Concept

Backpropping — where it appears

Props put back under a newly struck slab, snug rather than loaded, so that the floors below share the weight of the next floor cast above. Unlike shores, which carry wet concrete from the start, they carry only what arrives after they are installed.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

Also named here as early-age strength — the same set of essays touches all of them, so they are one junction rather than several.

More props, heavier slabs — unless they go in after the weight. The heaviest load any slab carries during construction, as a multiple of its own weight, against the number of levels of props under the slab being cast. Every level a shore that carried the wet concrete and moves up with the formwork: 2.00, 2.25, 2.37, 2.44 w for one to four levels. One level of shores and the rest backprops, put in snug after the formwork is struck: 2.00, 1.50, 1.33, 1.25 w. The same number of props, differently timed: a shore enters the load path before the weight it carries and passes it down, and a backprop enters after and shares only what arrives later.

Loaded twice over before it is a month old

A concrete floor in a building going up a storey a week carries, in its first weeks, the wet weight of the floors being cast above it, handed down through the props. By Grundy and Kabaila's arithmetic that is twice its own weight on a single level of shores, and more with more levels — 2.25 on two, 2.37 on three — because every prop that carried wet concrete passes its load down the stack. The same props put in after the formwork is struck carry nothing until something new arrives, and the peak falls to 1.5, 1.33, 1.25. The heaviest day of a floor's life is decided by when its props went in, not how many there are.

materials · Maturity

Where in a slab's first month its heaviest load lands. The load on the sixth floor of a frame of 250 mm slabs on a 7.5 m grid cast a floor every 7 days at 20 °C, on one level of shores and one of backprops, as a multiple of its own weight, against its age. Rigid and snug, no creep: a peak of 1.51 w at 14 days; steel springs and creep: a peak of 1.71 w at 21 days; the same, 1.0 mm of slack: a peak of 1.78 w at 7 days. Snug and rigid, the slab carries its heaviest load twice, a week apart, as the struck slab and then as the one below it. With creep the second is the heavier, because in the intervening week the slab above has sunk onto the backprops; with slack, the first.

The backprop that takes load nobody gave it

The arithmetic of propped construction says a backprop put in snug carries nothing until the next floor is cast, and then only a share of it. Count the props as the springs they are and that barely changes, because a steel prop is about seven times stiffer than the slab per square metre. Two other things change it. The young slab just struck creeps onto its backprops within a day and hands a quarter of a slab's weight down to the one below, which then carries 1.71 times its weight rather than 1.5. And a millimetre of slack at installation — about half the slab's own deflection — gives the struck slab back everything a backprop was meant to save.

materials · Maturity

Named alongside it

The objects these essays reach for when they reach for this one.

Construction sequence Early-age strength Flat slab Load sharing Propping Creep Elastic modulus Maturity Self-weight

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