How to get it right: Using reinforcement in your foundations

07.08.2018
News
Concrete reinforcement - how to get it right

One of our surveyors had a bit of a shock recently when visiting a site for a domestic extension.

They were called out to inspect reinforcement prior to concreting foundations but hadn’t been to site previously for an excavation or commencement inspection. The ‘builder’ proudly stood back and informed the officer that he had dug down 450mm but was still into filled ground so had decided to construct a reinforced raft foundation instead.

Better still, he was helping the environment by recycling shopping trolleys for the reinforcement.

"Every little helps" replied a bemused officer before explaining what was wrong. The project was subsequently abandoned because of the additional cost of doing it correctly and it reverted back to a patio area.

If you’re involved in constructing a raft foundation then there are some key factors that need to be considered to ensure that the reinforcement fabric is correctly installed. It’s an alternative if you can’t use a traditional strip or trench fill foundations but it's important to note that raft foundations aren’t suitable in all cases and usually need designing by a structural engineer.

Unlike suspended floors strip foundations where mesh is just placed in the bottom of the concrete to act in tension, rafts usually have mesh in the top to resist compression from heavy point loads like internal walls and in the bottom for tension to spread the load across a wider surface.

Reinforcement key points

  • Reinforcement comes in different sizes and grades but the most commonly used are A and B fabric reinforcement. The table below shows you the bar sizes and centres for those commonly used:

Foundation reinforcement table

  • Reinforcing fabric should be free from loose rust, oil, grease, mud and any other contaminants that may affect the durability of the concrete.
     
  • Sufficient cover needs to be provided around the steel to protect it within the concrete. 40mm is the minimum cover required to all faces of the concrete slab. On the bottom this can be achieved using proprietary stools/meshmen/styrofix/ raisers (not odd bricks) at 20 per sheet with hystool or wire spacers between any layers at 5 per sheet to ensure the top layer stays where it should and doesn’t just sink through the concrete (particularly when its being poured or tamped down and walked over) and keeps the minimum cover to the surface.
     
  • B grade fabric can be identified by the size of the longitudinal and transvers bars with the longitudinal bars spaced at 100mm centres and are always placed in the direction of span. Transverse bars are spaced at 200mm centres, as indicated in Table 3 of the LABC Warranty technical standards manual
     
  • Where reinforcement fabric overlaps the rule of thumb is a minimum overlap of two bars plus 50mm i.e. 200 + 200 + 50 = 450mm but this can sometimes be reduced through engineered design to Eurocode 2 Table 4 of the LABC Warranty technical standards manual provides minimum lap dimensions for B fabric.

Laps should be all be tied using wire binding.

Please note: LABC do not advocate the use of shopping basket/trolley mesh in foundations!

Further information

Raft foundation basics

Comments

Supermarket shopping trolley

Submitted 3 months ago

Did the Building Control officer notify the supermarket from where the shopping trolleys were being taken? 'Every little helps' especially for the local supermarket prices......

Competency lacking

Submitted 3 months ago

Once again we are back to the question of builders being registered for competency. How anyone thought this work was adequate is beyond comprehension.

Everyone registered for competency, vat and insurance. This situation cannot go on.

Wire binding

Submitted 3 months ago

I was asked the other day on site if it is ever acceptable to tie individual rebars together (to make up a ground beam for example) using plastic pull fasteners (i.e. the ones that once pulled through the loop cannot be untied), this is a much faster and safer option than using wire binding and once in the ground and surrounded by concrete what difference does it make? Also if this is not allowable are we saying that the wire binding is therefore a structural item that somehow gets designed/is included in the design codes? Thanks.

Reply

Submitted 3 months ago

Hello there

The function of the tie is to hold the reinforcement in position while the concrete is being poured. Materials other than steel wire can be used providing they are compatible with the concrete and the steel.

Thanks,
John Allen, LABC

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