Research On Steam Curing Timeliness And Thermal Damage Effect Of Manufactured Sand Concrete In Dry And Cold Regions

With the shortage of river sand resources and increasing environmental protection,the manufactured sand has become the main source of sand for construction.To promote the application of manufactured sand in high-strength concrete and to improve the curing technology of precast beams in dry and cold regions,this paper systematically investigates the performance and microstructural changes of the manufactured sand concrete in the steam curing stage and subsequent curing stage.The evolution mechanism of macroscopic properties of steam cured manufactured sand concrete is analyzed in the aspect of hydration kinetics and microstructure,the reasonable control time of constant temperature stage is proposed,and the calculation model of compressive strength considering the effect of thermal damage is established.Meanwhile,the effect of fly ash and stone powder on the steam cured manufactured sand concrete is analyzed.The manufactured sand concrete and steam curing system are applied to the production of precast beams at the project site,and the stressing performance of precast beams is tested and analyzed.The main research results of the paper are as follows.(1)Based on the analysis of the hydration kinetic process of cement in the steam curing stage,it was found that the nucleation rate(I_B)and growth rate(G)of hydrated calcium silicate(C-S-H)both increased exponentially with the increase of steam curing temperature,with I_Bbeing more sensitive to temperature.Changes in I_B had a greater effect on the length of the main hydration period,the peak heat release rate for hydration and its occurrence time,while the activation energy of I_B was higher than that of G.Therefore,the formation of C-S-H in the steam curing stage was mainly controlled by I_B.(2)By researching the properties and microstructure of manufactured sand concrete at the steam curing stage,it was found that the higher steam curing temperature and longer duration of constant temperature easily caused the growth of C-S-H into a mesh structure with larger pores,the growth of CH(calcium hydroxide)into a lamellar structure with spacing,and the increase of pore space between hydration products,leading to the decrease of compactness in the matrix and interface transition zone.It would also increase the content of pores above100nm in concrete,and coarsen the pore size.Further,it adversely affects the strength development in the later stages of steaming curing.Comprehensive consideration of the hydration process and temperature changes in the steam curing stage,the strength at the end of steam curing,as well as the late strength,the constant temperature duration at 40℃,50℃and60℃should not be less than 15h,9h and 6h.Considering the economic efficiency and heat damage effect of steam curing,the curing temperature of precast beams in dry and cold regions was 50℃and the curing time should be controlled at 18h.(3)Through studying the late performance and microstructure of steam cured manufactured sand concrete,it was found that both the steam curing temperature and duration would adversely affect the late performance and microstructure,with the thermal damage effect of curing temperature being greater.Uneven distribution of hydration products,decreased microstructural compactness and pore structure deterioration under steam curing were the main causes of late strength,dynamic elastic modulus and chloride ion impermeability damage.The increased pore connectivity was the main reason for the greater damage in chloride ion impermeability.The strength calculation model considering the thermal damage effect could accurately estimate the strength of concrete under steam curing.(4)Combined with the dry and cold climatic conditions in the north of China,the effect of the subsequent curing temperature on the late performance and microstructure development of the steam cured manufactured sand concrete was studied.The development of late strength and chloride ion impermeability of steam cured manufactured sand concrete was hindered under subsequent-5°C curing,mainly due to the inhibition of cement hydration reaction and the appearance of a large number of microcracks.The late strength and chloride ion impermeability of steam cured manufactured sand concrete essentially stopped developing under subsequent-15°C curing,and the microstructure deterioration was serious.(5)Analyzing the behavioral characteristics of fly ash and stone powder in steam cured concrete,it was found that adding appropriate amounts of fly ash or stone powder could improve the late performance of concrete and reduce the thermal damage effect.The pozzolanic reaction of fly ash under steam curing was advanced,and the filling effect and pozzolanic effect reduced the negative effect of steam curing on microstructure.The amount of fly ash admixture in concrete should be controlled at 20%.The major role of stone powder was to fill,the filler effect of stone powder in low strength grade concrete was better.The content of stone powder in C40,C50 and C60 should be controlled at 10%～15%,5%～10%and 5%or less respectively.(6)Relying on actual projects in dry and cold regions,equal proportions of precast beams with manufactured sand concrete were made and cured using the steam curing system proposed in the previous study.The concrete strength of the precast beams at the end of the steam curing satisfied the construction and design requirements.The deflection and concrete deformation of the manufactured sand concrete precast beam under steam curing were less than the river sand concrete precast beam under the same conditions,and the static load test results satisfied the standard requirements.