Composites realized by hand lay-up process in a civil engineering environment: initial properties...

ORIGINAL ARTICLE

Composites realized by hand lay-up process in a civilengineering environment: initial properties and durability

Sofiene Marouani Æ Laurence Curtil ÆPatrice Hamelin

Received: 28 February 2006 / Accepted: 19 July 2007 / Published online: 12 September 2007

� RILEM 2007

Abstract The repair, the reinforcement as well as

the setting in safety of buildings and existing

reinforced and/or pre-stressed concrete structure is

a real technological stake and a socio-economic

problem for the near future. The introduction of

composites in civil engineering is an interesting

answer to these goals, but they brought an important

amount of new problems that have to be solved for

safe structural applications under combined mechan-

ical and environmental loadings. In fact during the

past five years, we have witnessed exponential

growth in research or field demonstrations of fiber-

reinforced composites in civil engineering. Manufac-

turers and designers have now access to a wide range

of composite materials. However, they face great

problems with forecasting the reliability of composite

materials. Their introduction in civil engineering

applications is a difficult operation due to working

environment and weathering conditions. The objec-

tive of this paper is to study the effect of these

conditions and their consequences on the mechanical

properties of the final composite. An analysis of the

rheological (viscoelastic) properties was carried out

in order to observe the glass transition temperature

evolution according to reactive mixture stoichiometry

and weathering conditions.

Resume La reparation, le renforcement ainsi que

la mise en securite des ouvrages en beton arme et/ou

precontraint est un enjeu technologique et un

probleme socio-economique dans un futur proche.

L’introduction des composites est, certes, une solu-

tion interessante mais pose plusieurs problemes. En

effet, les conditions climatiques et materielles, dans

lesquelles s’executent les chantiers de genie civil de

reparation ou de renforcement, rendent la mise en

œuvre difficile et delicate des composites a matrice

polymere. Pour ces raisons, nous allons par

l’intermediaire de cette etude, essayer de mettre

en evidence l’importance de ces conditions afin

d’evaluer leurs repercussions sur les proprietes

finales du composite et par la suite sur sa durab-

ilite. Pour conduire cette etude, des mesures de

proprietes rheologiques (visco-elasticimetrie) sont

effectuees pour suivre l’evolution de la temperature

de transition vitreuse Tg en faisant varier des

parametres tels que la stœchiometrie du melange

reactif et les conditions d’humidite et de

temperature.

Keywords Composites � Hand lay-up process �Repair � Implementation conditions �Durability � Civil engineering

S. Marouani (&) � L. Curtil � P. Hamelin

Laboratoire de Genie Civil et d’Ingenierie

Environnementale (LGCIE) – Site Bohr, Universite Lyon I,

Villeurbanne Cedex, France

e-mail: [email protected]

Materials and Structures (2008) 41:831–851

DOI 10.1617/s11527-007-9288-z

Mots cle Composites � Moulage au contact �Reparation � Conditions de mise en œuvre �Durabilite � Genie civil

1 Introduction

According to the geographical localization, physical

properties and, more particularly, the mechanical

behavior of concrete structures are modified under

various environmental conditions [1]. This can be

observed in the deterioration of buildings, bridges

and other structures due to the corrosion of the steel

reinforcement. In addition, there are many instances

where concrete structures need strengthening. These

may include increase in seismic, wind or snow load

bearing capacity due to the revision of the building

codes. In such cases, extending the service life of

structures is usually less expensive than replacement

and can be accomplished in a shorter period of time

with fewer inconveniences.

Considering the limited resources available to

solve the deterioration problems, it is important to

find innovative and cost effective techniques and

explore new materials for the strengthening of

existing structures [2]. Conventional strengthening

and/or repair techniques often used include section

enlargement, polymer concrete overlays, post-ten-

sioning, and bonded steel plates [3]. However, within

the last 10 years Fiber Reinforced Polymer materials

(FRP) have been introduced and are becoming

increasingly popular choices for many repair/

strengthening projects. In this context, the repair,

the reinforcement and the protection of concrete

structures can be achieved by hand lay-up of

composite materials [4].

The reliability and the durability of civil

engineering structures reinforced by FRP depend on

parameters that can be divided into two categories:

the first consists of the durability of the concrete

structure; the second is related to the durability of

composite materials, which depends on the raw

material, quality of interfaces, implementation con-

ditions and service constraints.

Difficult workability conditions (Fig. 1) might

have a significant influence on initial properties of

composite; for example it’s durability during service.

The quality of processing of a component governs its

initial physico-chemical [5] and mechanical proper-

ties [6] as well as its long-term behavior [7]. Process

optimization requires control of the curing process

(i.e. curing parameters).

The implementation conditions include several

parameters (Fig. 2), in particular the:

(1) proportion of components (resin and hardener)

in the reactive mixture (stoichiometric ratio).

(2) postcuring operation

(3) environment temperature and relative humidity

(4) temperature of the concrete structure at the time

of stratification.

This paper analyses the implementation conditions

and their influence on the durability of the reinforced

structure. The objective is to examine the recom-

mendations published by the AFGC [9] regarding the

control and the characterization of polymers as a

function of their in situ transformation conditions, in

other words, to evaluate the influence of initial

properties on the durability of composite materials

under environmental conditions specific to civil

engineering applications.

The parameter studied to answer the raised ques-

tions is the glass transition temperature Tg. The

objective is to systematically study the variation of Tg

as a function of workability parameters (Fig. 2) and

to analyze their influence on the mechanical

characteristics.

In order to understand the methodology that we

followed during these researches, we will present:

(1) the mechanism of polymerization of the epoxy

and vinyl ester resins.

repair of a bridge (IREX) [8] mixing of resin and hardener on civil engineering building site

(IREX)

Fig. 1 Examples of composites implemented in civil engi-

neering building sites

832 Materials and Structures (2008) 41:831–851

(2) the experimental procedure.

(3) the results recorded as well as the comments

carried out.

2 The implementation of the composites: cure of

vinyl ester (VE) and epoxy (EP) resins

The cure process plays a key role in determining the

properties of a thermoset material. The changes occur

during cure: the stiffness develops and the volume

decreases, determining the final structure and thereby

the properties of the polymer matrix itself. In order to

use the full potential of a thermosetting material a

thorough understanding phenomena and changes in

the material during cure is required. An important

aspect of thermoset cure is the reaction mechanism.

Polymer networks may be formed by two principally

different mechanisms; step-wise reaction (EP resin)

and chain-wise reaction (VE resin). Both the structure

of the resulting network and the events during the

reaction depend strongly on the reaction mechanism,

on the process of composite elaboration and espe-

cially on implementation conditions (role of

weathering conditions).

2.1 Cure of Vinylester (VE) resin

Vinylester resins are formed by the copolymerization

of styrene monomer and dimethacrylate monomer

based on the diglycidyl ether of bisphenol A (Fig. 3).

The styrene monomer is a reactive diluent. The

percentage of the styrene rises up to 36%.

Curing of the VE is governed by a free-radical

polymerization. The chain-wise mechanism proceeds

by way of active species that initiates chain poly-

merization reactions; each species rapidly forms a

large number of primary bonds on different locations

in the system. The reacting system is composed of a

catalyst, an accelerator and an inhibitor. The role of

catalyst (organic peroxide R–O–O–R0) is to raise the

resin to an energy level which makes the reaction of

copolymerization possible under selected implemen-

tations conditions.

The accelerator is necessary, especially in the case

of polymerization at low temperatures to induce free

radical production by chemical reduction of the

peroxide. It is used according to its compatibility

with the catalyst. The inhibitor adapts the reactivity

of the catalytic system at some implementing

processes (composite implemented at high tempera-

ture). In our case, the retained catalyst is the

methylethylketone peroxide (MEKP), the accelerator

is a cobalt salt (Co) associated with dimethylaniline

(DMA).

2.2 Cure of epoxy (EP) resin

The crosslinking of epoxy resins can be achieved

through two different reaction mechanisms, polymer-

ization by addition and by steps. The most common

curing process is based on the addition reaction of a

hardener (amines or anhydrides). There are several

works in which the chemistry and technology of

epoxy resins is discussed in detail [10–12]. With cure

Stoechiometric ratio r=[Hardener]/[resin]

Postcuring Temperature of concrete support

Environmental temperature and moisture

∆ of Tg

∆ initial properties of composite

∆ durability ofcomposite

Fig. 2 Implementation parameters

O

CH2

HC OH

CH2

O

C

C

O

CH3

CH2

O

CH2

HC OH

CH2

O

C

C

O

CH3

CH2

O

CH2

HC OH

CH2

O

C

C

O

CH3

CH2

CH2 CH2

n

Fig. 3 Chemical structure of Vinyl ester monomer system

Materials and Structures (2008) 41:831–851 833

involving reaction of epoxy groups with curing agent,

there is a reduction in the volume occupied by the

system, the free volume decreases which leads to an

increased ‘‘resistance’’ to conformational changes.

The importance of various factors that affect the

Tg of an epoxy network has been evaluated by Chang

[13]. Their estimates were based on a DGEBA resin

cured with catalyst that promotes the homopolymer-

ization of epoxy groups to form ether linkages.

3 Tg as an indicator of the evolution of composite

properties

Studies of implementation parameters can be con-

ducted by a number of methods, among these, the

follow-up of the glass transition temperature ‘‘Tg’’ can

be an effective method of characterization. Tg is

defined as the transition between the entropy and the

elastic state energy at its largest difference. Relations

exist between Tg and the evolution of global properties

of composites. The polymeric matrices of fiber

reinforced composites exhibit a glass transition, Tg, a

temperature above which the properties of the material

degrade significantly. Mechanical properties depend

on the rigidity of polymeric chains. The Eqs. (1) and

(2) (theory of rubber elasticity) relate the mechanical

properties to the crosslink density q, whereas, accord-

ing to the laws of DiMarzio [14] and DiBenedetto [15],

the Eq. (3) establishes a relation between Tg and the

degree of crosslinking. It is well known that the

crosslink density of the formed network has a

pronounced effect not only on the Tg but also on the

lower temperature processes, especially at low

crosslink extent, although the influence of other

factors—such as free volume at high crosslink densi-

ties—remains to be clearly evidenced [16–18].

E0 ¼ 3 R T qm ðelastic modulusÞ ð1Þ

G0 ¼ R T qm ðshear storage modulusÞ ð2Þ

q (mol/cm3): the crosslink density expressed in moles

of elastically effective chains per cubic centimeter of

sample.

T(K): the absolute temperature

R (J mol�1 K�1): the gas constant (R = 8.314)

m (mol/kg) = 1/Mc: the concentration of elastically

effective network chains.

Mc (g/mol): molecular weight between junction

points

In the case of thermoset polymers Tg is related to

the crosslink density q which is related to m (3).

Tg ¼TgL

1 � K Fq; q ¼ 2m=3 : cross - linking density

ð3Þ

TgL: glass transition temperature of a linear hypo-

thetical polymer comprising all linear segments of the

network.

K: universal constant = 3 for tri-functional

junction

F: parameter of polymer rigidity

Equations (1) and (2) show the relationship

between thermo-rheological and mechanical

properties.

Than, the equations [3] determine the existence of

a relation between Tg and the mechanical properties.

Indeed the variation of Tg is an indicator on the

evolution of the mechanical properties of composites.

Tg is important both as a measure of the degree of

cure and establishes a maximal temperature for use,

which defines the material’s operational limit. When

the Tg of the polymer is exceeded, certain mechanical

properties may be compromised severely. In addition,

it is well known that an increase in the moisture

content will lower the Tg of thermosetting polymers

[19]. It is necessary that the application temperature

for a polymeric composite is below the Tg in order to

assure that the mechanical stiffness and creep resis-

tance of the material is satisfactory.

The Tg, or typically a temperature well below the

Tg, is often used as an upper limit for the use of

polymeric composites in structural applications [20].

In setting levels for use temperatures the ‘‘moist’’ or

‘‘saturated’’ Tg is often used to yield a more

conservative limit. For composite infrastructure

repair, the Tg of the material may limit the maximum

service temperature of the repair because the ability

of the matrix to adequately transfer load to the fibers

is compromised above the Tg, making the repair less

effective [21]. Not only is the repair rendered less

effective due to a decrease in the composite’s initial

modulus, but near and above the Tg, the level of creep

for polymer matrix is greatly increased. As a result,

the long-term viability of a composite repair also

becomes an issue. As a result, current Tg systems


need to be evaluated to determine their temperature

limits.

4 Experimental procedure

4.1 Materials

We have considered two composite materials formu-

lations with the same textile reinforcement and

different polymer matrix: Epoxy and Vinyl ester

polymer.

Unidirectional carbon fabric was used for rein-

forcement (Table 1), the sizing is made with epoxy-

polyurethane (15–85%), 1% by weight.

The composites are obtained by wet-lay up

technique and their characteristics are presented in

Tables 2 and 3.

The samples were then cut into small specimens

using a diamond saw, suitable for thermal scans and

mechanical characterization.

4.2 Instrumentation

4.2.1 Experimental glass transition temperature

The glass transition temperature was determined by a

‘‘kinemat’’ apparatus (Fig. 4a, b) on 50 mm · 10

mm · 1.2 mm samples which were tested in torsion

with an angle varying between 5� and 25� (Fig. 5).

The heating rate is 3�C/min and the required torque

was measured instantaneously and after 2 s of

relaxation (Fig. 6).

Tg is measured at the maximum difference between

the instantaneous and relaxed torques (Fig. 7).

The imposed strain is in the range of small

deformations (linear viscoelasticity), the temperature

varies between 20�C and 240�C. The glass transition

temperature, Tg, of the composite was determined on

a first Kinemat scan, performed at 3�C/min, Tg value

was measured on the extrapolated onset (glass

transition range, i.e. the onset of the change in the

slope of the instantaneous torque) Tg-onset (Fig. 7).

Legrand and Bellenger [22] have determined the Tg

of prepreg glass/epoxy material by DMTA and

Kinemat. The mean discrepancy between these two

measurements techniques is about 2�C.

Tg-onset (�C), Tg-midpoint (�C), Tg-endset (�C): the

temperatures corresponding respectively to the onset,

the midpoint and the endset of glass transition.

L (�C): size of the glass transition range.

4.2.2 Experimental mechanical characteristics

To determine ultimate mechanical characteristics (ff,

rf, ef), tensile mechanical tests were carried on a

universal testing machine ‘‘ZWICK/1475’’. Test

results are expressed in terms of stress–strain curves

(Fig. 8a).

The ends of the specimens are reinforced by glass-

epoxy laminate heels (Fig. 8b).

Table 1 Technical characteristics of the carbon fiber

reinforcement

Linear density (tex) 1600

Number of filaments 24 k

Filament diameter (lm) 7

Twist (t/m) 0

Running length per kg (m/kg) 625

Density 1.76

Tensile strength (MPa) 4300

Tensile modulus (GPa) 238

Elongation at break (%) 1.8

Thermal conductivity (W/mK) 17

Table 2 Technical characteristics of the Epoxy/Carbon (EP/

Carbon) composite for the reference formulation (40 g of

amine hardener for 100 g of epoxy resin)

% in weight (matrix/reinforcement) 60/40

% in volume (matrix/reinforcement) 74/26

Tensile strength (MPa) 909 ± 33

Young’s modulus (GPa) 79

Elongation at failure (%) 1,28

Table 3 Technical characteristics of the Vinyl ester/Carbon

(VE/Carbon) composite for the reference formulation (100 g of

Vinyl ester resin 1.5% MEKP; 0.3% Co and 0.5% DMA)

% in weight (matrix/reinforcement) 62/38

% in volume (matrix/reinforcement) 75/25

Tensile strength (MPa) 805 ± 36

Young’s modulus (GPa) 65

Elongation at failure e (%) 1,5


The displacement rate is fixed at 1 mm/min

according to standard NF EN ISO 527-5.

5 Results and discussion

5.1 Viscoelastic properties of VE and EP

Figure 9 shows kinemat analyze of the two compos-

ites system VE/Carbon and EP/Carbon after 2 weeks

of stratification at room temperature.

The composite with EP matrix has a single Tg

(Tg-onset) near 52�C (Fig. 9a).

The composite with EP matrix has a single Tg

(Tg-onset) near 54�C (Fig. 9a). Whereas, in the case of

composite with VE matrix, the graph of the Fig. 9b

shows the existence of two apparent Tg values, as

shown by the two apparent Tg values, as shown by

the two changes in the slope of the Ci

(Tg1�onset � 51�C and Tg2�onset � 141�C). These

transitions are related to thermosets morphology.

With most room-temperature-curing epoxies, the

Table 5 Tested formulations reactive mixture of the VE/Carbon system

Reference

formulation

Variation of the % of MEKP

(% of DMA and Co are maintained

constant 0.3% and 0.5%)

Variation of the % of Co

(% of MEKP and DMA

are maintained constant

1.5% and 0.5%)

Variation of the % of

DMA (% of MEKP

and Co are maintained

constant 1.5% and 0.3%)

Weight-percent MEKP = 1.5% 0.5; 1; 1.5; 2; 2.5; 3; 3.5; 4 and 5% 0.1; 0.2; 0.3 and 0.5% 0.1; 0.2; 0.5 and 0.8%

Co = 0.3%

DMA = 0.5%

Hand lay-up conditions T = 20 ± 2�C

RH = 50 ± 4%

Table 4 Tested formulations reactive mixture of the EP/Carbon system

mHardener ðgÞMResin ðgÞ

16/100 24/100 32/100 40/100 48/100 56/100 64/100

r 0.4 0.6 0.8 1 1.2 1.4 1.6

Fig. 4 The ‘‘Kinetech’’

apparatus


resin does not go to complete reaction, and Tg of such

epoxies is no more than 50–60�C.

The hardening at room temperature of the VE

resins-contrary to the EP resins- generates the

formation of a heterogeneous 3D network comprising

fluctuations on the composition and the crosslink

density. These inhomogeneities should be highly

crosslinked nodules immersed in an internodular

matrix of lower crosslink density; indeed, the

internodular crosslink density should be responsible

for the onset of molecular motion corresponding to

the Tg1�onset: The existence of high crosslink density

regions corresponds to Tg2�onset:

We can explain this phenomenon by the fact that

the hardening process of VE resin is governed by a

radical chemistry (chain-wise reaction), whereas, the

curing of EP matrix is based on polycondensation

mechanism (step-wise reaction).

The trend of the two Tg values associated with

room-temperature-cured vinyl ester resin has been

observed elsewhere. Cook et al. [23] observed by

DSC and DMTA tests the same anomalous behavior

exhibited by the room temperature cured Vinyl ester

resins (45 wt.% styrene and 55 wt.% of a dimethac-

rylate based on an oligomerized diglicydil ether of

bisphenol A). This phenomenon can be explained in

terms of the additional cure of the sample when the

temperature is being scanned in the DMTA. Herzog

et al. [19] concluded that the results recorded by the

DMTA concerning the existence of two Tg suggests

one of the two things: the sample is being cured

additionally during the DMTA scanning or the water

in the sample makes precise measurements difficult.

In the former case, an increase in the thermal energy

would provide an undercured material with sufficient

molecular mobility to restart the curing process. In

the latter case, drying would occur primarily at the

outside surface during the temperature scan.

The formed 3D networks started from a free-

radical polymerization are more subject to problems

of inhomogeneities in the morphology and phase

separation (multiphase networks) compared with

those formed by step polymerization reactions. The

heterogeneity of free-radical cross-linking systems is

an inherent result of the physical aspects of the free-

radical reaction and is often explained through micro-

gel theory. Micro-gels are zones of high crosslink

density dispersed in a pool of non crosslinked

material. In the case of VE resins, the dominating

of styrene-styrene reaction at high cure levels is an

indication of phase separation or the formation of a

polystyrene matrix or that micro-gel structures are

located within a polystyrene matrix.

In the literature, several studies treated the forma-

tion of 3D structures by a radical chemistry. Based on

the study of several unsaturated polyesters (UPE)

instantaneous Torque

relaxed torque

relaxation time

Time

Torque (mN)

Fig. 5 Imposed movement and transmitted couple

Ci

Cr

R

Temperature (°C)

Torque (mN)

Ci : instantaneous torque Cr :relaxed torque R : relaxation

Fig. 6 Experimental curve: results

Ci

Cr

R

Temperature (°C)

Torque (mN) Tgd

Tgm

Tgf

L

Fig. 7 Experimental curve: exploitation


[24–26] networks, it was possible to show that the

two transition detected could be attributed to the

chain formations comprising the crosslinking nodes

in the structure of the prepolymer chain rather than

the formation of polystyrene between microgels or

between crosslinking nodes.

These studies converged to the same conclusions:

heterogeneity of the formed network. In reality, the

transition zone detected corresponds to infinity of

microdomains which are set in motion one by one

under the effect of a rise in temperature. In parallel,

the view of the researchers diverges concerning a

multiphase network.

We note that the comparison of the values of Tg

recorded by the DMTA in the studies of [23] as well

as [19] shows for the same reactive system formu-

lation (same % of catalyst and accelerators) a light

difference (3�C) compared to the values recorded by

the kinemat.

To conclude, we remind that the hand lay-up of

composites in civil engineering application gener-

ally is not conducted in an oven or in an

autoclave, but instead a catalyzed resin is used to

permit room-temperature curing. If these ambient

conditions do not lead to a Tg associated with a

fully cured composite matrix, the system either

gels or vitrifies before a complete cure is achieved

and the obtained networks present lower Tg.

Therefore, epoxies, as well as vinyl esters, needs

to be postcured to achieve a high degree of

crosslinking.

5.2 Influence of errors in the reactive system

formulation

First of all, a preliminary study was conducted in situ

on civil engineering building sites in order to estimate

induced incertitude on the percentages of catalyst and

accelerator. This incertitude is estimated from 0.5%

to 5% in the case of catalyst, and from 0.1% to 0.8%

in the case of accelerator. Concerning the EP/Carbon

system, the incertitude applied to stoichiometric

epoxy-amine ratio ‘‘r’’ reveals a variation between

0.4 to 1.6.

To simulate these errors inherent to the transfor-

mation conditions of composite, essentially induced

by difficult implementation conditions, we intention-

ally varied the reactive mixtures formulation

(Table 2, 3).

ff

εf

Ef

Stress (MPa)

Strain (%)

ff : failure strength

E f : Young modulus

ε f : failure strain

a-Experimental curve: results of tensile tests

b-Specimen after tensile test

Fig. 8 Mechanical test


5.2.1 Effect of epoxy-amine stoichiometry on cured

composite materials properties

Seven stoichiometric ratios of amine to epoxy were

analyzed. The systems studied consisted of three

excess amine systems (r = 1.2; 1.4 and 1.6), a

stoichiometric system (r = 1), and three excess epoxy

systems (r = 0.4; 0.6 and 0.8).

Figure 10 is a plot of the Tg as a function of the

stoichiometric ratio ‘‘r’’. The maximum value occurs

at the stoichiometric point. The Tg depends strongly

on the used hardener (amine) quantity. Amounts in

excess or paucity of the stoichiometric composition

result in drastically lower value of Tg. The crosslink-

ing density of epoxy-amine thermosetting system is

affected by the amount of curing agent used. The use

of more or less amine than the stoichiometric quantity

required for stoichiometrically balanced cure, leads to

a final structure having lower crosslink density and

therefore lower Tg. If the quantity of amine is in

excess or in paucity of the stoichiometric composi-

tion, gelled networks will not form. Such limits can

be predicted using Flory’s theory of gelation.

For the epoxy-amine system used for this inves-

tigation limits are r = 0.4 and r = 1.6.

Under such conditions the concept of network

stiffening crosslinks is not applicable. The decrease

in Tg in amine-rich mixtures can be attributed to the

increasing numbers of chain ends. This can be

explained by the free volume argument. Chain ends

have in excess free volume associated with them that

can ease the movement of molecules and therefore

lower the value of Tg. The significant decrease in the

Tg of epoxy-rich mixtures with r < 0.8, compared

with mixtures having a composition near to stoichi-

ometry could be attributed to the fact that unreacted

epoxy groups can have some plasticization effect on

the crosslinked networks.

Our purpose for investigating the effects of

stoichiometry on material properties of epoxy-amine

systems is to eventually relate these properties to

predicted interphase composition profiles. Existing

methods for the prediction of Tg for crosslinked

off-stoichiometry compositions epoxy systems are

based on empirical and theoretical considerations.

However, in order to obtain reliable Tg values for

epoxy-amine systems with varying amounts of curing

agent, direct measurement is recommended.

5.2.2 Influence of catalyst (MEKP) percentage

The variation of Tg as a function of the percentage of

MEKP is presented in Fig. 11. The percentage of

accelerator (Co and DMA) is unchanged and repre-

sents respectively 0.3% and 0.5% of weight-percent.

In the phase I, the evolution of Tg1�onset with

respect to the percentage of catalyst can be divided in

two stages as shown in the Fig. 11a. During the first

stage, Tg is quasi proportional to the percentage of

MEKP; the increase is approximately 10�C. During

a-Epoxy /Carbon system

b-Vinyl ester /Carbon system

Tg-onset

Temperature (°C)

Torque (mN.m) Relaxation (mN.m)

Tg2-onset

Tg1-onset

Temperature (°C)


Fig. 9 Analyze Kinemat of VE/Carbon & EP/Carbon com-

posites polymerized at 20�C and 50% RH

15

30

45

60

Stoichiometric amine/epoxy ratio "r"

Ttesno-g

)C°(

0 0,4 0,8 1,2 1,6

Fig. 10 Tg versus stoichiometric epoxy-amine ratio


the second stage (Fig. 12b), Tg2�onset is almost

constant. In phase II, Tg2�onset shows a light increase

for MEKP between 0.5% and 1% than decreases

abruptly by loosing about 20�C for percentages of

MEKP between 1% and 5%.

These result can be explained by the fact that the

increase of the percentage of catalyst leads to the

growing of the concentration of free radicals, thus to

the increase of the reactivity of the mixture, and of

the reticulation density. Hence Tg is increased. It is

well known that one of the factors that affect the Tg,

is the crosslinking introduced into the polymer to

form a network. An increase of the crosslinking

density reduces the chain mobility and, consequently,

increases the Tg. Nevertheless, when the percentage

of catalyst continues to increase, the value of Tg

stabilizes or decreases lightly.

This phenomenon suggests two things: on the one

hand, an increase in the MEKP concentration,

generate an excess in free radicals which can cause

some secondary reactions and homopolymerization

of styrene and/or the Vinyl ester prepolymer which

create an effect of plasticization [27].

On the other hand, with an increase in the MEKP

concentration, the exotherm peak (corresponding to

the maximum reaction rate) will be shifted to lower

temperature as would be expected for a faster

reacting system, and this may cause the early (but

limited) initiation of vinyl resins [28].

5.2.3 Influence of accelerator percentage variation

Cure of Vinyl ester resin involves initiator (MEKP)

and accelerator (Co and, as an option, DMA). The

chemistry of the initiation is quite complex, but in the

simplest terms, the initiator (catalyst) is the active

element and the accelerator, having no direct effect

on the resin system, catalyzes the initiator process.

The percentage of catalyst is fixed at 1.5% by

weight.

The graphs a and b of the Fig. 12 enable us to

deduce that Tg varies slightly as a function of

accelerator (Co or DMA) percentage.

These results are logical. In fact the cobalt salt

accelerates the catalyst by debonding the MEKP

oxygen–oxygen bond (O–O) through redox decom-

position and the debonding of the prepolymer carbon-

carbon double bonds (C=C) allows creation of free

radicals. Li and Lee [29] state the same conclusions

after studying cure reaction of Vinyl ester resin

‘‘Derakane 411–350’’ containing bisphenol A.

In general, the amount of accelerator is related to

its compatibility with the catalyst, hence its presence

in the reactive mixture does not seem to affect the Tg

of the composite. These results agree with the fact

that the accelerator accelerates the curing process and

20

30

40

50

60

70

0 2 4 6

catalyst percentage (% MEKP)

Tg

)C°(

tesno-1

a-Tg1-onset versus catalyst percentage

110

120

130

140

150

160

0 2 4 6

catalyst percentage (% MEKP)

)C°(tes

no-2gT

b-Tg2-onset versus catalyst percentage

Fig. 11 Tg versus percentage of catalyst (MEKP)

0 0,2 0,4 0,6 0,8 1% of accelerator (Co and DMA)

a-Tg1-onset versus accelerator percentage

100110120130140150160

0 0,2 0,4 0,6 0,8 1

% of accelerator (Co and DMA)

Tg

2-on

set

(°C

)T

g 1-

onse

t (°

C)

Tg (°C) = f (% of DMA)Tg (°C) = f (% of Co)

b-Tg2-onset versus accelerator percentage

30354045505560

Tg (°°C) = f (% of DMA )Tg (°°C) = f (% of Co)

Fig. 12 Tg versus percentage of accelerator (Co and DMA)


enable it to take place at room temperature. Never-

theless, the kinetic process of chemical reduction and

the possibility of solely partial initiator decomposi-

tion imply that the material formed will be partially

cured.

This conclusion is confirmed by the low variations

recorded in size of the glass transition zone (Fig. 13)

with respect to the percentage of accelerator in the

reactive mixture.

These results contradict those of Yildiz and

Hazar [30] and Pryor & Hendrickson [31] which

affirms that the use of accelerator in Vinyl ester/

styrene polymerizations has significant conse-

quences. The increase in the DMA concentration

causes an increase in the Mc (average molecular

weight of the chain sections between crosslinks)

values of the network.

The explanation is that the increase in the DMA

concentration further retards the crosslinking level

which results in the decrease of the swelling. The

same conclusions were advanced by Cook et al. [23]

concerning the inhibition effect of the cobalt.

From these experimental results it can be con-

clude that for a very low amount of catalyst the

curing reaction takes place and the varying of the

amount of catalyst (high quantity), produces a poor

crosslinked material. The results obtained agree

with the fact that the accelerator accelerates the

curing process and enable it to take place at low

temperatures. Without post-cured thermal treatment,

the VE matrix composite specimen can remain

partially cured.

The properties of the cured VE matrix composites,

as well as the EP matrix composites appear to depend

on the initiator system, but these require a more detail

study of the free mechanism.

5.3 Evolution of mechanical properties

5.3.1 Sensitivity of the mechanical properties to

implementation parameters

The mechanical and sometimes chemical adhesion

which characterizes the fiber–resin interface is liable

for the transfer of load or stress between fiber and

resin.

The mechanical adhesion, often associated with

surface inhomogeneities, is also the result of the

normal pressure on the fiber due to chemical shrink-

age of the resin during the crosslinking process, as

well as the shrinkage of the resin via thermal

contraction after cooling.

It is common practice in the civil engineering

applications for composite parts to be cured at room

temperature or to realize only a low-temperature

postcure. Under-cure of the resin will not generally

produce optimal tensile properties and in applications

where corrosive environments are experienced by the

material, further degradation of the mechanical

properties can occur [32]. The loss of strength or

stiffness over time can produce unexpected material

failure resulting in, at least in costly downtimes or

needless expenses.

It was thought that the initial cure of the resin may

limit the degradation of mechanical properties.

Research that varied the cure conditions has shown

that the level of the residual stress depends on

implementation conditions (cure temperature, curing

agent concentration...). In the literature this aspect

of current industrial practice is slightly examined

[33, 34].

In summary, the normal pressure exerted on

reinforcement fibers embedded in a polymeric matrix

depends on implementation parameters as well as

implementation conditions of the composites. In the

first case, it has shown qualitatively that failure of the

interface between the fiber and resin is a function of

the following parameters: materials, sample loading,

Young’s modulus, Poisson’s ratio of the two mate-

rials and mismatch between the thermal expansion

coefficients of the two materials [35]. On the other

hand, the interfacial shear stress and coefficient of

friction are related through the pressure in the system.

Considering the sensitivity of the mechanical

properties of composites to their stratification condi-

tions, mechanical tests were carried out in order to

10

20

30

40

50

60

% of accelerator ( Co and DMA)

L (

°°C)

L (°C) = f (% of Co)L (°C) = f (% of DMA)

0 0,2 0,4 0,6 0,8 1

Fig. 13 Width of the glass transition zone ‘‘L’’ versus the

percentage of accelerator


evaluate the tensile strength, strain, and Young

modulus as a function of variation in catalyst and

accelerator percentage at the time of implementation.

5.3.2 Results and discussion

Figure 14a shows that the failure stress of the

composite increases with increasing percentage of

catalyst. For the variation of catalyst from 0.5% and

2%, we observe an increase of 250 MPa (from

800 MPa to 1050 MPa) i.e. an increase of approxi-

mately 31%. At 2% of MEKP, the failure stress

decrease slightly to reach 900 MPa at 5% of catalyst.

The Young modulus increases slightly in the first

time to reach 80 GPa at 1.5% of catalyst, then

decreases and stabilizes to 65 GPa for 3.5% of

catalyst, that represents a fall of approximately 20%

(Fig. 14b).

As reported in the Fig. 14, the mechanical prop-

erties of the composites in term of modulus and

failure stress containing matrix VE evolve differently

according to the quantity of catalyst. Whereas failure

stress starts to decrease from 2% of catalyst, the

Young modulus changes behavior when the percent-

age of MEKP reaches 1.5%. This phenomenon can be

explained by modification in the rigidity of the

composite.

The effect of the increase in the quantity of

catalyst appears certainly by an improvement of

chemical adhesion i.e. the improvement of the

interface matrix-reinforcement, which explains the

variations’, recorded on the mechanical properties. A

network with greater intermolecular cross-links will

require the rupture of many more bonds, and

therefore require more energy.

Results shown in Fig. 15 indicate that the Young

modulus is not influenced by the variation of

accelerator’s percentage (Co and DMA) in the

reactive system. Increasing the DMA content, subse-

quently increases failure stress (from 800 MPa to

1000 MPa); accelerated initiation with DMA results

in maximum failure stress value between 0.1% and

0.5%, 200 MPa was recorded when the percentage of

DMA reach 0.5%. In this DMA concentration, tensile

strength reaches 1000 MPa. This phenomenon can be

explained by the fact that employing DMA for the

curing at lower temperatures increase elastic energy

stored, which could be the result of greater stress

within system. The excess of elastic energy is stored

in the sample at the moment of failure and expended

in the creation of increased fracture surface area

giving rise to rough surfaces.

The quantity of Co in the reactive system seems

without effect in the mechanical properties.

Figure 16 shows the evolution of failure stress and

Young modulus as a function of amine used for cure.

For the two plots one maximum is apparent: one at

600700800900

100011001200

0 1 2 3 4 5 6

% of catalyst (MEKP)a-Failure stress versus catalyst percentage

55000

6000065000

70000

7500080000

85000

% of catalyst (MEKP) a-Young modulus versus catalyst percentage

You

ng m

odul

us (

MP

a)F

ailu

re s

tres

s (M

Pa)

0 2 4 6

b-

Fig. 14 Evolution of mechanical properties as a function of

catalyst percentage for the composite with VE matrix

0

200

400

600

800

1000

1200


Failure Stress = f( % of DMA)Failure Stress = f( % of Co)

40000

50000

60000

70000

80000

90000

100000


E (MPa) = f (% of DMA)E (MPa) = f (% of Co)

0 0,2 0,4 0,6 0,8 1

0 0,2 0,4 0,6 0,8 1

You

ng M

odul

us (

MP

a)F

ailu

re s

tres

s (M

PA

)

Fig. 15 Evolution of mechanical properties as a function of

accelerator percentage for the composite with VE matrix


r = 0.8 for the failure stress plot with a value of

2500 daN, and the other at stoichiometry (r = 1) for

the young modulus having a value of 82000 MPa.

The variations of the mechanical properties as a

function of composition are significant: 67% for

failure stress and 82% in the case of Young modulus

(% variation from minimum).

Evidently, the change in internal energy with

respect to strain contributes strongly to the determi-

nation of the modulus in the glassy state. Hence, the

molecular interactions must play an important role in

determining the mechanical properties.

It is apparent that the mechanical properties of

composite system with EP matrix are very sensitive

to the amount of amine used for cure when ‘‘r’’ is

different from the stoichiometry. While explanations

exist for the anti-plasticization behavior, a theory

capable of predicting the mechanical properties in

glassy state as a function of composition or structural

attributes is not available. It appears that the approach

to adopt is to consider that the internal energy of the

system is predicted on the basis of structure and

composition. Ideas based on free volume can then

used to help the description of the material’s

structure.

The mechanical properties of the composites in

term of modulus and failure stress containing VE and/

or EP matrix evolve differently according to the

quantity of catalyst (hardener for EP matrix). For the

composite containing VE matrix, the failure stress

starts to decrease from 2% of catalyst, the Young

modulus changes behavior when the percentage of

MEKP reaches 1.5%. In the case of composite with

EP matrix, failure stress reaches a maximum at

r = 0.8 and the Young modulus at the stoichiometry.

This phenomenon can be explained by modification

in the rigidity of the composite.

5.4 Evolution of composite rigidity

A complementary study was carried out in order to

detect the influence on rigidity and homogeneity of

the Vinyl ester and epoxy networks due to any errors

made in the reactive system formulation. The study

consists in observing the evolution of composite

hardness which can be used to follow the change in

mechanical properties of composite during the cure.

The Barcol hardness is a standard industrial method

of assessing the effects arising from cure conditions

and corrosive media [36].

The composite hardness was monitored because it

is easy to determine and it is non-destructive for the

samples. A ‘‘Hand-Held Portable Hardness Tester’’

was used for measuring the Barcol hardness (The

Impressor).

The impressor (Fig. 17) is intended for handheld

testing of hardness for composite material. As a

general rule, the number of readings increases with

the softness of the materials being tested. The

recommended number of readings for VE/carbon or

EP/carbon composite (reinforced plastics) was equal

to 29.

The hardness was determined 2 weeks after the

composites had been cooled down to room temper-

ature, the time necessary for the state of cure to reach

90%.

The analysis of Fig. 18 allows us to conclude that

the errors made on the percentage of catalyst tend to

increase the rigidity of the composite (Fig. 18a),

Hardness value ranging from 30 to 43 are obtained

when the percentage of MEKP reaches 1.5%. The

hardness remains constant when the quantity of

catalyst is up 1.5%. Hardness values decreases from

50 to 35 when the quantity of accelerator increases in

the reactive mixture. Indeed, an inverse trend for the

500

1000

1500

2000

2500

3000


a- Failure stress versus stoichiometric ratio "r"

2210032100421005210062100721008210092100


b- Young modulus versus stoichiometric ratio "r"

0 0,4 0,8 1,2 1,6

0 0,4 0,8 1,2 1,6You

ng m

odul

us (

MP

a)F

ailu

re s

tren

gth

(da

N)

Fig. 16 Mechanical properties as a function of stoichiometric

amine-epoxy ratio ‘‘r’’ for the composite with EP matrix


case of accelerator (Fig. 18b) is recorded. An impor-

tant reduction of hardness is recorded when the

quantity of acceleratuor increases from 0.1% up to

0.8% (16 in the case of DMA and 11 in the case of

Co).

Hardness test is very localized, nevertheless

Fig. 18 shows a slightly superior scattering compared

with values given in the standard of the apparatus

(average variance of 0.78). The dispersion in

hardness values can be explained by an important

heterogeneity of VE and EP network in the absence

of postcuring. So we need to study the effect of this

operation.

Figure 19 shows that Barcol hardness increases

when the quantity of amine in the reactive mixture

increases up to a threshold value r = 1.2. The value of

hardness is multiplied by 2 between r = 0.2 and

r = 1.2. From this threshold value, hardness falls

brutally to reach 28 for r = 1.6.

The significant decrease in the harness values of

epoxy mixtures with r > 1.5 and r < 0.8 could be

attributed to the fact that unreacted epoxy groups can

have some plasticization effect, thus the rigidity of

composite matrix decrease also the Barcol hardness.

5.5 Effect of post-curing operation

Depending on the crosslinker agent, the stoichiome-

try and the cure schedule, the final properties of EP

Fig. 17 The Impressor

0

10

20

30

40

50

6

% of catalyst (MEKP)

Bar

col H

ardn

ess

a-Barcol Hardness versus catalyst percentage

0

10

20

30

40

50

60

0 0,2 0,4 0,6 0,8 1

% of accelerator (Co & DMA)

Bar

col H

ardn

ess

Barcol Hardness = f (% of DMA)Barcol Hardness = f (% of Co)

b-Barcol Hardness versus accelerator percentage

0 1 2 3 4 5

Fig. 18 Variation of Barcol hardness versus the concentration

of catalyst and accelerator for the VE/carbon composite.

10

20

3040

50

60

Stoichiometric Amine-Epoxy ratio"r"

0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6 1,8

Bar

coll

Har

dnes

s

Fig. 19 Variation of Barcol hardness versus the stoichiometric

amine-epoxy ratio ‘‘r’’ for the composite with EP matrix


and VE network are different. When the curing

process occurs at room temperature, it is slow and the

resin (the matrix) does not go to complete reaction,

and Tg is no more than 50–60�C. Therefore the

obtained networks present a high ability to carbonate

and to absorb water. For these reasons, epoxies, as

well as vinyl esters, need to be postcured to achieve a

high degree of crosslinking.

The postcuring of VE/carbon sample is realized at

100�C for 3 h and 1 h at 150�C. The EP/specimen is

postcured at 100�C for 4 h.

The effect of post-cure is shown in Fig. 20. For the

VE/carbon sample cured at room temperature, two Tg

appear. In contrary, the sample cured at room

temperature and postcured exhibit one glass transition

region and a reduction of the glass transition zone.

The room temperature cured material was only partly

cured, having vitrified during the curing process. As

the temperature is raised further, the thermal energy

provides sufficient molecular mobility to restart the

curing process, causing a shift in the transition region

and allowing a restructuration of the network.

Thereafter Tg increases and reaches a value between

Tg1�onset and Tg2�onset (&123�C). Morchat et al. [37]

share the same reasoning; they showed that post-

curing directly influences the amount of residual

styrene and the density of reticulation, thus it

improves the mechanical properties of material. In

absence of post-cure, Zang et al. [38] and Lietard

et al. [39] also confirm that the VE resin has two

distinct glass transition temperatures corresponding

to two different phases.

In the case of EP/carbon composite, it is evident

that the post-curing (Fig. 21b) of the material above

its Tg allows for additional crosslinking due to

thermally activated diffusional mechanisms. The

result is an increase in stiffness and a decrease in

viscous losses due to decreased chain mobility caused

by additional crosslinking in the post-cured specimen.

Through additional curing via postcuring, Tg

increased to approximately 78�C. This was in agree-

ment with values found in the literature.

a-VE/carbon before post-cure

a-VE/carbon after post-cure

Tg 123 °C

Tg1 51° C

Temperature (°C)


Tg2 141° C

Temperature (°C)


Fig. 20 Influence of the post-cure operation on the VE/carbon

composite

a-EP/carbon before post-cure

Tg 54°C

Temperature (°C)


Tg 78.1 °C

Temperature (°C)


≈

≈

Fig. 21 Influence of the post-cure operation on the EP/carbon

composite


5.6 Influence of the temperature of cure (Tcure) on

Tg and on the adhesive properties of

composites

In this paragraph, we present the effect of the cure

temperature Tcure for different implementation con-

ditions. The main objective is to determine the

influence of civil engineering environment on the

initial properties, the strength and the durability of

composite joints.

Generally, the implementation of composite mate-

rial on a building site requires in situ hardening of

thermosets resins. The polymerization reaction (hard-

ening or cure) is extremely sensitive to temperature

and relative humidity at the time of implementation

and at the time of curing. For this reason, we studied

the variation of Tg as a function of cure temperature

as well as concrete slabs temperature.

5.6.1 Influence of environment temperature

During their cure reaction, VE and EP resins undergo

an exotherm and so it is instructive to study how the

cure temperature (Tcure) determines the degree of

cure and the resulting Tg.

Five values of cure temperature (Tcure) are con-

sidered: 10, 20, 30, 40 and 60�C

In the case of VE/carbon composite, Fig. 22a

shows that Tg increase with Tcure. Starting from 20�C

of cure temperature, the increase of Tg is constant:

when the Tcure is increased about 20�C, Tg increases

by approximately 10�C. This can be explained by the

fact that if the VE/carbon composite is isothermally

cured at a temperature above its maximal Tg, the

material remains in the liquid-like state during cure

and the enthalpy of the system is at its equilibrium

value. However, if the composite vitrifies during cure

and remains in this glassy state, the material may age

by slow segmental motion. Similar results have been

obtained by Cook et al. [23], an increase of 11�C was

observed, but from cure temperatures above 80�C.

In the case of EP/carbon, as also seen in Fig. 22b,

Tg increases first for cure temperatures Tcure between

5�C and 20�C. Then it is practically invariable

between 20�C and 40�C and reaches 70�C when the

material is cured at 60�C. This can be explained by

the fact that in the early stage of reaction, Tg

increases with the increase of Tcure, the reaction rate

is chemically controlled and thus depends on the

concentration of unreacted monomers. As the length

of the polymer chain grows, the rate of curing

reaction is increasingly affected by the diffusion rate

of unreacted monomers and/or unreacted functional

groups attached to the growing polymer chain. This

diffusion control typically becomes dominant near to

the glass state of polymers as Tg approaches Tcure

with the progress of the cure reaction. When Tg &Tcure, the polymer chains are subsequently vitrified

and the diffusion rate becomes all but negligible.

5.6.2 Influence of the concrete structure temperature

and humidity

The variation of environmental temperature and

relative humidity during the application of composite

produces condensation effect on the concrete struc-

ture. It occurs at a temperature and humidity which

can be predicted by the evaluation of the dew point.

During the stratification of composite on the struc-

ture, temperature can be favorable for condensation

which would affect matrix polymerization. Generally,

the temperature of concrete structure must be 3–5�C

higher than the dew point at the time of composite

implementation. With the Skew-T Diagram, we can

control the risk of condensation on the concrete

structure [40].

20

40

60

80

100

T cure (°°C)

T cure (°°C)

Tg

-ons

et (°°

C)

Tg

-ons

et (°°

C)

cure on Tg of the VE/carbon composite a-Influence of T

20

40

60

80

100

b-Influence of Tcure on Tg of the EP/carbon composite

0 10 20 30 40 50 60 70

0 20 40 60 80

Fig. 22 Influence of cure temperature Tcure on the Tg of

composites


It is necessary to know three parameters to

determine the temperature of the dew point: the

ambient temperature and relative humidity as well as

the temperature of the concrete support.

The concrete is a heavy and bulky material, thus it

has a good thermal capacity, and i.e. it is able to store

and to restore important quantities of heat and/or of

freshness. However, this capacity is not dissociable of

certain slowness: more the inertia is important; the

material takes a long time to be heated or cooled.

This thermal inertia plays an important role at the

time of the Hand Lay-up of composite and its bounds

on the concrete structure. Indeed, the polymerization

reaction is exothermic; therefore the temperature of

composite material at the time of the cure is

controlled by: the environmental temperature, the

temperature of the concrete structure as well as the

temperature released by the polymerization reaction.

Concrete slabs (100 · 100 · 5 cm) were tested at

three different temperatures Ts: 5, 20 and 40�C. For

the first case, the slab remained 1 month in a

refrigerator regulated at 5�C. In these conditions we

cannot avoid condensation because the temperature is

lower than the dew point. For the second case, the

slab remained at ambient temperature and for the

third case; it remained in an oven at 40�C during

1 month.

Figure 23a shows for the VE/carbon composite

that Tg increases with increasing temperature of

concrete slab. Tg for 40�C is practically double than

for 5�C. The same trend was observed in the case of

EP/carbon composite (Fig. 23b), nevertheless a dif-

ference equal to 32�C in the value of Tg was recorded

between Ts, 20�C and Ts, 40�C.

These variations of Tg modifie the viscoelastic

properties of composites and influence their adhesive

behavior.

5.6.3 Influence of environmental conditions on

adhesive properties of VE/carbon and EP/

carbon composites

The adhesion properties and durability depends on

many and opposites parameters, such as mechanical,

thermodynamical adhesion. Therefore, it is necessary

to study the effect of the nature of the adhesive (VE

or EP matrix), and the properties of the adherent: the

roughness, superficial characteristics, the environ-

mental conditions, etc.

In this paragraph, the influence of the temperature

of the concrete support on the adhesive capability of

the composite is monitored.

The adhesive behavior was studied by the pull-off

method.

This approach has been developed to measure the

in-situ tensile strength of concrete by applying a

direct tensile force. The method may also be useful

for measuring the bonding capability of the surface to

be repaired or reinforced. Procedures are covered by

BS 1881: Parts 207. The loading equipment is a

dynamometer having 10 KN capacity. A rod screwed

axially into a 50 mm diameter disk allows to apply a

tensile force normally to the composite surface.

Before the gluing of the disk (Fig. 24) on the

composite surface, a circular surface, having the

same diameter as the disk, is cut out from this latter

with a diamond coring equipment. The disk is glued

to this surface with an epoxy resin. Then the force

necessary to pull-off the composite or the concrete

beneath the composite is measured.

It is obvious that the temperature of the concrete

support influences considerably the Tg of the com-

posite independently from the type of the used

matrix. At 5�C, Tg is too low and the interface

properties are very poor particularly in the case of the

composite with EP matrix (Fig. 23). The polymeri-

zation reaction did not take place and the rupture is

30405060708090

Temperature of slab "Ts" (°°C)a-Influence of Ts on Tg of the VE/carbon composite

1030507090

110

Temperature of slab "Ts" (°°C)

Tg-

onse

t (°°

C)

Tg-

onse

t (°°

C)

b-Influence of the Ts on Tg of the EP/carbon composite

0 10 20 30 40 50

0 10 20 30 40 50

Fig. 23 Influence of the temperature of slab Ts on the Tg of

composites


localized in the composite (Fig. 25a). Nevertheless,

in the case of the composite with VE matrix, the

reaction of polymerization is engaged and the rupture

is localized in the concrete (Fig. 25b).

When the initial temperature of the concrete

support increases, pull-off strength and depth

increase too. We noticed a difference of 1 N/mm2

(i.e &2 KN/surface of disk) between Ts at 20�C and

Ts at 40�C (Fig. 26a), which represents an increase of

35%. Figure 26b also shows a difference of the pull-

off depth equal to 4 mm in the case of the composite

realized with VE matrix and respectively 2 mm for

the composite made with an EP matrix. This means

an improvement of the interface behavior and there-

after adhesion properties.

6 Conclusions

For the application of composites in civil engineering

application (repair and/or reinforcement) by hand

lay-up method, the influence of implementation

conditions (implemented and cure temperatures,

errors of formulation (stoichiometry), concrete struc-

ture temperature and postcuring operation...) were

studied by the follow-up of the glass transition

temperature Tg and mechanical tests. These methods

are used for quality control and inspection of

Concrete support

Epoxy resin adhesiveComposite

50 mm diameter steel disk

Failure surface

Concrete slab

Composite

50 mm diameter steel disk

Dynamometer

Failure surface

Fig. 24 Pull-off test: surface and partially cored

a-EP/carbon (Ts = 5°C)

b-VE/carbon (Ts = 5°C)

Fig. 25 Pull-off test: the localization of the failure surface

0

1

2

3

4

5

0 10 15 20 25 30 35 40 45

Temperature of slab Ts (°C)

Pul

l-of

f st

ess

(N/m

m2)

EP/ carbon

VE/ carbon

a- Pull-off stress versus Ts

02468

1012

0 10 20 30 40 50

Temperature of slab Ts (°C)

Pul

l-of

f de

pth

(mm

)

EP/ carbonVE/ carbon

a- Pull-off depth versus Ts

5

Fig. 26 Influence of the temperature of slab Ts on the

adhesive properties of the composites


composites properties. These following conclusions

are retained:

(1) The composites with Vinyl ester matrix imple-

mented at ambient temperature have two glass

transition temperatures. This implies the exis-

tence of two different phases: a postcuring

operation is recommended to stabilize and to

ensure a whole reticulation of the obtained

network.

(2) Implementation of composites at building site is

a difficult operation. Errors in the reactive

system formulation can be made during the

hand lay-up procedure. The material properties,

including the Tg, modulus and failure stress, of

vinyl ester/carbon and epoxy/carbon systems

used in external strengthening have been mea-

sured. The results of this study show that the

materials properties are significantly affected

even by relatively small variations in stoichi-

ometry. A reduction of 20% of Tg due to errors

in reactive system formulation, corresponds to

the same reduction in tensile modulus.

For the reliability of implemented composites,

the AFGC (French association of civil Engi-

neering) [9] recommended that Tg should not go

below 45�C; and operating temperature ‘‘T’’

should remain in the range of 20�C < T < Tg

10�C. The newly approved ASME post-Con-

struction Repair Standard, which governs the

use of composites in construction, requires that

the Tg of the repair material be at least 20�C

above the temperature of the repair [41]. The

MIL-HDBK-17 utilizes the Tg reduced by some

temperature margin Tg � DT, for composites

with epoxy resin (DT = 20�C). In all cases, the

maximum use temperature for a polymeric

material used in a structural application is often

set a certain number of degrees below Tg. That’s

why errors made in system formulation should

be controlled.

From the point of view of the durability, the

formulation errors result not only in loss of

mechanical properties, but also in an increase of

the macromolecular network sensitivity to water

absorption. For these reasons, the safety factors

af and aad (Eqs. (4) and (5)) suggested by the

AFGC (French Association of Civil Engineer-

ing) [2] and which takes into account the

durability and reliability of composite should

be modified.

ff ;d ¼af ffcf ;u

ð4Þ

sad;d ¼ aad �sad;e

cad

ð5Þ

ff,d: tensile strength of the composite

sad,d: shear stress in the interface composite-

concrete

a (af,aad): reduction factors to take account of

the long-term effect (safety and reliability

factors).(3) The post-curing improves physico-chemical

properties of polymeric matrix which increases

the performances of composite and interface

properties, hence the lifetime of the rehabili-

tated or strengthened civil engineering structure.

In practice, this operation is difficult to achieve.

Nevertheless, this could be done by heating the

structure to a higher temperature or by applying

an electrical heating around the structure for a

period of time.

(4) Other parameters are tested e.g. the implemen-

tation temperature and the temperature of the

concrete structure. The characterization by

Kinemat apparatus showed that Tg and adhesion

properties of composite increase proportionally

with these two parameters.

The use of ambient cure based process for the

rehabilitation and/or strengthening concrete

structure through hand lay-up based placement

of external polymeric matrix composite can be

an extremely flexible and efficient method.

However, the sensibility of overall initial prop-

erties of theses materials to implementation

conditions, rise the question of the estimation of

long-term durability and service life.

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