Mass Concreting

Thermal cracking in mass concrete results from temperature gradients between the concrete's surface and interior regions. During cement hydration, heat is generated internally, but the surface cools faster, creating differential cooling rates. This temperature difference induces tensile stress that exceeds the concrete's strength, causing types of non-structural cracks in concrete to develop.

Mass concrete mixes incorporate low-heat cement and pozzolans such as fly ash to minimize temperature rise during hydration. Pre-cooled angular aggregates and water-reducing admixtures further manage heat generation. Fresh concrete is mixed using chilled water to achieve an initial temperature of approximately 45 degrees Fahrenheit, reducing thermal stress development.

Temperature control after placement involves multiple strategies: formwork made of thermally insulating materials retains heat, plastic-coated quilts cover surfaces to prevent heat loss, and cooling pipes embedded in the structure facilitate active cooling. Thermocouples positioned throughout the concrete mass monitor temperature continuously, ensuring insulation effectiveness and allowing adjustments as needed.

For concrete sections thicker than twenty inches, formwork and insulation must remain in place for at least two weeks. This extended duration allows the concrete to cool gradually and uniformly, reducing thermal stress and minimizing the risk of cracking. Premature removal could expose the concrete to rapid temperature changes.

For large externally restrained concrete sections, construction proceeds in lifts or layers rather than as a single pour. Movement joints are strategically positioned throughout the structure to accommodate stress and strain caused by temperature changes. These joints allow the concrete to expand and contract safely, preventing the development of uncontrolled cracks.

Thermocouples are temperature sensors positioned at strategic locations throughout the concrete mass to continuously monitor internal and surface temperatures. This monitoring ensures that insulation systems are functioning effectively and that the concrete is cooling at acceptable rates. Temperature data guides decisions about formwork removal timing and additional cooling interventions.

Pozzolans like fly ash reduce the heat generated during cement hydration by partially replacing Portland cement in the concrete mix. This lower heat generation decreases the temperature rise within the concrete mass, reducing the thermal gradient between surface and interior regions. Lower temperature differentials result in reduced tensile stress and decreased cracking risk.