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Heat of Hydration

Heat generated during the chemical reaction between cement and water as concrete hardens

Heat of hydration is the heat generated during the exothermic chemical reaction between cement and water as concrete hardens. Temperature rise can reach 50-100°F in mass concrete, causing thermal expansion, subsequent cooling shrinkage, and potential cracking. Managing heat of hydration is critical in large pours and hot weather.

Why It Matters

Thermal cracking from heat of hydration damages mass concrete: foundations, thick walls, bridge piers. The concrete heats internally, expanding. Surface cools faster, restraining interior. Tensile stress exceeds concrete's early tensile strength, causing cracks. These cracks may be structural concerns and provide paths for water and chemical intrusion.

For typical residential slabs and foundations (12 inches thick or less), heat of hydration rarely causes problems. For thick elements (24+ inches) or large mass pours, heat management becomes essential.

Technical Details

Factors affecting heat generation:

Cement content:

  • More cement = more heat
  • Each 100 lbs cement generates 500 BTU
  • Typical: 500-700 lbs cement per cubic yard

Cement type:

  • Type I: highest heat
  • Type II: moderate heat
  • Type IV: low heat (rarely available)
  • Type III: very high heat (for cold weather)

Supplementary materials:

  • Fly ash, slag reduce heat generation
  • 25-50% replacement significantly reduces heat
  • Dilutes cement and reacts more slowly

Placement temperature:

  • Higher initial temperature increases peak temperature
  • Ice substitution or cooling reduces initial temperature

Temperature rise:

Thin sections (less than 12 inches):

  • 10-20°F temperature rise
  • Cools quickly
  • Minimal thermal cracking risk

Moderate sections (12-24 inches):

  • 20-40°F rise
  • Monitor and plan control measures
  • May require attention in hot weather

Mass concrete (greater than 24 inches):

  • 50-100°F+ rise possible
  • Insulation can increase further
  • Requires careful heat management

Thermal cracking mechanism:

  1. Hydration generates heat
  2. Concrete expands (thermal expansion)
  3. Surface cools, interior remains hot
  4. Differential creates tensile stress
  5. Early concrete has low tensile strength
  6. Cracks form if stress exceeds strength

Prevention strategies:

Reduce heat generation:

  • Use Type II or IV cement
  • Replace 25-50% cement with fly ash or slag
  • Lower cement content if strength allows

Reduce placement temperature:

  • Place during cooler weather/times
  • Use ice or liquid nitrogen in mix
  • Cool aggregates
  • Shade stockpiles

Control temperature differential:

  • Insulate surfaces to slow cooling
  • Limit differential to 35°F between interior and surface
  • Extended insulation duration

Structural measures:

  • Construction joints allow independent movement
  • Reduce restraint where possible
  • Reinforcement controls cracks if they occur

For residential concrete, heat of hydration becomes relevant for thick foundations, large footings, or mat slabs. For elements over 24 inches thick, discuss heat management with ready-mix supplier—fly ash replacement and placement timing can prevent thermal cracking.

  • Hydration - Process generating heat
  • Curing - Heat affects curing rate
  • Cracking - Heat of hydration can cause cracks

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