Page 75 - Handbook of Thermal Analysis of Construction Materials

Section 9.0 - Models of Hydrated Cement 57

8.0 DIMENSIONAL CHANGES

Aging, within the context of surface chemical considerations,
refers to a decrease in surface area with time. For hydrated portland cement,
this definition can be extended to include changes in solid volume, apparent
volume, porosity, and some chemical changes (excluding hydration) which
occur over extended periods of time.
The volume of cement paste varies with its water content, shrinking
when dried and swelling when re-wetted. It has been found that the first
drying shrinkage (starting from 100% RH) for a paste is unique in that a
large portion of it is irreversible. By drying to intermediate relative
humidities (47% RH), it has been observed that the irreversible component
is strongly dependent on the porosity of the paste, being less at lower
porosities and w/c ratios. [23]
Concrete exhibits the phenomenon of creep, involving deforma-
tion at a constant stress that increases with time. Creep of concrete (basic
creep) may be measured in compression using the ASTM C512 method.
There are two types of creep: basic creep in which the specimen is under
constant humidity conditions and drying creep when the specimen is dried
during the period under load.
Creep of a cement paste increases at a gradually decreasing rate,
approaching a value several times larger than the elastic deformation. Creep
is, in part, irrecoverable, as is drying shrinkage. On unloading, deformation
decreases immediately due to elastic recovery. This instantaneous recovery
is followed by a more gradual decrease in deformation due to creep
recovery. The remaining residual deformation, under equilibrium condi-
tions, is called the irreversible creep. Creep increases with w/c ratio and is
very sensitive to relative humidity and water content. It may also be affected
by admixtures.
Many theories have been proposed over the years to account for
creep mechanisms in cement paste, and each is capable of accounting for
some of the observed facts. The descriptions and mechanisms are based on

seepage, change of solid structure, and interlayer space.

9.0 MODELS OF HYDRATED CEMENT

In order to predict the performance of concrete, it is important to
have a model of cement paste that incorporates its important properties and