As a general rule of thumb, the placement of structural concrete with a minimum dimension equal to or greater than 1 metre would be considered as mass concrete.

Historically, limiting the temperature differential between the interior and concrete surface so that it is less than 20°C has been found to prevent or minimize thermal cracking.

The limit on temperature difference aims at reducing excessive cracking due to volumetric change.

The purpose behind establishment of maximum temperature for mass control is twofold:

• The control of maximum temperature effectively means that it also controls the time taken to reach the maximum temperature and reduces the temperature differential.
• To avoid the effect of delayed ettringite formation.

If the internal temperature exceeds 70°C, the long term durability of some concrete could be impaired by delayed ettringite formation.

Delayed ettringite formation is a type of internal sulfate attack in concrete, which is common in many concrete elements that have been heat-treated beyond a certain temperature and have restrained the normal ettringite formation, or, in concrete which is made by a high-sulfate portland cement. 

Rather than normal formation of ettringite by cement hydration in the plastic state, ettringite forms at a delayed state after the hardening of the concrete. 

In the continued presence of moisture, components of ettringite slowly dissolves out and forms ettringite in the confined spaces in hardened paste, thereby causes expansion and subsequent stresses in the paste due to restrained expansion and this leads to the eventual cracking of concrete.