Mass concrete refer to very thick structure (> 80mm). These structures often have large volume, which generally means that large volumes of concrete have to be installed in a short time. This requires extremely goof planning and efficient processes.
Mass concrete is used for:
- Foundations for large loads
- Foundations for buoyancy control
- Massive walls (e.g radiation protection)
These massive structures create the main following problems:
- High temperature differences between internal and external concrete layers during setting and hardening.
- Very concrete maximum concrete temperatures
- As a result of curing from the outside to inside, big differences in the humidity occur and therefore shrinkage is forced.
- Secondary consolidation (setting) of the concrete and therefore cracking over the top reinforcement layers and also settlement under the reinforcement bars.
All these problems can causes cracks and cement matrix defects:
So called “skin or surface cracks” can occur if the external/internal temperature difference is more than 150C or outer layers can contract due to their drying out. Skin cracks are generally only a few centimeters deep and can close again later.
Measures to be taken:
- Low cement content and cements with low heat development
- Largest possible maximum aggregate sized (e.g 0-50 mm instead of 0-32 mm)
- If necessary, cool aggregates to obtain a lower initial fresh concrete temperature
- Place the concrete in layers (layer thickness <80 cm)
- Retard the bottom layers to ensure that the whole section can be re-compacted after placing the top layer.
- Use curing with thermal insulation methods
- Ensure correct design and distribution of joints and concreting sections, to facilitate heat dissipation and accommodate temperature fluctuations