In-situ concrete

What is in-situ concrete and how does it differ from precast concrete?

In-situ concrete, also called cast-in-place concrete and known in Slovak as monolitický betón, is concrete placed into formwork on the building site and left to harden in its final position. The steel is tied first, then the mix is poured, compacted and protected while it gains strength. The element is never moved, which gives the designer freedom of shape and continuity, but its final quality depends on what happens on site.

The alternative is precast concrete, where elements are cast in a factory and delivered for assembly. Both rely on the principles described in reinforced concrete. They differ in where the casting happens, who controls it and how the parts are joined.

Aspect	In-situ concrete	Precast concrete
Where the element is cast	In formwork on the building site	In a factory, then transported
Formwork	Built on site for each element, often the largest cost item	Reusable factory moulds, spread over many units
Weather during casting and curing	Direct exposure, so timing and protection decide the result	Mostly controlled under cover
Shape flexibility	High, suits curves, cantilevers and stair runs	Best for repeated shapes and sizes
Joints	Few, because the structure is continuous	More, at every connection between elements

How does formwork drive the cost and schedule of in-situ work?

Debnenie, or formwork, is the temporary mould that holds fresh concrete in shape. It is often the largest single cost in the concrete package, and it can take longer to prepare than the pour takes to execute.

The cost depends on three things: how often the panels can be reused, how complicated the geometry is, and how many separate setups the element needs. A flat foundation slab with straight edges needs little formwork. A cantilevered balcony or a stair with a stepped profile needs far more labour for the same volume of concrete. Reusable system panels cost more to hire or buy but cut the labour per pour, while site-built timber forms are cheaper to start and slower to erect and strip.

Formwork also sets the schedule. Forms come off only when the concrete is strong enough, so the forms for one floor often cannot move to the next until the curing period is complete.

Which parts of a family house are usually cast in place?

In a typical Slovak family house, in-situ concrete appears in a handful of recurring places.

Element	Why it is often in-situ	What drives the formwork effort
Základová doska, the foundation slab	One continuous mass spreads the load and suits sloped or weak ground	Edge forms around the perimeter, with large flat areas that pour quickly
Stužujúci veniec, the ring beam	Ties masonry walls together at each floor level	Narrow forms along every wall line, plus the openings to leave out
Monolithic ceiling slab	Continuous, and can be thinner over longer spans than many prefabricated floors	Full-area shuttering and propping until the slab gains strength
Oporný múr, the retaining wall	Resists soil pressure and needs continuous mass and steel	Forms on the soil side, plus drainage and access behind the wall
Schodisko, the stair	Monolithic flights suit a fixed geometry and can be cast with landings	Each flight is a custom form with a stepped profile

Is in-situ concrete the right choice for a family house?

In-situ concrete is not inherently weaker than precast. Its quality depends on conditions the designer cannot fully control, because the element is cast outdoors and its strength develops under whatever weather follows the pour. Poor work shows up as honeycombing, cold joints and uneven surfaces, and these are usually site-control problems rather than material problems.

Choose in-situ when:

• the plan has curves, cantilevers, stairs or irregular foundation geometry that repeated precast units cannot follow easily;
• the element must be continuous, such as a foundation slab on difficult ground or a ring beam;
• the site can take a pump and a concrete truck, and the schedule allows for the weather;
• the total forming cost, including the crew, compares well with the alternatives.

Precast floor units are often the better answer on sites with easy crane access and a tight schedule. Many houses combine both, which is why the connection details matter as much as either system.

How does weather decide when a pour can go ahead?

Weather sets both the window for the pour and the protection needed after it. The chemistry of hardening is explained in concrete curing, so this section covers the planning decisions made before the truck is booked.

In cold weather the central risk is that water in the mix freezes before the concrete has gained enough strength to resist it. The protection method has to be chosen before the mix arrives, because a decision taken on the morning of the pour is usually too late. If the forecast cannot be met, moving the pour is the safer option.

In hot weather the surface dries quickly, the mix stiffens before it can be placed, and plastic shrinkage cracks can appear soon after finishing. Pours are usually scheduled for the cooler part of the day, and protective covers should be on site before the truck arrives.

What must be fixed in place before the concrete arrives?

An in-situ pour is effectively irreversible. Chasing a conduit into hardened concrete or coring a drain hole afterwards risks cutting the reinforcement, so everything that must pass through or sit inside the element is fixed at the formwork stage. Before the pour, check that the following are fixed, tied down and marked against the drawings:

• sleeves and ducts for pipes, drains and cables, positioned from the services drawings;
• electrical conduits, boxes and any heating loops, secured so vibration cannot float or displace them;
• anchors and fixings for railings, timber bearers and the first course of masonry;
• openings for windows, doors and stair voids, with the correct edges built into the forms;
• a walk-through with the plumbing and electrical trades, because a missed item is rarely recoverable once the pour is done.

How is the reinforcement inspected before the pour, and what goes in the construction diary?

Before any concrete is ordered, the reinforcement must be checked against the structural drawings. The inspection covers bar diameters and spacing, lap lengths, starter bars, the ties holding the cage together and the spacers that hold the cover. A cage that sags onto the formwork is a durability defect that cannot be seen once the concrete is placed.

The inspection should be recorded before the pour starts. In Slovak practice this goes into the stavebný denník, the construction diary, kept on site by the stavbyvedúci, or site manager. The entry should state what was checked, by whom, and what was corrected before the concrete was ordered. A dated photograph attached to the entry is cheap insurance, because later questions about steel placement are answered from the record rather than from memory.