C660 introduced a more sophisticated—and practical—risk model. It shifted focus from absolute temperature to , not just stress. The feature’s core innovation is recognising that cracking happens when the developing tensile strain capacity of the young concrete is overtaken by the restrained thermal contraction during cooling.
For a 600mm thick raft with 35% ggbs and a 15°C differential limit, C660 might let you pour without any active cooling. The saving? Tens of thousands in pipes, pumping, and labour. The risk? Quantified, not guessed. CIRIA C660 transformed early-age thermal cracking from a "black art" into an engineering calculation. It recognises that young concrete is not a weak version of old concrete—it's a different material entirely , one that generates its own heat, changes stiffness by the hour, and needs to be managed dynamically. early-age thermal crack control in concrete ciria c660
The practical takeaway? You can pour the same mix in two locations on the same site—one against existing rock (high restraint) and one on a slip membrane (low restraint)—and one cracks, the other doesn't. That’s not bad luck. That’s predictable physics. The most interesting feature of C660 is what it doesn't force you to do. It doesn't mandate cooling pipes, special cements, or post-cooling. Instead, it provides a validated path to waive thermal controls when the analysis shows they aren't needed. For a 600mm thick raft with 35% ggbs