| Summary: | Structural lightweight concrete presents a concrete of an equilibrium density between 1120 and 1920 kg/m3 and a minimum 28-day compressive strength of 17.24 MPa (ACI 213R-03). It has been widely used since centuries ago because it makes use of local industrial waste that can be economically utilized and the reduction in concrete weight helps for easy removal, transport and erection of precast products. It also has many other advantages including lower thermal connectivity as well as maximized heat and sound insulation properties for its porous structure.
Fire, as one of severe conditions, has a significant influence on concrete infrastructures. With increasing requirements of capacity, safety and performance of building construction as well as the construction productivity, innovative materials are preferably used to achieve high-strength and light self-weight properties in the structural design. It is reported that the lightweight aggregate concrete has a low conductivity and hence leads to a lower temperature on the reinforcing bars (ACI 213R-03). Therefore, it possesses better fire resistance comparing to the normal weight concrete. Nevertheless, researchers from Norway pointed out that this conclusion would be capable if subjected to a cellulosic fire. When exposed to a hydrocarbon fire, on the contrary, the lightweight aggregate concrete would experience a more severe spalling than the normal density concrete (Lindgard & Hammer, 1988). What is critical, high-strength concrete is more susceptible to spalling and may result in lower fire resistance. Therefore, maintaining lightweight concrete at a high strength for the structural use and preventing it from explosive spalling during fire exposure become primary importance among priorities.
There are two main reasons for spalling: pore pressure and thermal stress. This can be effectively dealt with lowering water content of the concrete mix and increasing the permeability of the concrete.
It is reported that the addition of polypropylene (PP) fibres can effectively negate the effects of spalling because the melting of PP fibres creates channels to allow effective release of vapour pressure out of the concrete. However, the addition of PP fibres might lead to a reduction in compressive strength. Therefore, this study proposed a hybrid fibre reinforced high strength lightweigh concrete that can prevent spalling and perform robust residual mechanical properties after fire exposure.
At the same time, the study proposed concrete involving a type of lightweight filler so-called cenosphere (CS), a by-product of coal combustion, as a greener alternative to sand. The presence of cenosphere also increased the permeability of a concrete, further negating the effects of explosive spalling.
To achieve high-strength lightweight concrete, the relationship between the compressive strength and the density of concrete was first investigated by adjusting the proportion of replacements in sands. Based on the test results, an optimised mix design (25% replacement of sands by cenosphere) that could meet the requirements of high-strength lightweight concrete (fck> 40 MPa and the density is less than 1850 kg/m3) were chosen for fire tests. It was observed that spalling occurred in all the samples. Based on the understanding of the mechanism of spalling, different fibres like steel fibre or/and PP fibre were added. Finally, the residual strength of LECA concrete were discussed after heated up to 200℃, 400℃, 600℃ and 800℃, respectively. It was concluded that the addition of hybrid fibres (0.2% PP and 0.5% steel fibres) could minimise the effects on compressive strength and improve fire resistance of the lightweight concrete.
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