Heat Transfer Characteristics of GGBS Concrete in Fire
Keywords:concrete, fire, GGBS, heat transfer
Partial replacement of high clinker content cements by Ground Granulated Blastfurnace Slag (GGBS) can reduce the carbon footprint of concrete, with consequent benefits in respect of sustainable construction. Furthermore a state-of-the-art report indicated that use of GGBS as a binder replacement may increase the fire resistance of concrete. If so this could lead to thinner sections in fire compartment elements, especially non-loadbearing walls, leading to further gains in respect of sustainable development. Concrete is considered an effective material in protecting against the detrimental effects of fire in structures. However, exposure to high temperatures can degrade concrete performance. Fire resistance performance is typically defined relative to three failure criteria: load bearing resistance (R), insulation (I) and integrity (E). Heat transfer is a critical element of insulation and integrity performance.
An experimental programme was developed to examine the potential beneficial influence of GGBS on fire resistance by examining the heat transfer performance of concrete, with and without GGBS. Test panels of concrete were subjected to heating to high temperatures. The panels included either limestone or sandstone aggregate, and a binder content of either CEM II/A-L only or a CEM II/A-L and GGBS combination at a cement replacement level of 70%. Concrete panels were heated in accordance with the standard fire curve of Eurocode 2.
It was found that the heat transfer behaviour was in line with published data in the Eurocode for structural fire design. When exposed to elevated temperatures, such as those experienced in a fire situation, the performance of concrete containing GGBS exhibited a marginally lower rate of heat transfer than that of CEM II/A-L concrete. This resulted in a marginal improvement in the separating function (EI) performance. The marginally lower transfer of heat exhibited by GGBS concrete improved performance in terms of EI, such that a 5% reduction in the thickness of reinforced concrete separating elements could be considered. In addition, increased resistance to the effects of actions on a member, designed in accordance with the 500˚C isotherm ‘simplified’ method detailed in Eurocode 2, demonstrated a potential increased resistance of up to 10%.