fire protection of concrete structures exposed to fast fires

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Fourth International Symposium on Tunnel Safety and Security, Frankfurt am Main, Germany, March 17-19, 2010 1400 Modified HydroCarbon (HCM) fire curve 1400 Temperature (°C) 1200 1000 800 600 400 200 0 0 20 40 60 80 100 120 140 160 180 200 Time (min) Temperature (°C) 1200 1000 800 600 400 200 0 0 2 4 6 8 10 12 14 16 18 20 Time (min) Figure 5. Evolution of the inner furnace temperature under a HCM fire curve (on the right, focus on the 20 first minutes of fire). 4. Results In this paragraph, the main results of the study are presented. Firstly, we present the analysis of the concrete spalling risk depending on different parameters (thermal protection thickness, type of aggregates, mechanical loading). Then we present an experimental and numerical study attending to establish the link between concrete spalling and the thermal gradients that is induced in the first centimetres of the fired slab. At last, we present the results of the assessment of the residual compressive strength of concrete in the fired slabs, from drilled cores. 4.1 Concrete spalling The Table 3 summarizes the results of the maximal concrete spalling depths that were measured depending on different parameters such as the thickness of the thermal protection, the type of aggregates and the type of mechanical loading. The main observations are as following. When concrete slabs are not thermally protected, concrete spalling occurs for any type of our tested concretes (i.e. for both types of aggregates) and for any type of mechanical loading (i.e. with no loading or for compressive loading). In that case, concrete spalling involves the disappearance of concrete cover and then the direct exposure to fire of the reinforcement. For the under sized thermal protection boards ( from 8 to 15 mm), concrete spalling can also occur. This has been particularly observed during loaded tests. It is very important to note that when concrete spalling occurred for protected slabs, concrete spalling was deeper than for slabs with no protection (see Figure 6). This unexpected result clearly indicates that an under sizing of the thermal protection can involve an important damage of the concrete structure (with in particular an important exposure of the reinforcement to fire). For thick enough thermal protection (20 and 25 mm in our study), concrete spalling is avoided in the case of loaded slabs. This results in a continuous protection of the reinforcement during the fire and good residual mechanical performances (see § 4.3). 240
Fourth International Symposium on Tunnel Safety and Security, Frankfurt am Main, Germany, March 17-19, 2010 Maximal spalling depth Protection thickness [mm] Calcareous concrete Unloaded slabs Silico-calcareous concrete Calcareous concrete Loaded slabs Silico-calcareous concrete 0 55 30 49 85 8 50 0 63 77 12.5 0 0 137 / 15 0 0 130 / 45 / 20 / / 0 / 25 / / 0 / / = no test; 0 = no spalling Table 3. Summary of the measurement of the maximal spalling depth. In our study, the influence of the type of aggregates on concrete spalling is not clear. Indeed, for tests with no mechanical loading, concrete spalling is deeper for calcareous concrete. Moreover, we observed for slabs protected with 8 mm boards that concrete spalling only occurs for calcareous concrete. On the contrary, for tests with mechanical loading, concrete spalling is deeper for silicocalcareous concrete. One important aspect of fire concrete behaviour is the spalling that can occur during the cooling down of the structure and even for a long period after the fire (actually, it can be better seen as a concrete departure or a concrete fall than a real spalling). In the § 4.3, we will see that the type of aggregates can influence this particular type of concrete departure. Observing the results in Table 3, we can say that the initial presence of a compressive stress at the inferior bed of the slab is unfavourable regarding to concrete spalling. However, for slabs with no protection, the influence of loading is not that clear: the presence of load only involves deeper spalling for silico-calcareous concrete. In the case of the calcareous concrete, spalling depth does not seem to be influenced by the loading. In the case of under sized thermal protection boards (from 8 to 15 mm), the presence of the loading leads to deeper spalling. In particular, for thermal protection of 12.5 and 15 mm, concrete spalling only occurs in the case of loaded slabs. However, we want to emphasize that making a direct link between concrete spalling risk and the presence of a compressive loading is not that easy. Indeed, observations during the fire tests led us to distinguish two modes of rupture resulting on concrete spalling: - 1- concrete spalling firstly occurs leading to the fall of the protection board. In such a case, we could directly link concrete spalling to the presence of a mechanical load, - 2- thermal protection board first falls resulting in a sudden exposure of "virgin" concrete zones to fire. This mechanism also leads to concrete spalling but the influence of the mechanical loading seems to be more linked with the mechanical resistance of the thermal protection system (board and anchorages) itself. This behaviour has been also observed by [13]. One main difficulty of the study is that these both types of rupture mode can occur during the same fire test. Further studies have to be performed in order to better analyse the influence of the mechanical loading on concrete spalling (e.g. by using a high velocity camera during fire tests). 241
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