Pullout test as a nondestructive test method in structural engineering

UNIVERSITY OF GAZIANTEP
GRADUATED SCHOOL OF NATURAL AND APPLIED SCIENCE
FACULTY OF ENGINEERING
DEPARTMENT OF CIVIL ENGINEERING
NON DESTRUCTIVE TESTS CE 550
Dr. Esra . M. Guneyisi
REPORT NO 7.
Pullout test as a nondestructive test method in
structural engineering
Submitted by
Master’s Student
AHMED ASSIM ABDULLAH

BACKGROUND
The pullout test measures the force required to pull an embedded metal insert
from hardened concrete. The earliestknown description of the pullout test method
was reported in 1938 by Skramtajeu of the Central Institute for Omdistroa in the
Soviet Union. Figures 1 and 2 show the schematics for some of the first pullout
equipment.
In 1962, Kierkegaard-Hansen initiated a research program to determine the
optimum geometry for the pullout test. The objectives of this research program
were to develop simple equipment that could be used in the field and also have a
high correlation between the ultimate pullout force and the compressive strength.
According to Kierkegaard-Hansen, the embedment depth should be sufficient to
assure that more than the outer-most surface of the concrete is tested and some
coarse aggregate is included within the failure cone. This suggests that a deeper
embedment is better. However, with increasing depth, the force required to pull
out the insert also increases, which would require bulkier equipment and a larger
failure area. Based on these factors, an embedment depth of 50 mm (1 in) was
chosen arbitrarily/6
'
Kierkegaard-Hansen also performed a number of tests in an effort to establish the
optimum diameter to be used for the insert head and the bearing ring. Because a

suitable tension loading system did not exist, a lab compression machine was used
to apply the load. The insert was tested by applying a compressive load to the
bottom of the embedded disk. Figure 3 graphically shows that the test can also be
considered as a punching-type test. The Danish word for punching is "lokning."
Therefore, the quantity measured by the test wascalled "lok-strength" ratherthan
the pullout strength. This is why the pullout test is sometimes referred to as the
"LOK" test.(6)
As a result of this research, the correlation between pullout strength and
compressive strength was found to be nonlinear. In 1962, Kierkegaard-Hansen
improved on the initial research by introducing a bearing ring. This modification
resulted in a failure cone with a well-defined geometry that resulted in linear
correlation between pullout strength and compressive strength

TEST EQUIPMENT AND PROCEDURE
Figure 4 shows a schematic of the pullout insert. A force is applied to the insert by
a loading ram that is seated on a bearing ring and is concentric with the insert
shaft. The bearing ring transmitsthe reaction force to the concrete. As the insert is
pulled out, a conical-shaped fragment of concrete is extracted from the concrete.
In the pullout test, a 25 mm (linch) diameter steel disc on a conical shaped stem is
embedded at least 25 mm (1 inch) below the surface of the concrete during
casting. A pull bolt is screwed into the stem of the disk and pulled by hydraulic
force against a surface mounted reaction ring. The disk is loaded to failure by
meansof a hand operated portable hydraulic jack and the total force is measured
on a gauge attached to the jack.
The pullout test can be used during construction to estimate the in-place strength
of concrete to help determine whether construction activities such as form
removal, application of post-tensioning, early opening to traffic, or termination of
cold weather protection can proceed. Because compressive strength is usually
required to evaluate structural safety, the ultimate pullout force measured during
the in-place test is converted to an equivalent compressive strength by means of a
previously established correlation relationship.

LABORATORY CALIBRATION PROCEDURE
Development of the correlation relationship between pullout force and
compressive strength should be performed for each specific concrete mix to be
used. To date, there are no standard procedures to establish the correlation data.
However, suggested correlation procedures are provided below.
Various techniques have been used to acquire companion pullout strength and
compressive strength data. Kierkegaard-Hansen placed pullout inserts in the
bottoms of standard cylinder specimens. A pullout test was performed on the
cylinder, and then the same cylinder was capped and tested for compressive
strength. As long as the pullout test was stopped at the point of maximum load, a
cone was notextracted, and the cylinder could be tested in compression without a
significanteffect on the results. However, for compressive strength above 40 MPa
(5800 psi), lower pullout strengths resulted for 150x300 mm (6xl2-in) cylinders
compared to companion slabs.
A recommended alternative method is to place inserts in slabs and cast
companion standard cylinders. At designated ages, replicate pullout tests are
performed on the slab and replicate cylindersare compression tested. A drawback
to this approach is to assure that the pullout tests and compression tests are
performed at the same maturity. Because of their different masses and shapes,
the slab and cylinders are not likely to experience the same temperature history
during the critical early stages when strength changes rapidly with age and is
strongly dependent on temperature history. Failure to account for possible
maturity differences can lead to inaccurate correlation relationships. Either
maturity meters should be used to ensure companion testing at equal maturities,
or compression tests should be performed on coresdrilled from the slab. While the
latter approach helps ensure equal maturities, it is time consuming. Refer to
Chapter 7 in this guide for information on the maturity concept.
For commercially available pullout systems, having embedment depths of 30 mm
(1.2 in) or less and apex angles of 70° or less, the preferred approach is to place
inserts on the side faces of 200-mm (8 in) cubes and cast companion standard

cylinders. Because of similar surface-to-volume ratios, the early-age temperature
histories of the two types of specimens will be similar. The cubes and cylinders
should be compacted similarly, and the use of an internal vibrator or a vibrating
table is recommended.
American Concrete Institute (ACI) Committee 228 recommends in its "In-Place
Methods for Determination of Strength of Concrete," ACI 228.1R-89, performing
eight replicate pullout tests and two cylinder compression tests at each test age.
These numbers of tests ensure that the average pullout strength and average
compressive strength are determined with about the same degree of certainty. By
placing four inserts in each cube, this recommendation requires two cubes and
two cylinders at each age. The specimens should be moist-cured until the time of
testing.
The number of pullout and compression tests chosen to establish the correlation
relationship should satisfy two needs: (1) the tests should span as wide a range of
strength as possible, and (2) there should be enough points to define the
relationship with a reasonable degree of accuracy. Based on field experience,
Bickley suggested that the range of compressive strength should be at least 20
MPa (3000 psi) but preferably greater than this. ACI Committee 228 recommends
performing companion tests at a minimum of six evenly spaced strength levels.
Generally, if test ages are increased by a factor of two there will be about the
same strength increase between successive tests. For example, tests at ages of 1,
2, 4, 8, 16, and 32 days should result in approximately evenly spaced test points.
This assumes a constant temperature during the curing period. If pullout tests will
be used to estimate in-place strengths at very low levels, the first test age should
be reduced to 12 hours. This will require special care in handling low-strength
specimens. Thus the recommended correlation testing program involves casting at
least 12 cubes, with 4 inserts per cube and 12 cylinder specimens. The inserts in
two cubes and two cylinders are tested at each test age so as to produce evenly
spaced points when correlation data are plotted. The average of the pullout
strengths and compressive strengths are used in a least squares fit analysis to
develop the correlation relationship

FAILURE MECHANISM
The pullout test subjects the concrete to a static load. Therefore, it should be
possible to calculate the internal stresses in the concrete and predict the onset of
cracking and the ultimate pullout force. This is desirable so that the ultimate
pulloutforce can be related to the strength properties of concrete. Unfortunately,
the stress distribution is not easy to calculate because the stresses are altered by
the presence of coarse aggregate particles. There is not a consensus on the failure
mechanism at the ultimate load. One theory is that the ultimate load occurs as a
result of compressive failure of concrete along a line from the bottom of the
bearing ring to the top face of the insert. The other theory is that the ultimate
failure is governed by aggregate interlock across the secondary crack system, and
the ultimate load is reached when a sufficientnumber of aggregate particles have
been pulled out of the matrix.
Based on review of various analytical and experimental investigations that had
been conducted, Krencheland Bickley concluded thatthe failure mechanism of the
pullout test involves the following stages (see figure 5):
Stage 1 - At a load of about 30 to 40 percent of the ultimate, "tensile cracks"
originate at the corner of the insert head and propagate into the concrete for a
distance of 15 to 20 mm (0.6 to 0.8 in) forming an apex angle between 100 and
135° (see figure 5 (a)). This cracking concentrates subsequent straining of the
concrete so that "all load is taken up in the truncated zone" between the insert
head and the bottom of the bearing ring.
Stage 2 - A large number of stable micro-cracks develop in the truncated zone,
These cracksrun from the top of the insert head to the bottom of the bearing ring,
forming an apex angle of about 84° (see figure 5(b)). This second-stage cracking
occurs as the load increases up to and just past the ultimate load. These stable
microcracksare analogousto the vertical microcracksobserved during an ordinary
uniaxial compression test of a cylinder or prism.

Stage 3 - Beyond the ultimate load, a circumferential "tensile/shear" crack
develops that forms the final shape of the extracted cone (see figure 5(c)).
Despite the lack of agreement on the exact failure mechanism, it has been shown
that the pullout strength has good correlation with the compressive strength of
concrete and the test also has good repeatability.
WITHIN-TEST VARIABILITY
Within-test variability, also called "repeatability," refers to the scatter of results
when the test is repeated on identical concrete using the same test equipment,
procedures, and personnel.<6)
For a given concrete, the repeatability of a test
affects the number of tests required to establish, with a desired degree of
certainty, the average value of the property being measured by the test.
If pullout tests are repeated on the same concrete at the same maturity, the
ultimate pullout loads would be expected to be normally distributed about the
average value and the standard deviation would be the measure of repeatability.
However, if replicate tests were performed on the same concrete but at different
maturities, so that there would be different average pullout strengths, it has been
generally agreed among several investigators that the coefficient of variation is
the better statistic for quantifying the repeatability of the pullout test. Reported

values of the coefficient of variation, for different aggregates and test
configurations, have ranged from about 4 to 15 percent with an average value of
about 8 percent. The maximum size of the aggregate in relation to the
embedmentdepth appearsto be a significant factor. Tests on concrete made with
coarse aggregate having a maximum size less than the embedment depth tend to
have lower variability
FIELD TESTS
To estimate in-place strength, pullout tests are performed on a particular part of
the structure and the correlation relationship is used to convert the test results to
a compressive strength values. To determine if sufficient strength has been
attained, the estimated compressive strength is compared with the required
strength in the specifications. However, to provide for a margin of safety, the
pullout test results need to be looked at statistically rather than simply comparing
the average estimated in-place strength with the required strength.
Number of Tests
ASTM C 900 requiresa minimum of five pullout tests for every 115 m3
(150 yd3
) of
concrete, or in case of slabs or walls for every 465 m2
(5000 ft2
) of the surface area
of one face. However, a greater number of inserts is recommended for added
reliability and as a safety measure in the event testing is begun too soon or some
of the tests were considered invalid.
Some investigators have suggested that a minimum of ten pullout tests should be
performed for a given concrete placement. As a practical matter, Bickley
advocated the placementof 15 inserts per 100 m3
(130 yd3
). When the anticipated
desired strength level is reached, five inserts are randomly selected for testing. If
the results indicate less than the required strength, testing is discontinued and
additional curing is provided. At a later age, the remaining 10 inserts are tested.
This technique provides for a reserve in the event that testing is done too soon. It

is recommended that the maturity method be used to determine the appropriate
time to perform the pullout tests, therefore reducing the possibility of doing the
pullout tests too soon.
TYPICAL APPLICATIONS
The pullout test has been adopted as a standard test method in many parts of the
world, including North America, and hasbeen used successfully on numerouslarge
construction projects. Primary use of the system has been in either controlling
formwork removal and the time of post-tensioning, or determining the minimum
amount of curing needed in cold-weather concreting. The system has been used
on cooling towers, chimneys, multi-story building frames, pipelines, bridges, and
other forms of construction.
CAPO (CUT AND PULLOUT)
A disadvantage of the standard pullout test is that the locations of the inserts
must be planned in advance of concrete placement so that the inserts can be
fastened to the formwork or floated in from the surface. In an effort to extend the
application of pullout testing to existing structures, the CAPO technique has been
developed. It should be noted however that the CAPO test method has not been
approved as an ASTM standard test method.
This technique involves drilling a 18 mm (0.7in) diameter hole into the concrete. A
special concrete router bit is then used to route a 25 mm (lin) diameter slot at a
depth of 25 mm (1 in). An expandable metal washer is placed in the hole and the
ring is expanded. Figure 6 shows a schematic of the equipment. The expanded
washer is then pulled out of the concrete using the same loading system as that
used for an ordinary pullout test. Attempts at using the CAPO test in the field have
indicated that the test is cumbersome and hashigh variability. This high variability
is probably due to the need for a flat concrete surface that is perpendicular to the

drilled hole. If these conditions are not achieved, the bearing ring will not seat
properly and test results will be erratic.

Pullout test as a nondestructive test method in structural engineering

More Related Content

What's hot (20)

Viewers also liked (13)

Similar to Pullout test as a nondestructive test method in structural engineering (20)

Recently uploaded (20)

Pullout test as a nondestructive test method in structural engineering