Mass Concrete

Mass Concrete

Mass concrete refers to very thick structures (> 80 cm). These structures often have a large volume, which generally means that large volumes of concrete have to be installed in a short time. This requires extremely good planning and efficient processes.

Mass concrete is used for:

·     Foundations for large loads

·     Foundations for buoyancy control

·     Solid walls (e.g. radiation protection)

·     Infill concrete

These massive structures create the following main problems:

·     High internal and external temperature variations during setting and hardening

·     Very high maximum temperatures

·     High internal and external temperature variations and therefore forced shrinkage

·   Secondary consolidation (settling) of the concrete and therefore cracking over the top reinforcement layers and also settlement under the reinforcement bars

Risks

All of these problems can cause cracks and cement matrix defects:

So-called “skin or surface cracks” can occur if the external/internal temperature difference is more than 15°C or the outer layers can contract due to their drying out first. Skin cracks are generally only a few centimeters deep and can close again later.

Measures to be taken

·     Use cements with low heat development

·     Low water content (reduction in w/c ratio)

·     Largest possible maximum particle size (e.g. 0–50 instead of 0–32)

·     If necessary, cool the aggregates to obtain a low 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 of the top layer

·     Use curing with thermal insulation methods

·     Ensure the correct design and distribution of joints and concreting sections, to allow heat dissipation and to accommodate the temperature developments and differences