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 competent structural engineer.

Unlike suspended floors or 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' grade 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.
     
  • Carbonation reduces the corrosion protection of the reinforcement by increasing porosity and decreasing alkalinity. Such corrosion can be reduced by providing as much concrete cover as possible. The appropriate level of concrete cover for reinforcement depends on the exposure of the concrete and its application – and is summarised below:  
Application (concrete position) Minimum cover 
(mm)
Concrete in direct contact with the ground 75
All external applications e.g. shuttered walling 50
Floor slabs and other applications where concrete is cast onto a membrane  40
Concrete over blinding  40
Internal conditions 25

Table courtesy of LABC Warranty Technical Manual (version 10)

  • Reinforcement should be supported by proprietary chairs or spacers and can be made of concrete, plastic or steel. The thickness and depth of a concrete spacer should not exceed 50mm x 50mm. Spacers should be placed at a maximum of 1m centres, and when supporting mesh should be staggered to avoid planes of weakness in the concrete. 
  • Supports for top layers of reinforcement should also be chairs, or other proprietary products, and positioned to ensure that the top layer is sufficiently held in place and doesn’t just sink through the concrete (particularly when it's being poured or tamped down and walked over) and keeps the minimum cover to the surface.
  • 'A’ grade reinforcement is usually specified where loading is limited and anti-cracking measures are needed. Whereas ‘B’ grade reinforcement is intended for structural uses where the loading is higher. ‘B’ grade reinforcement can be identified by the size of the longitudinal and transverse 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.
  • Where reinforcement fabric overlaps the rule of thumb is a minimum overlap of two bars plus 50mm; however, laps should be designed by a competent structural engineer  or suitable guidance followed such as Table 2 in the LABC Warranty Technical Standards which provides minimum lap dimensions for B fabric reinforcement.
  • Laps should 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

Technical Standards Manual V9 or the specific section on Foundations.
 

 

Please Note: Every care was taken to ensure the information was correct at the time of publication. Any written guidance provided does not replace the user’s professional judgement. It is the responsibility of the dutyholder or person carrying out the work to ensure compliance with relevant building regulations or applicable technical standards.

This article was updated on 2 August 2022

Comments

Supermarket shopping trolley

Submitted 3 years 11 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 years 11 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 years 11 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 years 11 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

B Mesh

Submitted 2 years 1 month ago

If a B mesh is added to a foundation base, which way does the rectangle face? The longer length going with the trench length or with the trench width? And why does it have to be that way? Thanks

In reply to by Jane Doyle (not verified)

Reply

Submitted 2 years ago

Hello there

You'll see the answer is in the article above, but here it is:

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.

Kind regards
Barry, LABC

A393 Mesh

Submitted 1 year 12 months ago

is there a limit to the number of A393 mesh pieces that can be used for a pad support and what is the minimum overlap ( the area is tight for accessabilty )

In reply to by Paul Hughes, C… (not verified)

Reply

Submitted 1 year 11 months ago

Hello there - thanks for getting in touch.

Any steel reinforcement added to pad foundations including overlap should be installed strictly in accordance with the structural engineer’s design details and calculations. Guidance in respect of the minimum overlap is included in the article namely ‘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’.

Kind regards
Martin, LABC

Depth of reinforcement mesh for bottom support in 400mm footings

Submitted 1 year 11 months ago

I am constructing a 600mm footing for a single storey extension its sat on a consistant clay bed at 1000mm + deep I have set reed bars to a levelled 450mm firstly will these need to be covered by the minimum 40mm concrete(expected answer yes) what depth should I set carriers for reinforcement mesh for bottom of foundation reinforcement & as Im expecting would mesh need to have internal clearance of a minimum 40mm for concrete coverage thanks in advance daz
P.s thanks LABC for providing in depth information to get the job right!!!

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