Study On Mix Design Method Of Self-compacting Concrete

Self-compacting concrete (SCC) , which is a new kind of high performance concrete(HPC) with excellent deformability and segregation resistance , can flow freely underits own weight during the placing process and flow through the gaps of reinforcementfilling up the formwork , while it is cohesive enough to be handled without anysegregation or bleeding . SCC in its fresh state is homogeneous and after hardenedhas excellent mechanical performance and durability.In view of lack of unified mix design criterion of SCC in China , the author proposeda new mix design method for SCC based on exhaustive investigation and summary ofexisting method in the literature . The rationality and practicability of this methodwere validated by experiments . A preliminary experimental study on thecompatibility of cement , mineral admixture and superplasticizer was also made andthis was instructive for research on mix design of SCC .Through many experimental researches , the following achivevements andconclusions were obtained:(1) This paper proposes a new mix design method for self-compacting concretenamed parameter design method .(2) In this method , the parameter α ,β and γ represents the modulus of coarse,fineaggregates and mineral admixture respectively , which provide the workability ofSCC at fresh state , while the parameter W/B is mostly related with the strength anddurability of SCC after hardened. For the raw materials used in this study , two valuesof 0.63-0.65 and 1.50-1.60 were recommended for α and β , while the value of γshould be selected according to the existing condition and based on the result of flowtest on paste . As far as deciding the value of W/B , we should take strength anddurability of SCC into consideration .(3) The SCC with 40MPa-90MPa compressive strenth at 28 days have beenprepared successfully using this parameter method .(4) A flow test of binding paste can be used to instruct the trial mix of SCC .Through this test the kind and content of superplasticizer and mineral admixture canbe preliminarily establishted .(5) Generally speaking , the quantity of superplasticizer put into the binding pasteshould be much enough to make the flow value measured through standard testingprocedure reach 300mm , thus SCC confected with such paste of binders has thepossibility to comply with the flowability requirements , otherwise moresuperplasticizer ought to be used . But when increasing the dosage ofsuperplasticizer is still helpless for the flowability of the fresh concrete or segregationand bleeding occur because of excessive superplasticizer , it is supposed to adjust themix proportion of concrete . And it is not right to entirely rely on superplasticizer tosolve all the problems of workability .(6) Mineral admixture can improve the compatibility of cement andsuperplasticizer . According to the result of flow test of binding paste and mix test ofSCC , the optimum content of original fly ash should be about 30% with top amountreaching 60% when used solely, while the content of ground granulated blast-furnaceslag is about 45% . Silica fume should be used together with other type of mineraladmixture and its dosage is about 5% .