Magazine of Concrete Research, 2005, 57, No. 8, October, 463–468
Concrete-to-concrete bond strength: influence
of an epoxy-based bonding agent on a
roughened substrate surface
E. N. B. S. Júlio*, F. A. B. Branco† and V. D. Silva*
University of Coimbra, Portugal; Technical University of Lisbon, Portugal
An experimental study was performed to evaluate the bond strength between two concrete layers, using different
techniques for increasing the roughness of the substrate surface and a commercial epoxy-based bonding agent. A
total of 40 slant shear half specimens and 40 pull-off half specimens first had the substrate surface prepared by
wire-brushing, sand-blasting, chipping with a light jackhammer, or were left as-cast against steel formwork. Three
months later, the bonding agent was applied and the new concrete was added. Pull-off tests and slant shear tests
were performed to evaluate the bond strength in tension and in shear. Analysis of the results indicates that the
application of an epoxy-based bonding agent does not improve the bond strength since the adopted method for
surface preparation adequately increases its roughness.
Introduction
Adding new concrete to an existing concrete substrate is a usual technique for repairing and/or strengthening structures. The usual practice consists of first
increasing the roughness of the substrate surface and
frequently, the application of a bonding agent is also
adopted. However, this procedure is empirically based
and the efficiency of the second step has not been
proved.
The authors performed an experimental study with
the objective of quantifying the influence of the application of a bonding agent on the bond strength between
two concrete layers with different ages. The selected
bonding agent was a commercial, widely used, twocomponent epoxy resin. The methods adopted to increase the roughness of the substrate surface, before the
application of the bonding agent, were those most commonly used in practice.
From the conclusions of this study, designers can
choose the best preparation technique for their sub-
* Department of Civil Engineering, Faculty of Sciences and Technology, University of Coimbra, Portugal.
† Department of Civil Engineering, Instituto Superior Técnico, Technical University of Lisbon, Portugal.
(MCR 41246) Paper received 20 February 2004; last revised 21
March 2005; accepted 29 March 2005
strate surface in order to obtain higher bond strengths
at lower costs.
Previous research
There are some published works on adhesion of
repairing materials to a concrete substrate where bonding agents are used.1–7 Nevertheless, the results obtained by different researchers are not always in
agreement.3–6 Furthermore, due to the variability of the
parameters that influence the bond strength, it is not
possible either to generalise or to extrapolate the conclusions drawn.
According to Garbacz et al.7 the adhesion in the
repair system depends on the surface roughness of the
concrete substrate, the presence of micro-cracks and
the properties of the materials to be used for the repair.
The authors state the increasing necessity of using a
bond coat as the violence of surface treatment increases.7 Cleland and Long8 concluded that the principal function of a bonding agent is to develop a bonding
bridge between the repairing material and the concrete
substrate. Based on published works, Talbot et al.9 have
stated that the use of a bonding agent reduces the
variability of results.
In terms of the characteristics of the bonding agent,
Emmons4 states that it should be easily absorbed by the
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Júlio et al.
pore structure of the substrate and must be compatible
with both the substrate and the repairing material. This
author indicates three main types of bonding agents
that are frequently used: cement-based slurries, epoxies,
and latex emulsions.4
With regard to the bond strength, the perceived advantage of considering a bonding agent is not unanimous. Austin et al.5 have reported that bond coats can
significantly increase the adhesion between the concrete substrate and the repairing material, remarking,
however, that a misuse can lead to much lower
strengths than without a bond coat. Cleland and Long8
also indicate that, for some repairing materials, the
values of bond strength in tension are greatly reduced
if no bonding coats are used. Emmons4 points out that
adequate bonding may be developed by placing the
repairing material directly against the prepared concrete
substrate and that the use of an epoxy bonding agent
may produce a vapour barrier, resulting in the failure of
the bonding. Saucier and Pigeon3 concluded that casting new concrete directly on an old concrete base was
better than using a cement slurry with a high water/
cement ratio as a bonding agent.
test provides a sensitive means of assessing the effect
of variations in resin formulation’. The pull-off test
(Fig. 2) was chosen to evaluate the bond strength in
tension because it can be carried out in situ, which is
contrary to the slant shear test.16,17
The adopted geometry for the slant shear specimens
was a 20 cm 3 20 cm 3 40 cm prism with the interface
line at 308 to the vertical. The specimens were tested
under compression using the standard procedure for the
testing of cubes or cylinders for compressive strength.
The adopted geometry for the pull-off specimens
was a 20 cm cube with the interface line at the middle.
A core of 75 mm diameter was drilled into the added
concrete and extending 15 mm beyond the interface
into the substrate. A circular steel disc was bonded with
an epoxy resin to the surface of the core. A tension
force was applied to the disc with a commercial device
at a steady rate of 0.05 MPa until failure occurred.
For each situation considered, five slant shear specimens and five pull-off specimens were built as well as
six standard specimens to characterise the compressive
strength of the concrete substrate and of the added
concrete (three cubes for each).
All parameters that could influence the bond strength
were kept constant, apart from the roughness of the
substrate surface and the method to promote it: (a) con-
Experimental investigation
The following objectives were defined for this experimental research: (a) to quantify the influence of
applying a bonding agent on the bond strength of the
interface, considering different methods for increasing
the surface roughness; and (b) to examine the correlation between the bond strength in shear and the bond
strength in tension.
The slant shear test (Fig. 1) was chosen to measure
the bond strength in shear because, according to different researchers, this test is roughness sensitive.2,4,10–13
In 1976, Kriegh14 published that the Arizona slant
shear test was probably the most ‘meaningful and discriminating test method available’ to select an epoxy
resin. Furthermore, in 1978, Tabor15 declared that ‘this
Fig. 1. Slant shear test
464
Fig. 2. Pull-off test
Magazine of Concrete Research, 2005, 57, No. 8
Concrete-to-concrete bond strength
crete substrate mix; (b) added concrete mix; (c) age of
both concretes; (d ) temperature and relative humidity of
the environment; (e) method used for cleaning the interface surface; ( f ) moisture condition of the interface
surface; (g) bonding agent, including type and thickness;
(h) method adopted for applying the bonding agent,
including the time gaps between preparation/application
and application/casting of new concrete; and ( i ) tests
used to measure the bond strength. As a consequence of
keeping the first four parameters constant, also the
strength of both concretes and the differential shrinkage
between them were kept constant.
Preliminary tests were conducted to define the following parameters: concrete substrate mix; added concrete mix; and their ages.10 To avoid monolithic
rupture modes, it was decided to adopt the same concrete mix for both the substrate and the added concrete,
although this would never be the situation in the field.
In order to obtain shrinkage deformations between the
substrate and the added concrete, their ages were set,
respectively, at 112 days and 28 days, at the time of the
test.
A concrete mix with an estimated compressive
strength of 50 MPa was used. The constituents of this
concrete were (per m3 ) 360 kg of type I:32.5 Portland
cement, 1.6 l of a modified lignosulphonate admixture,
168 l of water, 813 kg of siliceous sand with 2.84 fineness modulus, 469 kg of limestone crushed aggregates
with 6.16 fineness modulus and 567 kg of limestone
crushed aggregates with 6.93 fineness modulus.
The adopted roughening techniques were the most
commonly used in practice. The following situations
have been considered: (1r) and (1) surface cast against
steel formwork; (2r) and (2) surface prepared with a
steel brush (Fig. 3); (3r) and (3) surface partially
chipped (Fig. 4); and (4r) and (4) surface treated with
sand blasting (Fig. 5).
For situations (1) to (4), the substrate surface preparation was followed by the application of a bonding
agent (Fig. 6). A commercial epoxy resin that is widely
used in practice was chosen. The chemical composition
of this product is: bisphenol A-epichlorohydrin resin,
with 4,4-methylene bis (cyclohexylamide), as curing
component. According to the manufacturer, this product has 90 MPa of compressive strength and 45 MPa
of bending strength. In all cases the bonding agent was
applied immediately before casting the strengthening
layer of concrete.
Fig. 3. Surface prepared with steel brush
Results and discussion
Table 1 presents the average values of the compressive strength of the original concrete and of the added
concrete, used in all eight situations. These results were
obtained with tests performed on standard cubic specimens. The average value of the bond strength in shear,
determined with the slant shear test is also given in
Magazine of Concrete Research, 2005, 57, No. 8
Fig. 4. Surface partially chipped
465
33.85
7.95
8.90
15.44
20.67
11.47
8.56
2.59
1.30
11.20
10.67
12.63
6.24
11.16
14.13
11.57
Variation
coefficient: %
Bond strength in
shear: MPa
14.17
13.54
12.05
7.48
19.69
6.42
21.63
Variation
coefficient: %
3.81
3.75
3.78
3.97
3.69
3.86
3.58
3.64
2.40
1.92
2.24
1.47
1.93
2.65
2.08
Bond strength in
tension: MPa
Added concrete:
MPa
Tensile strength
Pull-off test
Slant shear test
Júlio et al.
466
47.42
45.22
46.11
49.38
45.46
46.61
45.14
45.30
50.95
50.08
49.66
50.88
51.40
49.69
50.60
50.80
Sand-blasting
Partially chipped
Wire-brushing
As-cast against
1r
1
2r
2
3r
3
4r
4
No
Yes
No
Yes
No
Yes
No
Yes
Added concrete:
MPa
Original concrete:
MPa
Substrate surface
treatment
Table 1 for each of these situations. The corresponding
average value of the bond strength in tension, measured
with the pull-off test, and the theoretical value of the
tension strength of the added concrete are also indicated. It should be mentioned that the rupture mode,
observed in all specimens, tested with both methods,
was always an adhesive failure at the interface. For all
five specimens of situation (1r), de-bonding of the core
occurred when executing the hole and so the corresponding average value of the bond strength is not
presented.
As mentioned previously, with the exception of the
Table 1. Test results
Fig. 6. Application of the epoxy resin on the substrate surface
Situations
considered
Epoxy resin
application
Compressive strength
Fig. 5. Surface treated with sand blasting
Magazine of Concrete Research, 2005, 57, No. 8
Concrete-to-concrete bond strength
Magazine of Concrete Research, 2005, 57, No. 8
The second objective defined was to investigate the
possibility of correlation between the slant shear test
results and the pull-off test results. In Figs 7 and 8, the
average values obtained with the slant shear test are
plotted versus the corresponding average values measured with the pull-off test, without and with epoxy resin
application, respectively. Taking into account the linear
trend line that correlates the values of the graph in Fig. 7,
one concludes that there is a good correlation between
the slant shear and the pull-off tests, with a correlation
coefficient of 0.948, for the situations without epoxy
resin application. Taking into account the corresponding
values of the situations with epoxy resin application
shown in Fig. 8, one concludes that, due to the reduced
variation of test results and in spite of the occurrence
previously referred to, that correlation is approximately
constant.
Conclusions and future research
The major conclusion drawn from the experimental
study described in this paper is that the application of
Bond strength in tension: MPa
6·00
5·00
4·00
3·00
y 5 0·1886x
R2 5 0·9614
2·00
1·00
0·00
0·00
2·00 4·00 6·00 8·00 10·00 12·00 14·00 16·00 18·00
Bond strength in shear: MPa
Fig. 7. Correlation between slant shear and pull-off tests results of situations considered without epoxy resin application
6·00
Bond strength in tension: MPa
surface roughness, all variables that could influence test
results were kept constant, including the concrete mix,
the concrete constituents, their ages, and all the laboratory procedures involved. However, it should be noted
that the average value of the compressive strength of
the concrete specimens was 50.51 MPa, for the substrate, and 46.33 MPa, for the added concrete (see
Table 1). This difference was probably due to the fact
that the first halves and the second halves of the composite specimens were cast during different times of the
year and differences in temperature and relative humidity were registered in the laboratory. When analysing
the results, it must be remembered that the compressive
strength of the added concrete of situation 2 presented
an
average
value
(49.38 MPa)
that
was
significantly higher than the average value used for
reference.
Examining the results of the slant shear tests (Table
1), it can be seen that the bond strength in shear was
approximately constant, and slightly superior to
11 MPa, for all situations with epoxy resin application.
The value of 12.63 MPa, corresponding to situation 2,
may probably be justified by the fact cited above.
Analysing the results of the pull-off tests (Table 1), it
can be seen that values varied within a small range,
between 1.93 and 2.51 MPa, for all situations considered with epoxy resin application. Noting that the upper
limit corresponds to the situation with no preparation
of the substrate surface, it is probable that this difference is inherent to the test itself and not to the roughening method used.
The results indicate that there is no influence of surface roughness on the bond strength in shear, when an
epoxy resin is used as bonding agent.
The comparison between results of the slant shear
tests (Table 1), with and without application of the
epoxy-based bonding agent, demonstrates that the application of the latter on the substrate surface does not
improve its bond strength if a surface preparation method is adopted that adequately increases its roughness.
In fact, using sand blasting to prepare the substrate
surface, the bond strength in shear, registered with the
slant shear specimens, was 14.13 MPa without epoxy
resin application, and 11.57 MPa with epoxy resin application.
When the epoxy resin was applied to the substrate
surface cast against steel formwork, partially chipped,
wire brushed, and sand blasted, the visual aspect became identical. It can be said that the epoxy resin
application eliminated the roughness differences between the substrate surfaces in these situations. Therefore, it is not surprising that the bond strength in shear
of these situations was also identical. This may also
explain the higher value obtained with the slant shear
specimens treated with sand blasting without epoxy
resin application relative to the value registered with
the slant shear specimens treated with sand blasting
followed by epoxy resin application.
5·00
4·00
3·00
2·00
1·00
0·00
0·00 2·00 4·00 6·00 8·00 10·00 12·00 14·00 16·00 18·00
Bond strength in shear: MPa
Fig. 8. Correlation between slant shear and pull-off tests results of situations considered with epoxy resin application
467
Júlio et al.
the adopted bonding agent on the substrate surface does
not improve the bond strength of the interface if a
surface preparation method is chosen that adequately
increases the roughness of the substrate surface. It can
also be stated that surface roughening by sandblasting
provides a better method than applying the adopted
bonding agent as the achieved bond strength of the
interface was higher and the operation cost was lower.
As the adopted bonding agent was a commercial
epoxy resin, which is widely used, attention should be
given to this conclusion. Further experimental research
is planned to investigate the influence on the bond
strength of the method of application of this bonding
agent and other epoxy-based bonding agents will also
be considered in this study.
It was also concluded that the results of the slant
shear test and of the pull-off test may be correlated.
This is of special interest since the slant shear test can
only be performed in the laboratory whereas the pulloff test can be performed in the field and so if the
correlations have been previously defined, the bond
strength in shear can be assessed in the field.
Acknowledgements
We are grateful to SIKA, HILTI, BETÃO LIZ,
FIVINTE, DYWIDAG, PREGAIA, CIMPOR and
SECIL for their collaboration in this research project.
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Discussion contributions on this paper should reach the editor by
1 April 2006
Magazine of Concrete Research, 2005, 57, No. 8