Concrete house construction in Batumi: cast-in-place walls and slabs

In Batumi a house is called monolithic when its load-bearing walls and floor slabs are reinforced concrete poured right on the plot. If only the columns, beams and slabs are concrete and the walls are built of blocks, that is a concrete frame with block infill, which has its own rules. This page is about solid concrete walls: where they are needed, how to pour them in the local weather and what to do so that salt from the sea doesn't reach the reinforcement.

Where solid concrete walls are needed

A concrete wall is heavier and more expensive than masonry, and building a whole one- or two-storey house from it usually makes no sense: a frame with block infill passes the same seismic calculation and costs less. We suggest solid concrete where a wall has to carry more than the weight of the floor slabs:

• the lower ground floor and basement of a house on a slope, where the wall holds back the soil and keeps water out at the same time;
• retaining walls that form part of the house itself, for example under a terrace on the sea side;
• a pool shell and the plant room beneath it;
• the stairwell and lift shaft: a concrete core makes the house stiffer in a tremor;
• a house of three storeys or more, a mini-hotel or a low-rise apartment building, where the structural engineer specifies concrete shear walls.

The opposite case is a summer house, a guest cabin in the garden or a plot that a concrete mixer truck can't reach. There solid concrete turns out expensive, and we suggest a light frame or masonry. If you're unsure whether to use concrete or steel for a long span, the comparison of concrete and steel frames goes through it point by point.

How a cast-in-place concrete house works

At the bottom is a foundation slab or strip footing with reinforcement starter bars sticking out of it, and the wall reinforcement is tied to them. Each wall is reinforced with two layers of mesh, with extra bars around openings and at the corners, and the floor slab is poured over the walls of the storey, so the joint is continuous. The wall thickness, bar spacing and concrete class are set by the structural engineer from the seismic calculation and the ground survey, and we don't change them on site.

Reinforced concrete carries loads well, but it is a poor insulator. An uninsulated concrete wall is cold in winter, and in the local damp, condensation collects on its inner face. So we cover the external walls of the living floors with insulation and a facade. Which facade to choose, ventilated or rendered, we decide by the budget and by how exposed the house is to wind off the sea.

Formwork and reinforcement: what we check before the pour

Almost all the mistakes in cast-in-place concrete are made before the concrete arrives. After the pour they can only be put right with a jackhammer, so before every pour the site foreman goes through a checklist:

• the formwork panels stand true to the gridlines and level, they are tied together and the gaps are sealed, otherwise the cement paste leaks out and leaves voids (honeycombing) in the wall;
• the reinforcement sits on spacers, with a gap between the bar and the panel for the concrete cover specified in the design;
• sleeves for pipes and cables and cast-in fixings for railings are in place in the wall, and the openings match the drawing;
• there is no debris, wire offcuts or rainwater inside;
• the props under the slab formwork stand on firm ground, not on soaked soil.

We photograph and film these points before the pour, and you see them in the weekly report. We usually hire system formwork: it is more accurate than home-made board formwork, the wall comes out flatter, and less plaster is needed afterwards.

Concrete house construction in Batumi in the rainy season

The heaviest rain here falls from September to January, and if construction started in summer, that is exactly when the slabs of the upper floors come due. A downpour on fresh concrete washes the cement out of the surface, and water that collected in the formwork before the pour weakens the bottom of the wall. So the pour schedule is tied to the forecast: we confirm the date with the concrete plant a day or two ahead and keep plastic sheeting and covers on site in advance.

We don't start a slab that can't be interrupted without a settled forecast: postponing a pour by a week costs less than deciding later what to do with concrete poured in a downpour. If rain does start halfway through, we bring the concrete up to the point where the structural engineer has allowed a construction joint and cover it. A slab must never be abandoned in the middle of a span.

In summer the problem is the reverse. In the heat a thin floor slab quickly loses water from its surface and gets covered in a network of fine cracks, so we pour slabs in the morning and then keep them damp for several days: we water them and cover them with plastic sheeting or hessian.

Gaining strength: when to strike the formwork

Concrete hardens over weeks. We usually strip the wall panels after one to three days, once the wall holds its shape, but the props under a floor slab stay in place until the concrete reaches the strength the structural engineer has specified. In summer this usually takes two to three weeks, longer in the cooler months. Concrete reaches its design strength in about 28 days, and until then we don't load the slab with pallets of blocks.

So as not to judge by eye, we take samples during the pour, and a laboratory tests them for compressive strength. We start the next storey when the slab below can bear the new formwork and the workers, and sometimes we leave some of the props in place under two floors at once. That sets the pace: one storey of a cast-in-place concrete house, including the time the concrete needs to cure, usually takes us 3–5 weeks.

Concrete by the sea: how to hold back the salt

Reinforced concrete near the shore ages because of salt, which over the years works its way through the concrete to the reinforcement. It can only be held back at the pour; after that it is a repair job. On sites by the sea we stick to a few rules:

• the concrete class, its water resistance and the thickness of the cover are set by the design with an allowance for the nearness of the sea, and we don't substitute a cheaper mix on site;
• we don't add water to the concrete on site, even if the crew asks: it makes the concrete easier to place, but it becomes porous;
• we compact every layer with a poker vibrator, especially in corners and around congested reinforcement, where voids tend to be left;
• for the first few days we keep the concrete damp, so there are fewer shrinkage cracks on the surface;
• on balconies, parapets and canopies we form a fall and a drip, so that salty water doesn't run down the edge of the slab.

Basement walls on a low-lying plot near the shore stand in groundwater, and it can be brackish. For these walls the structural engineer selects the concrete and the external waterproofing based on a water analysis, which we order together with the ground survey. What else changes when the sea is next door is covered on the page about a house right on the shore.

How much a cast-in-place concrete house costs in Batumi

The shell of a cast-in-place concrete house costs from 1,110 ₾ per m² of floor area. This price includes the foundation slab, concrete walls and floor slabs, stairs and roof, together with the concrete, reinforcement and formwork. Windows, building services and finishing are not part of the shell, and with them a turnkey house costs from 2,100 ₾ per m².

If only the columns, floor slabs and stairs are concrete, a frame like that, foundations included, costs from 650 ₾ per m². Walls and roof are not in that price: they are priced separately, and block walls usually cost less than concrete ones. That's why we recommend solid concrete where a wall holds back the soil or acts as a shear wall, and more often build the rest of the house as a frame with infill.

The price goes up with a basement whose walls are in the ground and need waterproofing, with cantilevers and tall storeys, and with pumping concrete up a slope or along a narrow street. It comes down with a simple shell and repeated storeys, where one set of formwork is used several times. We send a preliminary estimate from photos of the plot and a plan of the house within 24 hours on business days, and the final cost is set on the basis of the design, after a visit to the plot and once the estimate is agreed.