Research On Microstructure And Mechanical Properties Of Ready-mixed Concrete And Application In Mine

For the problems of artificial cast-in-situ shotcrete in coalmine roadway, such as inaccurate in raw material measurement, poor shotcrete quality, and large dust concentration, ready-mixed materials and ready-mixed concrete technology have been put forward to solve those problems. By mixing cement, sand and gravel uniformly in design proportion, the ready-mixed materials are formed and will be stored in some periods. When adding water and additives in ready-mixed materials, ready-mixed concrete is produced after hardening. Ready-mixed concrete gets many virtues, such as accurate mix proportion, good concrete quality, and low dust concentration. But the hydration reaction will occur between cement and a little moisture on sand in ready-mixed materials, which will deteriorate the performance of raw materials and affect the performance of ready-mixed concrete.Combining with the prior development project of doctoral fund of Ministry of Education, research on microstructure and mechanical characteristic of ready-mixed shrinkage-compensating shotcrete, the microstructure of ready-mixed materials and ready-mixed concrete are analyzed in moisture contents,0%,0.5%,1.0%,1.5%,2.0%, and storage periods,0d,3d,7d,10d,15d,20d,30d. Then the influence mechanism of microstructure on compressive strength and porosity of ready-mixed concrete is revealed. The main research contents and conclusions are shown as follows.(1) The microstructure analyses on ready-mixed materials and ready-mixed concrete show that larger sand moisture content and longer storage period lead to more amount of failure cement, more serious deterioration of raw materials, and more loosen microstructure of concrete. When sand moisture content is equal to or less than1.0%and storage period is within15d, there is little change of hydration production and microstructure in ready-mixed materials, therefore the degree of deterioration is low and the microstructure of ready-mixed concrete is dense. However, when sand moisture content is equal to or exceeds1.5%, the extending of storage period leads to the amount of failure cement increasing and the deterioration of raw materials intensifying.(2) The compressive strength tests of ready-mixed concrete show that when sand moisture content is equal to or less than1.0%and storage period is within15d, the strength loss rate of ready-mixed concrete with water-binder ratio of0.45is no more than8.64%and the compressive strength meets the requirements, reaching42.3MPa. When sand moisture content is equal to or exceeds1.5%, there is a big influence of sand moisture content on compressive strength. The results show that the valid storage period of ready-mixed materials in the condition of1.0%sand moisture content is15d.(3) The quantitative relation between compressive strength and porosity for ready-mixed concrete is established in various sand moisture contents and storage periods. Smaller sand moisture content and shorter storage period result in less pore number, smaller pore diameter, denser microstructure, and higher compressive strength of ready-mixed concrete. With storage period extending, the porosity increases slowly and shows little influence on concrete strength when sand moisture content is equal to or less than1.0%. While the porosity increases fast and presents large influence on concrete strength when sand moisture content is equal to1.5%or2.0%.(4) The influence mechanism of sand moisture content and storage period on compressive strength of ready-mixed concrete is obtained. The sand moisture content and storage period affect the hydration degree of ready-mixed materials. With high hydration degree, the hydration reaction consumes more cement leading to less cement in later concrete hardening, and some cement particles are packaged by dense hydration production which will stop or postpone the hydration reaction of cement. With low hydration degree, the hydration production of ready-mixed materials is loosen, and these loosen microstructures are much hard to hydrate densely which will become the weak parts of ready-mixed concrete. The higher the hydration degree, the more the amount of failure cement, the more serious the deterioration of ready-mixed materials, and the more the loss of concrete strength.(5) Engineering application indicates that ready-mixed shotcrete presents a better mechanical performance, better deformation performance, lower rebound rate and smaller dust concentration.