Study On Mechanical Properties Of Concrete Under Different Water Pressure Conditions

Concrete structures are often subjected to different water pressure conditions.One kind of service condition for concrete structures is relatively low water pressure,such as with spray splash,rainfall,and low water head soaking,which can be regarded as non-pressure water environment;the other kind of service condition for concrete structures is relatively high water pressure,such as in some parts of high dams(e.g.dam heel),which belongs to high pressure water environment.Concrete under different water pressure conditions is subjected to the combined actions of humidity and pore water pressure.The mechanical characteristics of concrete under different water pressure conditions exhibit significant differences.In this paper,based on the physical testing and theories of micromechanics,the variations in the mechanical characteristics of concrete under no-pressure and high-pressure water conditions were studied systematically.This study is expected to provide a basis for the study of hydraulic fracture mechanism of concrete and safety evaluation of structures such as high dams.The main contents and results of this paper include the following aspects:(1)Heating-ventilation drying method was used and the drying temperatures were set as 60℃,85℃,105℃,120℃ and 150 ℃.Two temperature modes were considered,namely,continuous and intermittent cyclic drying.Hence,the drying and dewatering characteristics of concrete was obtained and the tensile and compressive strengths of concrete were measured.After continuous and intermittent cyclic drying,the compressive strength of concrete decreased first and then increased with increase in temperature,with temperature of 105 ℃ being the turning point.Splitting tensile strength of concrete shows a trend of decrease in an approximately linear manner with increase in the drying temperature.After drying at 105 ℃ for 115.5 h,concrete reached to a completely dry state,which is the drying process for obtaining approximately no strength loss with moderate efficiency.Furthermore,an optimum drying technology without damaging hydrate structure and approximately with no strength loss was finally established based on the synchronous thermogravimetry(TG)and differential thermal analysis(DTA),scanning electron microscope(SEM)and industrial CT scanning,as well as based on the investigation of hydrate morphological characteristics,the pore structure characteristics of concrete after the optimum drying process,and the dewatering forms.These studies make up for the deficiency of controlling the concrete drying in the existing testing regulations for hydraulic concrete.(2)Based on the established optimum drying technology and the free water absorption method,the variation in humidity of the concrete with different strength grades,maximum aggregate sizes and specimen sizes were studied.In addition,based on the pore structure measured through mercury intrusion method and the adsorption theory of porous materials,this study established concrete humidity prediction models under different relative humidity conditions.Based on the variation in the humidity of concrete,concrete with three strength grades were made to reach six states of humidity with rational humidity gradients.Besides,the compressive strength,splitting tensile strength and fracture toughness under different humidity conditions were measured,and the effect of humidity on the mechanical characteristics of concrete was studied systematically.The results show that both the tensile and compressive strengths and fracture toughness of the concrete decreased with an increase in humidity,and the mechanical characteristics of low-strength concrete were relatively more sensitive to the humidity.The present study expands the present research scopes for mechanical characteristics of concrete with only two humidity conditions including dry and saturated.(3)The compressive strength and splitting tensile strength of concrete under different humidity conditions were measured,with the uniaxial compression loading rates being 0.05 MPa/s,0.10 MPa/s,0.30 MPa/s,1.00 MPa/s and 3.00 MPa/s,splitting tensile strength loading rates being 0.01 MPa/s,0.03 MPa/s,0.05 MPa/s,0.20 MPa/s and 0.50 MPa/s.The variation in strength under different humidity conditions and loading rates were obtained,and the reciprocal influences of the humidity effect and rate effect in the dynamic strength of the concrete were studied.The dynamic strength of concrete in the hygrometric state was determined by the leading role of humidity damage effect and rate effect.Under the relatively low loading rate,the gain in strength due to humidity damage effect exceeded that from rate effect.(4)Based on the theory of micromechanics and equivalent inclusion theory,concrete was regarded as four-phase composite material composed of aggregate,mortar,interfaces and pores.The proportion of active pores filled with water was used to represent the humidity of concrete,and the multi-step equivalent method was used to establish a model for predicting the elastic modulus of the wet concrete.In this model,the direct influence of grading of aggregates on the elastic modulus of concrete was considered,and the mortar humidity and interface humidity were distinguished.The elastic modulus of the concrete increased with an increase in humidity,and elastic modulus of concrete with high strength grade was more sensitive to the humidity.Through a contrastive analysis of the model predicted values and the trial values,the accuracy of the prediction model was verified.(5)Based on a custom-made constant hydraulic loading system,C15,C20 and C30 concrete under dry states were subjected under high pore water pressure conditions of 1 MPa,2 MPa and 3 MPa,respectively,with continued compression for 3h,10 h,24h,72 h and 240 h.Thereafter,the humidity,compressive strength and splitting tensile strength of the specimens after decompression were measured.The variation in the mechanical characteristics and interior humidity of the concrete after the action of different high pore water pressure conditions were thus obtained.Based on mercury intrusion method,the changes in the interior pore structure before and after compressive loading of concrete were measured,which provided a basis for explaining the mechanism of strength degradation under high pore water pressure.The humidity damage and hydraulic fracturing damage were decomposed based on the variation in strength of concrete under different humidity conditions.The independent damage effect of hydraulic fracture was obtained,which laid a foundation for the study of the failure mechanism of concrete under the action of high pore water pressure.