Concrete frame house in Batumi

You can see a cast-in-place concrete frame, or monolithic frame as it is called locally, on any high-rise building site in Batumi. In a private house on your own plot the scheme is the same, but the questions are different: where the columns will go, what the floor slabs will be like, how to make joints that hold the house together in a tremor, and what to do about parking under the house and balconies above the sea.

How a cast-in-place concrete frame works

The frame is made up of rigid frames: a column, the beam above it and the slab, tied together by reinforcement into a single joint. When the house sways, it is these joints that take the load, so the stirrups are spaced more closely where a column meets a beam, and the bars extend into the joint by the length set in the calculation. If the house is tall or elongated, the structural engineer adds concrete shear walls, usually around the stairs.

The infill walls don't carry the floor slabs, so the blocks for them are chosen on the basis of budget and facade. The foundation for the frame is usually a slab or strip footings tied together with ground beams, and the type is chosen from the ground survey.

The column grid and the layout

The column grid is set at the sketch stage, and it determines what the house will be like inside. In a private house we usually keep the spacing at 4–6 metres: that keeps the beams and slabs to a moderate depth, and a column ends up in the corner of a room or inside a wall rather than in the middle of the living room.

Floor slabs are made in two ways. With beams, the beam projects below the ceiling, but the slab is thinner and uses less concrete. A flat slab without beams gives a level ceiling, makes it easy to run ventilation underneath and to take windows right up to the top, but it is thicker and needs more reinforcement around the columns. For a living room glazed towards the sea the second option is often chosen, while the bedrooms keep the beams.

A column next to a panoramic window is worth spotting on the plan in advance, together with the architect. On paper it can be moved in an evening; after the pour, not at all.

Frame joints: what we photograph before the concrete goes in

In an earthquake the frame is held together by its joints, and the joints are then covered by concrete, so after the pour they can no longer be checked. That's why, before every pour of columns and slabs, the site foreman photographs what nobody will see afterwards: the stirrup spacing at the top and bottom of the column, where the stirrups are closer together than in the middle; the length by which the column bars extend into the slab and lap with the starter bars; and the spacers that hold the reinforcement at the right distance from the formwork panel.

We pour a column in one go over the full storey height, in layers, with a vibrator. If the concrete is dropped from a height without a chute, the coarse aggregate sinks to the bottom, cement paste is left at the top, and the joint under the beam ends up weak. We place a construction joint where the structural engineer put it, usually at the underside of the beam or the top of the slab, and before continuing we clean the cement film (laitance) off it.

How blockwork is tied to the columns, and why a flexible joint is left under the beam, is explained on the page about aerated concrete walls. The rules are the same for pumice and clay blocks, and the design sets the tie spacing for the particular block.

Parking under the house, balconies and cantilevers

On a slope and by the sea, people often want the house raised: parking or an open terrace on columns below, and two living floors with walls above. For a frame, this is the most awkward option. The floors filled in with masonry come out stiff, the lower one on bare columns is flexible, and in a tremor almost all the racking is concentrated there. The structural engineer calculates a storey like this separately: the columns are thicker, there is more reinforcement, and sometimes concrete walls are added at the ends, where they don't get in the way of cars.

A balcony or cantilever is the floor slab continued outwards. Out in the open, the edge of such a slab is cold and wet, so where it meets the house we build a thermal break into the slab, if the calculation permits, or insulate the cantilever above and below. Otherwise in winter the ceiling of the room next to the balcony gets damp. For the same reason we cover the columns on the facade with insulation. A long cantilever towards the sea is calculated for deflection, and the longer it is, the thicker the slab at its root.

Build sequence and shell timeline

The frame rises storey by storey: columns, then the formwork for the beams and slab, the pour and curing under props. We start the infill masonry on the lower floor once the slab above it is in place, and carry on while the concrete crew works higher up. That way the shell of a 150–200 m² two-storey house usually goes up in 3–5 months.

We bring the blocks in batches, one storey at a time, and keep them on pallets under plastic sheeting: a block soaked before it is laid goes on releasing water through the plaster for a long time. We try to get the roof on before the autumn rains, otherwise water stands on the slabs and runs down the fresh masonry.

You don't need to come out for every storey. Before each pour you get photos of the column and beam joints, and if you like, your own technical supervision engineer can inspect the frame before the concrete goes in.

When a different scheme is needed

A concrete frame is heavier than a steel one and needs access for a mixer truck. On a steep slope without a road, when adding a storey to an old house, or for a column-free living room 10–12 metres wide, steel is often simpler; this is covered in the comparison of a concrete frame and a steel frame. If you want brick walls or a brick facade, see the house with a concrete frame and brick walls: brick is heavier than block and changes the frame calculation.

For a small guest cabin or a summer house a concrete frame is overkill. There a light frame or masonry with reinforced ring beams is cheaper and faster.

How much a concrete frame house costs in Batumi

The shell of a concrete frame house costs from 690 ₾ per m² of floor area. The price includes the foundations, concrete columns, beams and floor slabs, block infill walls and the roof. Windows, building services and finishing are not included. For comparison, the shell of a house with solid concrete walls starts from 1,110 ₾ per m²: where a wall doesn't have to hold back the soil, block between the columns costs less than concrete.

After that the house goes through the stages in the table on the house construction page: white frame, ready for finishing, from 1,050 ₾ per m², and a mid-range turnkey house from 1,680 ₾ per m². Of the individual items, masonry stands out most in the estimate: a 30 cm aerated concrete wall with materials costs from 145 ₾ per m² of wall.

The price goes up with flat slabs and longer spans, cantilevers and balconies, brick or clay blocks instead of aerated concrete, and a stepped foundation on a slope. It comes down with a simple column grid at even spacing, repeated storeys and aerated concrete in the walls. We send a preliminary estimate from photos of the plot and a plan of the house within 24 hours on business days. The final cost is set on the basis of the design, after a visit to the plot and once the estimate is agreed.