| | | (drilling) cylinders with a diameter of 100, 150, 200 and 300 mm and double their respective diameter. Test load and test load distribution The large specimens require large test loads, usually a test load from 600 to 6,000 kN. The test load distribution over the entire (large) compression surface of the specimen is a substantial aspect for the test results, i.e. whether test results with a limited range of variation correspond to the actual strength of concrete or if considerably lower values with a large range of variation are output. Due to the high rigidity and the low deformability even slight unevenness (roughs, grooves, impurities etc.) of the compression surfaces, the bending of the compression platens or unsymmetrical deformations of the machine frame (angular position of the compression platens) lead | | |
| | | to locally different compressive stresses and to a premature break when having reached the peak of load application. The reduction in force during the first crack leads to an immediate elastic resilience of the load frame and accelerates the break process. Different load application speeds or speeds changing under load application will also influence the test results. Such influences lead to uncertain test results with a range of variation being more than 20 % below the actual strength. The actual strength of concrete is not reached. These influences can be limited decisively by taking the following measures: a) The load frames of the testing machines manufacturer must have high longitudinal and transverse rigidities so that unsymmetrical elastic deformations are negligible even if the specimen is | | not inserted exactly centrically for example. b) The compression platens must be extremely rigid, the surfaces must be hardened (3 55 HRC) and polished (average peak-to-valley height £ 0,0016 mm, flatness deviation £ 0.03 mm over 250 mm). c) The upper compression platen must be positioned in a way that it sits allover close to the specimen and without lateral offset (center of rotation on a level with the compression surface) when approaching the specimen, but with the test load increasing the angular position must not change any further. d) Due to the very small deformations of the specimens, the speed of load application is subject to a force-dependent control and must therefore be reproducible. | | |