| Concrete has quickly become the most widely used building material in the construction field due to its abundant raw materials,easy availability,low price,high compressive strength,good plasticity and durability.People put forward higher requirements for the construction level,and also put forward higher requirements for the strength.As the most important mechanical property of concrete,strength plays a key role in the quality of concrete structure.The water-cement ratio has an important influence on the strength of concrete.If we can know the two important indicators of concrete’s water-cement ratio and strength to evaluate the structural quality,we can accurately judge the quality of concrete in time.This is conducive to early detection of problems and timely adjustment and remediation of the problems that have occurred,which is of great significance to the development of concrete engineering.This thesis takes concrete as the research object,and uses the dielectric properties to determine the water-cement ratio of fresh concrete and predict the compressive strength of concrete for the research purpose.The following parts are carried out:(1)Four strength grades of concrete were prepared,and the cement paste(without coarse aggregate and fine aggregate)and mortar(without coarse aggregate)in the same proportion as the concrete were prepared.The relative complex permittivity of them was measured by the open coaxial probe method 30 minutes after stirring.(2)Study the changes of the real and imaginary parts of the relative complex permittivity of fresh concrete,cement paste and mortar within the measurement frequency range,as well as the real and imaginary parts of the relative complex permittivity of cement paste and mortar In the measurement frequency range,the relative complex permittivity of concrete,mortar and cement paste is compared with the Cole-Cole model,and the influence of coarse aggregate and fine aggregate on the relative complex permittivity of fresh concrete is analyzed.Establish the relationship model between the real and imaginary parts of the relative complex permittivity to determine the water-cement ratio of fresh concrete.After comparison,it is found that the effect of the water-cement ratio measured by the real part of the relative complex permittivity at different frequency points is roughly the same.The effect of determining the water-cement ratio by the imaginary part of the relative complex permittivity at different frequency points is not the same.The water-cement ratio determined by the real part is more accurate and more suitable for determining the water-cement ratio of fresh concrete.(3)Four strength grades of concrete were prepared,and the compressive strength tests were carried out on the four strength grades of concrete at 3d,7d,14 d and 28 d of demoulding.The relationship between the compressive strength and age of the four types of concrete has been established.The relative complex permittivity of concrete was measured at 30 min,24h,48 h,72h and 96 h of fresh concrete,and the real part and imaginary part of the relative complex permittivity of concrete changed with age.It is found that as the age increases,the real and imaginary parts of the relative complex permittivity of concrete continue to decrease,and the relationship between the real part and age,and the relationship between the imaginary part and age of the relative complex permittivity are established respectively.(4)The Debye model is used to fit the measured data,and it is found that the change law of the dielectric constant of the early-age concrete with frequency is suitable for the Debye model,but only within 6 GHz is suitable for the Debye model.With age as an intermediate variable,the real part,imaginary part and compressive strength of the relative complex permittivity of concrete are respectively connected,and finally a unified dielectric constant characteristic prediction concrete compressive strength model is established. |
