Page 144 - Handbook of Thermal Analysis of Construction Materials

Section 9.0 - Durability Aspects 127

polypropylene fiber exhibits an endothermic effect at 160–170°C caused
by melting. This is followed by an exothermal effect around 230°C due to
oxidation and an endothermal peak at 300°C for the decomposition. The
polyacrylonitrile did not melt but showed an exothermic effect around
300°C caused by the nitrile group polymerization and other side reactions.
The non-melting of this fiber together with its rapid exothermic degradation
effect may increase the risk of dangerous spalling in mortar exposed to rapid
thermal exposure.

9.4 Freezing-Thawing Processes

Concrete is vulnerable to cracking when subjected to increasing
number of freezing-thawing cycles. The freezing processes occurring in the
various types of pores in the cement paste are complex and low temperature
thermal analysis methods involving DTA and DSC have been applied with
some success in explaining the mechanism of expansion and contraction.
The increased damage that occurs when concrete is exposed to the
freeze-thaw cycles and salts has been investigated by many workers. [79]
Experiments involving measurements of length changes and heat changes
that occur when the cement paste is exposed to temperatures between 15 and
-70°C have yielded information on the mechanism of salt scaling. [79][80]
Figures 39 and 40 present results of length changes and DTA of cement
pastes saturated with NaCl (brine) of different concentrations (0, 5, 9, 13,
18, and 26%). During cooling cycles (Fig. 39), especially at lower concen-
trations, there is an expansion of 0.5% at 5% concentration compared to
0.08% in water and 0.01% at 26% concentration. On warming a significant
contraction occurs with highly concentrated salts at about -21°C, the
melting point of the eutectic. In the cooling cycle (Fig. 40), the first
exothermic peak is attributed to the commencement of formation of ice on
the external surface. According to Litvan, [79][80] this represents a freezing
of the disordered liquid from the pores of the cement paste. The second
exothermal effect at -22°C is ascribed to the freezing of the solution of
eutectic composition. On warming, two processes occur, one at about -21°C
and another at a higher temperature, increasing in magnitude with increas-
ing salt concentration. The peak at the lower temperature represents melting
of eutectic mixture and that at the higher temperature, the melting of pure
ice. In addition to the physical mechanism of increased volume at low
temperatures due to the presence of salts, chemical reactions may also
occur, especially at higher temperatures. For example, at a calcium chloride