Crack mitigation in structural fibre reinforced in-situ concrete toppings to beam and block suspended ground floors

1 February 2023

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This article draws attention to crack mitigation in structural fibre reinforced in-situ concrete toppings to beam and block suspended ground floors.

It should be read in conjunction with previous articles ‘Suspended beam and block concrete floors – guidance on selection of products and materials’ TE15, October 2014 and ‘Structural reinforced in-situ concrete toppings for use with beam and block suspended ground floors in residential construction’ TE20, April 2016. Further reference should also be made to the MPA British Precast Flooring Federation ‘Application Guide for the Specification and Installation of Concrete Toppings to Beam & EPS Block Suspended Floors’, September 2017.

Guidance

Structural fibre reinforcements (be it steel fibres to BS EN 14889-1 or Class II polymer macro fibres to BS EN 14889-2, with both needing to be certified for structural uses in concrete mortar or grout) have different properties and structural characteristics. They are also applied at different dosage rates in in-situ concrete toppings to beam and block suspended ground floors, as identified on relevant flooring system or fibre independent technical approval body assessments accepted by NHBC.

These assessments will also identify relevant concrete mix design requirements appropriate to the assessment, however there is still potential for variability of mix constituents.

 

Because of the permutations and varied magnitudes of reinforcement provided by the different structural fibres, it is difficult to predict performance with respect to mitigating cracks occurring in relatively young structural fibre reinforced concrete due to plastic or drying shrinkage.

If not properly considered, cracks or cracking could still develop to an extent whereby structural remedial work is required to reinstate the toppings integrity.

It is possible to mitigate potential for this occurring by not only referring to published industry guidance and guidance provided on assessments relative to the practical aspects and bearing in mind pour layouts, sizes, joints, curing techniques, etc., but also by seeking advice from fibre and concrete producers and suppliers as well as Chartered Engineers.

You need to...

  • be aware structural fibres have different properties and structural characteristics and are applied at different dosage rates in accordance with independent technical approval body assessments accepted by NHBC 
  • recognise these assessments do not remove the need for consideration still to be made of performance of the concrete topping, the design of which should still be undertaken by a suitably competent Chartered Engineer 
  • follow published industry guidance and guidance provided on assessments, as well as liaise with fibre and concrete producers and suppliers and Chartered Engineers to ensure details or construction proposals mitigate the risk of cracks occurring due to plastic or drying shrinkage that could be of structural significance and warrant remedial work. 

Who should read this:

Technical and construction directors and managers, architects, designers, fibre and concrete producers, specifiers and purchasers and site managers.

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