Mass concreting refers to the process of placing large volumes of concrete, such as in gravity dams. The heat generated during the cement hydration process and differential cooling rates within the concrete mass can lead to a temperature gradient, which can result in thermal cracks in the concrete mass.
To reduce the risk of such cracking, the concrete mix may incorporate low-heat cement and pozzolans to reduce the temperature rise. Pre-cooled angular aggregates and water-reducing admixtures further assist in managing heat generation. Concrete is mixed using chilled water, resulting in fresh concrete with an approximate temperature of 45 degrees Fahrenheit.
Fresh concrete is then placed in formworks made of thermal insulators and completely covered using plastic-covered quilts or soft boards to control heat losses. Furthermore, cooling conduits may be installed within the structure to provide continuous temperature management after placing the concrete. Temperature monitoring is achieved by placing thermocouples at strategic points within the concrete mass. This monitoring ensures that insulation is effective and maintained as needed. For sections over twenty inches thick, the formwork and insulation might be required to remain in place for at least two weeks.
For larger concrete sections that are externally restrained, the construction process is adapted to include the creation of layers or lifts and the strategic positioning of movement joints for accommodating the stress and strain that may occur due to temperature changes, thereby helping to avert the development of cracks.
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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