Experiment No: 11
Determination of Rebound Number of Hardened Concrete
Introduction
The rebound hammer, a
spring-loaded steel hammer, is essentially a surface hardness tester. It
operates on the idea that the rebound of an elastic mass relies on the hardness
of the surface against which the mass impinges. The approach simply measures the
modulus of elasticity of the near surface concrete. The principle is based on
the absorption of part of the stored elastic energy of the spring through
plastic deformation of the surface concrete and the mechanical waves
propagating through the stone while the remaining elastic energy generates the
real rebound of the hammer. The distance travelled by the mass, represented as
a percentage of the initial extension of the spring, is termed the rebound
number. There is little apparent theoretical correlation between the strength
of concrete and the rebound number of the hammer. However, within limitations,
empirical connections have been found between strength properties and the
rebound number.
Scope
This
test method covers the determination of a rebound number of hardened concrete
using a spring-driven steel hammer.
Purpose
To determine the rebound number of
hardened Portland cement concrete.
ASTM Designation
ASTM
C805—Rebound Number of Hardened Concrete
Terminology
Concrete —
a
composite material that consists essentially of a binding medium within which
are embedded particles or fragments of aggregate; in hydraulic-cement concrete,
the binder is formed from a mixture of hydraulic cement and water.
concrete, fresh —
concrete
that possesses enough of its original workability so that it can be placed and
consolidated by the intended methods.
concrete, hardened —
concrete
that has developed sufficient strength to serve some defined purpose or resist
a stipulated loading without failure.
Consistency —
the
relative mobility or ability to flow.
Curing —
action
taken to maintain moisture and temperature conditions in a freshly-placed
cementitious mixture to allow hydraulic cement hydration and (if applicable)
pozzolanic reactions to occur so that the potential properties of the mixture
may develop.
Pozzolan —
a
siliceous or siliceous and aluminous material, which in itself possesses little
or no cementitious value but will, in finely divided form and in the presence
of moisture, chemically react with calcium hydroxide at ordinary temperatures
to form compounds possessing cementitious properties.
cementitious material (hydraulic) —
an
inorganic material or a mixture of inorganic materials that sets and develops
strength by chemical reaction with water by formation of hydrates and is
capable of doing so under water.
concrete, hardened—
concrete
that has developed sufficient strength to serve some defined purpose or resist
a stipulated loading without failure.
hardening—
gain
of strength, and other properties, of a cementitious mixture as a result of
hydration after final setting.
Significance and Use
This test method is used to assess
the in-place uniformity of concrete, identify regions of poorer quality or
deteriorated concrete, and estimate in-place strength. The relationship between
rebound number and concrete strength is established by correlating rebound
numbers measured on the structure with the strengths of cores taken from
corresponding locations. At least two replicate cores should be taken from at
least six locations with different rebound numbers. Test locations should be
chosen to obtain a wide range of rebound numbers in the structure. Factors such
as moisture content, method of obtaining the test surface, vertical distance
from the bottom of a concrete placement, and depth of carbonation should be
considered when interpreting rebound numbers. Different hammers of the same
nominal design may give rebound numbers differing from 1 to 3 units, so tests
should be performed with the same hammer to compare results. If multiple
hammers are used, tests should be performed on a range of typical concrete
surfaces to determine the magnitude of differences.
Apparatus
Rebound
Hammer:
A rebound hammer is a spring-loaded tool that strikes a steel plunger on a concrete surface with a consistent velocity, measured on a linear scale attached to the instrument's frame, to determine the rebound distance from the plunger.
Figure: A typical rebound hammer
Abrasive
Stone:
Consisting of medium-grain texture
silicon carbide or equivalent material.
Test
Anvil:
The test anvil is a 150-mm diameter
tool steel cylinder with a hardened impact area of 66 ± 2 HRC, equipped with an
instrument guide to center the rebound hammer and maintain perpendicularity to
the surface.
Verification:
Rebound hammers must be serviced and
verified annually, and if their proper operation is questioned, using a test
anvil. The test anvil should be supported on a bare concrete floor or slab, and
the manufacturer must report the rebound number obtained by a properly
operating instrument.
Test
Area and Interferences
Selection
of Test Surface:
Concrete members must be at least
100mm thick and fixed within a structure. Smaller specimens must be rigidly
supported. Avoid areas with honeycombing, scaling, or high porosity. Test
results should not be compared if the form material is different. Troweled
surfaces have higher rebound numbers than screwed or formed finishes. Test
structural slabs from the underside.
Preparation
of Test Surface:
The test area must be 150mm in
diameter and must be ground flat with abrasive stone. Smooth-formed or troweled
surfaces can be tested without grounding. Results should not be compared
between ground and unground surfaces. Free surface water should be removed
before testing.
·
To
compare readings, the direction of impact must match or be adjusted using
established correction factors.
·
Do
not conduct tests directly over reinforcing bars with cover less than 20 mm
Procedure
·
Hold the instrument firmly to ensure
the plunger is perpendicular to the testing surface.
·
Slowly push the instrument toward the
test surface until the hammer makes contact.
·
After impact, maintain pressure on the
instrument and, if required, click the button on the side of the instrument to
secure the plunger in its retracted position.
·
Observe the rebound number on the
scale, rounding it to the nearest whole number, and note the rebound number.
·
Conduct ten readings from each testing
location. No two impact tests should be spaced closer than 25 mm [1 in.].
·
Assess the impression created on the
surface after impact; if the impact crushes or fractures a near-surface air
pocket, ignore the measurement and obtain a new value.
Calculation
·
Discard
readings differing from the average of 10 readings by more than 6 units and
determine the average of the remaining readings.
·
If
more than 2 readings differ from the average by 6 units, discard the entire set
of readings and determine rebound numbers at 10 new locations within the test
area.
Report:
·
General information:
i.
Date
of testing,
ii.
Air
temperature and time of testing,
iii.
Age
of concrete, and
iv.
Identification
of test location in the concrete construction and the size of member tested.
·
Information about the concrete:
i.
Mixture
identification and type of coarse aggregate, and
ii.
Specified
strength of concrete.
iii.
Description
of test area:
iv.
Surface
characteristics (trowelled, screeded. formed),
v.
If
applicable, type of form material used for test area,
vi.
If
surface was ground and depth of grinding,
vii.
If
applicable, curing conditions, and
viii.
Surface
moisture condition (wet or dry).
·
Hammer information:
i.
Hammer
identification or serial number, and
ii.
Date
of hammer verification.
·
Rebound number data:
i.
Orientation
of hammer during test,
ii.
On
vertical surfaces (walls, columns, deep beams), relative elevation of test
region,
iii.
Individual
rebound numbers,
iv.
Remarks
regarding discarded readings,
v.
Average
rebound number, and
vi.
If
applicable, description of unusual conditions that may affect test readings.
Lab
Questions:
Basic Understanding
1. What is the rebound
number, and how is it used to assess concrete?
2. Why is the rebound
hammer test considered a non-destructive test (NDT) for concrete?
3. What is the significance
of the rebound number in determining the quality of hardened concrete?
Procedure and
Equipment
4. What equipment is
used to determine the rebound number of concrete?
5. Can you describe the
procedure for performing the rebound hammer test on hardened concrete?
6. How does the rebound
hammer work to measure the surface hardness of concrete?
7. Why is it important
to take readings from multiple points on the concrete surface?
8. What is the
importance of calibrating the rebound hammer before testing?
9. How is the rebound
number recorded and interpreted?
Factors Affecting
the Rebound Number
10. What factors can
influence the rebound number in the test?
11. How does the age of
concrete affect the rebound number?
12. What role does the
surface texture and moisture condition of concrete play in the rebound number?
13. How does the
orientation of the hammer during testing (horizontal, vertical, inclined)
impact the results?
14. What effect do
surface carbonation and hardening of concrete have on the rebound number?
Calculation and
Interpretation
15. How do you relate the
rebound number to the compressive strength of concrete?
16. Why is it important
to use a correlation chart when converting rebound numbers to compressive
strength values?
17. Can the rebound
hammer test provide a reliable estimate of in-situ concrete strength on its
own? Why or why not?
18. What should you infer
if the rebound number shows significant variation across different areas of the
same concrete structure?
Standards and
Specifications
19. Which standards
(e.g., ASTM, IS etc.) are followed for determining the rebound number of
concrete?
20. What is the
acceptable range of rebound numbers for good-quality concrete?
21. How is the rebound
hammer test result affected by the type of aggregate used in the concrete?
Application and Practical
Considerations
22. What are the
practical applications of the rebound hammer test in the field?
23. What are the
limitations of the rebound hammer test in assessing concrete strength?
24. How does the rebound
hammer test compare with other non-destructive tests like the ultrasonic pulse
velocity test?
25. In which situations
would the rebound hammer test be particularly useful?
26. What are some common
sources of error in the rebound hammer test, and how can they be minimized?
27. How can the rebound
hammer test be used to monitor the uniformity and quality of concrete in large
structures?
References
ASTM C805: Standard
Test Method for Rebound Number of Hardened Concrete.