As water leaves the pores the cement paste contracts, and ordinary concrete shrinks around 0.04 to 0.08 percent, roughly 1/2 in to 1 in over 100 ft. Restraint from subgrade, foundations or reinforcement turns that movement into cracking, which joints are laid out to control. Water content is the biggest driver, so wet mixes, small aggregate and high paste volume shrink the most.
Drying shrinkage on a 200-foot slab runs over an inch, so of course it cracked where you didn't saw.
Also heard as
- shrinkage
Do not mix it up with
- Plastic shrinkage cracking: Plastic shrinkage strikes in the first hours before set; drying shrinkage plays out in hardened concrete over months.
Related terms
Control joint
A planned groove or sawcut that makes a slab crack in a straight line exactly where the crew wants it.
Curling
Upward warping of slab edges and corners, caused by the top drying and shrinking faster than the bottom.
Plastic shrinkage cracking
Cracks that open in fresh concrete before it sets, when the surface dries faster than bleed water replaces it.
w/cm
Water-to-cementitious-materials ratio: the weight of mix water divided by the weight of cement plus SCMs.
Wild crack
An uncontrolled crack that wanders across a slab panel instead of forming under a control joint.