Floors: reinforcing beam and block floors
26 February 2018
Paul Cribbens urges that care is taken when reinforcing beam and block floors
One of the major innovations in the construction of new homes in the past 25 years has been the increasing prevalence of beam and block suspended floors. This can be traced back to the widespread failures of poorly designed ground-bearing floors affected by movement in unstable soil, which peaked in the late 1970s and early 1980s.
Beam and block floors are typically constructed from pre-stressed concrete beams with concrete blocks between them to form the structural floor, using sand and cement screed to form the finished surface. They have proven to be reliable, quick to install and cost-effective. This makes them the first choice for designers, particularly where ground movement could be an issue.
More recently, free-flowing concrete mixed with steel or polypropylene fibres has been used instead of welded wire mesh reinforcement, which has traditionally been incorporated into the screed to provide resistance to plastic shrinkage cracking.
Driven by the need to provide homes that are better thermally insulated, a further development has been the use of insulated infill blocks, typically manufactured from non-load-bearing expanded polystyrene (EPS) or lightweight concrete to replace the load-bearing dense aggregate concrete infill blocks.
Although at NHBC we encourage innovation and the development of new and better ways of constructing homes, we have questioned some of the recent developments in suspended beam and block floor constructions. More specifically, we have queried the structural capacity of floors constructed from EPS or lightweight infill concrete blocks where they are used with screeds incorporating micro-polypropylene fibres.
Figure 1: A 5mm crack
The lack of test data on the use of micro-polypropylene fibres as reinforcement has meant that we cannot be confident of its use for structural purposes. Furthermore, we have received reports from sites of cracking in screeds containing micro-polypropylene fibres.
Where non-load-bearing blocks are used with a reinforced screed, cracking in the screed may disrupt its capacity to span the pre-cast concrete beams, which are the primary supporting elements of the floor construction.
Floor toppings intended to have structural capacity should either be designed in accordance with recognised standards or have their likely performance independently verified.
BS EN 15037 provides design guidance for an array of different products that can be used to construct beam and block floor systems. The standard also acknowledges that EPS infill blocks categorised as R1 and concrete infill blocks with a compressive strength of less than 7.3N/sq. mm have insufficient strength to fulfil a structural function in the finished floor. Therefore, the structural capacity of the floor has to be provided by other means, for example by a reinforced concrete screed known as a structural topping that is placed over it.
Structural toppings can be made of sand and cement incorporating welded wire mesh reinforcement or concrete reinforced with macro-polypropylene fibre or steel fibre. Non-structural screed, incorporating micro-polypropylene fibre to reduce plastic shrinkage cracking, can be used to provide the finished surface to a floor, but only where the infill blocks have sufficient structural capacity to take the predicted loads in accordance with BS EN 15037.
Working with the industry-wide task group, we have concluded that structural performance of suspended beam and block floors in habitable parts of the home can be achieved either by using:
- EPS or concrete infill blocks with sufficient structural capacity; or
- EPS infill blocks categorised as R1 and concrete infill blocks with a compressive strength less than 7.3N/sq. mm, combined with a reinforced concrete topping.
Beam and block floors with non-structural infill blocks and using a structural topping, reinforced with macro-polypropylene fibre or steel fibre, should be independently assessed and certificated as a complete flooring system.
Certification should be carried out by an independent body accredited to undertake such assessments. NHBC has been working with the manufacturing sector, trade bodies and certification bodies to ensure that there are products available that meet these criteria.
As part of the task group, British Precast has published design and application guides for beam and block suspended ground floors. These documents, which complement guidance previously published by NHBC, can be found here.
Since October 2017, NHBC’s building inspectors have been informing site managers that, for homes where the foundations are begun on or after 1 January 2018, beam and block floors incorporating non-structural blocks should only be used in conjunction with a suitably reinforced structural topping.
We hope that by bringing industry together and providing clear guidance, we will improve the quality of new homes, and ensure that they perform as intended, while allowing industry to harness innovations in construction technology.
Paul Cribbens is NHBC Standards Manager
- Image © NHBC
- Related competencies include Construction technology and environmental services
- This feature is taken from the RICS Building control journal (February/March 2018)
- Related categories: Building control, Modern methods of construction, Health and safety in construction