REBOUND Hammer Test

This is Report for Rebond hummer test it is usufull and respect report . used in work and good for student .non destractive test
and for any one like this branch

1. Using The Rebound Hammer
By Luke M. Snell, PE
Senior Materials Engineer, Western Technologies Inc., Phoenix AZ
This article was published in the Proceedings of The 11th Annual Mongolian Concrete Conference,
June 2012. Contact Luke Snell at [email protected] if you have questions about this article.
The rebound hammer is one of the most popular nondestructive testing methods used to
investigate concrete. Its popularity is due to its relatively low cost and simple operating
procedures. The rebound hammer is also one of the easiest pieces of equipment to misuse;
thus, many people do not trust the rebound test results. This article is to discuss the rebound
hammer and how to successfully use it to evaluate concrete.
Principles of The Rebound Hammer
Ernst Schmidt, a Swiss engineer, developed the
modern rebound hammer in 1948. The rebound
hammer measures the surface hardness of the
concrete. This is accomplished by placing the
rebound hammer plunger against the concrete
surface and releasing a spring loaded weight. The
amount the plunger rebounds or bounces back is
measured. This rebound number is shown on a
scale and will be between 10 and 100. The Impact
Hammer is another name for Schmidt Hammer.
The surface of concrete gets harder as concrete gains strength; thus, we have a method of
estimating the strength of concrete. A low rebound number will indicate that the surface of
the concrete is soft and the concrete is weak. A high rebound number will indicate that the
concrete is hard and strong. Unfortunately, there is no theoretical relationship between
surface hardness and the strength of concrete. Many things can affect concrete surface
hardness, this is discussed later in this article. However, ACI 318-11, Building Code
Requirements for Structural Concrete and Commentary (R5.6.5) states:
“Nondestructive tests of the concrete in place, such as by probe penetration, impact hammer,
ultrasonic pulse velocity, or pullout may be useful in determining whether or not a portion of
the structure actually contains low-strength concrete. Such tests are of value primarily for
comparisons within the same job rather than as quantitative measures of strength.”
Factors That Affects Rebound Hammer Numbers
Since the rebound hammer measures the surface hardness of the concrete, it is important
to understand all the items that might affect surface conditions of the concrete and thus,
the rebound hammer numbers. These factors include:
1. Smoothness of the surface 7. Coarse aggregates
2. Size and shape of the concrete sample 8. Type of cement
3. The rigidity of the test area 9. Forms used
4. Age of the concrete 10. Carbonation
5. Surface moisture 11. Location of the reinforcement
6. Internal moisture (moisture gradient) 12. Frozen concrete
For these reasons, the user of the rebound hammer must follow exact procedures and use
engineering judgment. To illustrate this, the following chart shows how the effects of the
coarse aggregates in concrete of the same strength can have on the rebound hammer.
CONCRETES OF SAME STRENGTH
Aggregates Rebound Hammer
River Rock 40
Granite 37
Limestone 32
Lightweight 31

2. Standard Procedures for Rebound Hammer Testing
ASTM C805, Standard Test for Rebound Numbers of Hardened Concrete, provides some
standard procedures so that the user can have consistency when using the rebound hammer.
Some of these standard procedures are:
1. Do not test frozen concrete.
2. The test area must be at least 150 mm (6 inches)
in diameter and fixed rigidly within the structure.
3. The surface to be tested must be flat with no loose mortar.
4. The surface to be tested must be free from water.
5. If the layer of carbonated concrete is thick,
it should be removed before testing.
6. The hammer must be held in the same direction —
horizontal, upward, downward and it should always
be at a right angle to the surface being tested.
7. Do not test over reinforcement with a cover of less
than 20 mm (3/4 inch).
8. If estimating concrete strength takes at least two cores from
six locations that have different rebound hammer number.
9. Take 10 rebound hammer readings at each test area.
All individual readings should be at least 25 mm (1 inch) apart.
10. Discard any reading that is over six units from the average
and calculate the average of the remaining readings.
11. If two units are over six units from the average,
discard the entire set of reading and redo the test.
Using The Rebound Hammer To Locate Requiring Additional Investigations
One of the ways to use the rebound hammer is to locate those areas that may need additional
investigation. In this procedure the round hammer is used at several locations to identify those
areas that have a lower rebound number. Since the structure would have the same mixture,
curing history, moisture content, etc., the rebound hammer can identify those areas that
appear to have the weakest concrete (lowest rebound hammer number).
Test On A Wall
(Rebound Hammer)
Area to be cored
to test strength.
35
24
22
37
30
26
38
34
30
As illustrated above, the area that needs additional investigation is shown in the circled area.
This might be done by taking cores, visual inspection and/or by a structural
evaluation of the impact of a low strength concrete in this particular area.

3. Comparison of Rebound Numbers Results
Another procedure used is to compare rebound numbers of the concrete that you know is
acceptable from a recent placement. This part of the structure has the concrete already
evaluated by cores, cylinders or cubes and the concrete strength met the project requirements.
In this procedure you would determine the rebound numbers of the concrete known to be
acceptable. The investigator would then test the concrete with the rebound hammer that
needed to be investigated. If the rebound numbers for concrete being investigated are
approximately the same or higher than the concrete that had met the project specifications,
the tested concrete can be determined to be acceptable. If the rebound numbers in the area
being tested are lower, then additional investigations would need to be done by the engineer.
New Developments In The Rebound Hammer
The rebound hammer has had several changes over the years. Some of the latest
improvements have been to make the rebound hammer lighter; use some of the aerospace
higher strength metals; install computer chips to calculate automatically the averages and
standard deviations of the readings. Although the rebound hammer has gone through several
changes and is an extremely useful nondestructive testing tool, the user must recognize that
the rebound hammer measures only surface hardness of concrete. Engineers must determine
how to use this information in their investigation of the concrete structure.
Engineering Judgment and Concluding Remarks
The rebound hammer must be recognized for what it is able to measure — the surface
hardness of concrete. When used as part of an investigative process that includes an
understanding of concrete being tested, a visual inspection, and documentation from cylinders,
cubes or cores, it can be an excellent nondestructive testing method. It is an instrument that
requires engineering judgment to interpret the reading and to accurately assess the concrete.
Engineering judgment can only be used when an exact procedure sought as the one outlined
in ASTM C805 is followed.
REFERENCES
1. ASTM C805 / C805M-08, Standard Test Method for Rebound Number of Harden Concrete,
ASTM International, West Conshohocken, PA, 2008.
2. Malhotra , V.M., Testing Harden Concrete: Nondestructive Methods, The Iowa State University
Press, Ames, Iowa and American Concrete Institute, Detroit, Michigan, 1976.
3. Bungey, J.H., The Testing of Concrete In Structures, Surrey University Press,
Distributed in the USA by Chapman and Hall, New York City, New York, 1982.
4. ACI Committee 228, In Place Methods to Estimate Concrete Strength (ACI 228.1R-03),
American Concrete Institute, Farmington Hills, Michigan, 2002.
5. ACI Committee 318, Building Code Requirements for Structural Concrete and Commentary
(ACI 318-11), American Concrete Institute, Farmington Hills, Michigan, 2011.
6. Proceq website, www.proceq.com, 2012.