Research On Detecting Strength Of Pumping Concreteby Ultrasonic-Rebound Combined Method

As a convenient and reliable non-destructive testing method, ultrasonic-rebound combined method is widely used in inspection of engineering quality, it uses multiple indicators to calculate strength of concrete and has a higher precision than rebound method. Nowadays pumping concrete used in the construction is mostly mixed with fly ash, while the national unified curve given by Technical Specification for Detecting Strength of Concrete by Ultrasonic-rebound Method(CECS02:2005) is particularly for neither specific province nor pumping concrete, that leads to a great error when we use unified curve to test the pumping concrete in particular regions, therefore it is necessary to do some research on inspection of pumping concrete in particular regions by ultrasonic-rebound combined method.This paper aims at establishing the strength curve of pumping concrete in Lanzhou by ultrasonic-rebound combined method, local materials and commonly mixtures are chosen to make different strength grade of concrete standard specimens, which is used for rebound test, ultrasonic velocity test and compressive strength test in different age, so we can analyze the development law of test data and its influencing factors by comparison, and finally establish optimal regression curve by error analysis and proper amendment.Test data shows that compressive strength, rebound value and velocity increase with age or strength grade growing, the development age generally extends to180days, during this period, compressive strength all meet the design requirement, while increase degree of rebound value and velocity is lower than expected when strength grade grows. Through check experiment, we found significant reduce in compressive strength and increase in rebound value when the content of fly-ash achieve a certain percentage; and by contrast with ordinary concrete under same compressive strength, we found that less coarse, larger sand rate and thicker coating layer can reduce rebound value and velocity of pumping concrete, these two factors make the test data of low-strength pumping concrete close to ordinary concrete, while rebound value and velocity of high-strength concrete is relatively lower and insensitive to the compressive strength. Univariate regression analysis shows correlation between compressive and rebound value is good enough to get strength curve met the accuracy requirements, where proper amendment is needed; it also shows velocity affected by different mixtures and moisture content distributes in a smaller interval and fluctuates acutely, so the accuracy and error can’t meet the requirement of regulation unavoidably. In multiple regression analysis, rebound value play a greater role in evaluating the strength of pumping concrete, the velocity still transfers information about the internal structure of concrete and contribute to improving the correlation and reducing errors. This paper suggest proper amendment of rebound value and ultrasonic velocity of high-strength concrete to get a strength curve whose accuracy and error can meet the requirements, and to provide a scientific method and basis for identifying or strengthening strength of concrete structure.