Raft Foundations

Specialised Raft Systems

Raft Foundations

A Raft foundation (sometimes known as Mat Foundation) is a large concrete slab which can support a number of columns and walls.

The slab is spread out under the entire building or at least a large part of it which lowers the contact pressure compared to the traditionally used strip or trench footings.

Because of the speed and volume of houses required after the second world war, the raft foundation was widely used. The raft foundation was cheaper, easier to install and most importantly, did not require as much excavation as the usual strip foundations.

When the Building Regulations were introduced in 1965 there were no generic rules for raft foundations as there were for strip foundations.

The implications for the use of a raft foundation was that it had to be designed and approved by Building Control. This resulted in the diminished popularity of the raft foundations –  almost overnight.

How does raft foundations work?

A raft foundation spreads the weight of the building over the whole ground floor area of that building. The raft is laid on a hard-core, or scalping bed and usually thickened at the edges, especially in very poor ground. Rafts are most suitable when the ground is of good load bearing capacity and little work is required to get a solid foundation.

The following steps are used to build Raft Foundations:

  1. Remove the soil down to correct depth
  2. The foundation bed is then compacted by ramming
  3. Lay reinforcement on spacers over the foundation bed
  4. Pour the concrete over the reinforcement

The foundation may stiffened by ribs or beams built in during construction which will add extra strength and rigidity.

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Different Raft Systems

Rafts in South African context have the following advantages over normal strip foundations :

  • Costing very accurate
  • Unsuitable soil conditions not an issue
  • Cost effective
  • Speed
  • NHBRC approved
  • Engineer designed, certified and supervised

In a highly progressive technological environment with its rapid advances so did raft foundations  evolve  enhancing the awesome qualities without compromising the existing rational of utilizing a raft foundation system. One of these is called the modular raft system :

You can’t run before you learn how to walk; similarly you can’t start your projects before your foundations are laid. A seemingly easy choice, the foundations bear the responsibility for the health of your structure, for many years (hopefully) to come.

Innovative technologies push professionals out of the “structural comfort zone”,  but the advantages cannot be ignored.  We have compiled the top 5 reasons why Modulo system by Geoplast adds incredible value to your structure.

What is Modulo? This permanent crawl space formwork in recycled polypropylene by Geoplast, is the perfect solution to physically separate the building from the ground. Agrément certified, Modulo creates a technical space for natural ventilation.

1. Elimination of harmful vapours

The void space created under Modulo allows natural ventilation and the consequent elimination of rising damp, humidity and Radon Gas mitigation.

2. Improvement of buildings’ structural performance

The reinforced concrete structure obtained with Modulo permits a uniform stress distribution all over the surface, thus producing an excellent load-bearing capacity.

3. Easy installation

Installing the system is up to 80% faster, when compared with traditional systems. It is the ideal product for community projects, where different skill sets and levels can contribute to high- quality outcomes

4. Concrete savings

Modulo allows a substantial reduction of the amount of concrete used, up to over 40%, in comparison to conventional systems.

5. Energy Performance of the Structure

Modulo creates a mechanical ventilation system to optimise energy efficiency and limit working consumption, making it a green building.

Lastly, Modulo can also be used as a filler element when building pile foundations or when raising the existing floor slab; the formwork replaces traditional backfilling material (sand), beating project deadlines, offering logistics advantages as the product is stackable, ensuring the lightenss of the structure and curbing the use of water on site. Read here about Modulo used as a filler in the creation of a new gymnasium at University of the Western Cape, designed by Noero Architects.

Make your structure Green and Efficient! Get in touch for more information about Modulo permanent formwork.

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