Preparation Of Self-compacting Concrete For Prefabricated Building Elements

The self-compacting concrete applied to prefabricated building elements can improve production efficiency, reduce noise, reduce the labor intensity, and actualize factory automation. However, mineral admixtures are often used in self-compacting concrete and the strength grade of the prefabricated building elements is not high, so the development of early strength of concrete is slow and often can not meet the requirements of the demoulding strength. Therefore, the mix design method study of self-compacting concrete which with less cementitious material, low cost, high early strength and moderate-intensity is important. Firstly, prediction method and research method of early strength of cementitious materials under the steam curing condition are carried out based on mortar early strength test. Then under the limiting conditions of the early strength meeting the demoulding requirements and late strength is not very high, mix design method of self compacting concrete is studied. Finally, the mechanism of cementitious materials affecting the working ability of self-compacting concrete is analysed. Through the research, the main conclusions are obtained as follows:(1) The addition of mineral admixtures has a great influence on mortar early strength and flow properties. A certain amount of fly ash is more favorable for mortar flow performance, slag powder is more favorable for early strength, and the effect is optimal with equal amount of complex mixing. Limestone powder can be added to improve the flow properties of the mortar, but it is less favorable on the strength development as to avoid the late strength superior measure.(2) In steam curing, water-cement ratio, the volume ratio of C-S-H to unhydrated particles of cementitious materials and a bulk density are the main factors of strength. Two cementitious material strength prediction models are established and they have a strong applicability. The prediction model established by GM (1, N) tests 108 groups and average relative error is 5.63% which is less than 10% relative error on the totally, as the precision is high. The average relative error of model established by neural network is 8.03%, and relative error of all test groups is less than 10% with a high precision. The most suitable dosage of mineral admixture and combination of gelling materials can be getting from two prediction models. Using the model obtained, stacking density of cementitious material, apparent density and reaction variable of cement and mineral admixtures, moles of C3S and C2S, chemical composition of mixed material and the mass fraction of silicon oxide are only needed to input to do cementitious material composition design and obtain the early strength requirements.(3) The improved self-compacting concrete mix design method which can meet the demoulding strength limit conditions is based on absolute volume method, early strength and stripping strength predictive model of mortar and water-reducing superplasticizer. The slump flow degree of prepared self-compacting concrete can reach 600 mm or more, and the stripping strength of 14 h can release up to 30 MPa with less cementitious materials and less water. Limestone powder is added to avoid late strength superior. In order to verify the universality of the method, raw materials such as blending materials and aggregate are changed. The slump flow of self-compacting concrete is higher than 550 mm and the early demoulding strength is higher than 30 MPa which is greater than 75% of the design value of the concrete strength grade. Also,28 d strength is lower than 48 MPa which is less than 125% of the design value of the concrete strength.(4) Particle morphology of the fly ash and limestone powder is favorable for workability and particle morphology of slag powder is irregular, so content is needed to be controlled. The combination of fly ash, slag powder and limestone powder is designed from particle packing density, mobility, and segregation performance, consistency and several aspects. Reasonable combination of fly ash, slag powder and limestone powder is conducive to improve the particle packing density and slurry relative density; the combination of fly ash and slag powder can improve the fluidity of slurry, but can not improve the segregation resistance of slurry. And combination of a certain amount of lime stone powder not only improve the fluidity of slurry, but also increases the anti segregation performance of paste. Also, mortar consistency is increased and flow performance is better.