Precast concrete house in Batumi

Many people know precast concrete from Soviet panel blocks, but a private house made of factory-made parts works differently. It needs a design in which every panel and slab is drawn in advance, a plant that will make them, and a site they can be delivered to and lifted into place by crane. If any of this doesn't come together, cast-in-place concrete poured on the plot makes more sense.

What the house is assembled from

• wall panels: load-bearing internal and external ones, sometimes with insulation and an outer layer already built in;
• floor slabs: hollow-core, which are lighter and cheaper, or solid ones for heavy loads and openings;
• columns, beams and stair flights for a precast frame;
• foundation blocks and pads, although for a house in a seismic area we more often pour a cast-in-place slab or strip foundation.

In private houses precast concrete is more often used in part: a cast-in-place concrete or brick house with floors of hollow-core slabs. A fully precast house makes sense when there are several identical houses, for example a row of townhouses or rental cottages on one plot, and the panels repeat from house to house. If you need a house that arrives from the factory complete, with finishes and services, that is a modular house, with its own delivery logic.

Where precast concrete fits and where it doesn't

For a warehouse or workshop we discuss the precast frame together with how the building will operate: spans, height, an overhead crane and doors. There is more on this on the page about warehouse construction.

Where to get panels and slabs in Georgia

Precast concrete is manufactured in Georgia, but the choice is narrower than in markets where panel construction never stopped. Columns, beams and slabs for warehouses and industrial buildings are easier to order than wall panels for a private house, which a plant makes for a specific design. We don't see ready-made series of panel houses on the market that you could pick from a catalogue, so we order panels and slabs from the manufacturer to our own drawings.

Before ordering, we check three things with the plant: whether there are certificates and test results for its products, whether it can provide cast-in fixings and openings to the drawings, and what delivery to the plot will cost. The plant may be a long way from Batumi, and then transporting long panels shows noticeably in the estimate. We also find out the manufacturing lead time in advance: the whole build schedule depends on it.

Joints and seismic loads

A cast-in-place concrete house works as a single whole, while a precast one is assembled from separate parts, and its reliability in an earthquake lies in the joints. Panels are connected by welding the cast-in plates or with reinforcement starter bars, and then the joint is filled with concrete. Floor slabs must bear on the walls by the length set in the design and be tied to each other and to the ring beam. Otherwise, in a tremor, the floor behaves like a set of separate planks rather than a rigid diaphragm.

That's why we take on a precast house only with a design in which every joint is calculated to the Georgian seismic code. Over hollow-core slabs we usually add a cast-in-place topping with mesh and a ring beam around the perimeter. We photograph the welds and joint reinforcement before grouting, just like the reinforcement before any pour. How else the structural engineer prepares a house for tremors is covered in the article on earthquake-resistant construction.

Panel joints and water

Many people know leaking panel joints and cold corners from old panel blocks. In Batumi the autumn rain is driven into the wall by the wind, and a joint with a single layer of sealant ages within a few years and starts letting water through.

In a new house we try not to leave any joints on the facade at all: we cover the panels with continuous insulation and a facade, ventilated or rendered, and the joints end up inside the warm envelope. If the design leaves a joint exposed, it is made in several layers: an insulating backing rod, an elastic sealant and a protective outer layer. Such joints are inspected every spring.

By the sea, cast-in plates that end up near the outer surface after erection are protected with zinc, a coating and dense mortar packing. Exposed metal in a joint rusts and spalls the concrete around it.

Crane erection and site preparation

Assembly goes quickly if everything is ready for it. Before the first lorry arrives, the plot must have foundations with accurate levels, a standing area for the crane outriggers and an access road along which a long-load lorry can pass and turn. A levelling error of a couple of centimetres in the foundations later turns into a crooked joint on the facade, so we accept the foundations against a setting-out survey.

Erecting the panels and slabs for a 150–200 m² house usually takes 2–4 weeks. Strong wind stops the crane, and grouting the joints, like any concrete work, waits for a dry day.

All openings, holes for pipes and ventilation, and cast-in fixings for railings and canopies are made at the plant. A new opening in a load-bearing panel or a cut-out in a hollow-core slab is only allowed afterwards on the structural engineer's calculation, so we finalise the building services completely before the panels are ordered.

How much a precast concrete house costs in Batumi

The shell of a house made of factory-made precast concrete panels and slabs costs from 990 ₾ per m² of floor area. The price includes the foundations, delivery and crane erection of the panels, grouting of the joints to the seismic calculation, and the roof. Windows, building services and finishing are priced separately, and a turnkey house costs from 1,980 ₾ per m².

The difference shows in the floors alone: hollow-core slabs, installed, cost from 240 ₾ per m², and a 200 mm cast-in-place floor slab from 305 ₾ per m². A precast shell is cheaper than a cast-in-place one, which starts from 1,110 ₾ per m², but the difference melts away if the panels come from far away, or there are only a few of them and each one is different.

The price goes up with delivery from a distant plant, a heavy crane on a cramped site, and non-standard panels and openings. It comes down with repeated elements and a level plot by the road. We send a preliminary estimate from the design and photos of the plot 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.