Specification - Concrete Mix Design, Quality Control and Specification - page 111

6.1 TEMPERATURE RISE
The specifier may want to restrict temperature rise or differential to
limit thermal restraint cracking, strength loss, or development of delayed
ettringite formation (DEF) in precast or mass concrete applications. Thermal
restraint cracking is a common problem. There are two types of thermal
restraint: (1) internal restraint due to the temperature differential between
the interior and the exterior of the concrete; and (2) external restraint due
to thermal shrinkage being restrained by a previously cast element. Most of
the significant thermal cracking observed by the authors over the years has
been external thermal cracking. However, most specifications focus on the
internal thermal restraint and include a limit on temperature differential
of 20°C. There are a number of problems with this approach. The value
of 20°C relates to the estimated internal restraint for a smooth gravel as
shown in Table 6.1 from BS 8110.2-1985, whereas most coarse aggregate
used would be crushed and a limit of 27.7°C would be more realistic.
Mass concrete elements are often heavily insulated to achieve this
temperature differential limit, which tends to increase the peak temperature
and the temperature near the surface increasing the probability and extent of
external restraint cracking. As the insulation tends to obstruct the work, the
contractor wants to remove it as soon as possible. Therefore as soon as the
monitoring of thermocouples is discontinued, the insulation will be removed
causing a large thermal differential at the concrete surface and possibly
resulting in thermal shock cracking. The authors have found that, except for
concrete placed in freezing conditions, ponding with around 50 and 75 mm
of water is the great method to facilitate heat loss from a massive element,
particularly if it contains large replacements of fly ash or ground-granulated
blast-furnace slag (GGBS), as well as preventing thermal shock of the
concrete surface. It also ensures excellent curing and helps limit autogenous
shrinkage. Some limit on the maximum allowable temperature differential is
prudent but needs to consider the type of aggregate used as well as its possible
influence on practical construction procedures and peak temperature. One
author was involved in the casting of a 15000 m³ self-consolidating con-
crete raft that took 3 days to cast. With the initial concrete having achieved
its peak temperature while fresh concrete was still being placed, clearly
the temperature differential within the raft was high but the gradient was
low. This is an important point as the temperature differential required to
induce cracking is due to the gradient not the absolute differential within a
large element. Limiting pour size to achieve some arbitrary differential will
generally increase cracking due to additional external restraint.
Specifications sometimes require a peak temperature limit to control
external thermal restraint cracking, often around 70°C. However, the
peak temperature required to limit cracking will depend on the restraint
conditions and the average ambient temperature. In heavily restrained