Concrete Guidance
Chris Hudson, Managing Director for Asphalt and Readymix at Aggregate Industries, draws on the National House Building Council’s (NHBC) recent change in guidance, around the technical considerations housebuilders must factor in before installing block and beam flooring
There is no doubt that the methods used to construct new floors have come on in leaps and bounds in the last decade. However, block and beam floors still remain the most popular solution for housebuilders when building a ground floor.
Suspended concrete flooring systems were originally developed as a means of overcoming ground problems such as unstable sloping sites, where it made sense to bridge deep gaps rather than fill them. Today, nearly all new houses employ block and beam construction for ground floors.
With a greater understanding of the benefits of block and beam flooring, including minimal ground preparation, quick installation and significant savings in labour, more and more housebuilders are turning to this method of construction.
However, little has been said about the consideration that must be taken when selecting materials; in particular when it comes to reinforced concrete topping. In April this year NHBC published Technical Extra 20, announcing changes to the construction of suspended block and beam concrete floors, and more notably, changes to the use of structural reinforcement of in-situ concrete toppings.
The latest guidelines will be applied to all homes registered with NHBC where the foundations are begun on or after 1st January 2017. In order to stay ahead of the game and iron out teething problems housebuilders should adopt these changes at the earliest opportunity. However, there are a number of consideration that must be taken into account - and housebuilders must be aware of the implications.
NHBC REQUIREMENT
Where in the past, micro fibres have been allowed instead of steel mesh reinforcement, this is now no longer acceptable to the NHBC. Although micro fibres may help to enhance certain properties of hardened concrete - such as reducing plastic shrinkage cracking, reducing plastic settlement and increasing impact resistance - the NHBC claims that there is no evidence that micro fibres will provide the increase in residual (post cracking) flexural strength required in structural concrete topping.
Therefore, as of 1st January 2017, NHBC will not accept the use of Class I synthetic polymer fibres as a replacement for welded steel mesh reinforcement in a structural in-situ suspended concrete topping.
However, approved macro structural fibres or steel fibres will be acceptable for use as reinforcement. According to the guidance, structural toppings on insulating block and beam systems requiring approval by the NHBC must use one of the following reinforcement options; macro ‘structural’ synthetic fibres, steel fibres or steel mesh reinforcement.
In all situations where the cast in-situ concrete topping is required to perform the function of a structural screed, the reinforcement of the topping should consist of a welded mesh in accordance with BS EN 15037-1.
Alternatively, cast in-situ structural topping reinforced with polypropylene macro fibres or steel fibres may be acceptable to the NHBC for residential suspended ground floor construction, providing that the adequacy of the complete structural floor system - including beams, blocks and structural concrete screed - has been satisfactorily verified by calculation to BS EN 1992-1-1. In all cases, the structural capacity of the concrete topping will need to be verified through appropriate testing by an independent third- party assessor, in accordance with the NHBC.
AGGREGATE INDUSTRIES
TOP TIPS:
Chris gives his top three tips for housebuilders when selecting concrete block and beam flooring:
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Make sure you understand the latest guidelines. The NHBC changes will be in effect before you know it. Use this time wisely to source out the correct materials and train employees on how this needs to be used.
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Speak with your materials supplier of choice for advice. They can help you to understand the product limitations and unique benefits, and in some cases create a bespoke product where required.
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Choose proven brands. Like any construction material, cheaper alternatives are available, but it is important to understand the impact that a low quality concrete will have on the long-term structure of the property.
TEL: 01335 372222 59
WWW.AGGREGATE.COM
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