Precast Concrete: Production and Erection

No, and the confusion is historical rather than technical. The bad reputation comes from a period when prefabrication meant repeating the same unit as often as possible, because almost all the cost sat in the mould: few moulds, many identical buildings. That economic logic has changed. With numerically controlled cutting and adaptable moulds, dimensional variation costs far less than it did, and variety can also be achieved without changing the mould at all, by working the surface: matrices that imprint texture, exposed aggregate obtained with retarders, pigments through the mass, blasting and treatments that would be impossible or ruinously expensive in situ. There is in fact a reversal that is rarely told: genuinely successful fair-faced concrete is much easier to obtain in a factory than on site, because there the mould is steel, the pour is controlled and the unit is rejected if it is not good. In situ, a botched fair-faced pour simply stays there.

It depends almost entirely on the connections, which is why the question has no single answer. A precast building with simply seated units can have perfectly sound elements and still behave badly, because under horizontal action the pieces tend to move relative to one another and the structure has no continuity through which to redistribute forces. A building with nodes able to transmit tension and moment behaves comparably to one cast in situ. The historically critical point, which has produced serious damage in real events, is precisely the seating of beams and roof units left to friction or to connections conceived only for vertical loads: when horizontal action exceeds friction, the element slides, and if the seating length runs out it falls. Two practical implications follow: on existing buildings, the connections are the first thing to check in a vulnerability assessment; on new ones, node capacity must be explicitly required and never assumed just because the element is certified.

The problem arises because tolerances belong to different parties who do not talk to each other. The producer works to very high precision, in the order of millimetres, because the mould is steel and repeatable. The structure receiving the units, typically foundations or in-situ works, has precision an order of magnitude coarser, because it was made by other means and other logics. Erection adds a third. The joint is the only place where these three imprecisions meet, and it must absorb all of them together in the worst case, not in the average case. The recurring error is designing the joint from the producer's data sheet, which quotes the smallest of the three tolerances. The practical countermeasure is equally simple: survey the existing works before sending the units into production, not after, because it is the only moment when a discrepancy is corrected by changing a drawing rather than by demolishing something.

Precasting is in fact the setting where these materials work best, and the point is worth grasping because it is counter-intuitive. Concretes with recycled aggregate and those with alternative binders, such as geopolymers, all share the same practical problem: they are more sensitive to curing conditions and more variable, so on site they add uncertainty to a process that already has too much. In a factory that sensitivity stops being a defect, because curing is controlled, the mix is consistent batch by batch and every unit can be tested before it leaves. Some alternative binders in particular perform best precisely with the heat curing that is already standard practice in a plant. Two conditions remain unavoidable. The first is consistency of recycled aggregate supply, because a heterogeneous material produces inconsistent units. The second concerns fair faces: recycled aggregates bring colour variation that shows on a facade, so either that variability is accepted as a character, or the recycled content goes into the load-bearing wythe and not the outer one.