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Last edited 06 May 2022
Types of concrete specification
 BS8500 standard for concrete specification.
- Designed concretes,
- Designated concretes,
- Prescribed concretes,
- Standardised prescribed concretes
- Proprietary concretes.
The Concrete centre guide to specifying sustainable concrete, describes each of these in turn, the essence of which is given below, to refer to the original text please visit the concrete centre at www.concretecentre.com.
Designed concretes are for the informed specifier, where the designer considers all the requirements for the hardened concrete such as strength and durability to derive the necessary strength class and other properties such as cement type, minimum cement content and maximum water/ cement ratio. Normally the designer will assess the exposure conditions and consider the recommendations set out in BS 8500: Part 1:2015+A2:2019 Annex A to determine the concrete properties and minimum cover to reinforcement required to achieve the structural performance and service life.
Due to its inherent durability properties most reinforced concrete is designed for a minimum intended working life of at least 50 years, but often performance of 100 years can be achieved with zero or modest increases in cover or concrete quality. The longevity of concrete in use is an important sustainability consideration. The flexibility of designed concretes, makes them suitable for specifying the most sustainable concrete, that is with the minimum of embodied carbon using low carbon cements or combinations, together with the option of recycled or secondary aggregate where considered beneficial.
Designated concretes are types of designed concretes that allow scope to be achieved using a range of predetermined mixes, where for a range of applications the specifier only calls up the required designation, e.g. FND2 is a concrete suitable for use in ground assessed as ‘DC-2’, Design Chemical Class 2. Similarly, RC28/35 is a designated concrete of C28/35 strength suitable for an internal suspended floor, or RC25/30 is a designated concrete of C25/30 strength suitable reinforced foundations and GEN1 suitable for unreinforced foundations.
Designated concretes are quality-assured designed concretes that conform to a specification detailed in BS 8500-2. These concretes have been selected to be fit for their intended use and they can only be supplied by ready-mixed concrete producers who have third-party product conformity certification. A QSRMC or BSI Kitemark logo on the delivery ticket provides this confirmation. Purchasers can be confident that the concrete will be delivered as specified and ordered, and as the concrete is optimised by the producer it will be the most cost effective, which can often also be the most sustainable.
Designated concrete is the preferred method of specifying concrete when working to the NHBC Standards. Guidance entitled Concretes for Housing - Designated concrete  is available from the British Ready-Mixed Concrete Association (BRMCA). Designated concretes are not appropriate where concrete needs to resist the risk of corrosion by the ingress of chloride in such cases the specifier should specify a designed concrete.
Prescribed concretes allow the informed designer to specify concrete by prescribing the composition. This method is rarely used but is useful where a particular ratio of constituents is required for exposed aggregate concrete finishes.
Standardized prescribed concretes are intended for small building sites where concrete is either mixed by hand or in a small, less than 150 litre, concrete mixer. They are designated ST and are accepted by NHBC for some applications. There is no requirement to demonstrate the strength of ST concrete but BS 8500 Part 1:2015+A1:2019 Annex A provides some indicative values for the strength class that may be assumed for structural design, e.g. the highest grade of standardized prescribed concrete is ST5, where this may be assumed to be at strength class not greater than C20/25.
To ensure the ST designation recommendation is safe for the indeterminate range of materials and site supervision the prescribed cement content is very high, and the use of ST concretes should be avoided where a ready- mixed concrete in the form of a designated or designed concrete can be used. Standardized prescribed concretes have a much higher embodied carbon than the equivalent designated concrete. The GEN series of designated concretes are the more sustainable option because the cement content is optimised to that required for strength. For example ST1 as a standardized prescribed concrete equates to GEN0 as a designated concretes, ST2 as GEN1, ST3 as GEN2, ST4 as GEN3 and so on.
Proprietary concretes are developed by the concrete producer and are marketed on the basis of their enhanced fresh or hardened properties. The producer will normally guarantee the performance of these products and provide test certificates. Proprietary concretes may be covered by third-party product conformity certification. Proprietary concretes are often used for high performance applications where the sustainability benefit is in the reduction in the total material volume used rather than the value per volume. For commercial reasons the producer may not disclose the exact composition and is not required to do so by the concrete Standards, EN 206 and BS 8500.
Additional aids to specification of concrete include the National Structural Concrete Specification (NSCS) Edition 4  and the National Building Specification (NBS) . More details of these resources aswell as specific examples of each specification type can be found in the concrete centre guide to specifying sustainable concrete available via their website.
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