Exploration On Microstructure And Structural Theory Of Crumb Rubber Concrete

Crumb Rubber Concrete （CRC） is one type of“young”building materials, with less than thirty years of history. CRC exhibits many different mechanical behaviors from those of conventional concrete. Current studies reveal that （1） CRC improves its anti-cracking behavior; （2） CRC increases in the resistance to water and chloride penetration; and （3） CRC exhibits large deformability and energy-absorbing capability. These macro properties of CRC have a close relationship with its microstructure and would affect the structural properties. In this regards, it is important to“read”CRC in the view of microscopic scale and explore its influence on structural properties.The main methods applied in this paper are: （1） experimental research, including scanning electron microscope （SEM）, X-ray diffraction （XRD）, and mercury intrusion porosimetry （MIP） as well as some mechanical experiments; （2） numerical analysis; and （3） theoretical analysis.The conclusions are made as follows:（1） XRD results show that although the chemical constituents of cement is complicated, no new phase is found in the hydration products of the cement paste incorporating crumb rubber; The peak intensity of calcium hydroxide （CH） from the hydration products of the cement paste incorporating crumb rubber is reduced significantly compared to that from the control blends. This suggested that cement-based materials incorporating crumb rubber would have a larger growth in strength in late time, and this suggestion is preliminarily supported with a set of experimental data by other searchers.（2） Pore structure and fractal characteristic of CRC is analyzed quantitatively using MIP. The results show that the Mode Pore Diameter, the Mean Pore Diameter, the Median Pore Diameter and the porosity of CRC are increased compared to those of conventional concrete, and the pore distribution scope of CRC becomes wide. The fractal dimension D of the pores in CRC decreases with the increase of the crumb rubber content, which indicates that the pore distribution tends to becomes regular when rubber is added. （3） A finite element model for a rectangular plane containing an elliptic hole in the center is established based on stress concentration theory. This model illustrates the mechanism of rubber’s delaying the multiplication of the crack in CRC and explains why CRC exhibits high ductility. Stress concentration factor （SCF） is introduced to measure the stress concentration degree. It is found from the results that SCF increases significantly as the shape factor of the elliptic hole increases. Soft filler decreases the SCF and reduces the affected area of the hole along x and y directions. This research is helpful in explaining why CRC can exhibit ductile failure mode even in low temperature.（4） A modified stress-strain relationship for CRC is proposed. Based on this, theoretical formula for the flexural capacity of CRC beam, which take into account the peak compressive strain and the ultimate compressive strain of CRC, is established. The neutral axis moves downward remarkably because of the large ultimate compressive strain of CRC, which results in the compressive zone of CRC beam increases significantly. The calculation results show that with the same grade of concrete strength the flexural capacity of CRC beam is increased by 0¹0% （when rubber content is 0¹2%, namely,λ₁≤1.5;λ₂≤2.0）. An example shows that the flexural capacity of the CRC beam is increased by 7.6%.（5） Based on the modified stress-strain relationship of CRC, ductility evaluation method is proposed by means of curvature ductility factor （CDF）. The method indicates that the peak compressive strain and the ultimate compressive strain of concrete affect the ductility of concrete beam significantly as well as reinforcement ratio and yield strength of steel. In the end, a comparison shows that the calculated ductility of a reinforced CRC beam is about as 1.64².59 times as that of a conventional concrete beam.