Raft Foundations (sometimes known as Mat Foundations) are a large concrete slab reinforced with Steel 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. It can also be defined essentially as a continuous slab resting on the soil that extends over the entire footprint of the building, thereby supporting the building and transferring its weight to the ground.

Foundations provide support for structures, transferring their load to layers of soil or rock that have sufficient bearing capacity and suitable settlement characteristics.

Very broadly, foundations can be categorized as shallow foundations or deep foundations. Shallow foundations are typically used where the loads imposed by a structure are low relative to the bearing capacity of the surface soils.

Deep foundations are necessary where the bearing capacity of the surface soils is not adequate to support the loads imposed by a structure and so those loads need to be transferred to deeper layers with higher bearing capacity.

Shallow foundations include:

• +Strip foundations (or footings).
• +Pad foundations.
• +Raft foundation.

Raft foundations (sometimes referred to as raft footings or mat foundations) are formed by reinforced concrete slabs of uniform thickness (typically 150 mm to 300 mm) that cover a wide area, often the entire footprint of a building. They spread the load imposed by a number of columns or walls over the area of foundation, and can be considered to ‘float’ on the ground as a raft floats on water.

They are suitable where:

• -Floor areas are small and structural loadings are low, such as in one or two-storey domestic construction.
• -A basement is required.
• -Ground conditions are poor and strip or pad foundations would require significant excavation, for example on soft clay, alluvial deposits, compressible fill, and so on.
• -Settlement, or differential settlement is likely.
• -Where it may be impractical to create individual strip or pad foundations for a large number of individual loads. In very general terms, if strip or pad foundations would cover 50% or more of the floor area, then a raft may be more appropriate.

Raft foundations can be fast and inexpensive to construct, as they tend not to require deep excavations compared to strip or pad foundations and they may use less material as they combine the foundation with the ground slab. However, they tend to be less effective where structural loads are focused on in a few concentrated areas, and they can be prone to erosion at their edges.

They are generally constructed on a compacted hardcore base (perhaps 100 mm thick). A layer of blinding concrete may then be laid to allow formation of the raft (typically 50 mm) with a waterproof membrane above.

The concrete raft tends to include steel reinforcement to prevent cracking, and may incorporate stiffening beams or thickened areas to provide additional support for specific loads, for example, below internal walls or columns (which may require punching shear reinforcement). Beams may stand proud of the raft, either above or below it, or may be 'hidden' beams, formed by reinforced areas within the depth of the raft itself. These thickened areas are particularly useful where there are poor ground conditions, as the required thickness of the raft itself might otherwise be uneconomic.

Typically, a thickened reinforced area is created at the perimeter of the raft to form an edge beam supporting the external walls of the building. A concrete toe often supports the external leaf of the wall.

Insulation will generally be laid on top of the raft, with a concrete floor, or raised floor above.

Drainage may be required in some circumstances under raft foundations, and geotextile barriers may be required to prevent free-draining materials from becoming clogged up by the surrounding soil.

Types of raft foundation include:

Solid slab raft, sometimes referred to as a plain raft, and including; flat rafts, mats, wide toe rafts, slip plane rafts, blanket rafts, and so on.

• Slab beam raft.
• Cellular raft.
• Piled raft.

The design of raft foundations involves a number of disciplines, as consideration must be given not only to the structure itself, but also to; integration other constructions (such as external walls), insulation, damp proofing and complex ground conditions such as the presence of groundwater, trees or contamination.

Where soil is compressible, a raft foundation may be formed as a compensated foundation. In this case, the raft slab is provided to a depth that the weight of the excavated soil is equal to the raft slab weight plus that of the structure to be supported. This can be appropriate when constructing buildings on soft clay or loose sand, as settlement can be significantly reduced.

Raft foundations (sometimes known as Mat Foundations) are a large concrete slab reinforced with steel 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. This is basically a material that holds the wet reinforced concrete of the raft foundation in place until it cures. This material is mostly of wood, but this idea of formwork become obsolete internationally and it is gradually becoming obsolete in the country with the introduction of our steel shutters, steel panel or steel formwork.

STEEL SHUTTERS (ADVANTAGES)

1) ITS CHEAPER

It is cost effective and economical. The average cost of formwork using wood for a raft foundation is about five hundred thousand naira (500,000.00) while with steel shutters it’s a about half the cost of wood for an average building.

2) BETTER QUALITY END PRODUCT

The finished work when steel shutters are used gives perfect quality with no honey combs on the surface of raft foundation unlike wood that will bulge with lots of honey combs.

3) IT ELIMINATES WASTE

Steel shutters are reusable as justified by the fact that it is made of steel, therefore it can be use subsequently as against wood that is condemned after use due to water and weather.

4) IT MAINTAINS A CLEAN ENVIRONMENT

Steel shutters when dismantled after use is easily removed from site with no extra cost unlike wood that litters around site thereby making the entire surroundings unkempt. On other hand, cold-formed thin-walled profiled steel decking sheets with embossments on top flanges and webs are widely used in many composite slab constructions. Profiled steel deck performs two major functions that act as a permanent formwork during the concrete casting and also as tensile reinforcement after the concrete has hardened. The only additional nominal light mesh reinforcement bars that needs to be provided is to take care of shrinkage and temperature, usually in the form of welded wire fabric.