1. Materials and Structures/Matdriaux et Constructions, 1989, 22,280-286
Evaluationofconcretespall repairsbypullout test
F. G. COLLINS
Taywood Engineering Ltd, 7/F,275 Alfred StreetNorth, North Sydney, N.S. W. 2060, Australia
H. ROPER
School of Civiland Mining Engineering, Universityof Sydney, 2006N.S. W., Australia
Spalling concrete wassimulated in the laboratory by utilizingpullout testmethods generally
usedfor the determination of in situ concrete strength. The crackingpatterns displayed by
pullout testspecimens typify the damage of concrete by spalling. All specimens were damaged
bypullout testing, repaired with epoxy mortar and subjected to asecond pullout test at a later
time. The testprogramme showed that the overridingfactor which governs successful repairs to
concrete is thesoundness of the repair plane.
1. INTRODUCTION
A spall is characterized by the breaking away of a
fragment, detached from a larger concrete mass by a
blow, the action of weather, or by pressure which may
result from expansion of porous aggregates, occluded
organic material, reactive contaminants, or corroding
steel reinforcing bars and/or embedments.
For this study, spalling concrete was simulated in the
laboratory by utilizing pullout test methods generally
used for the determination of in situ concrete strength.
The radial and circumferential cracking patterns dis-
played by pullout test specimens typify the damage of
concrete by spalling. The pullout cavity shape is repro-
ducible using constant apparatus geometry.
2. TEST METHOD
In order to conduct a pullout test a metal insert is
embedded in fresh concrete. After the concrete has
hardened, the pullout strength is determined by measur-
ing the maximumforce required from a loading device to
pull the insert and adjoining concrete from the concrete
insideDiameterof ReactionRing__I
l~
Reaction
Ring
V7
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Failure [
Surface
I Reaction
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V--,-----_.Apex
_I Ang(e 2o~
[
Disk Diameter
Fig. 1 Schematicrepresentation ofpullouttest.
0025-5432/899 RILEM
mass. The configuration of the embedded insert and
bearing ring of the loading system determine the
approximate shape of the pullout fracture within the
concrete mass. A typical configurationis shown in Fig. 1.
3. THE PULLOUT TEST AS A MEANS OF
EVALUATING CONCRETE STRENGTH
The concept of a pullout test as a means of determining
the concrete strength in a completed structure was sug-
gested by Skramtajew [1] in 1938. Kierkegaard-Hansen
[2] conducted a further series of tests to investigate the
effect which a bearing ring of finite radius would have on
the ultimate pullout force. These tests indicated that for
smaller reaction rings (and thus smaller apex angles, 2a)
the pullout force increased. Since less material was
involved in the failure surface, Kierkegaard-Hansen
concluded that the increase in ultimate load was due to a
change in the internal state of stress within the failure
region. Subsequent studies were aimed at developing
calibration curves for various concretes using a constant
test geometry.
In the early 1970s a series of tests were conducted by
Malhotra [3], Richards [4], and Rutenbeck [5] to estab-
lish correlations between the pullout strength of con-
crete and strength parameters derived from other non-
destructive test methods. These tests form the basis of
the current ASTM requirements for the pullout test,
ASTM C900 [6]. The ASTM requirements define the
apex angle to fall between 54 and 70~ A linear correla-
tion was found to exist between pullout strength of
concrete and compressive strength of concrete cylin-
ders.
Stone and Giza [7] sought to obtain a lower coefficient
of variation for the pullout test by examining different
geometries, aggregate types and aggregate sizes. Failure
surfaces were found to exhibit a conic frustum geometry
at low apex angles and a trumpet-shaped geometry at
higher apex angles.

2. Materials and Structures 281
4. PREVIOUS STUDIES ON THE DEFORMATION
AND FAILURE OF CONCRETE USING THE
PULLOUT TEST
4.1 Non-linear finite-element analysis of test conditions
Ottoson [8] analysed the pullout test by means of an
axisymmetric, non-linear, finite-element computer
program. This analysis followed the progression of ra-
dial and circumferential cracking by means of an itera-
tive smeared-cracking procedure.
The analysis showed that circumferential cracks,
forming the surface of the pullout cone, begin at the disk
edge at 15% of ultimate load and propagate to the
reaction ring by 65% of ultimate load. The analysis also
indicated the formation of radial cracks which begin at
low load levels at the intersection of the top concrete
surface and the pulling stem and propagate towards the
circumferential failure surface (Fig. 2).
I
I jj
I B
I
t' II
[
I 0
Disk~
Ill Support
C
Fig. 2 Crack developmentwith increasing load (results of
Ottoson [8]); loading (A) 15%, (B) 25%, (C) 64% and (D)
98% ultimate.
4.2 Experimental measurements of deformation and
failure
Stone and Carino [9] performed pullout tests on two
large-scale specimens. A reverse modelling procedure
was employed, resulting in the adoption of a pullout disk
diameter measuring 305 mm and a reaction ring inside
diameter of 660mm. The depth of embedment was
varied so that apex angles fell within the upper and lower
bounds specified in ASTM C900 [6]. Micro-embedment
strain gauges were placed in the concrete to measure the
internal strain distribution in critical regions. The study
showed that beyond 65% of ultimate, the load is be-
lieved to be carried entirely via aggregate interlock
across the failure surface.
Because of this failure mechanism, analytical solu-
tions based on a continuum theory are not applicable for
predicting the ultimate pullout force, since beyond 65%
of ultimate, the load appears to be carried via a non-
continuous discrete mechanism. Some of the scatter
associated with measured pullout strengths is likely to he
caused by the random manner in which the aggregates
are located with respect to the failure surface. Stone and
Carino concluded that the governing strength parameter
leading to excellent correlations between pullout
strength and compressive strength of concrete is mortar
tensile strength.
5. LABORATORY STUDY
Spall characteristics were simulated in the laboratory by
utilizing pullout strength test methods on concrete.
Damaged specimens were repaired with epoxy mortar
and subjected to a second pullout test at a later time.
Strength characteristics of specimens were examined as
a function of concrete maturity and strength. Emphasis
was placed on assessment of the influence of damage to
the repair surface on the repair strength.
5.1 Pullout assembly
The dimensions of the embedded pullout insert used are
shown in Fig. 3. A 12 mm diameter thread through the
insert axis accommodated a high-tensile rod for testing.
The insert was pulled out of the hardened concrete via
the rod by means of a tension ram which exerted pres-
sure on the specimen through a bearing ring. The assem-
bly set-up is shown in Fig. 3.
The inside diameter of the bearing ring, the diameter
of the insert head and the distance between them,
control the size and apex angle of the concrete frustum
that will be pulled out. Bearing ring inside diameters of
90 and 150 mm were adopted for initial pullout damage
and repair pullouts, respectively. These diameters
achieved apex angles (defined as 2a) of 69 and 114~
respectively.
5.2 Pullout specimens
The cylindrical concrete pullout specimen dimensions
were 200 mm diameter by 140 mm depth. Pullout inserts
were bolted to the bottom plate of the cylindrical moulds
during casting to ensure correct centring and embed-
ment depth, and a smooth, true bearing surface. A steel
ring was cast concentric to both the mould and insert to
achieve confinement. Confinement was deemed neces-
sary to simulate the boundary conditions in a large
concrete mass.
Mix quantities for the concrete used were specified in
the ratios 2:1:1.6:1.06 of 20 mm basalt, 10 mm basalt,
Nepean sand and cement, respectively. The cement
content was 360 kg m -3. A water/cement ratio of 0.5 was
maintained throughout this series of tests.
The conic frustum created by the pullout test was
repaired using a commercially available epoxy resin
mortar. Resin mix proportions were 3:1:4.5 of resin,
curing agent and quartzite sand, respectively. An iden-
tical pullout insert to that used for forming the damaged
area was adhered into the repair patch for subsequent
testing.
All specimens were continually subjected to environ-
mental conditions of temperature 23 _+2~ and 100%
(fog-room) humidity except when the epoxy was curing.

3. 282 Collins and Roper
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Fig. 3 Loadingarrangementofpullouttesting.
A record of standard concrete cylinder strengths was
maintained for each batch at relevant stages of pullout
testing.
8. REPAIR STRENGTH AS A FUNCTION OF
CONCRETE STRENGTH AND MATURITY
8.1 Procedure
An initial series of pullout tests were performed on 60
specimens at ages of 1~,3, 7, 14, 28, 60 and 90 days. The
frustum cavity, possessing an apex angle of 69~, was
repaired with an epoxy mortar, and the specimens were
subjected to standard curing conditions until subsequent
pullout testing. A total of 44 specimens were repaired.
The testing sequence is summarized in Table 1.
A series of pullout tests were also conducted on 44
concrete specimens using an apex angle of 114~. These
specimens facilitated a direct comparison with results
from repaired specimens. Frustum cavity volumes and
masses were monitored throughout the testing se-
quence.
8.2 Correlation between pullout load and
cylinder strength
Pullout force is shown in Fig. 4 as a function of cylinder
compressive strength; this relationship is linear as indi-
cated in the literature, and linear regression curves have
been fitted to the data. Correlation coefficients (Rz) of
0.803 and 0.841 were calculated for results of specimens
having apex angles of 69 and 114~ respectively.
Table 1 Testing sequence
A B C D E F G
Age of specimen (days) at 1~ 3 7 14 28 60 90
initial pullout
(2a = 69~
Age of repaired specimen (days)
at subsequent pullout 3 7 14 28 60 90 -
(2a = 114~

4. Materials and Structures 283
"5
o
1/+0 -
120 --
10080-- " ~ f
jr "'- * ~oo
20
I I I f 1 I I l l
0 10 20 30 z,0 50 60 70 80 90
CyfinderCompressiveSi'rengl-h(HPa)
Fig. 4 Pulloutforce against cylindercompressive strength: (e)
apex angle 69~ (o) apex angle 114~ (A) epoxy-repaire d
specimens with apex angle 114~
Results of specimens repaired with epoxy mortar
show a linear relationship until a maximum of 45.0 kN is
reached for a cylinder strength of 41.2 MPa. The linear
portion of the curve displays a correlation coefficient
(R 2) of 0.935. Pullout force diminishes at higher cylinder
strengths. Epoxy-repaired specimens consistently
yielded lower pullout loads than concrete pullouts of
identical loading geometry in the linear portion of the
loading curve.
8.3 Correlation between pullout stress and
cylinder strength
Pullout cavity shape was accounted for in stress calcula-
tions. An adjusted surface area of the failure surface was
calculated from measured water volumes of the cavity
and theoretical conic frustum surface area and volume
for the relevant geometry.
For concrete failures linearity is displayed in Fig. 5.
Correlation coefficients (R2) of 0.812 and 0.731 were
recorded for frustum apex angles of 69 and 114~ respec-
tively.
701 "/
2
1.0~ ~ 7~
I I I I I I I
0 10 20 30 L,0 50 60 70
Compressive Cytinder Strength (MPa)
I I
80 90
Fig. 5 Pullout stress against cylindercompressivestrength:(e)
apex angle 69~ (o) apex angle I14~ (A) epoxy-repaired
specimens with apex angle 114~
Results of specimens repaired with epoxy mortar once
again show a linear relationship until a maximum of
2.26 MPa is reached for a cylinder strength of 42.0 MPa.
The linear portion of the curve displays a correlation
coefficient (R2) of 0.870. Pullout stress diminishes at
higher cylinder strengths. Ultimate pullout stresses of
repaired specimens were consistently lower than con-
crete pullouts of identical loading geometry for the
linear portion of the curve. In general it is concluded that
little is gained by attempting to recalculate the results in
terms of stress rather than considering them as loads.
8.4 Correlation between pullout load and age
The pullout load-age relationship shown in Fig. 6 resem-
bles plots of compressive cylinder strengths against age.
Strength development is most pronounced .at an early
age as the rate of hydration is at a maximum and reaches
an asymptotic limit at about 90 days.
Epoxy-repaired specimens displayed similar strength
gains to concrete Specimens at early specimen ages. A
maximum of 46.0 kN was reached at an age of 25 days.
Specimens of greater age yielded strengths of dimi-
nishing magnitude.
120-
I00
Z
6(1
/,0
2O
I
100
!
~P
l l [ I I f I [
10 20 30 L,0 50 60 70 80 90
Concrete Age (Days)
Fig. 6 Pullout load against concrete age: (e) apex angle 69~
(o) apex angle 114~ (A) epoxy-repaired specimens with apex
angle 114~
8.5 Correlation between pullout stress and age
Fig. 7 illustrates a similar relationship to that found for
cylinder strength plots as a function of concrete age.
Strength development is most pronounced at an early
age, and reaches an asymptotic limit at about 90 days.
Epoxy-repaired specimens displayed similar strength
gains to normal concrete pullout specimens at early
ages. A maximum of 2.10 MPa was reached at an age of
23 days. Specimens of greater age yielded strengths of
diminishing magnitude.
8.6 Correlation between initial pullout damage force and
pullout force of repaired specimen
Fig. 8 illustrates the influence of the initial damaging
pullout force on the strength of the pullout of the

5. 284 Collins and Roper
_ ,.~F .
+ +.or
+.~
o
I+ I I I 1 I 1 I I I
0 10 20 30 t,O 50 60 70 80 90
Concrete Age (Days)
I
lOO
Fig. 7 Pulloutstress against concrete age: (*) apex angle 69~
(o) apex angle 114~ (A) epoxy-repairedspecimens with apex
angle 114~
A
Z
- ,----.
eo
(3C
X
&
uJ
u_
o=
a_
60--
50--
t,0--
30--
20-
10 -
I ! I I I I I I I I I
0 10 20 30 /+0 50 60 70 80 90 100 110
Put[out DamageForce (kN)
Fig. 8 Pulloutforceofrepairedspecimensagainstinitialdam-
age force.
repaired specimen. Epoxy-repair strength ascends to a
maximum of 43.5 kN, corresponding to initial damage
occurring at 78.0 kN. Repair strengths diminish for high-
er initial damage loads.
8.7 Correlation between pullout force and frustum mass
Specimens possessing an apex angle of 69~consistently
yielded linearly increasing pullout loads as the frustum
mass became greater for each of the ages tested. Speci-
mens possessing an apex angle of 114~ displayed no
consistent relation.
8.8 Correlation between pullout force and frustum
volume
Pullout load increases linearly as the frustum volume
becomes greater for specimens of apex angle 69~. Speci-
mens of apex angle 114~ exhibit no consistent relation.
9. REPAIR STRENGTH AS A FUNCTION OF
CONCRETE STRENGTH
9.1 Procedure
A further set of 25 specimens were subjected to initial
pullout damage and subsequent repair, utilizing existing
loading geometry. Aggregate, sand and cement propor-
tions were maintained as in the previous study, with
water contents varied to yield a set of differing concrete
strengths at a constant specimen age. Initial damage was
induced at 14 days, followed by repair testing at 21 days.
Water/cement ratios of concretes were 0.30, 0.35, 0.37,
0.40, 0.5 and 0.6.
9.2 Correlation between pullout load and cylinder
strength
Fig. 9 shows a linear curve which was fitted to the initial
pullout results for the apex angle of 69~ A correlation
coefficient (R2) of 0.829 was obtained for the regression.
Results from pullouts conducted on the epoxy-re-
paired specimens are also presented in Fig. 9. The curve
shows a maximum pullout force of 44.5 kN correspond-
ing to a cylinder strength of 39.9 MPa. Pullout strengths
were found to diminish at higher cylinder strengths.
9.2 Correlation between pullout stress and cylinder
strength
A linear curve was fitted to the data for the apex angle of
69~ as shown in Fig. 10. A correlation coefficient (R2) of
120
I00
ao
60
g co
20
1 0_
I I I I I I I t I
0 10 20 30 40 50 60 70 80 90
Compressive Cytinder Strength (MPa)
Fig. 9 Pulloutforce against compressive cylinderstrength
(specimens ofidenticalage): (9 apex angle 69~ (o) epoxy-
repaired specimens with apex angle 114~
7
r~
7.0 --
6.0--
50--
4.0--
3.0-
2.0-
1.0 -
iii
I [ I t ! 1 I I I
10 20 30 ~.0 50 60 70 80 90
Compressive Cytinder Strength (MPa)
Fig. 10 Pullout stress against compressive cylinderstrength
(specimens ofidentical age): (e) apex angle 69~ (o) epoxy-
repaired specimens with apex angle 114~

6. Materials and Structures 285
0.792 was calculated for the regression. Specimens re-
paired with epoxy mortar display a maximum as also
shown in Fig. 10. A maximum pullout stress of 2.30 MPa
corresponds to a cylinder strength of 54.5 MPa.
9.3 Correlation between pullout damage load and
pullout force of repaired specimen
Results of damaged and repaired specimens are plotted
in Fig. 11. Epoxy-repair strength ascends to a maximum
of 46.8 kN corresponding to an initial damage of
88.0 kN. Repair strengths diminish for higher initial
damage loads.
60-
5o-
40-
301-
20-I.I--
~ 10-0
a_ 0
4O
I f [ I I I [ I I I I
50 60 70 BD 90 100 110 120 130 140 150
Puttout Damage Force (kN)
Fig. 11 Pulloutforce (epoxypatch) against damage force.
10. DISCUSSION
A linear relationship was obtained between pullout
strength and compressive cylinder strength. A higher
correlation was found for specimens whose loading
geometry was of higher apex angle. This may be attri-
buted to the greater failure surface area of those speci-
mens, leading to a smaller percentage area of the failure
surface that an aggregate particle would occupy.
If the pullout force is directly proportional to the
number of aggregate particles crossing a failure surface,
then some of the scatter of results is likely to be caused
by the random manner in which the aggregates are
located along the failure surface.
Pullout strength was found to develop most rapidly at
early ages. As maturity reached 90 days, strength de-
velopment approached a limiting value as the rate of
hydration diminished.
Epoxy-repaired specimens possessed a similar rapid
strength gain at early concrete ages. A maximum load
was reached, beyond which pullout strengths were of
diminishing magnitude. Beyond that maximum load,
the failure planes were observed to occur at the epoxy-
concrete interface. A similar diminishing magnitude of
pullout strengths was witnessed for specimens of high
concrete strengths.
Epoxy-repaired specimens displayed a trumpet-
shaped geometry of failure for values below the max-
imum pullout load (Fig. 4). Beyond that maximum load,
the failure planes were observed to occur predominantly
at the epoxy-concrete interface.
The observation of a pullout strength maximum for
epoxy-repaired specimens raises questions about the
nature of the repair. A greater degree of damage yields a
less sound repair plane and a smaller likelihood of a
successful repair. The pullout test creates a zone of
cracking beyond the surface of the frustum cavity. As
this zone of cracking increases, the likelihood of a sound
epoxy-concrete repair plane diminishes. The zone of
cracking increases with increasing pullout load, until a
stage is reached where repairs to the frustum tend to fail
at the epoxy-concrete interface. As a greater degree
of damage is developed across the repair interface, the
epoxy-concrete bond diminishes, and repair strengths
correspondingly diminish. An alternative explanation,
that the bond strength of the epoxy has now been
reached, is probably not tenable as there continues to be
a fall-off in the values obtained.
Epoxy-concrete bond degradation with time may also
contribute to failure at the epoxy-concrete interface.
Repair strength maxima from Figs 4 and 9 were com-
pared to ascertain the likely cause of diminution of
repair strength. Almost identical maxima were achieved
for both series of tests with respect to concrete cylinder
strength. Pullout load and stress maxima differed by
0.5 kN and 0.04 MPa, respectively. Furthermore, these
parameters fell within 1.0 kN and 0.16 MPa of the max,-
imum achieved as a function of concrete age. The
epoxy-concrete bond could be concluded not to degrade
with time, at least for the duratiorr of the experiment.
As concrete is damaged, the soundness of the repair is
diminished by a zone of cracking which extends beyond
the failure surface. Concretes of higher strengths possess
a more extensive zone of damage, leading to an
observed failure at the interface between the concrete
substrate and repair medium.
11. CONCLUSION
The pullout test for concrete can be utilized to simulate
spalling concrete. The cracking patterns displayed by
pullout test specimens typify the damage of concrete by
spalling. The test programme showed that the overrid-
ing factor which governs successful repairs to concrete is
the soundness of the repair plane.
The pullout test creates a zone of cracking beyond the
surface of the frustum cavity. As this zone of cracking
increases, the likelihood of a sound epoxy-concrete
repair plane diminishes. The zone of cracking increases
with increasing pullout load, until a stage is reached
where repairs to the frustum tend to fail at the epoxy-
concrete interface. As a greater degree of damage is
developed across the repair interface, the epoxy-con-
crete bond diminishes, and repair strengths correspon-
dingly diminish.

7. 286 Collins and Roper
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3. Malhotra, V.M., 'Evaluation of the pullout test to deter-
mine strength of in-situ concrete,' Mater. Struct. 8 (43)
(1975) 19-31.
4. Richards, O., 'Pullout strength of concrete reproducibility
and accuracy of mechanical tests,' ASTM STP 626
(American Society for Testing and Materials, Phi-
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5. Rutenbeck, T., 'New developments in in-place testing of
shotcrete', in 'Use of Shotcrete for Underground
Structural Support', SP-45 (ACI, Detroit, 1974) pp.
246--262.
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dards, V.04.02 (American Society for Testing and
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aggregate on the reliability of the pullout test,' Concr.
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