Study On The Long-term And Post-fire Performance Of Steel Slag Concrete Considering The Wet Bulk Density Theor

Concrete is the most widely used building material worldwide.The demand of concrete is growing dramatically thanks to the bloom in infrastructure construction worldwide,which causes over-exploitation of natural aggregate that inevitably leads to serious environmental concern.As a result,it is exigent to find alternative materials for natural aggregate in the concrete production.Steel slag is a main waste material in the steel production process.China is the biggest steel manufacturer in the world,with a large amount of steel slag generated per year.However,the utilization rate of steel slag is less than 30%.The excessive stacking of steel slag not only wastes storge space,but also causes major irreversible environmental problems such as water and soil pollutions.Steel slag has been reported to be a potential substitute for natural aggregate.Using steel slag to replace natural aggregate to make concrete provides a win-win solution in alleviating environmental problems regarding huge depletion of natural aggregate and disposal of a large amount of slag.It has been reported that using steel slag to replace natural aggregate can enhance the mechanical behavior of concrete.Whereas,concrete structures will not only be affected by load,but also be eroded by various ions(e.g.,Cl~-)and be subject to fire during the service life,so the durability and post-fire behavior of steel slag aggregate concrete are also the important research points.However,little effort has been put on these two aspects.To fill up this research gap and promote the utilization of steel slag,an experimental study was conducted in which 41 concrete mixes(29 mixes for durability tests and 12 mixes for post-fire behavior tests)with steel slag partially or totally replacing natural coarse aggregate(crushed granite)and fine aggregate(river sand)and pozzolanic fillers(FA and SF)partially replace cement by volume were prepared.The wet packing density theory(WPD)is applied to quantify the void ratio of concrete.The performance at ambient temperature including slump,compressive strength(7-,28-,91-and182-day)and durability in terms of total charge passed and Cl~-penetration depth in rapid chloride permeability test were tested,while the behavior at and after elevated temperature including heating curve,weight loss,surface change and residual compressive strength were assessed.Besides,the chemical composition change of concrete was investigated using X-ray diffraction analysis.Results revealed that the slump of steel slag aggregate concrete decreased due to the porous structure of steel slag.Besides,the use of slag enhanced the compressive strength of concrete.By partially substituting cement by SF,the strength of steel slag concrete could be escalated further.At given composition of aggregate and filler type,the total charge passed and the Cl~-penetration depth of steel slag concrete decreased as the WPD increased due to the denser and impermeable microstructure of concrete that improved the resistance of chloride ion penetrability.Furthermore,using steel slag for partial/full replacement of crushed granite and/or adding FA or SF could improve the heat resistance and residual strength of concrete.Besides,the weight loss of FA concrete was lower than that of pure cement concrete,while that of concrete containing SF showed the opposite trend due to higher pozzolanic activity of SF.Moreover,the residual strength of concrete containing steel slag was larger than that of natural aggregate concrete while adding FA or SF could have further enhancement on the residual strength.Lastly,the post-fire design model stipulated by Eurocode 2:Structures fire design(EC2)was compared with the obtained test results of residual strength,which showed that the prediction of EC2 is conservative.