10.10
Mass Concreting
Mass concreting refers to the process of placing large volumes of concrete, such as in gravity dams. The heat generated during the cement hydration pr…
When large volumes of concrete are placed, if the temperature of the concrete on the surface and the interior region is different, thermal cracks may occur.
To mitigate thermal cracking, the concrete mix uses low-heat cement, pozzolans such as fly ash, cooled aggregates with angular particles, and water-reducing admixture.
The ingredients are then mixed using chilled water to produce fresh concrete at a temperature of about 45 degrees Fahrenheit.
Fresh concrete is then placed in formwork made of thermally insulating materials.
Covering the surfaces of the structure with plastic-coated quilts controls the heat loss.
Additionally, cooling pipes can be embedded into the structure to facilitate cooling post-concreting.
To ensure the structure is insulated, thermocouples are positioned at various locations to monitor the concrete's temperature.
The formwork and insulation are kept in place for up to two weeks for sections with thicknesses greater than twenty inches.
However, for externally restrained large concrete sections, construction should take place in lifts, and movement joints are to be provided at suitable locations to prevent cracking.
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Q1: Why does thermal cracking occur in mass concrete?
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.
Q2: What materials are used in mass concrete mixes to reduce heat generation?
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.
Q3: How is temperature controlled after concrete placement in mass concreting?
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.
Q4: How long should formwork and insulation remain in place for thick concrete sections?
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.
Q5: What construction techniques prevent cracking in large externally restrained concrete sections?
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.
Q6: What role do thermocouples play in mass concreting operations?
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.
Q7: How do pozzolans contribute to thermal crack mitigation in mass concrete?
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.