ICOLD Bulletin: The Physical Properties of Hardened Conventional Concrete in Dams
Section 8 (Frost resistance)
As submitted for ICOLD review, march 2008 Section 8-11
8.3.2.4 Casting technique
The concrete pore system is related not only to the concrete mix design but also to the
casting technique and in particular to the type of vibration. Low vibration can induce
large pores and cavities that are detrimental, as above explained, for the concrete frost
resistance. But also excessive vibration of concrete during placing, with the
consequence of reducing air bubbles generated through the use of air-entrainment
admixtures, has a detrimental influence on the frost resistance.
Measurements on concretes from some Swiss dams [8.21] showed that the air content,
as measured in the laboratory, decreased considerable when measured much later
(after 20 years) from cores. For example, at their Moiry and Sambuco dams the initial
laboratory air content value of 3.6% decreased down to 1.1% and 1.5% respectively in
the concrete cores. An evaluation of frost resistance by the method of "critical degree of
saturation" (paragraph 8.4) of the 20 year old dams concrete showed then also
insufficient frost resistance because of a low percentage of "not fillable" pores (1.2 to
2.3%).
8.4 METHODS FOR EXPERIMENTAL DETERMINATION OF THE CONCRETE
FROST RESISTANCE
Different test methods have been proposed by National or International Standards for
the experimental determination of the frost resistance of concrete, both with and without
de-icing salts. Some of them are here presented, with reference to the case without de-
icing salts, more frequent for the concrete dams.
Most of the tests include placing the concrete specimen, after an initial curing period, in
a freezing chamber where nominal freezing-thawing cycles are applied. They consist in
alternatively lowering the temperature of the specimens from about +5°C to about –
20/25 °C, in a period of time sufficient to freeze to centre of the specimens, and then
rising it from – 20/25 °C to + 5 °C. The rate of temperature changes varies between 5
and 15 °C/h. The specimens are repeatedly subjected to these cycles until visual
observed or objective damages are assessed. For example the length change and the
weight loss of the specimen is measured and the static or dynamic modulus of elasticity
of the concrete is calculated. The frost resistance is quantified by the number of cycles
at which the concrete properties show an appreciable variation. A concrete expansion
of 0.1% or a reduction of the modulus of elasticity to about 50-60% of the value of the
start of the freezing-thawing test are usually considered to represent concrete failure as
this degree of disintegration is associated to loss of structural usefulness and concrete
integrity [8.22]. Examples of results of this test are reported in Fig. 8.5 and in Fig. 8.7
Usually this test is not intended to provide a quantitative measure of the length of
service that may be expected from a specific type of concrete; satisfactory freeze-thaw
resistance is considered as a indicator for durable concrete. Therefore freeze-thaw
testing is also meaningful for climates without sub-zero temperatures.