Precast Concrete

Precast is concrete cast in a factory and delivered ready to install, rather than poured and cured on site. It suits repetition, long spans and constrained sites, because the work happens under cover while the site is still being prepared. Where a frame is one of a kind and geometrically awkward, in situ concrete often remains the better answer.

Not by the cubic metre. Precast mixes are usually richer, and the element carries the cost of the mould and the delivery with it. What precast saves is time and everything time costs: fewer trades on site, no formwork or curing in the critical path, and a programme that does not move with the weather. On long spans and repeated floors it also tends to be cheaper by the square metre.

Installation is measured in days where an in situ equivalent is measured in weeks, because the curing has already happened in the factory. The saving is real but it is not free: it is bought with design time up front, since the elements have to be finished and scheduled before the first one is cast.

Hollowcore is a prestressed floor unit with voids running through it, which takes weight out of the slab without taking away its capacity. The voids are what let it span far further than a solid slab of the same depth, so it suits apartment and commercial floors where the span is long and the layout repeats. It also uses less concrete and steel than the in situ floor it replaces.

The factory needs the design settled before it can schedule production, so the freeze point comes earlier than on an in situ frame. That is the single most important thing to know about building in precast: the programme saving on site is paid for with decisions made early, and the earlier the layout, the loads and the openings are fixed, the more of that saving survives.

Somebody has to, and it should be agreed in writing before anything is cast. A frame designed by one engineer and detailed by another is where connections go missing, because each assumes the other has it. Where the analysis, the detailing and the element schedules are done by the same office, there is no handover for a connection to fall into.

The architectural layouts, the loads, the stability strategy and the openings, plus whatever the mechanical and electrical design needs to pass through the frame. Getting the penetrations in before the elements are cast is the whole point: a hole drilled on site into a prestressed unit is a repair, not a detail.

The Eurocodes with their Irish National Annexes: IS EN 1990 for the basis of design and IS EN 1992 for the concrete. Alongside them, the product standards the factory makes the elements to: IS EN 13369, the common rules for precast concrete products, IS EN 1168 for hollowcore slabs, IS EN 14992 for wall elements and IS EN 14843 for stairs. Designing to the product standard as well as the Eurocode means the element on the drawing is the element the factory makes.

Frames are modelled in Allplan Precast and analysed in SCIA Engineer. The model is issued as IFC, so the architect, the services engineers and the contractor can bring it into their own software, and the drawings are issued as DWG and PDF.

A chartered engineer, in the same office that designed the frame. Every drawing is checked and signed before it goes out, so a manufacturer gets drawings with a named engineer behind them, rather than a detailing service working to somebody else's design.

Only at a cost, and the cost rises with every element already cast. A change to a layout that has reached the factory means re-scheduling, re-detailing and sometimes remaking units, which is why precast rewards decisions made early and punishes ones made late. Changes are handled far more cheaply while the model is still the only thing built.