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[P.K.Mallick]

A Comprehensive review of Rebound Hammer Test.
Rebound Hammer test is a surface hardness method for Non-destructive Testing of Concrete. In 1948, a Swiss Engineer, Ernst Schmidt, developed a test hammer for measuring the hardness of Concrete by the rebound principle. The Schmidt rebound hammer is principally a surface hardness tester with little apparent theoretical relationship between the strength of Concrete and the rebound number of the hammer. However, within limits, empirical correlation has been established between strength properties and rebound hammer number.

METHOD OF TESTING: A steel hammer impacts, with a predetermined amount of energy, a metal plunger in contract with a concrete surface. Either the distance that the hammer rebounds is measured or the hammer speeds before and after impact are measured. The test result is reported as a dimensionless rebound number. Rebound numbers produced based on two measurements principles are not comparable. The surface hardness and therefore the rebound is taken related to the compressive strength of the concrete. The predetermined amount of energy used for impact of steel hammer is called “IMPACT ENERGY FOR REBOUND HAMMER”. IS 516(Part-4/Sec-4) 2020(Hardened Concrete-Methods of Test stipulates Impact Energy for Rebound Hammer for different application and these are as follows:
a)     For testing normal weight concrete, approximate Impact Energy required for the Rebound Hammer is 2.25 Nm.
b)     For light-weight concrete or small and impact sensitive part of concrete, approximate Impact Energy required for the Rebound Hammer is 0.75 Nm.
c)     For testing mass concrete, for example, in roads, airfield pavements and hydraulic structures, approximate Impact Energy required for the Rebound Hammer is 30.00 Nm.
USE: The Rebound Hammer Test method may be used:
a)     To assess the likely compressive strength of the concrete with the help of suitable correlation between rebound number and compressive strength.
b)     To assess the uniformity of the concrete quality.
c)     To assess the quality of the concrete in relation to standard requirement.
d)     To assess the quality of one element of concrete in relation to another.
WHEN, WHERE AND HOW TO USE:
a)     This test can be used to estimate the concrete strength during the construction so that operations that require a specific strength can be performed safely or curing procedures can be terminated. To estimate the concrete strength during the construction, it is required to establish correlation between compressive strength of concrete and rebound number (Using Cube compressive strength). To establish a correlation between compressive strength of concrete and its rebound hammer number, it is required to measure both the properties simultaneously on concrete cubes. At least three cubes each for three different concrete grades shall be cast and tested for establishing the correlation. Cube specimens should be wet cured for 27days and they should be removed from wet storage and kept in the laboratory atmosphere for about 24 hours before testing, The concrete cube specimens are held in a compression testing machine under a fixed load, measurements of rebound number taken using the particular hammers for which conditions are to be established and then compressive strength is determined by following usual procedures. The fixed load required is of the order of 7N/mm2 when the Impact Energy of the hammer is about 2.25 Nm. The load should be increased for calibrating rebound hammer of greater Impact Energy and decreased for calibrating rebound hammers of lesser Impact Energy. The test specimens should be as large a mass as possible in order to minimize the size effect on the test result of a full scale structures. 150 mm cubes are preferred for calibrating rebound hammer of lower impact energy(2.25Nm), whereas rebound hammer of higher Impact Energy, for example 30Nm, the test cubes should not be smaller than 300mm. Only the vertical faces of the cube as cast should be tested. At least nine readings should be taken on each of the two vertical faces accessible in the compression testing machine when using the rebound hammer. The points of impact on the specimen must not be nearer an edge than 25mm and should be not less than 25mm from each other. The same point must not be impacted more than once.
b)     The Rebound Hammer Test can also be used to estimate concrete strength during evaluation of existing structure. To estimate the strength of concrete during evaluation of existing structure, a correlation between equivalent cube compressive strength of concrete cores and rebound number (using core compressive strength) is required to be established. At least two replicate cores shall be taken from at least six locations with different rebound numbers. The test conditions and surface conditions of locations where strengths are to be established using developed correlation shall be similar to the locations used for development of correlation. The locations where these tests are conducted and cores are taken should have ultrasonic pulse value greater or equal to 3.5 Km/s for grades  less than equal to M25 and 3.75 Km/s for grades above M25.
USE OF RELATIONSHIPS BETWEEN REBOUND NUMBER AND CONCRETE STRENGTH THAT ARE PROVIDED BY THE INSTRUMENT MANUFACTURER: As per IS 516 (Part-5/Sec-4):2020, Rebound Hammer Test, the correlation between rebound number and concrete strength that are provided by Instrument Manufacturers shall be used only to provide indications of relative concrete strength at different locations in a structure. To use this test method to estimate strength, it is necessary to establish a correlation between rebound number and strength for a particular concrete and particular instrument. It further states that for readings to be compared the direction of impact must be the same or established correlation factors shall be applied to the readings. In absence of data, manufacturer correlation for direction effect can be adopted. ASTM C805/C805M-18(Standard Test Method for Rebound Number of Hardened Concrete) states that relationship between rebound number and concrete strength that are provided by Instrument Manufacturer shall be used only to provide indications of relative concrete strength at different locations in a structure. To use this test method to estimate strength, it is necessary to establish a relationship between strength and rebound number for a given concrete and given apparatus.
USE OF PREDETERMINED CURVE: Predetermined curve prepared for similar concrete in the same region may be used for approximate estimation of strength of concrete used in the structural members tested for cases where correlation cannot be developed either by cube compressive strength or in-situ core strengths.
USE OF DIFFERENT INSTRUMENTS: Different instruments of same type may give rebound numbers differing from 1 to 3 units. Therefore, tests should be made with same instrument in order to compare results. If more than one instrument is to be used, perform comparative tests on a range of typical concrete surfaces so as to determine the magnitude of differences to be excepted in the readings of different instruments.
STEPS TO BE FOLLOWED: Rebound Hammer Test shall be conducted around all the points of observation on all accessible faces of the structural element. Concrete shall be thoroughly cleaned before taking any measurement. Around each point of observation, six readings of rebound indices are taken and average of these readings after deleting outliers becomes the rebound index for the point of observation. Further, the point of impact should be at least 25mm away from the edge or shape discontinuity.
FACTORS INFLUENCING TEST RESULTS OF REBOUND HAMMER TESTS:
a)     Type of Cement.
b)     Type of aggregate.
c)     Surface condition and moisture content.
d)     Curing and age of concrete.
e)     Carbonation of Concrete Surface,
f)     Vertical distance from the bottom of concrete placement.
g)     The influence of surface conditions used in development of correlation between compressive strength and rebound number is important. The direct  correlation between rebound numbers and strength of wet cured and wet tested cubes is not recommended. It is necessary to establish a correlation between the strength of wet tested cubes and strength of dry tested cubes on which rebound readings are taken.
INTERPRETATION OF RESULTS: The rebound hammer method provides a convenient and rapid indication of the compressive strength of concrete by means of establishing a suitable correlation between the rebound number and compressive strength of concrete. However, the rebound numbers are indicative of compressive strength of concrete to a limited depth from the surface. If the concrete has internal micro-cracking, flaws or heterogeneity across the cross section, the rebound hammer number will not indicate the same. It is stated that probable accuracy of predication of concrete strength in a structure can be up to ±25 percent depending on correlation curve and methodology adopted for establishing correlation between rebound number and likely compressive strength. Because of various limitations in rebound hammer test, the combined use of Ultrasonic Pulse Velocity Test and Rebound Hammer Test is a must for proper interpretation. The most important stipulation of  IS 516 (Part-5/Sec-4):2020, Rebound Hammer Test conveys that if the quality of concrete assessed by Ultrasonic Pulse Velocity Test method is 3.5 Km/s for grades ≤ M-25 and 3.75 Km/s for above M-25, only then the in-situ compressive strength assessed from the Rebound Hammer Test is valid. In cases the quality of concrete assessed by Ultrasonic Pulse Velocity is doubtIn ful, no assessment of concrete strength shall be made from the Rebound Hammer Test. In fact, as per amendment No-1 to Non-destructive Testing of concrete- Ultrasonic Pulse Velocity Testing, for concrete grade ≤ M-25, if the average pulse velocity by cross probing is below 3.5 Km/s, the concrete quality grading shall be considered as “DOUBTFUL” and similarly for concrete grade higher than M-25, if the average pulse velocity by cross probing is below 3.75 Km/s, the concrete quality grading shall be considered as “DOUBTFUL”.
POINTS FOR DISCUSSION:
1)     It is stated in IS 516 (Part-5/Sec-4):2020, Rebound Hammer Test that the estimation of strength of concrete by Rebound Hammer has an accuracy of ±25 percent. If that is the case, then how this test is helpful in estimating concrete strength during the construction so that operations that require a specific strength can be performed safely or curing procedures can be terminated?
2)     In case of a distress structure needing structural rehabilitation, if it is found by Ultrasonic Pulse Velocity Test that the concrete quality is “Doubtful”, then the in-situ compressive strength assessed from the Rebound Hammer Test is not valid. In the subject scenario, what residual value of distress concrete shall be considered for carrying out the Structural Repair?
3)     While defining concrete quality grading , the BIS publication IS 516(Part-5/Sec1):2018, Hardened Concrete-Methods of Test ,Part(5) Non-destructive Testing of concrete , Section-1, Ultrasonic Pulse Velocity Testing with Amendment-1 refers to Average value of pulse velocity by “CROSS PROBING”. But the code has not defined what is “CROSS PROBING”? The code has defined “Direct Transmission”, “Semi-direct Transmission” and “Indirect Transmission”. If we presume that “Cross Probing” means “Direct Transmission”, then how do we interpret the result from “Semi-direct Transmission” and “Indirect Transmission”?
REFERENCES:
1)     IS 516(Part-5/Sec-1):2018, Hardened Concrete-Methods of Test, Part(5) Nondestructive Testing of Concrete, Section-1, Ultrasonic Pulse Velocity Testing with Amendment-1.
2)      IS 516(Part-5/Sec-4):2020, Hardened Concrete-Methods of Test, Part(5) Nondestructive Testing of Concrete, Section-4, Rebound Hammer Test.
3)     In-place Methods to estimate Concrete Strength, ACI 228-1R-03
4)     Assessment of in-situ compressive strength in structures and precast concrete components. BS EN 13791:2019.
5)     Standard Test Method for Rebound Number of Hardened Concrete. C805/C805M-18
6)     Testing Concrete in Structures – Part-2: Nondestructive Testing –Determination of Rebound Number, EN 12504-2:2021(E)
7)     Handbook on Nondestructive Testing Concrete edited by V.M.Malhotra and N.J.Carino.

[vikram.jeet]

Respected P K Mullick sir,,
Seen your post after long time,, hope you would have retired from regular service,  may be carrying advisory assignment s.

Now a days writers on sefi are very few,  and if time permits,, give yr valuable respinses on sefi gof benefit of budding engineer s as well as veterans like me.

Best wishes and regards

[P.K.Mallick]

In fact, IS:516(Part 5/Sec1):2018, Hardened Concrete -Methods of Test, Part 5, Non-destructive Testing of Concrete, Section-1 Ultrasonic Pulse Velocity Testing has defined Direct Transmission as Cross Probing. It further states that "Surface Probing"(Indirect Transmission) is not as efficient as Cross Probing, because the signal produced at the receiving transducer has amplitude of 2 to 3 percent of that produced by Cross Probing and the test results are greatly influenced by the surface layers of concrete which may have different properties from that of concrete inside structural member.
The indirect velocity is invariably lower than the direct velocity on the same concrete element. This difference may vary from 5 to 20 percent depending largely on the quality of the concrete under test. For good quality of concrete, a difference of about 0.5 Km/s may generally be encountered.
The above explanation more or less addresses the issue at No-3 above.

IT IS REQUESTED TO ADDRESS THE ISSUES AT NO-2 AND NO-1.

[P.K.Mallick]

[Arunachalam]