Experimental Study On Strength Of Solid Concrete At Long Age

The existing strength of concrete is an important index in the safety review and evaluation of concrete structures built for many years.Surface concrete exposed to the atmosphere for a long time,carbonization will reduce the strength of concrete in different degrees,how to evaluate the strength of these long-age concrete structures is a problem worth studying.In this paper,an experimental study of "rebound valuecarbonization depth--solid concrete strength" of 12 pieces of concrete walls at 14 ages is carried out,in which the strength of solid concrete is taken as the concrete compressive strength of drilled core specimens in the test area.Combined with the test results of"rebound value--carbonization depth--solid concrete strength" at 14,28,60,90 and 740 days and the calculation results of grey system theory,the prediction formula of solid concrete strength at long age is established.The specific research contents are as follows:(1)The"rebound value--carbonization depth--solid concrete strength" of 12 pieces of concrete walls at 14 ages was measured,and the solid concrete strength was taken as the concrete compressive strength of drilled core specimens in the test area.Based on the test results and reference to the previous test data,two regression models,power function and quasi-exponential function,were selected to establish the strength measurement curve of rebound method,which is suitable for 14 days to 14 years and varies from C20 to C80 of concrete strength grade.The established strength measurement curve all meets the error requirements of regional strength measurement curve.(2)Two-sample mean t-test was used to calculate the difference between the strength conversion values and the measured values of ages 14 to 740 days and 14 days and ages 740 days and 14 days respectively.The calculation results showed that the influence of ages could not be taken into account when the strength curve of rebound method was used to predict the strength of long-aged solid concrete.Only the impact of rebound value and carbonization depth on solid concrete strength is considered.(3)The variation law of the converted strength of concrete with different strength measurement curves and the error distribution between the converted strength value and the measured value are analyzed.The results show that compared with the strength measurement curve given in literature and strength measurement curve given in Technical Specification for Testing concrete Compressive Strength by Rebound Method(JGJ/T 23-2011),The error between the intensity conversion value and themeasured value is relatively small and the distribution is relatively balanced.The results show that the greater the carbonation depth is,the greater the influence of strength curve selection on the presumed strength of concrete is.Comparing different age and different design strength grade discipline,literature study and the study of this test to establish strength curve of the average relative error of conversion strength the results show that the strength grade is high,long age,design procedures and the study of literature strength conversion error is bigger,the strength of the test the average relative error is less than the study procedures and literature study,the study It also meets the error requirement of regional strength measurement curve,indicating that the strength measurement curve established by this test has certain regional applicability,and can be used to estimate the strength of long-age solid concrete in Zhengzhou area.(4)Based on improved Euler’s formula calculation method of the grey system,respectively,on the development coefficient and grey function are optimized,and the optimized sequence boundary after 28 days for 12 pieces of wall,60 days and 90 days,740 days and 14 years of solid strength of concrete is simulated calculation,the calculation results coincide well with the test results,verify the validity of the calculation model of gray system.Based on the grey system model,the concrete strength of 12 walls in 20 years,25 years,30 years and 35 years was predicted.In order to facilitate engineering application,based on the test results of 14 days to 14 years and the prediction results of grey system,the prediction formula of strength change with age of solid concrete from 14 days to 35 years is established.