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Concrete In Australia : June 2011
Concrete in Australia Vol 37 No 2 25 Feature technical paper: hiGh StrenGth cOncrete (peer reVieWeD) 1.2 .2 Failure mode 2: Spalling due to restrained thermal dilatation Spalling due to restrained thermal dilatation was identified by Bazant (1997) and later adopted by Ulm et al (1999a) and Nechnech et al (2002). This failure mode considers that the spalling results from restrained thermal dilatation close to the heated surface, which leads to compressive stresses parallel to the heated surface. These compressive stresses are released by brittle fracture of concrete, i.e . spalling. Due to the volume expansion of a growing crack, and the slowness of release of additional water into the crack, the pressure in the crack must rapidly decay after the crack begins to open. As a result, the pore pressure can play only a secondary role as far as the growth of a larger crack is concerned. The pore pressure may affect the onset of instability in the form of explosive thermal spalling (Figure 6). Figure 7 shows a slab after a fire test exposing only the middle region of the slab to fire, and illustrates this failure mode (Hertz, 2003). 1.2.3 Failure mode 3: Thermal incompatibilities between cement paste and aggregates When subjected to increasing temperature, the cement paste initially expands and when it is heated beyond about 300°C, it starts to rapidly contract. Figure 8 shows the behaviour of four different cement pastes reported in the literature. Due to thermal gradients in concrete, parts which are still under 300°C would be experiencing expansion while the other parts which are more than 300°C would be experiencing contraction. This competition between simultaneous expansionand contraction damages the concrete matrix. This behaviour depends on the type of cement binders used. With increasing temperatures, most types of aggregates undergo expansion. Aggregates typically occupy about 60 Figure 6. Spalling due to restrained thermal dilatation. Figure 8. Contraction of Portland cement paste at high temperatures: (a) Phileo (1958); (b) Harada et al (1972); (c) Cruz and Gillen (1980); (d) Crowley (1956). Figure 9. Thermal incompatibilities in concrete at high temperatures. Figure 7. Spalling due to restrained thermal dilataion.