42907 Chap.11 Restrained shrinkage and early age cracking
Restrained shrinkage and early age cracking
Weeks 10 to 12 are the subject of the second design report. Restraint converts contraction into tension in a material with roughly a tenth of its compressive strength in tension, and cracking is where the developing stress crosses the developing strength. The bridge deck slab is cast in place on hardened precast girders, which is about as complete an external restraint as a slab can have.
The report asks for two numbers, the time to concrete cracking and the minimum required curing duration, and this chapter is honest that the calculator behind them is not in the published materials.
What this chapter covers
- 01
The four step mechanism: the element dries, shrinkage is restrained, internal stress is induced, the concrete cracks
- 02
Why this is an early age problem: stress arrives before tensile strength has developed
- 03
Where the restraint comes from on a deck slab cast in place on hardened girders
- 04
The race between developing stress and developing strength, and how curing moves both
- 05
The two quantities the second design report asks for, and that they are one calculation asked twice
- 06
What the published materials do and do not contain about the calculator
- 07
Preparing the environmental inputs, the restraint condition, the section and the mix
- 08
The thermal crack control trigger: least dimension over 500 mm and a cast volume over 5 cubic metres, or a restrained edge
- 09
The 20 degree limit on temperature differential across the section during curing
- 10
The 75 degree ceiling on concrete temperature, shared with the delayed ettringite control
- 11
Minimum shrinkage and temperature reinforcement, and why a 200 mm slab takes it at each face
- 12
Why an early age crack undoes the cover design of every previous chapter
Reading two clauses against one steam curing proposal
- +1The first clause is the ceiling on concrete temperature during curing, which is 75 degrees. The proposal exceeds it directly.
- +1The failure mode behind that ceiling is delayed ettringite formation, an internal sulfate attack that cracks the element months after the heat cycle ends and worsens in humid storage.
- +1The second clause is the limit on the temperature differential across the cross section during curing, which is 20 degrees. A steam cycle that heats the surface fast can breach it even if the peak temperature were acceptable.
- +1The failure mode behind that limit is thermal cracking: a hot core inside a cooler surface restrains the surface exactly as hardened girders restrain a drying slab. The two clauses are not alternatives, and meeting one does not satisfy the other.
Key terms
- Restraint
- Anything that prevents a member from contracting freely as it shrinks, whether an adjacent hardened element, a foundation, or the member's own cooler outer layer. It is what converts contraction into tension.
- Early age cracking
- Cracking that occurs in the first days or weeks, while restrained shrinkage stress is rising faster than the tensile strength that has to resist it. It is permanent, and it happens before the design life clock has started.
- Time to cracking
- The age at which the induced tensile stress first exceeds the developed tensile strength, and the first of the two quantities the second design report asks for.
- Curing duration
- The period for which moisture is retained in the member, which delays the onset of drying stress and accelerates strength development at the same time. It is the design variable rather than a site convenience.
- Thermal cracking
- Cracking driven by a temperature gradient rather than a moisture gradient, where a hot core restrains a cooler surface. The Standard controls it through a trigger, a differential limit and a temperature ceiling.
- Temperature differential
- The difference in temperature across a member's cross section during curing, limited to 20 degrees by the bridge code. Exceeding it produces the same restraint problem as differential drying.
Restrained shrinkage and early age cracking FAQ
Why is this called early age cracking rather than simply shrinkage cracking?
Because of the timing of the race. Shrinkage begins as soon as the concrete meets an unsaturated environment, which can be within hours, while tensile strength takes days to develop. The dangerous window is the one where the stress has arrived and the strength has not, and it closes as the concrete matures. A member that survives its first weeks is very much less likely to crack from restrained shrinkage later.
Why is the deck slab the member chosen for this task?
Because it is cast in place on top of precast girders that are already hardened and are not shrinking any further, so it is bonded along its whole length to an unyielding substrate. That is about as complete an external restraint as a slab can have. The columns and the footings are far less restrained, which is why the second design report is set on the deck slab rather than on them.
Why does the guide not give me a time to cracking formula?
Because the published subject materials available here contain the task statement, the topic titles for the three teaching weeks and the Standard's own controls, but not the calculator, its input list or a worked restrained shrinkage calculation. Printing a formula anyway would be inventing the examinable method.
Take the model from the week 10 and 11 material and the tool from the subject site, and use this chapter for the mechanism, the Standard requirements and the structure of the argument.
Does the thermal crack control clause apply to a 200 mm slab?
It can, and the reason is the second limb of the trigger rather than the first. The size limb requires a least dimension over 500 mm together with a cast volume over 5 cubic metres, which a 200 mm slab does not meet. The restraint limb applies where an edge of the member is held by concrete that hardened earlier, or by any other outside restraint, which a deck slab cast on hardened girders plainly is.
Read the clause as an or rather than an and.
How does this chapter connect back to the ingress models?
Directly, and it is the reason the subject ends here. A chloride design assumes a coefficient measured on sound concrete and a carbonation design assumes a front advancing through an intact pore network. A member that cracks at three days has neither, and the cover calculated so carefully now has a path straight through it that no coefficient in this guide describes.
The carbonation lecture states it in one line: cracking at early age has to be kept out if the compactness of the concrete is to mean anything.
Assessment move
Learn the mechanism as four steps and be able to recite them in order, because the whole chapter follows from the conversion of contraction into tension. Understand why curing moves both curves in the race rather than just one, since that is the argument for treating a curing duration as a design output.
Learn the four Standard numbers, the two limbs of the thermal trigger, the 20 degree differential, the 75 degree ceiling and the minimum reinforcement with its 150 millimetre thickness threshold, and be able to say which failure mode each one addresses. Then prepare the inputs the calculator will want before you ever open it: the environmental conditions, the restraint condition, the section thickness and the mix.
Do not memorise a formula this guide does not give you.
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