Study on mechanical properties and size effect of coal gangue concrete at mesoscale

Using solid waste coal gangue as building materials, especially the coarse aggregate of concrete, conforms to the concept of environmental protection and saves natural resources, such as sand and gravel. The traditional numerical model cannot describe the heterogeneous characteristics of coal gangue concrete. A numerical model of coal gangue concrete based on a mesoscale was established to study the influence of the internal component properties on the mechanical properties and size effect of the compressive strength of coal gangue concrete. The results show that the mesoscopic model can better describe the mechanical properties, failure modes and internal damage evolution process of the coal gangue concrete. With the increase of ITZ strength, the peak stress of coal gangue concrete increases first and then decreases, but the residual strength tends to be consistent. Improving the mortar strength can increase the peak stress and brittleness of coal gangue concrete. The degree of influence is related to the strength of the three-phase mechanical properties. When ƒcp-ITZ<ƒcp-M<ƒcp-Agg, the influence is the largest. When ƒcp-Agg<ƒcp-ITZ<ƒcp-M, the influence is weaker than the former case. When ƒcp-ITZ<ƒcp-Agg<ƒcp-M, the influence is small. The increase in the aggregate strength improve the peak stress of the concrete. When ƒcp-Agg<ƒcp-M, the peak stress of concrete can be effectively increased, and when ƒcp-Agg>ƒcp-M, the increase rate will decrease rapidly. Increasing the strength of the aggregate improves the brittleness of concrete. The size effect of coal gangue concrete is more significant than that of ordinary concrete, and it is recommended to consider conversion factors of 0.9 and 1.15 for cube sizes of 100 mm and 200 mm, respectively. Increasing the mortar strength made the size effect more pronounced. The size effect rate model based on the Bažant theory can describe the relationship between the nominal strength and the size of coal gangue concrete.