Structural Engineering & Statics
where the load goes
Concept

Maturity — where it appears

A measure of concrete's development combining the time it has cured with the temperature it cured at. Two pours of identical mix reach different strengths on the same day if they were kept at different temperatures, so a strength is a property of a history rather than of an age.

Named by 3 essays across one field — each of them below, with the objects they name alongside it.

Seventy per cent of the strength and ninety of the stiffness. Strength and modulus against age, each as a fraction of its own twenty-eight-day value. They do not move together: E follows f to the power 0.3, so at seven days the concrete has 78 per cent of its strength and 93 per cent of its stiffness, and at three days 60 and 86. A young structure is much nearer its final deflection than its final capacity. The 20 N/mm² a striking calculation asks for arrives at 2.2 days at 20 °C, 4.6 at five degrees and 1.7 at thirty-five.

The strength it had on the day

Every concrete strength a design quotes is a twenty-eight-day cylinder value, and a structure is loaded long before that — formwork struck at three days, the next storey cast at seven, a prestressing force transferred at two. The number that existed at the moment the load arrived is a different one.

materials · Maturity The stress that leaks away. A restrained shrinkage strain of 320 microstrain in concrete of modulus 34000 N/mm². Ignoring creep it produces 10.88 N/mm², which is above the tensile strength of 3.8 and predicts that every restrained concrete member ever cast has cracked. Counting creep by the superposition integral leaves 1.72 N/mm² after 55 years, and the one-line age-adjusted shortcut at the usually quoted ageing coefficient of 0.8 leaves 3.05. The two disagree — this creep function implies an ageing coefficient of 1.67, not 0.8 — and both are below the tensile strength, so the conclusion turns on counting creep at all rather than on how it is counted.

The stress that leaks away

Creep makes a load's deflection grow and an imposed strain's stress shrink, and the second is why restrained concrete does not crack as often as an elastic calculation says. The same material property runs both ways, and which way it runs depends on whether the structure was given a force or a movement.

materials · Creep 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

Named alongside it

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

Construction sequenceCreepAgeingBackproppingCharacteristic strengthEarly-ageEarly-age strengthElastic modulusEquivalent ageFlat slabFree bodyHydration

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