Materials Engineer, Roorkee
The standard method of determining strength of hardened concrete consists of testing concrete cubes in compression. The quality of entire concrete of a structure cannot be fully assessed by testing a few concrete cubes. The results obtained in testing cubes do not always reflect the actual strength of concrete in construction. In a whole day, concreting work cubes are cast in a few batches, the differences (unintentional and intentional) in the composition are not uncommon, their compaction and their hardening conditions always differ more or less from those of the structure. In addition, the number of test cubes is generally so small that they can only be considered as random tests. Some times, in case of failure of cubes, doubtful concrete, cracks, deterioration of concrete, etc. it becomes necessary to assess the quality and strength of concrete of the structure. As far back as early thirties, the necessity was felt to develop instruments by which in-situ strength of concrete may be obtained. Various non-destructive methods of testing concrete have been developed, which include, Firing method, Skramtayev’s method, Polakov’s method, Magnitostroy method, Fizdel ball hammer, Einbeck pendulum hammer, Ball indentation hammer, Rebound hammer, Pull out techniques, Windsor probe, Ultrasonic pulse velocity methods, Radioactive and nuclear methods, Magnetic and electrical methods. In all these methods of tests, due to simplicity, rebound hammer test based on surface hardness becomes most popular in the world for non-destructive testing of in-situ concrete.
A handy non-destructive testing instrument should be cheap, easy to operate and should have reproducibility for, fairly accurate results. In 1948, a Swiss Engineer, Ernst Schmidt developed a test hammer for measuring the hardness of concrete by the rebound principle.
Inspite of its popularity, this testing has not been standardized in any country till 1970 except in Bulgaria. In 1971 the British Standards Institution Standardized this test in recommendation for “Non-Destructive Methods of Test for Concrete” part 4 surface hardness methods (BS 4408 : part 4 : 1971). ASTM issued a tentative standard in 1975 “Tentative Method of Test for Rebound Number of Hardened Concrete” (ASTM C 805 : 75 T), and in 1979 ASTM standard of this test was issued “Test for Rebound Number of Hardened Concrete” (ASTM : C805-1979).
Bureau of Indian Standard did not published any standard for this test upto 1991. In 1992 they published IS: 13311 (Part 2) for this test. IS: 456-2000 specified the Non-destructive tests are used to obtain estimation of the properties of concrete in the structure, the methods adopted include Rebound Hammer. CPWD specifications 77 vo. 1 specified that in case the concrete cubes fails, concrete test hammer may be used to arrive at strength of the concrete Laid. Revised CPWD specifications 2002 page 104 specified that for the purpose of payment (Rebound Hammer) hammering test results only shall be the criteria.
According to A.M. Neville, in the book Properties of Concrete (Fourth Edition) on page 626, the rebound hammer is useful in the assessment of uniformity of concrete with in a structure. The test can also be used to establish whether the rebound number has reached a value known to correspond to the desired strength. This is of the help in deciding when to remove false work or to put the structure into service.
IS: 13311 (part 2): 1992 specified, the rebound hammer method could be used for assessing the likely compressive strength of concrete with the help of suitable co-relations between rebound index and compressive strength.
IRC Special Report – 17 on page 5 specified that rebound hammer test when properly calibrated on site with cubes, can be useful for measuring in structure magnitude and variability of...