Degradation Mechanism Under Elevated Temperature And Post-fire Performance Of Steel Slag Concrete Considering Wet Packing Density Theory

Concrete is the most consumed artificial material in the world,while granite gravel and natural river sand are the most commonly adopted natural coarse and fine aggregates,respectively,which have been frequently used since the advent of concrete.As the consumption of concrete increased rapidly in the past few decades due to the infrastructures,tall buildings and bridges construction requirements,natural aggregates are in great demand.Obtaining a mass of natural aggregates from the earth definitely impairs the environment.To resolve this problem,it is imperative to seek for new eco-friendly high-performance green concrete raw materials to replace natural aggregates.Steel slag is the waste by-product generated in the process of steelmaking,which not only seriously wastes the land storage space,but also causes a series of environmental problems such as water pollution.The research shows that steel slag has great potential in concrete production.Using steel slag aggregate(SSA)to replace natural aggregate in making concrete offers a sustainable solution for mitigating the environmental issues owing to the large-scale mining of natural aggregate and disposal of a large amount of steel slag.Over the past few decades,extensive research studies were conducted on the mechanical behaviour and chemical property of concrete made with steel slag at ambient temperature.The results show that compared with ordinary concrete,steel slag concrete has obvious advantages in microstructure,mechanical properties and resistance to chloride ion penetration.But the concrete structure is likely to be affected by fire during its service life.Although concrete itself is an excellent fire protection material,the significant changes of its physical characteristics and chemical composition under elevated temperature environment will lead to the progressive collapse of concrete structures or other adverse structural damage after fire,endangering the safety of our lives and property.However,relatively little effort has been put to study the behaviour of concrete with steel slag after exposure to elevated temperatures.To fill up this research gap,an experimental study was conducted in which nine concrete mixes with steel slag replacing natural aggregate were prepared.Taking the wet packing density as the quantitative index of concrete void ratio,the effect of steel slag replacement ratio by volume(0%,25%,50%,75% and 100%)on the heating behaviour,post-fire residual compressive strength,weight loss,surface change of concrete and failure mode under curing period of 28 and 91 days,after exposure to 25℃(ambient temperature),400℃,600℃,800℃ and 1000℃ was investigated.Besides,chemical deterioration behaviour was studied using X-ray diffraction analysis,which helps us clear the elevated temperature degradation mechanism of steel slag concrete from the micro level.From the test results,it is evident that with steel slag partially or totally replacing natural aggregate,the heat insulation ability of concrete could be enhanced and the development of surface cracks could be alleviated.Moreover,the residual strength of concrete with steel slag coarse aggregate replacement ratio of 100% after exposure to elevated temperatures≥ 600℃would be comparable to concrete with crushed granite only.Steel slag fine aggregate concrete presented larger or comparable residual strength after exposure to elevated temperatures from600-800℃ in contrast to river sand concrete.Lastly,the prediction model of post-fire residual strength by EC2(Eurocode 2: Structures fire design)was adopted to compare the residual strength of concrete with and without steel slag and results demonstrated that EC2 was conservative.