Thermo-physical And Mechanical Properties,Similar Materials Of Inner Shaft Lining Concrete During Construction Period

In recent years,with the increase in the depth and diameter of vertical shafts,the inner shaft lining of the frozen wellbore has cracked and water leakage has become increasingly serious.The mechanism is unknown,causing major safety threats and economic losses.Revealing the cracking mechanism of the inner shaft lining during the construction period is the prerequisite for scientific prevention and control of water leakage from frozen shafts.This article first simulates the actual temperature conditions of the new inner wall,studies the evolution of the inner lining concrete thermos-physical and mechanical parameters with age and temperature,and provides temperature data for subsequent research;Then,based on the principle of electric heating,a conductive concrete similar material that can break through the similar simulation problem of cement hydration heat release was developed;Finally,through a similar simulation test,the feasibility of using conductive concrete to make up the hydration heat gap of the inner wall shrinkage model was initially verified.The above research provides a guarantee for the further development of numerical calculation of the cracking mechanism of inner shaft lining concrete and similar simulation tests,and has important application value.The main findings are as follows:1)Using finite element software to build a newly built freezing shaft calculation model.Calculate the change law of temperature field of inner shaft wall under different inner shaft wall thickness,inside wind speed and inside air temperature.The research shows:(1)In the initial period after the inner shaft lining is poured,concrete undergoes three stages of change:Rapid heating(0.95℃/h～1.01℃/h),rapid cooling(≥0.02℃/h),slow cooling(≤0.01℃/h).(2)The temperature inside the wellbore develops faster than the surface of the wellbore,When the thickness of the inner lining is 1.6m,the maximum temperature of the inner shaft lining reaches 79.5℃and the maximum temperature difference reaches 38.0℃.(3)The highest temperature of the inner lining occurs at about 80～180mm near the inner edge,and gradually increases away from the inner edge of the inner shaft lining as the inner shaft lining thickness and the wind speed in the well increase.(4)Reducing the thickness of the inner shaft lining and reducing the wind speed in the well can effectively reduce the temperature of the inner shaft lining and reduce the occurrence of cracks.(5)The temperature field of the inner shaft lining is greatly affected by the thickness of the inner shaft lining..2)Simulate concrete pouring of inner shaft wall under real temperature environment conditions,study the evolution of thermo-physical and mechanical parameters of inner shaft lining concrete with age and temperature under different wall thickness.The research shows:(1)The pouring temperature curve presents five stages:Induction period,slow heating,rapid cooling,rapid cooling,and slow cooling.The maximum hydration temperature increases with the thickness of the well wall,and the time to reach the maximum temperature is postponed.The highest temperature reaches79.3℃,the maximum temperature difference between inside and outside is 16.4℃,and the arrival time is 16h after pouring(casting thickness 1.5m intermediate temperature).Compared with the frozen temperature field of the inner well wall in Chapter 2,it is summarized as follows:The change trend of the inner shaft lining temperature is the same,all experiencing rapid heating,rapid cooling,and slow cooling stages;The effect of the thickness of the inner shaft lining is the same.The higher the thickness of the inner shaft lining,the higher the hydration temperature of the inner shaft lining concrete.There are differences between numerical calculations in terms of heating rate and time to reach the highest temperature,but they can be consistent in the highest temperature.(2)The early compressive strength of concrete developed rapidly,the intensity on the1st day reached 61%～85%,the intensity on the 3rd day reached 81%～95%,and the intensity on the 7th day reached 85%～99%.The overall change range is 37MPa～58MPa.(3)The elastic modulus increases rapidly from the 1st to the 3rd day,and tends to grow slowly from the 3d to the 7th day,with a small change range.The overall value range is 3.19×10~4MPa～3.57×10~4MPa.(4)The splitting tensile strength showed a nonlinear increase with age,and the growth rate increased rapidly from the 1st to the3rd day,and decreased from the 3rd to the 7th day.The overall change range is2.78MPa～4.26MPa.(5)Both the thermal conductivity and the specific heat reached the maximum on the first day,and gradually decreased with age.The overall value range of thermal conductivity is 0.823W/m·K～2.386W/m·K,and the overall value range of specific heat capacity is 0.662k J/kg℃～0.984k J/kg℃.(6)Among the three casting thicknesses,the compressive strength gradually increases with the increase of the wall thickness,but the growth at different ages is different;the elastic modulus gradually increases with the increase of the wall thickness,and the overall growth rate increases with age It gradually decreases with the increase of the period;the splitting tensile strength is close to linear growth with the increase of the wall thickness,this phenomenon is more obvious in the 3d and 7d;the thermal conductivity shows a linear growth law with the increase of the wall thickness When the thickness of the shaft wall increases by 20 cm,the thermal conductivity increases by about 0.06 W/m·K;the specific heat capacity gradually increases with the thickness of the shaft wall.3)In order to solve the problem of the dissimilarity of the hydration heat of cement in the model of the shrinkage ratio of the new building,the steel slag powder is used as the conductive phase material to simulate similar materials of conductive concrete.By adjusting the amount of steel slag and voltage to meet the similar temperature field,and then adjusting the water-cement ratio,sand rate and sand dosage through orthogonal test to meet the similar mechanical parameters.The research shows:(1)The electric heating effect of steel slag concrete is obvious,and its temperature change law is similar to the prototype.After voltage is applied to the concrete test block,the maximum temperature is increased by 12.3℃～52.4℃,and the time to reach the maximum temperature is advanced by 10h～21h.(2)The heating rate of concrete(0.6℃/h～1.49℃/h)and the highest temperature(35.7℃～52.3℃)under the voltage of 10V and 20V are lower than the temperature data in Chapter 3(heating rate 1.95℃/h～3.12℃/h,the highest temperature is 58.9℃～79.3℃),and the arrival time of the highest temperature is delayed by 8h～28h.A similar voltage that can satisfy the temperature field is a 30V voltage(heating rate 1.94℃/h～3.38℃/h,maximum temperature 58.2℃～76℃).The content of steel slag satisfying the similar temperature field and mechanical field(the mass ratio with cement)is 0.7.(3)The mechanical parameters are obtained through9 sets of orthogonal experiments,under the premise of satisfying the similar temperature field,it is determined that the formula of conductive concrete that meets the similar mechanical parameters is the fifth group.(The water-cement ratio W/C is0.34,the sand rate is 38%,and the sand amount is 770m~3).4)A similar model of conductive concrete is used to establish a shrinkage model of the inner shaft wall.By applying a reasonable voltage at both ends of the inner wall,the electro-thermal effect is used to make up for the gap of cement hydration heat,The research results are as follows:(1)The electro-thermal effect of the similar material of conductive concrete is obvious,which can effectively make up for the shortage of hydration heat of shrinkage model cement.increase the maximum temperature of the inner wall of the well to above 80℃.(2)The temperature rising and falling curve of the inner wall shrinkage model is the same as the inner wall pouring test law in Chapter3,all experiencing rapid temperature rise(1.6℃/h～5.33℃/h),slow temperature rise(1.05℃/h～2.22℃/h),rapid temperature drop(0.1℃/h～1.1℃/h),the stage of slow cooling(0.1℃/h～0.2℃/h).Compared with numerical calculation and simulated pouring,the rate of heating stage is increased,but the overall average heating rate and cooling rate are close.(3)Compared with the simulation results of the inner wall in Chapter 3,the maximum temperature arrival time is delayed by more than 2.7h,and the maximum temperature difference between the inner and outer edges is reduced by 16°C,but it is basically close to the temperature increase rate and the highest temperature,which can meet the similar temperature field.(4)Compared with the inner wall temperature field in Chapter 3,the maximum temperature arrival time is delayed by more than 2.7h,and the maximum temperature difference between the inner and outer edges is reduced by 16℃,but it is basically close to it in terms of heating rate and maximum temperature.It is verified that conductive concrete is used to compensate the inner wall feasibility of shrinkage model hydration heat gap.