Precast concrete

Precast concrete is reinforced concrete cast into elements in a factory or a casting yard, cured under controlled conditions, and then transported and fixed in place on the site. It is one of the most common ways to apply prefabrication to a structure, because the concrete is shaped and hardened before the element reaches the foundation. In Slovak practice the term prefabrikovaný betón covers everything from a single stair flight to a complete floor system.

What is precast concrete and how is it made?

Precast concrete is made in moulds or formwork beds. Steel reinforcement is placed, concrete is poured and compacted, and the element cures under controlled moisture and temperature until it reaches the strength the design requires. Because this happens under cover and in a repeatable process, dimensions, surface finish, and cover to the reinforcement are usually more consistent than what a site crew achieves in the open. The finished element is lifted by crane, moved on a truck, and set onto bearings or into connections that were detailed in advance.

The essential difference from in-situ work is timing. The concrete is placed and checked in the factory, and the site only performs lifting, bearing, jointing, and finishing. That is why the design has to be frozen earlier than it would be for a house built with poured walls. It is also not the same as a panel construction system, which is a complete building method on a fixed grid rather than a choice of element type.

Which precast elements are used in a family house?

In a family house, precast elements usually appear in a few specific places rather than as the whole structure. The most common are:

• Stairs: precast flights or prefabrikované schodisko sets that bear on a wall or beam. Some systems allow the bearing edge to be trimmed on site to match the real height, so the final geometry should be fixed in the drawings before casting.
• Lintels: precast preklad beams that span window and door openings in masonry walls. They are quick to place, but the bearing length at each end is a design value and must not be shortened on site.
• Floor planks: thin filigree slabs, known in Slovak as filigránová doska, act as permanent formwork and bottom reinforcement. A cast-in-place topping, the monolitická nadbetonávka, is then poured over them so the floor works as one slab.
• Hollow-core slabs: prestressed floor units with longitudinal voids, laid side by side over longer spans and joined with grouted joints. They suit larger houses and garages where a long, flat span is needed.
• Basement wall elements: precast wall units for below-ground walls. They speed up the shell, but every joint needs the same external waterproofing a poured wall would receive.
• Garden walls and copings: precast wall elements, posts, and caps for garden walls and boundary walls. They still need a proper foundation and drainage behind them.

How does precast concrete compare with in-situ concrete?

Precast and in-situ (cast-in-place) concrete use the same material, but they differ in where the pour happens and how many joints the structure has to carry. The comparison below sets out the practical differences for a residential project.

Aspect	Precast concrete	In-situ concrete
Where the concrete is cast	Factory or casting yard, under controlled conditions	On site, in formwork built around the element
Continuity of the structure	Separate elements that rely on designed joints	Monolithic when the pour is planned well, with fewer interface details
Lifting equipment	High, every element has to be lifted into position	Lower, concrete is pumped or hoisted by bucket
Work on site	Mostly lifting, bearing, jointing, and sealing	Formwork, reinforcement, pouring, and curing
Late design changes	Difficult, changes may need new moulds or re-cut elements	Easier, the formwork can be adjusted until the pour
Typical use in a family house	Stairs, lintels, floor planks, individual walls	Foundations, irregular walls and slabs, complex shapes

What are the main trade-offs of precast concrete on a residential site?

Precast is not automatically the better choice for a single house. Its advantages come with constraints that are easy to underestimate.

• Crane access: the site needs a crane position, clearance, and a truck route. This can rule out precast on narrow or built-up urban plots, and lifting depends on the weather.
• Joints: every connection is a place where water, air, and sound can pass. Joint design is usually the most important detail in the whole structure.
• Factory quality: surfaces and dimensions are usually more consistent, but only when the supplier works to tolerances and inspections written into the contract.
• Lead time: elements are made to order, so the design must be final before manufacturing starts.
• Storage: finished elements need a flat, protected area on or near the site.

How do joints between precast elements affect waterproofing and airtightness?

Joints determine how continuous the building envelope is. A precast structure is a set of pieces, so the air barrier and the waterproofing are only as good as the connections between them, and air leakage through poorly sealed joints is a common reason a house misses its airtightness target.

Each joint therefore needs a designed sealing strategy: a continuous airtight layer that crosses the connection, compressible seals or sealant at the interface, and a plan for closing the joint after installation. Below ground, joints between basement wall elements need the same attention to external waterproofing that a poured wall would receive, and the drainage behind the wall must not depend on a joint staying perfect. The thermal design should also check the connections, because concrete can carry heat through the insulation layer at a junction.

Is precast concrete always cheaper or faster than in-situ concrete?

No. Precast can shorten the time spent on site, but the overall programme depends on design lead time, factory scheduling, and crane availability. Cost depends on element size, transport distance, how many elements repeat, and the lifting cost on the specific site. A quote for the actual project is the only reliable comparison.

A second misconception is that precast always means a plain, industrial appearance. Exposed surfaces and facade elements can be cast with texture or colour, and the structural logic of precast does not dictate a particular architecture.

What should a homeowner check before choosing precast concrete?

Settle these points before the design is frozen, because most of them are expensive to change later.

Question	Why it matters
Can a crane reach every element position?	Crane position, clearance, and truck route can rule out the method on a tight plot.
Who designs and signs off the connections?	Connections must be drawn on the structural plans, not left to the installer.
How are the joints sealed?	Airtightness, waterproofing, and sound all depend on the joint detail.
What tolerances and inspections does the supplier commit to?	Factory quality is only an advantage when it is written into the contract.
How is a late layout change priced?	Changes after production starts can need new moulds or re-cut elements.