Research On Structure And Performance Of Ceramsite Combined Aggregate Concrete

Combined aggregate concrete is one kind of moderate or high strength concrete, which is prepared by replacing part of normal weight aggregate with lightweight aggregate, and its density is between 1950kg/m3 to 2200kg/m3, its strength is between 40MPa to 80MPa. It can increase the elastic modulus and decrease the creep of lightweight aggregate concrete, as well as lighten the dead load of structure, so the application of combined aggregate concrete in modern construction, especially the heavy load-bearing, long span and high prestressed structure can bring good economic and technical meaning. In this paper, based on the study of the effect of ceramsite on the workability, mechanical property, early shrinkage cracking, durability and microstructure of concrete, the relationship among the composition, structure and performance of combined aggregate concrete is discovered, and the theoretical basis can be provided for preparing combined aggregate concrete.The main research work and compliments of this paper is:The influence of ceramsite on the microstructure of concrete was investigated using SEM, XRD, EDXA, microhardness measurement and pore structure analysis. The results show that, the addition of ceramsite into concrete can reduce the difference of microhardness and elastic modulus between aggregate and mortar, the internal structure of concrete seem to be more homogeneously. The ceramsite with different water absorption and releasing capacity has different influence on the hydration degree and structure of the adjacent cement paste, and the influence degree varies with the change of water-to-cement ratio and curing age. The ceramsite with high water absorption has strong effect on the refinement of the adjacent cement paste, especially the volume content of macropores within the range of 100nm~1000nm reduces sharply; however a weak interfacial zone can be observed surrounding ceramsite YH with a 1-hour water absorption of 2.6%, which is similar to that around normal weight aggregate. For concrete with a higher water-to-cement ratio, the dry ceramsite has strong effect on the densification of the adjacent cement paste. For concrete with a lower water-to-cement ratio, the ceramsite with high pre- wetting degree can also improve the structure of the cement paste around aggregate, and the improve effect becomes apparently with increasing the curing age. The concept and calculation formula of"interface influence factor"is put forward based on the interfacial structure research, and hence a quantitative and comprehensive comparison method for study the influence of aggregate on the interfacial structure can be provided.Through the study of the effect of ceramsite on the workability and early shrinkage cracking of combined aggregate concrete, it is found that, although the water absorption character of ceramsite can reduce the fluidity of fresh concrete, the increase of paste viscosity is advantageous to improve the concrete homogeneity. By mixing fly ash, air entraining admixture and water reducing admixture into concrete simultaneously, the combined aggregate concrete with a slump of 210mm and a segregation degree of 3.3% is prepared. The water releasing effect of ceramsite can improve the concrete early anti-shrinkage cracking, and the more content and higher pre-wetting degree of ceramsite, the higher the"interface influence factor"and the more obvious the improvement.The mechanical property and durability of concrete which is closely related to the requirement of structure engineering are investigated systematically. The results show that, in the engineering required range of the concrete strength and elastic modulus, mixing proper content of ceramsite into concrete can increase the specific strength of concrete. Under this study condition, for concrete with a water-to-cement ratio of 0.32, the ceramsite content is better no more than 50%, whereas the ceramsite content is better no more than 25% for concrete with a water-to-cement ratio of 0.49. The constitutive equations of combined aggregate concrete were put forward based on the study of the stress-strain relationship of concrete prepared with different ceramsite content. The ceramsite with higher particle strength has positive effect both on the concrete compressive strength and elastic modulus; the ceramsite with higher water absorption not only can increase the concrete splitting strength, but also has positive effect on the increase of concrete strength in long term; the ceramsite with high pre-wetting degree is negative to the early strength of concrete, but is advantageous to the later growth rate of concrete strength.The addition of ceramsite into concrete is advantageous to the concrete frost-resistance and the impermeability of concrete with a higher water-to-cement ratio, and the more the ceramsite content, the higher the"interface influence factor"and the more obvious the improvement. Furthermore, the improve effect of ceramsite on the frost-resistance of concrete with a lower water-to-cement ratio becomes more significant in the long term. However, the impermeability of concrete with a lower water-to-cement ratio has some decrease due to the addition of ceramsite, especially in early age, so under this condition the ceramsite content is better no more than 50%. In order to obtain good impermeability and frost-resistance performance of concrete, the ceramsite with low water absorption should be chosen for preparing combined aggregate concrete with a higher water-to-cement ratio, whereas the ceramsite with moderate water absorption should be chosen for that with a lower water-to-cement ratio. And in the premise of meeting concrete workability, the dry ceramsite can be chosen to improve the concrete impermeability and frost-resistance.The relationship among aggregate composition, concrete microstructure and performance is analyzed comprehensively, and the aggregate required for preparing combined aggregate concrete is put forward, furthermore, the proof test is carried out, and the selective preference of aggregate on the improvement of the structure and performance of combined aggregate concrete is testified.