Formwork And Propping Cycles

Formwork is temporary, which is why it gets treated as a hire item rather than a structure. It is the structure that carries every kilogram of a floor before the floor can carry itself.

Most concrete failures during construction are formwork and propping failures, not permanent works failures.

What the formwork carries

More than the finished slab weighs.

The wet concrete, at full density with no strength.

The reinforcement, placed before the pour.

The formwork itself.

Construction live load, meaning people, barrows, pumps, vibrators and stacked materials.

Impact and surge from placing, which is a dynamic load, higher than the static weight, and concentrated where the pump is discharging.

Wind, on an exposed deck or a wall form.

Those combine into a load case that is frequently more severe than anything the completed structure sees, and it is carried by a system assembled by hand on site.

When the props come out

This is the decision that matters, and the governing quantity is concrete strength, not time.

A slab can be stripped when it has enough strength to carry its own weight plus whatever construction load it will then take, at an acceptable deflection. That strength is confirmed by test, not by the calendar, using field-cured cylinders stored alongside the slab rather than laboratory-cured ones.

Striking early has two consequences beyond the obvious.

Deflection. A young slab has a lower modulus and creeps far more. A slab loaded at seven days deflects more permanently than the same slab loaded at twenty-eight, which feeds straight into [shrinkage and creep](/blog/shrinkage-and-creep-in-concrete-structures) and the long-term numbers in [deflection limits](/blog/deflection-limits-and-serviceability).

Cracking, because the slab takes load before it has the tensile capacity to resist the resulting moments.

Backpropping

When the next floor is poured, the slab below carries that load through props. If that slab is still young, the load has to be shared down through more than one level.

Backpropping is the designed system that does this: props left in or reinstalled below the slab being loaded, sized and positioned so the load distributes across two or three floors rather than resting on one that cannot take it.

It is a designed system with a drawing, a load path and a sequence, and it is covered properly under [backpropping and temporary works](/blog/backpropping-and-temporary-works). Reinstalling props by eye after striking is not backpropping, because a prop reinstalled without preload carries nothing until the slab has already deflected onto it.

The failure modes

Props not founded on anything. A prop bearing on a slab that is itself propped, bearing on fill, or bearing on a sheet of ply over soft ground.

Props out of plumb, which reduces capacity sharply. A few degrees of lean is a large reduction, not a small one.

Missing lateral bracing, so the prop system is stable vertically and has no resistance to a horizontal nudge. Several recorded collapses began as a sideways movement rather than a vertical overload.

Props removed early by a following trade to get access, with nobody structurally aware that it happened.

Overloading the deck with stacked material, which puts a point load where the formwork was designed for a distributed one.

Striking to the calendar when the weather was cold and the concrete had not gained the strength the schedule assumed.

Props landing out of line between floors, so the load does not travel down a continuous path and instead punches a slab mid-span.

Where it affects the permanent works

A formwork decision leaves a permanent trace.

Surface finish, because the form face, release agent and joints are what the concrete reads off, and a soffit that will be exposed is a formwork specification as much as a concrete one.

Tolerance and level, which is where [tolerances in concrete construction](/blog/tolerances-in-concrete-construction) is decided in practice.

Construction joints, where a pour stops and the next begins.

Long-term deflection, set largely by the age at first loading.

Who is responsible

On most contracts the formwork and propping design sits with the contractor, because it depends on their system, their plant and their sequence.

That does not make it unengineered. It needs a design, a drawing and a check against the permanent works, because the permanent structure is being used as part of the temporary load path. The structural engineer for the building needs to know the propping sequence, since the slab is being loaded in a way the permanent design has to accommodate.

Where that conversation does not happen, the question of [who is responsible when a design changes on site](/blog/who-is-responsible-when-a-design-changes-on-site) arrives after something has already moved.

ACSES provides structural engineering and temporary works review across Sydney. Talk to us about a project.