A mass pour is usually defined as one of sufficient size to demand special attention to be given to logistical and technical considerations such as concrete supply, casting sequence, cold joints, plastic settlement, heat of hydration and early age thermal cracking.
The major advantages of large-volume pours are the savings in cost and timescale resulting from the decreased number of joints. These are expensive to form, requiring stop ends when vertical and careful preparation of the concrete surface, regardless of direction, when new concrete is cast against old.
Additionally, heavily reinforced foundations required to carry significant design loads have tended to produce situations where the construction of joints or temporary stop-ends are virtually impossible to form.
The drawback of cracks that might transpire in situations where construction joints are not used appear to be relatively minor. The importance of such cracks should be considered in relation to the structural requirements of the section, together with considerations of durability.
For the protection of reinforcement, a maximum crack width should be considered. In addition, the elimination of joints removes potential cracks and zones of weakness, which are often highlighted in structures that are required to be water-retaining.
As in all construction projects the designer can have significant impact on the construction process and therefore it is essential that the designer works closely with the contractor. The designer must ensure that all aspects of the specification are compatible and will work with the construction process.
It is important to achieve a balance in the concrete mix between specified strength, durability, heat of hydration and the requirements for placing and compaction. In particular the strength and minimum cement content should not be over specified.
To control the occurrence of early age thermal cracking, it is usual to specify allowable limits on the peak core temperature and on temperature differentials. The differential is usually for a post construction period and typically relates to the difference between the core and the outside surfaces and between previous pours.
Hydration of cement is an exothermic reaction and in large-volume pours where heat dissipation is low, the temperature within the pour can rise significantly.
In the core of sections greater than 2 metres thick, the temperature rise will be nearly adiabatic. The cement content per cubic metre fundamentally governs the heat generated and values of peak temperatures of the order of 70–80°C are possible.
A concrete containing 330 kg/m3 of EverSure when placed at a temperature of 20°C is likely to have a peak adiabatic temperature rise of approximately 70°C which is a typical specification limit imposed. The replacement of 30% EverPlus Class C Fly Ash on a one for one basis will reduce this peak temperature by approximately 12°C and will have a significantly reduced temperature gradient.
The graph is based on data from temperature rise model C660 – prediction of the early age temperature rise of concrete.
Alternatively the option of using a preblended binder of EverSure and EverPlus Class C Fly Ash at specified replacement levels from our product solution range is available. This can provide logistical and operational advantages if silo infrastructure at a ready mix concrete plant is limited.
This is only one aspect in materials engineering for large-volume pours and close collaboration between designer, contractor and ready mix supplier are critical for a successful outcome.