chloride and carbonation resistance – to suit the various exposure classes – are not stated.

If essential spacer characteristics and correct application are not considered, weak areas may develop in the cover zone, together with increased threat of reinforcement corrosion. As the standards are considered as a minimum requirement only, a specification of spacers – with the same exposure class as the surrounding concrete – ultimately avoids this risk.

Construction reality and potential shortcomings.

As demonstrated, the topic of spacers is thoroughly 'covered' from a theoretical perspective. However, reality often shows a different picture. Common shortcomings range from not using enough spacers to using incorrect spacers. Examples for the latter is the use of normal or light duty plastic spacers for heavy load applications, e.g. where common foot traffic is present. In general, plastic spacers are more likely to allow water passage due to the differing thermal properties between plastic and concrete – concrete spacers provide the optimum bond with the in-situ concrete. Nevertheless, concrete spacers also vary in quality. In particular, dimensional tolerances, potential blowholes and compressive strengths need to be checked and verified.

Max Frank Ltd. provides a range of block and bar spacers for covers up to 100mm, compressive strengths up to 90N/mm² and in various colour options. As well as complying with BS 7973, our spacers can be produced to meet the requirements of exposure classes XC4, XS3, XD3, XF4, and DC4 to ensure compliance with all project requirements. The ISO 9001 Quality Management System and rigourous product testing and certifications ensure adherence to the above standards. If you are working on a new scheme where spacers are required, please contact us. We will be pleased to answer any technical or commercial questions!

Concrete quality and durability? Another problem solved by MAX FRANK.

References:
1. BRITISH STANDARDS INSTITUTION: BS EN 1992-1-1. Eurocode 2: Design of concrete structures. General rules and rules for buildings. BSI, London, 2004+A1:2014.
2. BRITISH STANDARDS INSTITUTION: BS EN 13670:2009. Execution of concrete structures. BSI, London, 2009.
3. BRITISH STANDARDS INSTITUTION: BS 7973-1:2001. Spacers and chairs for steel reinforcement and their specification – Part 1: Product performance requirements. BSI, London, 2009.
4. BRITISH STANDARDS INSTITUTION: BS 7973-2:2001. Spacers and chairs for steel reinforcement and their specification – Part 2: Fixing and application of spacers and chairs and tying of reinforcement. BSI, London, 2009.
5. BRITISH STANDARDS INSTITUTION: BS 8500-1:2015. Concrete – Complementary British Standard to BS EN 206. BSI, London, 2015+A1:2016.
6. THE CONCRETE CENTRE & CONSTRUCT: National Structural Concrete Specification for Building Construction – Fourth edition complying with BS EN 13670:2009. Blackwater, 2010.

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