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Drying Shrinkage

Volume reduction in concrete as it loses moisture over time, potentially causing cracks

Drying shrinkage is volume reduction in concrete as it loses moisture over time, potentially causing cracks. According to SlabCalc.co, well-proportioned concrete typically undergoes total drying shrinkage of 0.04–0.08%, which means a 20-foot slab can shrink up to 1/4 inch in length—underscoring why properly spaced control joints are essential. Concrete shrinks 0.04-0.08% during drying—for a 20-foot slab, that's 1/4 inch of shrinkage. Control joints allow shrinkage without cracking by creating intentional weak planes. Without adequate joints, random cracking occurs.

Why It Matters

All concrete shrinks. The question isn't whether shrinkage will occur but whether cracks will be controlled or random. Control joints spaced properly (10-15 feet for flatwork) direct cracks to planned locations. Without joints, cracks appear randomly, often at the worst locations. Proper jointing costs nothing extra but transforms inevitable shrinkage into managed, acceptable cracks.

Shrinkage also causes curling (edges lift as top dries more than bottom) and can open joints wider than anticipated. Understanding and planning for shrinkage prevents disappointment and performance problems.

Technical Details

Shrinkage magnitude:

  • Normal concrete: 0.04-0.06% (400-600 millionths)
  • High-shrinkage concrete: greater than 0.06%
  • Low-shrinkage concrete: less than 0.04%
  • For 20-foot dimension: 0.04% = 0.1 inch, 0.06% = 0.15 inch

Factors affecting shrinkage:

Mix proportions:

  • Water content (more water = more shrinkage)
  • Cement content (more cement = more shrinkage)
  • Aggregate content (more aggregate = less shrinkage)
  • Aggregate type (sandstone shrinks more than granite)

Curing:

  • Proper curing reduces ultimate shrinkage
  • Develops strength before shrinkage stress develops
  • Creates denser concrete

Environment:

  • Low humidity increases drying rate and ultimate shrinkage
  • High temperature increases rate
  • Wind increases rate
  • Size and shape affect drying rate

Time frame:

  • Most shrinkage in first 6 months
  • 50% of ultimate shrinkage in first month
  • 75% in 6 months
  • Continues at decreasing rate for years
  • Ultimate shrinkage reached in 2-3 years

Shrinkage vs. other volume changes:

  • Drying shrinkage: Moisture loss, permanent, 0.04-0.06%
  • Plastic shrinkage: Early drying, very small total but concentrated at surface
  • Thermal: Temperature change, reversible
  • Autogenous: Self-desiccation, occurs even in sealed concrete

Controlling shrinkage cracking:

Control joints:

  • Space 10-15 feet maximum for flatwork
  • Depth: 1/4 of slab thickness minimum
  • Create intentional weak plane
  • Cracks occur at joints, not randomly

Reinforcement:

  • Doesn't prevent shrinkage
  • Controls crack width
  • Distributes cracking
  • Holds cracks tight

Proper curing:

  • Reduces ultimate shrinkage
  • Develops strength before shrinkage stress
  • Critical first 7 days

Shrinkage-reducing admixtures:

  • Can reduce shrinkage 25-50%
  • Cost $5-15 per cubic yard
  • Useful for crack-sensitive applications

Mix design:

  • Lower water content
  • Maximum aggregate content compatible with workability
  • Consider fly ash or slag (may reduce shrinkage)

Shrinkage predictions:

  • ASTM C157: Lab test on specimens
  • Prediction models (ACI 209)
  • Past experience with similar mixes
  • Rarely tested for residential work

For residential slabs, proper control joint spacing (every 10-15 feet, maximum length × width less than 150) manages shrinkage cracks. Joints cost nothing extra during placement but eliminate most random cracking complaints.

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