Study On Preparation And Performance Of Early Strength Concrete For Power Transmission Project

As one of the most economically and industrially developed regions in China,the demand for electricity in the Yangtze River Delta is always high.In order to ensure the stability of regional electricity consumption,China’s "new infrastructure" clearly proposes to speed up the construction of ultra-high voltage transmission lines.However,due to the complex construction environment of power transmission and transformation project,the construction period is long,the construction amount is large but the construction time is short.Therefore,the concrete of power transmission and transformation project can not reach the expected strength value within the specified time.Winter is the most complex construction environment in the whole construction cycle,it is of great significance to clarify the developing law of concrete macroscopic performance in low-temperature and low-humidity environment to ensure the construction of power transmission and transformation project.the reasonable mixing amount of early strength agent and polycarboxylic acid water reducer was determined through the preliminary experiment.Different curing temperature(-5℃,5℃ and control group 20℃,60%RH),Water to cement(W/C,0.37 and 0.41),low temperature early strength agent(0% and 3%)were then set to determine the workability,compressive strength,volume stability,permeability resistance,pore structure of early strength concrete under low-temperature and low-humidity environment.Additionally,advanced charactering methods including XRD,TG,SEM-EDS and other methods were used to try to analyze the influence mechanism of various factors on concrete properties from the microscopic level.The main research conclusions are as follows:Under the enviroment of-5℃ and 60%RH,adding 3% of early strength agent and 1.2%of polycarboxylic acid water reducer could not only improve the growth of early compressive strength of concrete,but also ensure the continuous growth of late strength of concrete.Under low temperature,the increase in forming temperature,the decrease in W/C and the addition of3% early strength agent would lead to a decrease in slump and expansion of concrete,and the shortening of initial setting time and final setting time.The lower the temperature,the more obvious the influence of the low-temperature and early-strength agent on the workability of concrete.Under the condition of low-temperature forming and curing,the increase in concrete’s forming temperature,the decrease in W/C and the addition of 3% early strength agent could increase the compressive strength and shrinkage rate of concrete at various ages.With the addition of early strength agent,the increase in compressive strength of the specimen decreased with the increase in curing temperature.Under the curing conditions of-5℃,5℃ and 20℃-60%RH,the growth rate of compressive strength of concrete at 7d was 139%,46% and 9%,respectively.The experimental results of concrete transport indicated that the increase in transport time of concrete and difference in transport height led to a rapid decrease in concrete’s slump and deterioration of paste’s cohesion,leading to a slight decrease in the compressive strength of concrete at various ages in power transmission and transformation engineering.The microscopic testing results manifested that the hydration rate of the matrix was slow under the condition of low-temperature and low-humidity curing.The early hydration rate of cement could be accelerated by raising the curing temperature,reducing the W/C and adding low-temperature and early-strength agent.When the age reached 28 days,the specimens under the condition of 5℃ and 60%RH showed the highest degree of hydration and the densest microstructure.In the low-temperature and low-humidity curing environment,the chloride diffusion coefficient decreased gradually with the increase in age,decreased first and then increased with the increase in curing temperature,decreased gradually with the decrease in W/C,and decreased rapidly with additng 3% of early strength agent.When curing in low-temperature and low-humidity environment for 3 days,the increase in curing temperature,the increase in W/C and the addition of early strength agent could fill mesopores and harmful pores in the matrix,reduce the critical pore size and threshold pore size,reduce porosity and optimize pore structure.When curing to 28 days,the critical pore size,threshold pore size and porosity of the net paste decreased and then increased with the increase in curing temperature,slightly decreased with the decrease in W/C,and greatly decreased with adding 3% of low temperature and early strength agent.However,the reduction effect gradually decreasesd with the increase of curing temperature.