Study On The Recycled Concrete Strength By Ultrasonic Rebound Testing

The failure to effectively deal with construction waste has had a very bad impact on the environment.Based on the concept of environmental protection and sustainable development,the application of recycled concrete technology in practical engineering is becoming more and more popular.However,the mechanical properties and failure mechanism of recycled aggregate concrete are different from those of natural concrete,and there are many internal defects.In view of the above problems,this paper explores the correlation between compressive strength and non-destructive testing of recycled aggregate concrete,and chooses waste concrete blocks as recycled aggregate for research.The strength test and non-destructive testing were carried out on the C30 strength grade recycled concrete test block with 792 pieces of recycled coarse aggregate replacement rate of 30% and 50%,respectively,and fly ash content of 0%,10%,20% and 30% respectively.The effects of fly ash replacement rate and recycled coarse aggregate replacement rate on the strength of recycled concrete were analyzed.Fly ash can properly improve the later strength of recycled concrete.When the amount of fly ash replacing cement is 10%,the compressive strength value of recycled concrete is higher than that of the other three fly ash admixtures.The increase of supersonic velocity with age is smaller,but the increase of compressive strength and rebound value with age is larger.Comparing with ordinary concrete strength measurement formulas,there is a big difference.Using national strength measurement curve to estimate the compressive strength of recycled concrete has a big error,so it is not suitable for the strength estimation of recycled concrete.The strength regression curve of recycled aggregate concrete by ultrasonic rebound comprehensive method is established through the comprehensive test of two kinds of recycled aggregate replacement rate recycled concrete.The effects of the replacement ratio of recycled coarse aggregate and the amount of fly ash on the "rebound value-compressive strength value" and "ultrasonic sound velocity value-compressive strength value" were analyzed,and the corresponding strength curves were established.And the ultrasonic rebound comprehensive method to detect the intensity curve,the three detection intensity curves were compared and analyzed,and the error analysis was carried out.In order to verify the accuracy of the strength curve obtained by the ultrasonic rebound comprehensive method,the 72-recycled concrete test block with the same condition of 30% and 50% of the recycled aggregate was used to verify the obtained regression curve.Research conclusions: The strength measurement formula of the two-factor mathematical model of compressive strength-rebound value-sound velocity value of recycled concrete obtained in this paper has high fitting accuracy and good correlation.The strength measurement curve obtained by ultrasonic rebound is verified,and the results show that the average strength of recycled aggregate concrete calculated by using the strength measurement formula is good.Relative errors and relative standard deviations are within the allowable range of the regulations.The research results lay a good foundation for drawing up the strength measurement curve of recycled concrete,and provide basic technical data for exploring and studying the practical engineering of recycled concrete with fly ash.