Concrete Shrinkage & Crack Width Estimator
Estimate how much a concrete member wants to shrink over its length, and how much of that movement restraint holds back to drive cracking. It is a quick sanity check for spacing movement joints and sizing crack-control reinforcement — not a full crack-width calculation.
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Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the member length in metres (the run of slab, wall or beam between free ends or joints).
- Optionally set the design shrinkage strain in microstrain (default 800) to suit your mix and environment.
- Optionally set the restraint factor from 0 (free to move) to 1 (fully restrained); 0.5 is a common default.
How it works
Concrete shrinks as it dries. The free shrinkage movement equals the shrinkage strain (in microstrain, i.e. millionths) times the length: strain × 1e-6 × length in mm. When ends or a subgrade restrain the member, only part of that movement is accommodated — the restrained portion (restraint factor × free movement) is what builds tensile stress and can open cracks. Reinforcement and joints then control where and how wide cracks form.
Worked example
Worked example. For a 6 m member with 800 microstrain and a restraint factor of 0.5: length = 6,000 mm; free movement = 800 × 1e-6 × 6,000 = 4.8 mm; restrained crack-driving movement = 0.5 × 4.8 = 2.4 mm. That 2.4 mm must be absorbed by joints and distributed crack-control steel.
Common mistakes
- Reading the restrained movement as an actual crack width — real crack width depends on reinforcement, cover, bar spacing and concrete tensile strength.
- Using a shrinkage strain that ignores member size, humidity and mix — thin members in dry climates shrink far more than the default.
- Assuming full or zero restraint; most real members sit somewhere in between, so the restraint factor matters.
Frequently asked questions
What restraint factor should I use?
It depends on the boundary conditions. A member free to slide is near 0; one cast tight between stiff, fixed ends approaches 1. A value around 0.5 is a common planning assumption, but judge each case and be conservative where cracking is critical.
Does this replace the AS 3600 crack-control checks?
No. AS 3600 sets minimum crack-control reinforcement and bar-spacing rules to limit crack widths for the exposure class. This tool only estimates movement to help you plan joints and steel — the code checks and a qualified engineer still govern.
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