Regression Curves Of Compressive Strenth Of Concrete Tested By Rebound And Ultrasonic-Rebound Method For Highway Engineering In Yunnan

The rebound method and the ultrasonic rebound comprehensive method are two commonly used methods for on-site inspection of concrete strength of highway structures.The reference standardsare "Technical Specification for Inspection of Concrete Compressive Strength by Rebounded Method" JGJ/T 23-2011,"Technical specification for detecting strength of concrete by ultrasonic-rebound combined method"CECS 02:2005 and "Technical specification for strength testing of high strength concrete" JGJ/T 294-2013.The national unified strength curve used for estimating the compressive strength of concrete is given in these three standards.However,China has a vast territory,where different regions vary greatly in climate and concrete raw materials.different climates in different regions,and concrete raw materials vary widely.The accuracy and reliability of the unified strength curve are most likely to be affected when applied in different regions.large number of experimental results indicated that the unified strength curve showed large error when applied in many highway projects in Yunnan Province.In order to further improve theaccuracy and reliability of the detection results,to better control the quality of engineering projects and to guide engineering production,it is necessary to establish a local non-destructive strength curve of highway engineering in Yunnan Province.According to the typical climate zones of Yunnan Province,this study selected four highways under construction in each climate zone as representatives.For each highway project,theraw materials for concrete structures with strength grades from C30 to C55 were selected on site for this study The concrete cubic samples are then prepared at the mix ratio used in the project,according to the requirements of the current industry standard on the sample preparation of the local strength curve test,.The samples are cured under natural maintenance condition in the filed lab of each highway.After reaching the curing age,the concrete cubic samples of each strength grade are tested for the rebound value,ultrasonic value,compressive strength value.carbonization depth value,etc.The rebound method uses a normal rebound hammer with a nominal energy of 2.207J and a high-strength rebound hammer with a nominal energy of 4.5J.Finally,regression analysis is carried out on the data collected from the above tests.The fitting results are evaluated on the average relative error,relative standard deviation and correlation coefficient corresponding to the regression equation proposed in the current standards and specifications.The final research results show that:1)the rebound value of concrete gradually increases with the curing time,but decreases as the gas content increases.The rebound value also increases as the rebound position changes from the surface layer to the middle layer and inner layer.Among three influencing factors of rebound value,the gas content(factor A)has the most significant effect,followed by the curing time(factor B).The rebound position(factor C)has the least effect on the rebound value.2)after comparative research,it is found in the study.The compressive strength is in the range of 25 MPa-70 MPa.Within the compressive strength range between 25 MPa-70 MPa,the error of the strength curve obtained by the ordinary rebound hammer and the high-strength rebound hammer is lower than the national unified curve.the error difference between the curves obtained by the two rebound hammers is not significant.3)The the concrete strength curve equation by rebound method of Yunnan Province is fccu=0.5956Rm1.1903×10-0.0065dm,in which correlation coefficient r=0.8160,average relative error 5=7.6254%(National unified curve 5=17.6383%),relative standard deviation er=10.0494%(national unified curve ei=20.3994%).When compared with the national unified curve,The average relative error is reduced by 56.77%,and the relative standard deviation is reduced by 50.74%;4)The the concrete strength curve equation by ultrasonic rebound comprehensive method of Yunnan Province is fcuc=0.2321v1.3753Rm0.8478,in which the correlation coefficient r=0.8410,the relative error er=9.8319%(national unified curve er=19.8421%).the relative error is reduced by 50.45%when compared with the national unified curve.All of the curves fitted in this study meet the requirements on the accuracy of regional curves as specified in "Technical Regulations for Testing Concrete Compressive Strength by Rebound Method" JGJ/T23-2011,"Technical Regulations for Testing Concrete Compressive Strength by Ultrasonic Rebound Method" CECS02-2005 and "High Strength Concrete Strength Testing" Technical Regulations JGJ/T 294-2013.Moreover,the accuracy of those fitted curves is higher than that of the national unified strength curve.Therefore,they are applicable to the non-destructive testing and assessment of concrete strength of highway engineering in Yunnan Province.These results of this study are expected to provide more accurate reference for the concrete production control and on-site quality control in and the highway engineering of Yunnan Province.