Research On Testing Compressive Strength Of Concrete By The Ultra-small Diameter Concrete Core

As a direct method to test concrete compression strength, core-drill methodis widely used. With the increasingly strict testing requirements in practicalwork, the existing core-drill method can’t fully meet the demands. This thesiswill study the ultra-small core sample smaller than the70mm one allowed to useby now, with only50mm diameter, and find the way to solve practical problems.Now, the main difficult of using50mm small diameter core is due to thesize effect of small core sample and discreteness in statistics.This studyanalysed available literatures at home and aborad.522specimens are made sothat comparisions among strengths from cores with different diameters. The coresample is found different from the test cube of casting forming, which isinfluenced by various factors. Generally it won’t increase with diameterdecreases, and its size effect isn’t as obvious as casting forming cube.The ratioof diameter of the core sample to the largest grain diameter of the coarseaggregate and the position where the coarse aggregate is cut have significantinfluence on the compressive strength. The ratio of diameter of the concrete coresample to the largest grain diameter of the coarse aggregate should be at or morethan2.The influence of size effect presented by the different height-diameterratio is more remarkable to the compressive strength of concrete, so the shortcore of concrete is not suggested to evaluate thin components. The uncertaintyof the testing result of drilled core method comes from systematic, random anddetecting operation.Systematic deviation of concrete intensity by drilled coremethod is relatively small; the deviations in the operating process will reducecore compressive strength value, whose effects to small cores will be muchobvious.Thus, we must conduct strict control. It is not suitable for50mm-diameter ultra-small core samples with containing rebars.The maximumsize of coarse aggregate of pumped concrete and sprayed concrete can becontrolled. Concrete has good homogeneity, which can reduce the discretenessof the sample.It accords with the study condition of small core. The compressive strength values of pumped and sprayed concrete50mmsmall core samples follow normal distribution, and can be carried on analyticstatistics with the knowledge of statistics.The examine of significant hypothesisin statistics shows that there is no obvious difference in the average ofcompression strength of pumped concrete, no matter it is tested by the50mmcore sample or the standard test specimen, but an significant difference whenusing the sprayed concrete, at the meanwhile, there is an obvious relevance inthe compression strength between the sprayed concrete50mm core sample and100mm diameter one. We can consider that the compressive strength values ofthe50mm small concrete-pumping core sample and the standard test sample arealmost the same.As for the sprayed concrete, the conversion ratio of0.9can beadopted. Because the50mm small core is more obviously interfered by variousfactors and has a larger discreteness, under the situation of small samples, it isnot suggested to adopt50mm core to do single rating according to the method ofminimum value.According to the experimental data and engineering condition,the number of samples of50mm ultra-small cores’ batch testing is: when thedesigned grade is less than C50, the number of batch testing is no less than25;when the design grade is greater than C50, the amount of batch inspection ismore than20.