Concrete shrinkage is the gradual reduction in volume that occurs as concrete cures and excess water evaporates from the mix. This is a natural and unavoidable process, but if it is not properly accounted for during design and construction, it can lead to unsightly and sometimes structural cracks.
Types of Shrinkage
Concrete experiences several forms of shrinkage:
- Plastic shrinkage: Occurs within the first few hours after pouring, while the concrete is still wet. Rapid surface evaporation (caused by wind, sun, or low humidity) dries the top layer faster than bleed water can replace it, causing surface cracks.
- Drying shrinkage: The primary long-term form. As the concrete cures over weeks and months, water that is not chemically bound gradually evaporates, and the concrete volume decreases. Most drying shrinkage occurs in the first 6 to 12 months.
- Autogenous shrinkage: Caused by the chemical reaction (hydration) itself consuming water internally. More significant in high-strength concrete mixes with low water-cement ratios.
- Carbonation shrinkage: A very slow process caused by carbon dioxide from the air reacting with compounds in the hardened concrete.
How to Control Shrinkage
Several practical measures help manage shrinkage and prevent cracking:
- Proper curing: Keep the surface moist for at least 7 days by covering with plastic sheeting or applying a curing compound.
- Expansion joints (control joints): Cut or formed grooves at regular intervals that create planned weak points where cracks can occur in a controlled, straight line.
- Adequate reinforcement: Steel mesh or rebar does not prevent shrinkage but holds any cracks tightly closed.
- Mix design: Using the minimum amount of water needed for workability reduces the total shrinkage potential.
Application
For DIY builders pouring a concrete slab, cut control joints at intervals no greater than 24 to 36 times the slab thickness. For a 100 mm slab, that means joints roughly every 2.4 to 3.6 metres. Always cure the concrete properly — this single step prevents the majority of early-age cracking problems.
Related terms
- Concrete
- Crack
- Expansion joint
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