Specification - Concrete Mix Design, Quality Control and Specification - page 113

elements or cooler environments, a peak temperature of significantly lower
than 70°C may be necessary to limit cracking. Modeling is necessary to
determine the appropriate value, and guidelines can be found in documents
like CIRIA C660. In situations where there is significant restraint for
elements with a minimum dimension exceeding around 600 mm or for
water restraining structures, probably the best course of action for a
specifier is to require that such modeling be done.
Specifications for mass concrete or hot weather concrete usually require
a maximum concrete placement temperature, often 32°C but sometimes as
low as 20°C. ACI 305 specification recently revised this limit stating that
“the maximum allowable fresh concrete temperature shall be limited to
35°C (95°F), unless otherwise specified, or a higher allowable temperature
is accepted by architect/engineer, based upon past field experience or
preconstruction testing using a concrete mixture similar to one known to
have been successfully used at a higher concrete temperature”. Although
there are important effects of higher placement temperature on plastic prop-
erties of concrete, we would suggest that these are the responsibility of the
producer in consultation with the contractor. The primary performance goal
is limiting external restraint cracking due to the peak temperature. There are
different ways of achieving the primary goal. The specified peak temperature
for a 4 metre thick raft in Kuwait was 71°C, but the concrete producer did
not have access to flake ice to reduce the placement temperature below about
35°C. The solution proposed by one of the authors was to use a high percent-
age replacement of fly ash. A 55% fly ash replacement achieved the required
peak temperature in the raft as well as the strength and other properties.
The master specification for the MASDAR development in Abu Dhabi actu-
ally prohibited the use of flake ice in the concrete, as the embodied energy
involved was contrary to the sustainability goals of the project.
In the case of expected temperature-induced strength loss, temperature
will influence strength loss very differently depending on the chemistry
of the cementitious binder and therefore the temperature limit should
vary depending on the mixture proposed by the producer. Concrete with
Portland cement only binder exhibits progressive strength reduction as
temperatures increase above about 70°C, whereas concrete containing
significant quantities of GGBS or fly ash does not.
Delayed ettringite formation is a form of internal sulfate attack that has
been found in heat cured precast elements and in some in-situ mass concrete
(Thomas et al., 2008). It is generally accepted that DEF does not occur
in concrete when the peak temperature does not exceed 70°C. Guidelines
suggest a number of precautions to minimise the risk of destructive DEF in
the event of temperatures from 70°C to 85°C.
1. The cement should have a maximum fineness value of 400 m 2 /kg
2. Portland cement with 1-day mortar strength less than 20 MPa