Non-destructive Testing Of Concrete By Rebound Hammer

By
Kaushal Kishore
Materials Engineer, Roorkee

INTRODUCTION
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.

REBOUND HAMMER

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 strength. It is most commonly used due to its simplicity and low cost.

OBJECT
The rebound hammer method could be used for (IS: 13311 Part 2-1992):
a) assessing the likely compressive strength of concrete with the help of suitable co-relations between rebound index and compressive strength.
b) assessing the uniformity of concrete.
c) assessing the quality of the concrete in relation to standard requirements, and
d) assessing the quality of one element of concrete in relation to another.
Note: The rebound hammer method can be used with greater confidence for differentiating between the questionable and acceptable parts of a structure or for relative comparison between two different structures.

DESCRIPTION
The hammer consists of a spring controlled mass that slides on a plunger within a tubular housing. When the plunger is pressed against, the surface of concrete, it retracts against the force of the spring. When completely retracted the spring is automatically released. On the spring controlled mass rebound, it takes the rider with it along the guide scale. By pushing a button, the rider can be held in position to allow readings to be taken.

CALIBRATION
Each hammer is furnished with a calibration chart supplied by the manufacturer. This calibration chart can be used only when material and testing conditions are similar to those in effect when the calibration of the instrument was carried out. Each hammer varies considerably in performance and needs calibration for use on concrete made with aggregates produced from a specific source. A practical procedure for calibration of the hammer for use on a job in progress is outlined below:

• Prepare a number of cubes covering the strength to be encountered on the job. Use the same cement and aggregates as are to be used on the job. The cubes should be preferably as large a mass as possible in order to minimize the size effect on the test results of a full scale structure. 150 mm cube specimens are preferred. The cube size must be increased with the increase of hammer impact energy. For hammer impact energy of 0.225 kgm, 150 mm cubes size will be quite sufficient, but for hammer of 3 kgm impact energy the cube size shall not be less than 300 mm.

• The cubes shall be cast and cured as laid down in IS: 516:1959.

• After the curing period the cubes should be removed from wet storage to the laboratory atmosphere for about 24 hours before testing. It may be noted that the strength of wet-tested cubes will be normally 10% lower than that of dry tested cubes.

• After cleaning the faces of the cubes they should be gripped in the compression testing machine under a load of 7 N/mm2 (15.75 Tonnes for 150 mm cubes), when the impact energy of the hammer is about 2.2 Nm. The load should be increased for calibration rebound hammer of greater impact energy and decreased for caliberating rebound hammer of lesser impact energy.

• Atleast nine hammer readings should be taken on each of the two vertical faces accessible in the compression testing machine. The points of impact on the specimen should not be nearer on edge than 20 mm and should be not less than 20 mm from each other. The same points must not be impacted more than once.

• Immediately after taking the hammer readings, the cube should be tested to its ultimate load.

• Repeat this procedure for all cubes.

• After discarding the extreme values, average the reading of all the individual cubes and call this the rebound number.

• The values of rebound numbers and cube compressive strength should be plotted by fitting a curve or line by method of least squares.
The accuracy of the hammer reproducibility should be ascertained from time to time using a standard anvil, particularly before the testing of structure.

Calibration is an important stage in the use of every apparatus. The errors of the apparatus and the accuracy in determining the strength of concrete by non-destructive methods depend on proper calibration. Calibration should, therefore, be carried out with great care and on a larger number of specimens. It is interesting to note that 700 to 1000 tests are needed to plot calibration curves for rebound hammer.

Fig. 1 gives calibration curve of Test Hammer. The cubes were cast with OPC 43 grade river sand of Zone II and 20 mm graded crushed aggregate. The cubes were wet cured for 28 days, and then tested in SSD and room dry condition (dried for 24 hours prior to testing at room temperature).

TESTING OF A STRUCTURE
A concrete test hammer of impact energy of 2.207 N.m (0.225 kgm) is quite suitable for testing concrete in ordinary building and bridge construction. The procedure for testing a concrete structure is given below :

• All members and points of a concrete structure selected for testing should be marked for identification, they should also be in dry condition.

• Testing should be conducted on surfaces that are smooth and uniform, preferably surfaces created by casting against a form. Avoid rough spots, hony-comb or porous areas. Free or trowelled surface may also be satisfactory if appropriate corrections are applied or a special calibration is prepared. If loosely adhering scale, plaster work or coating is present, this should be rubbed off with a grinding wheel or stone.

• For concrete section less than 100 mm thick, the rebound of the hammer will be affected by the elastic deformation of the section, and it should be backed up by a heavy mass placed on the back side.

• At each of selected points, made smooth and clean, take six rebound readings. For each reading shift the hammer 25 mm and take care not to rebound the same spot twice. The point of impact should be at least 20 mm away from any edge or sharp discontinuity. Small air pockets near the surface under the point of impact cause low rebound, on the other hand, immediately over a hard aggregate the impact will result in a high rebound.

Note: Manufacturer of Schmidt hammer recommend at selected points 5 or better 10 impact reading. BS: 4408 ; Part 4 : 1971 specified at least 9 valid reading and not more than 25. It is normally better to confine the readings of a test (9 to 25 readings) to an area not exceeding about 300 mm x 300 mm rather than to carry out random testing extending over the whole structure or unit. Revised CPWD specifications 2002 specified the result should be the average of at least 12 readings. IS: 13311 (Part 2)-1992 specified around each point 6 readings. ASTM-C805 required 10 readings to be taken.

• The usual directions of test are either horizontal or vertically down, but any direction of test can be used a long as it is consistent. Calibration or corrections for a given direction of test are supplied with the hammer or can be derived.

• The rebound values usually are considered reliable when at least six readings deviate not more than +2.5 to 3.5 on the impact scale. The compressive strength is then determined by taking average of rebound reading.

• Compressive strength of the concrete can be determined from the relationship between the rebound number and the strength given by the curve. For reliable results the calibration curve shall be drived from the given set of materials and conditions. If cubes are available from the structure to be tested, the hammer should be checked first on Anvil then upon these cubes, if need be the hammer should be adjusted accordingly and re-checked for satisfactory performance. If it is found that hammer performance is doubtful, the hammer should be changed.

Factors Influencing the Results
Type of cement
Concrete made of high alumina cement can given strengths upto 100% higher, whereas supersulphated cement concrete can give 50% lower strength compared to a calibration obtained on Portland cement cubes. It is necessary to recalibrate the hammer for different types of cement.
Type of aggregate

Gravel and most crushed rocks give similar correlations, but lightweight aggregates and aggregates with unusual properties require special calibration.
Surface and internal moisture condition of the concrete

This method of testing is applied only on close textured concrete. Open texture concrete typical of masonary blocks, `honeycombed’ concrete, or no fines concrete cannot be tested using this method.

Trowelled and floated surfaces as in floors, are harder than moulded surfaces and in most cases will tend to overestimate the strength.

A wet surface will give rise to under-estimated of the strength of concrete calibrated under dry conditions. This influence can be considerable and in structural concrete it is about 10% lower on wet surfaces than on an equivalent dry surface.

Age of concrete
In very old and dry concrete the surface will be harder than the interior, giving rebound values some what higher than normal. New concrete with moist surface generally has a relatively softer surface, resulting in lower than normal rebound.

Carbonation of concrete surface
Surface carbonation of concrete significantly affect the rebound hammer test results. In old concrete where the carbonation layer can be upto 20 mm thick, the strength may be overestimated by 50%.
Limits

Testing concrete by test hammer has its own limitations. If all factors are taken into consideration the strength of concrete in a structure may be determined within an accuracy of +15%. The concrete test hammer is an excellent tool in the hands of experts. The operation of the hammer is very simple, yet it is not so simple as to entrust this tool to a raw hand for taking readings of a structure. Its operation, calibration, taking readings of a concrete structure, analysis and interpretation of the test data must always be carried out by specialists trained for this purpose.

REFERENCES
1. IS: 383-1970, Specification for coarse and fine aggregates from natural sources for concrete (second revision).
2. IS: 456-2000 Plain and reinforced concrete Code of Practice (Fourth revision).
3. IS: 516-1959 Method of test for strength of concrete.
4. IS: 8112-1989, Specification for 43 grade ordinary Portland Cement (first revision).
5. IS: 13311 (Pat 2)- 1992, Methods of non-destructive testing of concrete Rebound Hammer.
6. Revised CPWD Specifications, 2002 for Cement Mortar, Cement Concrete and RCC Works.
7. BS 1881: Part 202-1986, Recommendations for surface hardness testing by rebound hammer.
8. ASTM C805-85, Test for Rebound Number of Hardened Concrete.
9. Malhotra, V.M. – Testing hardened concrete, Non-destructive methos, Monograph 9, American, Concrete Institute, Detroit, 1976.
10. Zolness N.G. – Calibration and use of Impact Test Hammer ACI Journal, Proceeding V. 54, No. 2, Aug. 1957, pp. 161-165.
11. Akashi, T. and Amasaki, S., Study of the stress waves in the plunger of a rebound hammer at the time of impact, Spec. publ. SP82-2, American Concrete Institute Detroit, 1984, pp. 17-34.
12. Kolek, J., Non-destructive testing of concrete by hardness methods. In Non-destructive Testing of Concrete Timber, Institution of Civil Engineers, London, 1970, pp. 19-22.
13. Neville, A.M., Properties of Concrete (Fourth Edition) – 1996, pp. 625-626.
14. IRC Special Report 17, 1996. State of the Art: Non-Destructive Testing Techniques of Concrete Bridges.
15. Kishore Kaushal, Testing hardened concrete by surface hardness. Indian Concrete Institute Bulletin No. 20, Spet. 1987, pp. 17-20.

We at engineeringcivil.com are thankful to Sir Kaushal Kishore for submitting his research paper on “Non-destructive Testing Of Concrete By Rebound Hammer” to us. This will be of great help to all civil engineers seeking information on Rebound Hammer test.

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