Slab foundation in Batumi: base, reinforcement, pour

When a slab foundation beats the alternatives

A slab lies under the whole footprint of the house and spreads its weight over a large area. That is why it is chosen on weak and mixed ground, where a strip would have to go deep and piles are not needed: there is a dense layer, but not right under the surface, and the house is not heavy. The advantage of a slab is that it immediately gives a level base for the ground floor and ties all the columns into one rigid system, which helps in an earthquake.

On a slope a slab needs a level platform, and that is a different estimate. Where a slab wins over piles and where it loses is covered in the comparison of pile and slab foundations; here we describe the work itself.

The base under the slab

A slab only works on a well prepared base. We strip the topsoil and fill down to the ground named in the survey, lay sand or crushed stone in layers with compaction and check the levels. On top goes a thin concrete blinding: it makes it easy to lay the waterproofing and set the reinforcement on spacers. If you skip the blinding and tie the cage straight on sand, the reinforcement sinks into the bottom edge and the concrete cover under it disappears.

Waterproofing under the slab goes down as a continuous sheet with overlaps, and its edges are left sticking out so they can later be joined to the vertical layer on the slab edges. This detail is described on the page about foundation waterproofing.

Slab foundation in Batumi: reinforcement and thickness

The engineer sets the slab thickness and reinforcement from the weight of the house, the ground and seismic loads. Usually there are two meshes, bottom and top, joined by support chairs, with thickenings under the columns and internal load-bearing walls. Along the edge we place U-bars so the slab edge does not spall. Starter bars for columns and walls come out where the setting-out drawing shows, and we check them with a tape before the concrete; after the concrete they cannot be moved.

Before the pour we build sleeves into the slab for every service entry and every sewer outlet: the plumber walks the plan with the site foreman and checks each opening against the layout of bathrooms and kitchen. Moving a toilet after the pour means drilling and chasing the slab.

Poured in one day

We pour the slab in one shift without breaks, otherwise a cold joint appears in the foundation that the calculation does not allow for. For that we order the full volume of concrete and the pump from the plant in advance, check access for the concrete trucks and pick a day without a downpour. The concrete is compacted with a vibrator and the surface levelled to screed rails. Over the following days the slab is kept damp and shaded from the sun, and we start loading it after it reaches the strength set in the design.

Plinth, insulation and paved apron

We raise the top of the slab above ground so splash from the apron does not reach the wall, and insulate the edges with XPS boards. If the house will have underfloor heating, we decide in advance where the insulation goes: under the slab or in the screed on top. Insulation under the whole slab with pipes inside it is an insulated slab foundation, which has its own limits in seismic areas. Round the house we lay drainage and a concrete paved apron sloping away from the wall.

How much a slab foundation costs

A slab foundation costs from 390 ₾ per m² of house footprint: bed, waterproofing under the slab, reinforcement, concrete and service sleeves. If it is easier to price by volume, a concrete slab is from 880 ₾ per m³. Excavation with soil removal from 39 ₾ per m³, edge and plinth insulation from 75 ₾ per m², drainage from 87 ₾ per linear metre and the paved apron from 93 ₾ per m² are separate lines. The final cost is set from the design and ground survey, after a site visit and agreement on the estimate. The slab can also be ordered together with the house shell.