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Effect of curing conditions on strength development in an epoxy resin

for structural strengthening

Christoph Czaderski ⇑, Enzo Martinelli 1, Julien Michels, Masoud Motavalli

Empa, Structural Engineering Research Laboratory, Überlandstrasse 129, 8600 Dübendorf, Switzerland

a r t i c l e i n f o

Article history:Received 24 February 2011Received in revised form 17 May 2011

Accepted 3 July 2011

Available online 13 July 2011

Keywords:A. Resins

B. Cure behaviour

B. Stress transfer

D. Mechanical testing

FRP (fibre reinforced polymer)

a b s t r a c t

Civil structures such as bridges and buildings can be strengthened with prestressed fibre reinforced poly-mer (FRP) strips to enhance both their stiffness and load-bearing capacity. End anchorage is a crucial

issue for prestressed FRP strips. An innovative anchorage procedure, called the ‘‘gradient anchorage

method’’ and based on the possible accelerated curing of the epoxy-resin in the end region of the FRP

strip, has recently been conceived with the aim of avoiding more invasive mechanical fastening systems.

An in-depth knowledge of the actual development of the key mechanical properties of resins under dif-

ferent curing conditions (i.e., in terms of curing temperature) is of paramount importance for employing

the above mentioned gradient method in practical applications. This paper presents experimental results

and analytical investigations aimed at developing a better understanding of the strength development of

a commercial adhesive under different curing times and temperatures. Firstly, direct tensile tests on

epoxy specimens were performed at different curing temperatures. It was shown that the necessary cur-

ing time to reach the maximum tensile strength can be significantly reduced from several hours at room

temperature to approximately 30 min at 90 C. Furthermore, higher curing temperatures reduced the

activation time after which strength starts to increase. The experimental observations are shown graph-

ically with both the activation time and reaction duration at different curing temperatures. Secondly,

pull-off bond tests were conducted on 100 mm wide and 1.2 mm thick FRP strips bonded to concrete

using epoxy adhesives cured either at 90 C for different durations or at room temperature. An opticalimage correlation system (ICS) allowed the load transfer behaviour of the inhomogeneous cured adhesive

between the FRP strip(s) and concrete to be studied. Finally, using the experimental measurements, the

bond shear stress–slip interface relationships for the different test specimens were identified in order to

present the effect of elevated curing temperatures and curing durations.

2011 Elsevier Ltd. All rights reserved.

1. Introduction

The ageing of existing civil structures and the enhancement of

safety standards in structural codes are two important reasons

for the increasing interest of the structural engineering community

in developing cost-effective strengthening techniques. For in-

stance, active reinforcement of existing members through pre-stressing techniques can significantly enhance both stiffness and

strength of those members, with a more rational and efficient

exploitation of the mechanical properties of the reinforcing mate-

rials. Using fibre reinforced polymers (FRPs) for external prestress-

ing of existing reinforced concrete (RC) members is one of the most

innovative active techniques for structural strengthening.

The main advantages derived by employing FRP in structural

strengthening are their low density, ease in mounting and superior

durability. However, similar to the case of external prestressing

with steel cables, FRP prestressed reinforcement needs a suffi-

ciently strong anchorage in order to guarantee a correct force

transfer. Besides the conventional mechanical anchorages, which,

for example, use steel plates doweled against the concrete surface,a fully adhesive anchorage system may be considered to be a more

durable solution, as no permanent steel plates and dowels are

required.

The so-called ‘‘gradient anchorage method’’ [1–3] is a possible

technical solution for avoiding mechanical fasteners and connec-

tors in RC members externally prestressed by FRP strips. This

method reduces the prestressing force at the strip ends to zero over

a certain length while the FRP strips are placed by using a special

stressing and heating device. Currently, such a device consisting

of heating elements is being developed in collaboration with an

industry partner for practical applications. In order to achieve an

efficient application of the gradient anchorage method, detailed

1359-8368/$ - see front matter 2011 Elsevier Ltd. All rights reserved.doi:10.1016/j.compositesb.2011.07.006

⇑ Corresponding author. Tel.: +41 58 765 55 11 11; fax: +41 58 765 44 55.

E-mail address: [email protected] (C. Czaderski).1 Permanent address: University of Salerno, Dept. of Civil Engineering, via Ponte

don Melillo, 84084 Fisciano, Italy.

Composites: Part B 43 (2012) 398–410

Contents lists available at ScienceDirect

Composites: Part B

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o m p o s i t e s b

http://dx.doi.org/10.1016/j.compositesb.2011.07.006mailto:[email protected]://dx.doi.org/10.1016/j.compositesb.2011.07.006http://www.sciencedirect.com/science/journal/13598368http://www.elsevier.com/locate/compositesbhttp://www.elsevier.com/locate/compositesbhttp://www.sciencedirect.com/science/journal/13598368http://dx.doi.org/10.1016/j.compositesb.2011.07.006mailto:[email protected]://dx.doi.org/10.1016/j.compositesb.2011.07.006


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information on how to prepare, place and cure the adhesive is

essential. Although plenty of experimental and theoretical

studies have been carried out on the structural behaviour of

FRP-strengthened RC structures (see, for instance, Meier [4] and

Teng et al. [5]), the effect of the curing conditions on the develop-

ment of the mechanical properties of epoxy resins generally uti-

lised for structural strengthening is one of the less investigated

issues. In this context, quantifying the effect of curing temperature

and duration on the strength development of epoxy resins could

significantly improve the preparation and installation procedures

for the gradient anchorage or similar applications.

It must be pointed out that the present investigation studied the

effect of curing temperature and duration on the stiffness andstrength evolution of adhesives. As such, it did not consider the ef-

fect of test temperature on the properties of fully cured adhesives,

a subject investigated for example in Klamer et al. [6].

In the first part of this paper, a State-of-the-art section pre-

sents information available in the literature related to the

strength development of epoxy adhesives. Following this, the re-

sults of two different types of experimental tests are presented.

On one hand, a series of direct tensile tests were performed on

samples of epoxy resin cured under different conditions with

the objective of describing the evolution of the adhesive strength.

On the other hand, six pull-off bond tests were carried out on FRP

strips glued on concrete blocks and cured in different conditions

for investigating how the interface relationship between shear

stresses and interface slip is influenced by the curing conditionsin terms of temperature and duration (see also [7]). Full-field

3D-displacements of the test specimens were measured using

an optical image correlation measurement system. Finally, differ-

ent analytical models are developed and applied for deriving the

quantitative information related to the development of the rele-

vant mechanical properties of both the adhesive and the FRP-

to-concrete interface.

2. State-of-the-art on strength development of epoxy adhesives

In general, adhesives can be classified according to either their

origin, method of bonding, end use or chemical composition. De-

tailed information about their possible classification can be foundin Mays and Hutchinson [8]. The present work focuses on the

strength development of epoxy adhesives, which are the most

commonly used for applications in structural strengthening.

2.1. Curing temperature and duration effects on strength development

The current section gives an overview of previous investigations

on the effect of curing temperature and duration on the strength

development of epoxy adhesives as reported in the scientific liter-

ature. A summary of significant contributions is outlinedin Table 1.

Kwan et al. [9] investigated several methods for rapid curing of

structural adhesives. They carried out an experimental campaign

on two kinds of structural adhesives (namely, urethane- and

epoxy-based), considering both ultrasonic and electromagnetic(radio frequency and microwave) heating techniques. The study

was aimed at demonstrating the feasibility of heating procedures

for adhesive curing other than the traditional ones based on ther-

mal heating. Different behaviours were observed for the two types

of adhesives. In particular, all the rapid curing techniques were

successful for the urethane-based adhesive with a fast enhance-

ment of the joint strength observed within a few seconds, when

tested as a single lap shear joint according to ASTM D3163-96

[10]. On the other hand, the epoxy-based resin behaved worse,

resulting in arcing and smoking while heating.

Several researchers have investigated the effect of elevated cur-

ing temperatures on the strength development of structural

adhesives.

Lapique and Redford [11] studied the evolution of some rele-vant mechanical parameters (i.e., viscosity, strength and stiffness)

of a commercial epoxy-adhesive during the curing period. They

investigated the effect of the curing temperature and quantified

the time evolution of both the strength and the elastic modulus

at room temperature. It was demonstrated that when cured at

room temperature, the strength as well as the elastic modulus in-

creased with time. Furthermore, a faster strength evolution was

observed at higher temperatures, as the same mechanical proper-

ties obtained at 23 C after 28 days could be attained at 64 C after

only 4 h.

Dodiuk and Kenig [12] observed increasing flexural strength

and flexural modulus of fibreglass epoxy composites with higher

curing temperatures. For instance, the strength achieved after 1 h

curing at 60 C was higher than that obtained after 10–20 days of curing at room temperature.

Nomenclature

F maximum tensile force during the axial tensile testsF u maximum force at failure or at initiation of debonding

during the pull-off bond test f (t ) time dependent axial tensile stress of the axial tensile

tests f a axial tensile strength of the axial tensile testst a activation time (time which is necessary to start the

curing reaction, see Fig. 15)t r reaction duration (time which is necessary to reach the

end strength, see Fig. 15)m reaction rate (see Fig. 15)n virtual intersection of the ascending branch with the y-

axist curing durationt tot full curing duration, i.e., activation time plus reaction

duration (t a + t r )t n normalized duration for fully curing in reference to

room temperature testing at 22 C

t h heating durationt c cooling durationT curing temperatureT avg mean temperature during pull-off tests

max maximum shear strength (see Fig. 17)s FRP-to-concrete interface slipsel slip at end of elastic branch (see Fig. 17)smax slip at debonding (see Fig. 17)su ultimate slipsT the total number of specimens cured at temperature T E f strip elastic modulust f strip thicknessb f strip widthL bond lengthkel elastic stiffness (see Fig. 17)nm number of displacement measurements x abscissa of the coordinate axis system xel point on the abscissa at which s( xel) = sel

C. Czaderski et al. / Composites: Part B 43 (2012) 398–410 399


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Similar observations have been made by Matsui [13], who stud-

ied the effects of curing conditions on the development of both

shear strength and stiffness for two kinds of adhesive (namely,

epoxy–polyamideamine and cyanoacrylate, only the first one beingused for structural purposes). In general, the author observed that

the minimum curing duration needed for reaching the maximum

shear strain was clearly related to the curing temperature.

Tu and Kruger [14], after testing two different epoxy resins

(with and without silica filler) in pull-out tests according to ASTM

C882-87 [15], observed an increase in bond strength with higher

curing temperature at a constant curing duration (7 days).

A large number of torque tests with a circular FRP composite

laminate bonded to a concrete cylinder were presented by Dutta

and Mosallam [16]. The investigation presented four different

epoxy-based adhesives tested at various curing temperatures rang-

ing from 34 to 49 C with different curing durations. Similar con-

clusions as those made by the previous researchers regarding

strength evolution were drawn.Although all experimental observations pointed out the huge

influence of the curing conditions on the development of the rele-

vant mechanical parameters, no quantitative formulations have

been proposed so far for expressing this relationship between, for

instance, the actual value of the tensile strength f (t ) of epoxy resinsat curing time t and the corresponding curing temperature. Onlythe following ‘rule of thumb’ is given by Mays and Hutchinson

[8], who observed that, for curing temperature higher than 5 C,

the curing duration needed for achieving the final strength of the

resin is halved for every increase of 10 C in temperature.

2.2. Influence of curing temperature on the end strength of adhesives

In addition to studying the development of the relevantmechanical properties of adhesives, various researchers have ana-

lysed the end strength of resins and its possible relationship with

the curing temperature. For instance, Dutta and Mosallam [16] ob-

served higher end torque strengths for higher curing temperatures.

Cao and Cameron [17] confirmed this observation by investigat-

ing an additional effect. Instead of using isothermal curing, consist-

ing of rapidly heating the resin specimen to the desired

temperature, the heating was carried out in various steps, starting

at a low temperature and increasing it in a step-wise manner for

different curing durations. According to the authors’ observations,

this technique leads to a more uniform curing of the adhesive layer

and, thus, results in a better mechanical performance.

Dodiuk and Kenig [12] observed both higher flexural strength

and stiffness of specimens cured at 60 C and 120 C in comparisonto epoxies cured at room temperature.

Moreover, Tu and Kruger [14] obtained higher bond strengths at

20 C than at 10 C for the same curing duration. With further heat-

ing up to 40 C, however, no further enhancement in strength was

obtained. Islam et al. [18], by means of tensile tests, obtained opti-mal values for both tensile strength and elastic modulus at a curing

temperature of 70 C, while both decreased again approaching

120 C.

To summarize, one can say that the adhesive stiffness and

strength development is highly dependent on the curing tempera-

ture and curing duration. In general, stiffness and strength devel-

opment occurs faster with higher curing temperatures.

Furthermore, some indications of higher end strength with increas-

ing curing temperature can be found in the literature. The main

goal of the current research was to investigate and quantify the

influence of curing temperature and duration on strength develop-

ment for the particular epoxy adhesive used in this investigation.

For other adhesive types with different chemical compositions, as

seen in the literature, a different behaviour might be observed.

3. Materials

All experimental tests were performed with a commercially

available epoxy based adhesive called S&P resin 220. Its measured

glass transition temperature T g was 52 C and 58 C at room and90 C curing temperatures, respectively. According to the declara-

tion of the manufacturer, the tensile bond strength on steel is lar-

ger than 14 MPa.

The FRP strip had a thickness of 1.2 mm and a width of 100 mm

with the mechanical properties listed in Table 2. The elastic mod-

ulus was determined in a loading test for each strip before the

bond tests, which are also reported in Table 2. The measured mod-

ulus of 174 GPa was almost exactly that specified by the

manufacturer.

Aggregate with particle sizes ranging from 0 to 32 mm was em-

ployed for the concrete of the blocks used for the pull-off tests, to

which the FRP strips were glued. At 28 days, a cube compressive

strength f c ,cube of 51.5 MPa was measured, whereas the splittingtensile strength f ct was found to be 3.25 MPa.

4. Experiments

Two different types of tests were carried out with the aim of

investigating both the evolution and the final values of the key

mechanical properties of the adhesives for different curing condi-

tions in terms of both temperature and duration. A commercialepoxy adhesive which can be employed for the ‘‘gradient

Table 1

State-of-the-art summary on curing temperature and time effect on epoxy strength evolution and end strength (RT = room temperature).

Author Adhesives Experiments Curing

temperature

Curing

duration

Kwan et al. [9] Urethane- and epoxy-based Lap shear test Ultrasonic, radio frequency and

microwave heating

Lapique and Redford

[11]

Araldite Uniaxial tension and 3-point bending test 23–64 C 1–28 days

Dodiuk and Kenig[12]

High-temperature epoxies Bending and lap shear tests RT, 60, 120 C 60 min to20 days

Matsui [13] Epoxy (Araldite)/instant glue

(cyanoacrylate)

Lap shear test 18–150 C 3.3–50 h

Tu and Kruger [14] Unfilled and silica filled epoxies Slant shear test 10–40 C 1–14 days

Dutta and Mosallam

[16]

Four different epoxies Torque tests with circular FRP laminates bonded on concrete 34 to 49 C 24–72 h

Islam et al. [18] Hemp fibre/epoxy composite/neat

epoxy

Fiber pull-out with interfacial shear strength measurements and

uniaxial tension

25–120 C 0.08–13.8 h

Cao and Cameron

[17]

Glass fiber reinforced epoxy Bending and shear tests RT to 150 C 5 h s tep curing

400 C. Czaderski et al. / Composites: Part B 43 (2012) 398–410


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anchorage method’’ in RC beams externally prestressed by FRP

strips was used for all tests. The experimental campaign can be

subdivided into the two following groups of tests:

– axial tensile tests on adhesive samples, to measure the develop-

ment of the adhesive axial tensile strength at different curing

temperatures;

– pull-off bond tests on FRP-to-concrete joints, aimed at describ-

ing the evolution of the interface behaviour under variable cur-

ing conditions.

The former tests were carried out for curing temperatures rang-

ing between approximately 10 and 90 C, while the latter ones

were performed at a constant mean curing temperature of approx-

imately 90 C and variable curing durations. It should be noted that

the tests were performed approximately 5–10 min after stopping

the heat transfer, which means that the temperature of the test

specimens at testing was still significantly higher than room tem-

perature (Sections 4.1 and 4.2) and therefore could influence the

test results. However, this corresponds approximately to the real

situation at the time when the prestressing force is released during

application of the gradient method.

4.1. Axial tensile tests

A large number of axial tensile tests were performed on speci-

mens as those represented in Fig. 1. The uncured resin was applied

between an aluminium cylinder with a diameter of 20 mm and a

square aluminium plate with dimensions of 60 60 mm. These

test specimens were cured at different temperatures in an oven

and tested with the pull-off tester shown in Fig. 2. The tests were

aimed at measuring the tensile strength of the resin dependingon both curing temperature and duration. It is clear that, due to

the short length of the adhesive sample, this test specimen does

not deliver the real tensile strength. Nevertheless, the actual trend

in the evolution of the tensile strength of the resin can be captured

in these tests, whose specimens are particularly convenient and

can be easily and rather uniformly heated inside a simple oven.

The evolution of the temperature of the air in the oven as well as

the temperature inside the resin layer were monitored in some

of the specimens which underwent an accelerated curing process

by heat transfer (Figs. 3 and 4).

Table 2

Mechanical properties of the FRP strips – comparison between experimental investigation and product certificate.

Test No. Experimental investigation According to S&P certificate

Modulus of elasticity E f (GPa)

Modulus of elasticity E f (GPa)

Tensile strength f u(MPa)

Elongation at 2000 MPa e2000(%)

Width b f (mm)

Thickness t f (mm)

1 169 173.9 2975 1.142 100 1.23

2 173

3 179

4 174

5 171

6 177

Mean

value

174

Fig. 1. Specimens for axial tensile tests, on the left side (Nos. 10 and 11) the

aluminium stamps and plates withapplied adhesive in between can be seen, on theright side (No. 12) the stamp including the fixture for the curing process is visible.

Fig. 2. Set-up for axial tensile tests.

Fig. 3. Axial tensile test specimens inside the oven and cables for temperaturemeasurements.



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A complete overview of the tested resin specimens is reported

in Table 3. The number of specimens cured at room temperature

as well as those which underwent an accelerated curing process

at different temperatures is also shown in the table. For the tem-

perature curing, two different ovens were used. In particular, a to-

tal of 200 tests were carried out in several sessions with variable

curing conditions: 136 tests were carried out on specimens with

accelerated curing in an oven with temperatures ranging between

45 and 90 C, and 64 tests were carried out on specimens cured at

different room temperatures, with curing durations ranging from a

few hours to 7 days.

Most of the tests were performed on specimens cured at 90 C,

which is considered as the reference temperature for the present

investigation. However, this was only the ‘‘nominal temperature’’

of the air inside the oven. The temperature development in the

adhesive resin was generally slower than that in the air and the fi-

nal temperature was slightly lower than the nominal one. A typical

temperature development observed is shown in Fig. 4, emphasis-

ing the different measurements inside the adhesive and in the oven

air. Additional tests on specimens cured at 120 C were also carried

out, but the results are not reported herein as their failure was al-

ways controlled by the loss of bonding between the resin and the

aluminium plate, possibly as a result of the difference in relative

expansion of the two materials at that temperature.

Fig. 4 also shows the cooling curve for test specimens taken out

of the oven. All the axial tensile tests were carried out after a min-

imum of 5 min of cooling outside the oven at room temperature.

Therefore, from Fig. 4, it can be concluded that the resin of the

tested specimens had a maximum temperature ranging between

60 and 75 C at the time that the tests were actually performed.

4.2. Pull-off bond tests on FRP strips glued on concrete through a layer of epoxy resin

Six pull-off bond tests were carried out with the aim of measur-

ing the strength of FRP-to-concrete joints. In particular, the time

evolution of the mechanical properties of those joints cured at high

temperature was the key issue of this investigation. Fig. 5 shows a

view of the test layout: a 100 mm wide and 1.2 mm thick FRP strip

was glued to a concrete block and pulled off after either curing at

an elevated temperature (approx. 90 C) or at room temperature.

The bond length was 300 mm and the heating was performed

using three heating elements each with a dimension of

100 mm 100 mm. The pull-off bond tests were prepared follow-

ing the main phases represented in Fig. 6. The tested specimens

were always equipped with sensors for monitoring the tempera-

ture evolution within both the adhesive layer and the heating ele-

ments. The preparation of the adhesive layer between the FRP strip

and concrete surface was followed by the heating phase (t h), car-ried out by means of heating elements. After stopping the heating

procedure, a cooling time of 5–10 min was included before starting

0

30

60

90

120

0 100 200 300

Time [mins]

T e m

p e r a t u r e [ o C ]

Room Temperature Air temp. in the oven

Sensors on the adhesive

Oven in testing hall (SH)

0

25

50

75

100

125

0 50 100 150

Time [mins]

T e m

p e r a t u r e [ ° C ]

Room Temperature Air temp. in the oven

Sensors on the adhesive

Oven room (OR)

Fig. 4. Typical temperature development during curing of the axial tensile test

specimens, including the temperature decrease due to removal of the test

specimens from the oven.

Table 3

Overview of the experimental programme for the axial tensile tests on S&P resin 220

(SH: oven in testing hall, OR: oven in oven room).

Series Tests with heating Tests w/o heating Oven

[#] n. T (C) n. t max (h)

1 12 48 –

2 4 90 SH

3 12 90 SH

4 24 24 –

5 19 90 4 23 SH

6 7 90 SH

7 12 36 –

8 16 65 8 168 SH

9 16 45 4 168 SH

10 24 90 OR

11 24 65 OR

12 14 90 OR

Total 136 64

Fig. 5. Test setup for pull-off bond tests.



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the loading test in order to cool down the adhesive (t c ). Afterwards,one can observe the force increasing up to the maximum value F u.It can be observed that the temperature during the experiment was

not constant, but slowly decreased. The heating and cooling times,

as well as the average test temperature (T avg ), are given in Table 4.The cooling time was chosen so that the existing temperature in

the adhesive was safely below its glass transition temperature.

The full-field 3D displacements of the test specimen were mea-

sured during the experimental investigations with an optical

image correlation measurement system (ICS) [19]. A description

of the measurement system used can be found in Czaderski et al.

[20], Czaderski and Rabinovitch [21]. With the ICS, the shape of

the surface was also measured. Fig. 7 shows the isolines of the sur-

face of the strip and concrete before starting the test. It can be seen

that the surface of the strip was not planar, but was curved with a

maximum vertical value at the surface centre. This was the result

of the application process. Hence, a larger adhesive quantity can

be observed in the strip centre. Over the whole bonding area, an

average value can be obtained. The latter can subsequently be

transformed into the mean adhesive thickness by subtracting the

strip thickness. A summary of the measured adhesive thicknesses

is given in Table 4.

The heating duration range of 15–25 min was chosen based on

the results observed in the previous campaign, while the waiting

time for cooling was chosen so that the adhesive temperature

was lower than approximately 50 (Table 4). Comparing Figs. 4

and 6, a key observation is the temperature development, and con-

sequently, the rate of the heat transfer in the axial tensile tests and

pull-off bond tests. In particular, the temperature developed much

faster in the second series of tests as a result of greater heat trans-

fer by using heating elements in contact with the CFRP strips com-

pared to heating the air in the oven.

5. Results

The large number of axial tensile tests produced plenty of

experimental results useful for understanding the development

of the axial strength of the epoxy-resin used in this study at differ-

ent curing temperatures. The strength was simply obtained by

dividing the failure load by the nominal cross-sectional area of

the adhesive layer.

On the other hand, analysing the pull-off bond tests performed

on the FRP-to-concrete adhesive joints was much more demanding

because the properties of the adhesive interface could not be

Fig. 6. Adhesive temperature (T 1, T 2, T 3) and force development of Test No. 3 (pull-off bond tests), t h: heating duration, t c : cooling duration, T avg : mean temperatureduring pull-off test, F u: ultimate force, L = bond length, T 1, T 2, T 3 = temperatures inthe adhesive at the positions 1, 2, 3 (see schematic drawing).

Table 4

Overview of the experimental programme for the pull-off bond tests.

Test

No.

Mean

distance

concrete

surface-

upper

strip

surface

(mm)

Strip

thickness

(mm)

Mean

adhesive

thickness

(mm)

Heating

duration

t h (min)

Waiting

time to

failure

test t c (min)

Average test

temperature

T avg (C)

1 4.85 1.2 3.65 15 5.7 45

2 5.14 1.2 3.94 25 8.6 43

3 5.00 1.2 3.80 20 5.8 41

4 4.90 1.2 3.70 25 7.7 42

5 4.74 1.2 3.54 25 2 days 19

6 4.93 1.2 3.73 – 3 days 18

Fig. 7. Isolines of the surface of the CFRP strip before the start of the pull-off test determined by using the ICS.

Table 5

Results of the axial tensile tests on specimens cured at room temperature (n = number

of tests, T = curing temperature, t = curing time, F = maximum tensile force, f a = axial

tensile strength).

Series n T (C) t (h) F (kN) f a (MPa)

1 12 24–26 4.0–48.0 0.08–6.15 0.25–19.6

4 24 22 4.0–24.0 0.03–6.18 0.1–19.7

5 4 24 7.0–22.9 1.49–6.13 4.7–19.5

7 12 10 6.3–36.0 0.03–5.06 0.1–16.18a 4 10 48–168 4.40–4.95 14.0–15.8

8b 4 22 48–168 5.76–6.44 18.3–20.5

9 4 22 48–168 5.84–6.41 18.6–20.4



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directly measuredby those tests, but had to be derived by means of

an indirect identification procedure [20,22].

This section presents the ‘‘raw results’’ obtained by the experi-

mental tests; only a few phenomenological observations are re-

ported. An analytical model for deriving the key parameters of

the corresponding interface bond law is discussed in Section 6.


The results of the axial tensile tests carried out on the adhesive

specimens cured at room temperature are reported in Table 5. The

table reports both the minimum and maximum nominal curing

temperatures and the duration (in hours) of each series. Further-

more, the minimum and maximum axial forces at failure F andthe corresponding mean axial strengths f a are given for each series.Here, axial strengths were evaluated under the assumption of a

uniform distribution of axial stresses on the circular transverse

section of the adhesive layer. As discussed before, this mean axial

strength is not the correct tensile strength because of the short

length of the test specimen. However, it allows the strength devel-

opment at different curing temperatures to be studied. The results

for the specimens cured at higher temperatures are reported in

Table 6 following the same procedures explained above.

Several observations may be made on the basis of these exper-

imental results. Firstly, the maximum values of the axial strength f aobtained from the specimens cured either at room temperature or

within the oven are quite close to one another, being in both cases

about 18–20 MPa. However, a closer comparison between the dif-

ferent curing conditions reveals slightly lower end strength

reached at 10 C. This temperature resulted in an end strength of

16 MPa, whereas higher temperatures led to higher resistances of

about 18–22 MPa. This is in accordance with the findings of the

researchers mentioned above, especially Tu and Kruger [14], who

found a very similar bond strength enhancement from about 15

to 18.5 MPa in the same curing temperature range of 10–20 C.

However, in order to verify this finding, further detailed experi-

mental investigation is needed. In general, the presented strength

range corresponds to the declaration of the distributor of the mate-

rial, as previously described in Section 3.

On the other hand, the key difference between the various cur-

ing temperatures lies in the time span necessary for reaching the

end strength. For the case of specimens cured at room temperature

it is in the order of 12 h to a few days, whereas it drops to a few

hours in the case of specimens cured in the oven at higher temper-

atures (45–90), see Fig. 12. Consequently, the effect of heat trans-

fer on the curing process of resins is quantitatively relevant andthe

relationships between the curing temperature and the develop-

ment of strength in the resin should be approximated with expo-

nential or linear functions, as reported in detail in Section 6.

In all the tests, failure developed throughout the adhesive layer

with no significant loss of adhesion observed at the aluminium-to-

resin interface. Consequently, the strength values strictly refer to

the adhesive and are not significantly affected by the testing sys-

tem. Fig. 8 illustrates two relevant failure modes of the resin layer,

the first observed either at low curing temperature or short curing

duration (Fig. 8a), and the second observed after the activation of

the chemical reactions resulted in the hardening of resin ( Fig. 8b).

5.2. Pull-off bond tests

The results of the pull-off bond tests carried out on the six FRP-

to-concrete specimens are given in Table 7 as a function of the

heating duration and waiting time before the test started. Fig. 9

shows the maximum measured force or the force at initiation of

debonding as a function of the curing duration. The characteristic

‘‘initiation of debonding’’ stage was defined as the decisive stage

to determine the bond shear stress–slip relationship, although

the force can slightly increase afterwards. A discussion related to

that topic can be found in Czaderski et al. [20]. The result of Test

No. 6, carried out on a specimen cured at room temperature for

3 days, is considered as a reference. As indicated in Fig. 9 and Table

7, the absolute strength and displacement values of Test No. 2 have

to be excluded from a detailed analysis, as problems occurred with

the test setup (complete concrete block displacement occurred in-

stead of a relative movement between the concrete and FRP strip).

It can be observed that the specimens cured at an elevated temper-

ature for 15 or 20 min exhibit significantly lower strengths. More-

over, all the specimens cured for 25 min (tested either a few

minutes after the heating procedure or after 2 days) failed at forces

higher than that observed for the reference specimen cured at

room temperature.

The failure modes observed in the various tests confirm the

above quantitative observations. Fig. 10 shows the interfaces of

Table 6

Results of the axial tensile tests on specimens cured in an oven (n = number of tests,

T = curing temperature, t = curing time, F = maximum tensile force, f a = axial tensile

strength).

Series n T (C) t (h) F (kN) f a (MPa)

2 4 90 1.0–1.3 2.87–3.88 9.1–12.35

3 12 88–90 0.8–4.0 1.36–5.66 4.3–18.0

5 19 88–89 0.7–3.7 1.28–7.14 4.1–22.7

6 7 90 0.4–0.6 0.23–2.03 0.7–6.58 16 65 0.7–2.7 0.05–6.41 0.16–20.4

9 16 45 0.7–4.0 0.0–6.10 0.0–19.4

10 24 90 0.25–2.5 0.0–7.2 0.0–22.9

11 24 65 0.52–2.5 0.31–5.94 1.0–18.9

12 14 90 0.3–0.7 0–3.4 0–10.8

Fig. 8. Axial tensile tests: typical failure modes.



8/13

the test specimens after failure. For test specimen No. 1, the resinwas not completely cured after only 15 min, which resulted in a

failure process in the adhesive layer without any cracks developing

in the concrete. A substantially similar behaviour was observed for

the specimen cured for 20 min (Test No. 3), although a small con-

crete failure area is visible. On the contrary, the other specimens

failed with a fracture developing throughout the concrete sub-

strate (Test Nos. 2, 4 and 5) as was the case for the reference test

(No. 6).

Another interesting observation is presented in Fig. 11, where

the isolines of the displacements in the direction of the applied

pulling force ( x-direction) determined with the ICS for specimensNos. 4 and 6 are shown. One can observe a significant difference

between the behaviours in terms of displacements measured.

Whereas the displacement throughout the central axis is signifi-cantly lower than the one on the outer borders for Test No. 4, the

opposite occurs for the reference Test No. 6. A reason for this dis-

crepancy lies in the inhomogeneous heating process over the strip

width. During curing with the heating elements, a lower tempera-

ture develops at the strip edges leading to a lower stiffness and

strength compared to the central part of the strips. Therefore, high-

er displacements can develop at the strip edges. The reference Test

No. 6 shows the typical behaviour given in the literature, e.g., [23],

whereby the centre part of the strip exhibits higher displacements

than the edge parts of the strip.

In addition to the direct measurement of the ultimate load lead-

ing to the failure of the FRP-to-concrete joint, the pull-off bond

tests are analysed in the next section with the aim of deriving

the bond shear stress–slip interface law of the FRP-to-concreteinterface and their evolution depending on the heating duration.

6. Analytical calculations


The experimental observations derived by the axial tensile tests

show that the strength f a is negligible in the first stages of the cur-ing phase. The chemical reactions resulting in the hardening of the

resin and the development of its mechanical properties actually

begin after a certain time span, referred to here as the ‘‘activation

time’’ t a. After this time, the mechanical properties (i.e., the axialstrength f (t )) develop quickly, with rates depending on the curingtemperature, up to a maximum asymptotic value (the end

strength) which is almost independent of the curing conditions.

The following trilinear relationship was considered, as it clearly

represents the key features of the observed behaviour in terms of

activation time (t a = n/m), reaction rate (m) and asymptoticstrength value f a (end strength):

f ðt Þ ¼ m t þ n with 0 6 f ðt Þ 6 f a: ð1Þ

Table 7

Results of the pull-off bond tests (F u = maximum force at failure or at initiation of

debonding, smax = maximum bond strength, sel = slip at maximum bond strength,

smax = maximum slip at full debonding, wt : waiting time to failure test 2 or 3 days).

Test

No.

Heating

duration (min)

F u(kN)

smax

(MPa)

sel(mm)

smax(mm)

Failure

mode

1 15 18.4 0.8 0.602 0.652 Adhesive

2a 25 60.4 5.1 0.205 0.408 Concrete

3 20 34.4 1.4 0.803 0.803 Mainlyadhesive

4 25 75.7 4.4 0.426 0.712 Concrete

5 25 + wt 62.0 7.7 0.020 0.248 Concrete6 wt 57.6 7.9 0.031 0.218 Concrete

a F u is not the force at failure, due to problems with the test set-up.

0

20

40

60

80

15 20 25 25* 25 + 2 days

waiting time

no heating +

3 dayswaiting time

Curing duration [min]

M a x i m u m

f o r c e F u

[ k N ]

T e s t N o . 1

T e s t N o . 3

T e s t N o . 4

T e s t N o . 2

*

T e s t N o . 5

T e s t N o . 6

Fig. 9. Maximum force at failure or at initiation of debonding according to the

applied curing duration and waiting time before testing (⁄F u of Test No. 2 is not theforce at failure, due to problems with the test set-up).

Fig. 10. Pull-off bond tests at different curing temperatures: observed failure

modes at the concrete-FRP interface.



9/13

A least-squares calibration of Eq. (1) was performed for every set of

sT experimental results reported in Tables 5 and 6 and derived at thevarious temperatures T reported in Table 8:

ð m; nÞT

¼ argminðm;nÞ

XsT

i¼1

ð f ðt i;m;nÞ f a;iÞ2" #; ð2Þ

where t i is the duration of curing and f a,i the observed axial strengthof the ith specimen cured at temperature T (out of the total sT curedat the same temperature).

The results of the calibrations in terms of m, n, and other relatedparameters, are reported in Fig. 12 for the six relevant tempera-

tures T considered in the curing process of the test specimens.The diagrams are sorted in Fig. 12 from lower to higher curing tem-

peratures and the effect of the heating process on the time evolu-

tion of the axial strength f a is clearly visible. For instance, theactivation time, which is about 6 h in the case of roomtemperature

(T = 22 C), is sharply reduced to less than half an hour in the caseof the higher oven temperature (T = 90 C). Fig. 13 shows the expo-

nential reduction of the activation time t a as a function of the cur-ing temperature T ; the corresponding exponential interpolation is

Fig. 11. ICS image of displacement in x-direction (pulling direction) of the FRP strip and the concrete surface.

Table 8

Results of the calibration of the tri-linear model for the different temperatures

(T = curing temperature, m = reaction rate, n = virtual intersection of the ascending

branch with the y-axis, t a = activation time, t r = reaction duration, t tot = full curing

duration, t n = normalized duration for full curing relative to room temperature testing

at 22 C, f a = axial tensile strength).

T (C) m(MPa/h)

n(MPa)

t a = n/m(h)

t r (h)

t tot (h)

t n(–)

f a(MPa)

10 1.12 12.72 11.37 13.53 24.90 1.63 15.15

22 2 11 5.49 9.77 15.26 1.00 19.57

25 2.32 10.57 4.56 8.11 12.67 0.83 18.82

45 9.37 14.13 1.51 2.00 3.51 0.23 18.8

65 13.06 8.03 0.61 1.34 1.95 0.13 17.68

90 18.99 5.36 0.28 0.88 1.16 0.08 16.79



10/13

0

5

10

15

20

25

0 10 20 30 40 50 0 10 20 30 40 50

0 10 20 30 40 50

Curing Duration [h]

S t r e n g t h [ M P a ]

Test results

Tri-Linear Model

Nominal Temperature

~10o

C

0

5

10

15

20

25

Curing Duration [h]


Test results

Tri-Linear Model

Room Temperature

22o

C

0

5

10

15

20

25

Curing Duration [h]

S t r e n

g t h [ M P a ]

Test results

Tri-Linear Model

Room Temperature25

oC

0

5

10

15

20

25

0 1 2 3 4 5

Curing duration [h]

S t r e n

g t h [ M P a ]

Test results

Tri-Linear Model

Nominal Oven Temp.T=45ºC

0

5

10

15

20

25

0 1 2 3 4 5

Curing duration [h]


Test results

Tri-Linear Model

Nominal Oven Temp.

T=65ºC

0

5

10

15

20

25

0 1 2 3 4 5

Curing duration [h]


Test results

Tri-Linear Model

Nominal Oven Temp.

T=90ºC

Fig. 12. Axial tensile strength evolution and calibration of the tri-linear law for the specimens cured in an oven at different temperatures.

tstart = 20.828e-0.0574 T

R2 = 0.9843

0.1

1.0

10.0

100.0

0 10 20 30 40 50 60 70 80 90 100

T [oC]

t a [ h ]

Fig. 13. Axial tensile tests: calibrated relationship between curing temperature andactivation time.

m = 0.0332 T1.4143

R2 = 0.9545

0.1

1.0

10.0

100.0

100101

T [oC]

m [ M P a / h ]

Fig. 14. Axial tensile tests: calibrated relationship between curing temperature andreaction rate.



11/13

drawn as well as its analytical expression and the corresponding

coefficient of determination R2 with respect to the experimentaldata. In addition, Fig. 14 displays the relationship between the

reaction rate m and the curing temperature T , suggesting the expo-nential interpolation shown along with the corresponding value of

the coefficient R2. In both cases, the proposed interpolations are invery good agreement with the observed data and the correspond-

ing coefficients of determination are rather close to unity.

Finally, the relationship (1) between the actual strength f (t )developed in the resin at time t and the curing temperature can

be utilised for generating a complete chart representing the evolu-tion of the ratio f (t )/ f a as a function of time t at different curingtemperatures T (see Fig. 15).

The duration necessary for developing the final asymptotic

strength can be obtained by adding the activation time to the reac-

tion duration. Fig. 16 presents the normalised duration for com-

plete curing (room temperature of 22 C is taken as the reference

value) compared to the earlier mentioned ‘rule of thumb’ by Mays

and Hutchinson [8]. A good agreement between the experimental

results and the suggested rule of thumb found in the literature

can be observed.

6.2. Pull-off bond tests

The key objective of this part of the investigation was to identifythe general bilinear bond shear stress–slip relationship repre-

sented in Fig. 17 [24] . Three mechanical parameters completely

define the relationship between the shear stresses s f and the inter-

face slip s:

– the maximum shear strength smax;

– the interface slip sel at the end of the elastic branch of the inter-face relationship;

– the interface slip smax at debonding.

Displacements were monitored during the tests by means of an

advanced optical measurement system [19]. The slip was

calculated from the difference between the displacements of the

CFRP strip (section 0 in Fig. 11) and the concrete surface (Sections5 and 6 in Fig. 11) beside the strip.

The maximum observed displacement at the last stage before

failure or initiation of debonding was taken as smax. In particular,three load stages were considered in the present study at three dif-

ferent load levels F (i) (i = 1, 2, 3) (see Fig. 18).

Fig. 15. Calibrated relationship for strength development depending on heating duration at different curing temperatures (the activation time and reaction duration asindicated in the graph are only valid for T = 60 C).

Fig. 16. Normalized duration for complete curing (activation time plus reaction

duration) dependent on the curing temperature: experimental measurements incomparison with the rule ‘rule of thumb’ according to Mays and Hutchinson [8].

Fig. 17. General bi-linear relationship for the FRP-to-concrete interface.



12/13

The following equations and descriptions explain the modelling

approach for both the elastic and the post-elastic stages.

6.2.1. Elastic stage

The elastic stiffness kel =s

max/sel (Fig. 17) can be identified bymeans of a numerical calibration of the following relationship, rep-resenting the interface displacement field due to an axial force F applied at the end of a bonded joint behaving elastically through-

out its length L [22]:

sð x;x; F Þ ¼ F

xE f b f t f

coshx x

sinhxL ; ð3Þ

where E f is the Young’s modulus of the composite strip, b f and t f arethe width and thickness of its transverse section and the parameter

x is defined as follows:

x ¼

ffiffiffiffiffiffiffiffikelE f t f

s : ð4Þ

Considering the first load level F (1) andassuming that the behav-iour under this load remains elastic, the optimal value of x (and the

corresponding value of the elastic stiffness kel) can be derived bysolving the following least-squares minimization problem:

x ¼ argminx

Xnmi¼i

½sð xi;x; F ðiÞÞ si;exp

2

( ); ð5Þ

where nm is the number of displacement measurements si,exp avail-able at the same load stage.

6.2.2. Post-elastic stageThe post-elastic response of an adhesive joint can be modelled

by a more complicated analytical relationship, where the depen-

dent variables are smax and smax = su. The explicit analytical expres-sion of that relationship is omitted here, but can be found in

Czaderski et al. [20] or Faella et al. [22]; it can be written symbol-

ically as follows:

s ¼ sð x;smax; su; xelÞ; ð6Þ

indicating that the value of the interface slip s at a point on the ab-scissa x throughout the bonded length L depends on both the post-elastic branch of the bi-linear law represented in Fig. 17 (namely, on

the maximum shear strength smax and the ultimate slip su) and thepoint on the abscissa xel throughout the adhesive interface at whichs( xel) = sel.

The resulting external force F can be evaluated through anotherrelationship whose expression is reported below as a function of

the same variables:

F ¼ F ðsmax; su; xelÞ: ð7Þ

Consequently, the optimal values of the parameters smax and xelcan be derived imposing the following two conditions:

– the interface slips evaluated using Eq. (6) result in values as

close as possible to the observed values si,exp;– the external force F evaluated using Eq. (7) is as close as possible

to that applied during the experimental test.

The above conditions can be mathematically stated through the

following least-squares optimization problem with reference to

the third load stage F (3) (namely, resulting in specimen failure)[22]:

ðsmax; xelÞ ¼ argminðsmax ; xelÞ

Xnmi¼i

½sð xi; smax; su; xelÞ si;exp2

s2u

(

þ F ðsmax; su; xelÞ F

ð3Þ

F ð3Þ

" #29=;: ð8Þ

The results of the optimization procedure are reported in Table

7 in terms of the resulting values of smax, sel, and smax = su.The bilinear interface relationships corresponding to the above

calibrations are presented in Fig. 19, which demonstrates the effect

of accelerated curing on the mechanical properties of the FRP-to-

concrete interface. The relationships derived from Test Nos. 1

(15 min heating duration) and 3 (20 min) are characterized by a

very low stiffness and no decreasing branch, as the resin is not fully

cured. On the other hand, a significantly higher value of the

stiffness is observed in Test No. 4 (25 min heating duration), but

the stiffness is still clearly lower than that of the fully cured pull-

off tests. Also, the value of the strength smax in Test No. 4 is lower

than that for Test Nos. 5 and 6. Finally, the interface relationships

derived for Test Nos. 5 and 6 are rather close to one another, con-

firming the fact that the accelerated curing procedure (carried out

0.0

0.1

0.2

0.3

0 50 100 150 200 250 300

0 50 100 150 200 250 300

x [mm]

I n t e r f

a c e s l i p - s [ m m ]

62.0 kN 62.0 kN

40.5 kN 40.5 kN

20.1 kN 20.1 kN

Test no.5

Experimental

results

Calibrated

model

load increase

x

Pulling directionStrip

Concrete

0.0

0.1

0.2

0.3

x [mm]

I n t e r f a c e s l i p - s [ m m ]

57.6 kN 57.6 kN

40.1 kN 40.1 kN

20.1 kN 20.1 kN

Test no.6

Experimental

results

Calibrated

model

load increase

Fig. 18. Measured relative displacements for different loading steps in x-direction(longitudinal to the strip direction) between the strip and the concrete surface

referred to as interface slip and simulated slips after the calibration procedure.

0

1

2

3

4

5

6

7

8

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Interface slip - s [mm]

B o n d s h e a r s t r e s s - τ [

M P a ] Test. no.1

Test. no.3

Test. no.4

Test. no.5

Test. no.6

fully cured

25 mins

20 mins

15 mins

Fig. 19. Pull-off bond tests: resulting interface relationships between shear stressesand slips.



13/13

for Test No. 5) does not affect the final behaviour of the resin (as

obtained in Test No. 6, the reference test).

7. Conclusions

The presented experimental and analytical results lead to sev-

eral conclusions. As expected, the axial tensile tests revealed a con-

siderable influence of the curing temperature on strength evolutionof the epoxy adhesive. The necessary curing duration for com-

pletely developing the end strength of the adhesive, composed of

the reaction duration and the activation time, was significantly re-

duced at higher curing temperatures. On the basis of the axial ten-

sile tests, a complete chart representing the evolution of the

normalized tensile strength f (t )/ f a as a function of time t at differentcuring temperatures T was presented (Fig. 15). The tests demon-strated that to develop almost the same end strength, curing dura-

tions ranging from approximately 1 day at 10 C to a slightly more

than one hour at 90 C were required. Moreover, the values of cur-

ing duration obtainedat the various temperatures were foundto be

in good agreement with a ‘rule of thumb’ estimate available in the

scientific literature (see Section 2.1). No significant variation in end

strength was found at higher curing temperatures. However, fur-

ther experiments are needed in order to confirm this observation.

The investigations on the FRP-to-concrete interface at 90 C cur-

ing temperature showed that 15–20 min of curing resulted in an

adhesive failure, which means that the strength of the adhesive

layer had not completely developed for that duration and temper-

ature. On the other hand, the specimen cured for 25 min at 90 C

exhibited the same failure mode observed in the pull-off test on

the reference specimen, which was cured for 3 days at room tem-

perature. The experimental results in terms of the distribution of

interface slip throughout the bond length at different load levels

were used to identify the shear stress–interface slip relationships

for the various specimens cured at 15, 20 and 25 min and for the

reference one cured in the usual way. Besides the above mentioned

adhesive failure at the shortest two curing durations, curing for

25 min induced a lower elastic stiffness, a lower maximum bond

strength and a larger total slip displacement as compared to the

fully cured specimens. The displacement measured over the strip

width by optical image correlation revealed the need for optimis-

ing the performance of the heating elements in order to obtain a

more homogeneous heat transfer and temperature within the

adhesive layer.

Finally, the quantitative results obtained in the present study

are of key importance for enhancing the practical implementation

of the gradient anchorage method for prestressed FRP strips in RC

beams (Czaderski and Motavalli [1], Aram et al. [2]). Defining the

optimal anchorage length, the number of steps and the heating

temperature and duration to be used for implementing the above

mentioned procedure are among the next objectives of this

research.

Acknowledgements

The financial support of the Swiss innovation promotion agency

(CTI) of Switzerland is acknowledged (Project Number KTI Nr.

10493.2 PFIW-IW). Furthermore, the financial support and delivery

of the test materials of the industrial partner of the project, S&P

Clever Reinforcement from Seewen, Switzerland, is also

appreciated.

References

[1] Czaderski C, Motavalli M. 40 years-old full-scale concrete bridge girder

strengthened with prestressed CFRP plates anchored using gradient method.Compos Part B: Eng 2007;38(7–8):878–86.

[2] Aram MR, Czaderski C, Motavalli M. Effects of gradually anchored prestressed

CFRP strips bonded on prestressed concrete beams. J Compos Constr ASCE

2008;12(1):25–34.

[3] Motavalli M, Czaderski C. FRP composites for retrofitting of existing civil

structures in Europe: state-of-the-art review. In: ACMA, 2007. Tampa, USA,

October 17–19; 2007.

[4] Meier U. Strengthening of structures using carbon fibre/epoxy composites.

Constr Build Mater 1995;9(6):341–51.

[5] Teng JG, Chen JF, Smith ST, Lam L. FRP strengthened RC structures. Chichester

(England): John Wiley & Sons, Ltd.; 2002. p. 245.

[6] Klamer EL, Hordijk DA,Hermes MCJ. Theinfluenceof temperature on RC beams

strengthened with externally bonded CFRP reinforcement. Heron

2008;53(3):157–85.

[7] Czaderski C, Martinelli E, Michels J, Motavalli M. Effect of partly cured

adhesives on bond-slip-relationship of FRP-concrete interface. In: SMAR, first

middle east conference on smart monitoring, assessment and rehabilitation of

civil structures, 2011. Dubai, UAE, February 8–10; 2011. p. 8.

[8] Mays GC, Hutchinson AR. Adhesives in civil engineering. Cambridge University

Press; 1992.

[9] Kwan KM, Cheng K, Benatar A. Feasibility study of raid curing of structural

adhesives. In: Proceedings of the ANTEC ‘98. Atlanta; 1998. p. 1089–94.

[10] ASTMD3163-96. Standard test method for determining strength of adhesively

bonded rigid plastic lap-shear joints in shear by tension loading.

[11] Lapique F, Redford K. Curingeffects on viscosityand mechanical properties of a

commercial epoxy resin adhesive. Int J Adhes Adhes 2002;22(4):337–46.

[12] Dodiuk H, Kenig S. Low temperature curing epoxies for structural repair. Prog

Polym Sci (Oxford) 1994;19(3):439–67.

[13] Matsui K. Effects of curing conditions and test temperatures on the strength of

adhesive-bonded joints. Int J Adhes Adhes 1990;10(4):277–84.

[14] Tu L, Kruger D. Engineering properties of epoxy resins used as concrete

adhesives. ACI Mater J 1996;93(1):26–35.

[15] ASTMC882-87. Standard test method for bond strength of epoxy-resins used

with concrete.

[16] Dutta PK, Mosallam A. A rapid field test method to evaluate concrete

composite adhesive bonding. Int J Mater Prod Technol 2003;19(1–2):53–67.

[17] Cao Y, Cameron J. The effect of curing conditions on the properties of silicamodified glass fiber reinforced epoxy composite. J Reinf Plast Compos

2007;26(1):41–50.

[18] Islam MS, Pickering KL, Foreman NJ. Curing kinetics and effects of fibre surface

treatment and curing parameters on the interfacial and tensile properties of

hemp/epoxy composites. J Adhes Sci Technol 2009;23(16):2085–107.

[19] Gom, Software ARAMIS, version v.6.2. Gom GmbH: Braunschweig, Germany;

2009.

[20] Czaderski C, Soudki K, Motavalli M. Front and side view image correlation

measurements on FRP to concrete pull-off bond tests. J Compos Constr ASCE

2010;14(4):451–63.

[21] Czaderski C, Rabinovitch O. Structural behavior and inter-layer displacements

in CFRP plated steel beams – optical measurements, analysis, and comparative

verification. Compos Part B: Eng 2010;41(4):276–86.

[22] Faella C, Martinelli E, Nigro E. Interface behaviour in FRP plates bonded to

concrete: experimental tests and theoretical analyses. In: Proceedings of the

international conference on advanced materials for construction of bridges

and other structures – III. Davos (CH); 2005.

[23] Pellegrino C, Tinazzi D, Modena C. Experimental study on bond behavior

between concrete and FRP reinforcement. J Compos Constr 2008;12(2):180–9.[24] Faella C, Martinelli E, Nigro E. Direct versus indirect method for identifying

FRP-to-concrete interface relationships. J Compos Constr ASCE

2009;13(3):226–33.


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