Civil Engineering Reference
In-Depth Information
Figure 23.4
Compressive strengths of pastes made with different cements and
exposed to increasing temperatures. OPC, ordinary Portland cement;
BFSC=blast furnace slag cement with 50% of granulated blast
furnace slag; PFA, fly ash cement with 25% of pulverized fly ash;
Trass, trass cement with 25% of trass; SF, Portland cement with 10%
of added silica fume.
Source:
Zürz
et al.
(1986)
or most of its chemically bound water, some renewed thermal expansion of the residual
material may be observed. Figure 23.5 shows the shrinkage of a series of cements taking
place upon heating.
The shrinkage associated with the decomposition of the hydrated paste may cause
cracking within the material. The cracking may be even greater in concrete, rather than
plain cement mixes, as here the contraction of the hardened cement paste is countered by
the thermal expansion of the aggregate particles.
In thermally decomposed cement pastes that contain distinct amounts of residual
calcium oxide, this constituent rehydrates readily to calcium hydroxide, if in contact with
water or exposed to humid air. This reaction is associated with expansion, which may
cause further damage to concrete that has been exposed to fire, even in situations where
the concrete structure has withstood the exposure to fire without disintegration or visible
signs of destruction. Because of this, concrete structures made with blended cements, in
which the amount of free lime present after exposure to fire is significantly reduced, are
generally less damaged after being exposed to fire than those made with plain Portland
cement.
