Analyses On Several Issues Concerning Concrete Containing Composite Ferronickel Slag In Engineering Application

Ferronickel slag is a solid waste discharged from the smelting process of nickel or nickel-iron alloy.In China,the annual discharge of ferronickel slag reaches more than 30 million tons,causing serious environmental pollution and land resource waste.It is an effective way to utilize ferronickel slag by grinding it into fine powder and using it as a kind of mineral admixture to replace part of cement in concrete production.When ferronickel slag concrete is applied into structures,the following issues still need to be clarified or solved:(1)Mix design method of ferronickel slag concrete based on strength requirement;(2)Mechanical properties of ferronickel slag concrete;(3)Under the same compressive condition,the mechanism of the weaker bond strength between ferronickel slag concrete and horizontal rebar than that of normal concrete;(4)The influences of concrete strength and weaker bond level on the flexural performances of RC beams;(5)Whether additional structural measures are needed to meet the bearing capacity requirements of frame joints made with ferronickel slag concrete;(6)Evaluating the deformation of frame joints made with ferronickel slag concrete.Based on the above issues,the work of this paper is as follows.(1)With the goal of determining mixes of C30 and C35 composite ferronickel slag concrete with composite ferronickel slag content percentages of 0%,10%,20%,30%,40%,50%,a total of 12 mixes,take the mix design method of normal concrete as reference,the cement strength in the water-binder ratio formula was multiplied by the corresponding strength reduction factor,then,the mixes of composite ferronickel slag concrete were calculated.It was proved that the method is feasible and can be applied in engineering practice through trial matching and micro-adjustment.(2)The basic mechanical properties of composite ferronickel slag concrete,including the above 12 mixes,were studied.Tests included axial compressive strength,elastic modulus,Poisson’s ratio,splitting tensile strength and stress-strain relationship under compression.Results showed that under the same compressive condition and composite ferronickel slag content percentage within 50%,the basic mechanical properties of composite ferronickel slag concrete are compared to those of normal concrete,and have nothing to do with the content of composite ferronickel slag.(3)The pull-out tests of composite ferronickel slag concrete,including the above 12 mixes,were carried out.The effects of the composite ferronickel slag content and the concrete compressive strength on the bond strength between concrete and steel were studied,and the corresponding mechanism was analyzed.The research showed that under the condition of the equal compressive strength,due to the existence of bleeding effect,the bond strength between composite ferronickel slag concrete and the horizontal rebar decreases gradually with the increase of the composite ferronickel slag content.The fitting formula of bond strength between composite ferronickel concrete and the horizontal rebar was put forward,which is related to the concrete compressive strength and the composite ferronickel slag.(4)Based on the flexural test data of four RC beams made with composite ferronickel slag concrete,the effects of concrete strength and composite ferronickel slag content,i.e.,the decrease of bond strength on the longitudinal rebar,on the flexural capacity and ductility of RC beams were theoretically analyzed.Analysis showed that the strength of composite ferronickel slag concrete has an important effect on the flexural capacity and ductility of RC beams,while composite ferronickel slag content has little effect on the flexural performances of RC beams.The bond strength change only affects the flexural crack distribution,as well as the stress and strain distribution of rebar and concrete in the tensile region of RC beams.However,the stress-mean strain relationship of rebar in the concrete tension chord per unit length is not changed.(5)The failure mechanism of the interior beam-column joint made with composite ferronickel slag concrete(FCJ)was analyzed theoretically,and the corresponding joint bearing capacity formulus were deduced.The reliability of the theoretical analysis of bearing capacity in this paper was verified by using the results of five existing quasi-static tests of FCJs.In order to prevent bond failure of the beam longitudinal rebars in the joint area,the limit value of the relative length of the beam longitudinal rebar crossing the joint(h_c/d),as a function of the composite ferronickel slag content,concrete strength and the rebar grade,was calculated by using the proven formula of joint bearing capacity.(6)The deformation characteristic of the FCJ was analyzed theoretically.Analysis showed that with the increase of composite ferronickel slag content,the additional joint deformation(rotation)caused by the rebar slip effect at the beam-column interface increases,which accounts for a considerable proportion of the overall joint deformation.In order to evaluate the deformation,the equivalent plastic hinge model of the beam end was proposed,and the equivalent plastic hinge length,as a function of the beam length,the composite ferronickel slag content,and the rebar diameter was determined.