Study On Manufactured Sand And Properties Of Its Concrete Used In Prestressed Structure Of Railway Engineering

The railway engineering projects,especially the mountain railway projects of Yunnan,Guizhou and Sichuan provinces,are faced with a serious situation of the shortage of river sand resource caused by the circumstances of the large span of railway engineering,the uneven distribution of river sand resources and the policy implementation of river sand mining restriction.The manufactured sand(MS)is a kind of building material made by the rock minerals through technologies of removing soils,crushing,sieving and shaping.The MS plays a crucial role in solving the river sand issues of the shortage,insufficient supply and prices rising.The application technology of MS in prestressed concrete structure of railway engineering is imperfect because of the limit of the traditional technique of preparing MS,the restriction of railway prestressed structure standards and the complex composition of rock minerals.In order to promote the MS application in railway engineering,especially the prestressed structure engineering,this paper studied on some key issues which were the characterization techniques of MS key parameters,the effects of powders with different lithology and the properties of manufactured sand concrete(MSC).There are some mainly conclusions:(1)The similarities and differences of image analysis method and other methods in characterizing the MS key parameters such as powder content,particle shape and particle size distribution were compared,and the feasibility of image analysis method was researched.The image analysis method and the wet method had a good correlation in the aspect of characterizing powder content of MS(R2 > 0.976).The particle size distribution calculated by short diameter was closer than that calculated by equivalent diameter to the sieving results,and the average value of the difference of passing percentage between sieving method and short diameter method was less than 6%.The powder had a great influence on the shape parameters of MS.The image analysis method could characterize the powder content,particle shape and particle size distribution fast and conveniently when the powder content was lower than 15%.(2)The effects of the limestone,tuff,granite,basalt and quartzite powders on the properties of cement paste and cement mortar were studied.The higher superplasticizer adsorption of tuff powder and granite powder resulted in that their cement paste had lower fluidity and higher plastic viscosity.The workabilities of cement paste and mortar were affected by the fineness and the MB value of powder.The fluidity of paste was improved by the effect of fine particle filling and the improvement of particle size distribution.The powder with high MB value could increase the water demand of paste and decrease the fluidity of paste.Because of the high fineness of limestone powder and the low MB value of quartzite powder,the limestone powder and quartzite powder enhanced the fluidity of cement mortar when the contents were less than 12% and 28%,respectively.The activity indexes of mortars with different powders were more than 60% when the powder contents were less than 30%.The contribution rates of limestone powder and tuff powder to mortar strength and cement hydration were higher than other powders,and their activity indexes of mortars were 68.7% and 72.2%,respectively.(3)The effect of MS lithology on the properties of MSC was studied.The lower the crushing value of MS was,the higher the strength of concrete was.The compressive strengths of MSCs with different lithology from large to small in order were tuff > quartzite > basalt > granite > limestone.The bending strength and the elastic modulus of MSC are all related positively to the compressive strength.The resistance to chloride ion permeability and the frost resistance of MSCs were good and hardly affected by sand lithology.The dry shrinkage developments of MSCs were consistent,and the chemical composition of rock leaded to different shrinkage values in long-term age.The creep of MSC was related to the compressive strength under the same stress,and the ultimate creep strains of MSCs with different lithology from large to small in order were limestone > granite > basalt > quartzite > tuff.(4)The properties of MSCs used in the prestressed structures of railway engineering including beam,track slab and sleeper were studied and its properties met the requirements of relevant standards of railway concrete.The early age strength of MSC was higher than river sand concrete(RSC)because of the limestone powder promoting cement hydration,and the later age strength of MSC and RSC were similar.The surface roughness and strong angularity of MS restricted the paste deformation and aggregate particles sliding,which made the elastic modulus of MSC larger than that of RSC.The frost resistance of the MSC and the RSC were similar because of their same air content and strength grade.The MSC had higher surface resistivity,lower chloride ion diffusion coefficient and lower electric flux because of the filling effect and nucleation effect of limestone powder.The dry shrinkage and the creep of MSC were higher than those of RSC when the effect of the powder on increasing paste volume fraction was more obvious,while the dry shrinkage and the creep of MSC was lower when the effect of manufactured sand on deformation restriction was more obvious.(5)The pilot production of ?a type concrete sleeper was conducted,and the workability,mechanical properties and structure performance of the MSC sleeper and the RSC sleeper were studied comparatively.The MSC sleeper needed more superplasticizer content to achieve the same workability as RSC sleeper.The demoulding strength and the demoulding elastic modulus of MSC sleeper were higher slightly than those of RSC sleeper.The property of static load of the MSC sleeper met the requirements.The appearance quality and properties of MSC sleeper were good and its production cost was lower,which indicated MS was economic and applicative in concrete sleeper.