evaluation of bonding at the interface between cfrp ...€¦ · composite strips and concrete for...

Evaluation of bonding at the interface between CFRP

composite strips and concrete for hybrid structures

N. TARANU, G. OPRISAN, R. OLTEAN, V. MUNTEANU, C. COZMANCIUC

Department of Civil and Industrial Engineering

“Gh. Asachi” Technical University of Iasi

ROMANIA

[email protected] http://www.ce.tuiasi.ro

Abstract: Recent advances in fiber reinforced polymer (FRP) composite technologies have resulted in alternative

reinforcing materials that can be used efficiently as supplemental, externally bonded reinforcement, also contributing

to the development of performant hybrid structures. However, the application of these hybrid solutions is associated

with considerable difficulties due to the bond behaviour between FRP composites and traditional building materials. In

bonded application, the interfacial behaviour and strength between the FRP materials and the substrate plays a major

role in the structural behaviour and capacity of the hybrid system. An extensive experimental program has been

initiated at the Faculty of Civil Engineering, the Technical University of Iasi, Romania to study the interfacial

behaviour between carbon fiber reinforced polymer (CFRP) composite sheets and concrete as a preliminary required

step to develop hybrid structures made of polymeric composites and traditional building materials. The experimental

results obtained during the carried out tests provide an adequate characterization of the interfacial region regarding the

force-slip behaviour, ultimate shear strength of the interface and the types of failure. It has been proven that the

association of CFRP sheets and concrete is possible and a good bonding behaviour could be achieved.

Key-Words: bonding, CFRP sheets, structural response, failure modes, hybrid elements, bonding stress, slip

1 Introduction Hybrid structures made of fiber reinforced polymeric

(FRP) composites and conventional building materials

such as steel, concrete, masonry and wood, offer an

efficient alternative for solving numerous requirements

in civil and structural engineering.

The conceived hybrid structures fulfil structural and

functional requirements which cannot be provided by

any traditional material or by composites working

individually. The hybrid structures are recommended

since they provide substantial advantages compared to

traditional structures.

Current practice in civil and structural engineering

involves the use of FRP composites in all composite

structures, externally reinforced members made of

conventional materials, sandwich structures, all making

use of the integrity of the bond between the FRP and

traditional materials.

The bond between the traditional material and FRP

composite strips may determine the success and the

failure of externally bonded strengthened load bearing

elements. Initial considerations on bond between FRP,

concrete[1], masonry [2], wood [3] and steel [4] include

adequate surface preparation, quality of adhesive and

quality of workmanship in composite laminate

application.

Carbon fibre reinforced polymer (CFRP) sheets have

gained recognition as alternatives to traditional materials

in the strengthening field due to important advantages

such as: light weight, resistance to corrosion, high

tensile strength and simple construction. Bond behaviour

between CFRP sheets and concrete is crucial in load

bearing applications for transfer of stress between

concrete and CFRP external reinforcement. The bond

strength between CFRP and concrete is therefore the

most critical problem. The bond performance between

CFRP and concrete is determine by adhesive, concrete

strength bond length and width and the mechanical

characteristics of CFRP especially stiffness [5-8].

Extensive theoretical and experimental work has

been carried out by numerous research teams in an

impressive effort to implement the modern strengthening

techniques based on external bonding of CFRP strips to

concrete elements [9, 10]. An experimental program has

been initiated at the Faculty of Civil Engineering, the

Technical University of Iasi, Romania to study the

interfacial behaviour between carbon fiber reinforced

polymer (CFRP) composite sheets and concrete as a

preliminary required step to develop hybrid structures

made of polymeric composites and traditional building

materials.

Tests carried out on beams specimens confirmed the

difficulty of providing the full composite strength

capacity because of failure due to debonding before the

total strength capacity of concrete and reinforcements is

achieved [11, 12]. The interfacial behaviour is very

complex since the stress distributions are complicated by

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ISSN: 1792-4294 279 ISBN: 978-960-474-203-5

the non-linear relationships of parameters. Therefore the

experimental investigations are currently doubled by

analytical formulas, most of them semi empirical [13].

2 Experimental evaluation

2.1 Experimental setup

The proposed set-up shown in Fig. 1 consists of two

equal concrete prisms. A thin metal plate separates the

two concrete prisms. The height of this plate is at both

sides 15 mm less than the height of the prisms, so that

both prisms remain aligned during specimen

manipulation and application of the CFRP sheet. On one

section of the test specimen, extra fixation of the CFRP

sheet has been provided by means of steel clamps, Fig.1,

to determine the bond failure at the opposite side [14].

Fig. 1 Concrete prisms with attached CFRP sheets

Two steel bars were embedded into both concrete

prisms. These steel reinforcing bars do not connect the

concrete prisms, Fig. 2, which means that the two prisms

will be only connected through the surface bonded

CFRP composite sheets. The length of the protruding

part of the steel reinforcing bars have been selected so

that they enable an efficient clamping in the tensile

testing machine.

Fig. 2 Specimen mould with pulling steel bars to be

embedded in concrete prisms

A good bonding concrete surface preparation has

been performed; the concrete surface has been abraded

and the grinding dust has been removed. A detail of this

operation is given in Fig. 3.

Fig. 3 Preparation of the concrete surface for CFRP

sheet application

The CFRP reinforcing sheets have been bonded on

two opposite sides of the concrete specimen, following

the application procedures recommended by the

manufacturer, Fig 4. Over a central zone of 100 mm

(where the two concrete prisms connect each other), the

CFRP sheets were left un-bonded.

Fig. 4 Phases of CFRP sheet bonding on concrete

2.2 Properties of component materials

The mechanical properties of concrete, relevant to the

performed test have been determined experimentally

using the typical procedure for cylindrical and cubic

specimens; the CFRP sheets properties were provided by

the supplier.

2.2.1 Concrete

The target concrete cylinder compressive strength was

established at fc,cyl = 30 N/mm2; thus the concrete mix

was designed in order to satisfy this requirement, having

a maximum aggregate size equal to 16 mm.

The properties of the fresh and hardened concrete for

the designed mix are presented in Table 1. Three

cylinders (150 x 300 mm) and three cubes (150 x 150 x

150 mm) have been tested to determine the cylindrical

compressive strength and the cubic compressive strength

respectively.

Table 1 Properties of the concrete batch

Property Concrete

batch

Density (kg/m3) 2350

Slump (mm) 120

Compressive strength on cubes fc, cub(MPa) 34.68

Compressive strength on cylinders fc, cyl(MPa) 31.37

Elastic modulus of concrete Ec (GPa) 26.5


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2.2.2 FRP sheets

The laminates used at the experiment have the

dimensions 100 x 700 x 1.2 mm and 100 x 700 x 1.4

mm respectively.

The external CFRP reinforcing sheets have been

glued on two opposite sides of the concrete specimens,

observing the application procedures as provided by the

suppliers, who also give the main characteristics of the

FRP strips, presented in Table 2. Tests were performed

on 3 prisms specimens of the same type, using Sikadur

30 as adhesive (an epoxy-based on two-component

adhesive mortar- Sikadur 30 Technical Data Sheet,

2005)

Table 2 Laminates properties

Property Sika

Carbodur

S1012

Sika

Carbodur

M1014

Thickness (mm) 1.2 1.4

Ult. tensile strength, fu (MPa) 3100 3100

Long. elastic modulus E (GPa) 165 210

Ult. strain εu (%) 1.70 1.35

Table 3 Adhesive used in experimental program

(Sikadur 30 Technical Data Sheet, 2005 and BS EN 196-

1:2005)

Property Sikadur 30

Density (kg/l) 1.65

Shear strength (MPa) 15

Tensile strength (MPa) 25

Bond Strength(MPa) >4

Tensile modulus (GPa) 11.2

2.3 Instrumentation and loading procedure A special steel device was conceived and constructed to

facilitate the application of the tensile load, Figure 5.

The tests have been carried out using a universal testing

machine of 3000 KN. The rate of loading has been kept

constant during the test, selecting a strain rate of 1.4

microstrain/min.

Fig.5 The steel device for load application

Fig.6 Loading procedure

The relative displacements between CFRP

reinforcing sheets and the concrete have been recorded

with LVDTs placed on each monitored side, Fig.6; the

device was attached to the concrete and directly

connected to the CFRP sheet at the loaded end (at the

location of the transition between the central un-bonded

and the bonded zone), Fig. 7.

Fig. 7 LVDT positioning

Five strain gauges have been applied on both sides of

the specimens at 10mm, 80mm, 150mm, 220mm and

290 mm, respectivelly from the end of the CFRP sheet,

Fig. 8.

un-bonded areaLVDT

Strain gauges

Steel bars

FRP strip

steel clamps

10 70 70 70 70

960

80040080 400 80

80 80

Fig. 8 Location of strain gauges on the CFRP sheet

3 Results and comments

3.1 Experimental results Double pull-pull tests have been performed on the

prepared specimens. Experimental values for applied

loads, relative displacements and strains in the CFRP


ISSN: 1792-4294 281 ISBN: 978-960-474-203-5

sheets have been recorded during the test program. The

performed tests have enabled the evaluation of the

influence of the investigated parameters on performance

of the bond between CFRP composite sheets and

concrete.

For the first set of three specimens, namely the ones

with attached Sika Carbodur M1014 sheets, the average

relative displacements results obtained with the LVTD

transducers are illustrated in Fig. 9.

Fig.9 Load-displacement diagram: LVDT R – right side

transducer; LVDT L – left side transducer

In case of the second set of three specimens, namely

the ones for which Sika Carbodur S1012 sheets were

utilised, the results presented in Fig. 10 were obtained.

Fig. 10 Load displacement diagram: LVDT R – right

side transducer; LVDT L – left side transducer

The curves shown in Figs. 9 and 10 illustrate the

typical behaviour of all tested specimens. It can be seen

that it is quite difficult to apply centric loading for

double pull-pull test, in order to subject the specimen to

shear; in addition the debonding is not uniform along the

whole width of CFRP composite sheets. This

eccentricity causes different strains on left and right

side. Similar results were reported in [15]. Load versus

strain curves show an initial cvasi-linear load-strain

response on both sides of the instrumented specimens,

Figures 11 and 12 (with Sika Carbodur M1014 sheets)

and Figures 13 and 14, respectively (with Sika Carbodur

S1012 sheets).

Fig.11 Load strain curve diagrams for the strain gauges

located on the right side of the sample


located on the left side of the sample


located on the right side of the sample


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located on the left side of the sample

3.2 Comments on experimental results The strain recordings in gauges placed along the CFRP

sheets enable the calculation of the shear stresses and

slips at different loading values. If it is accepted that the

composite sheets have an elastic behaviour for the whole

range of loading the average shear bond stress (τ, in

MPa) values for any two points along the laminated

sheets can be determined with [16]:

, 1 ,( )f f i f i fE t

L

ε ετ

+−

=∆

(1)

where:

Ef is the elastic modulus of CFRP composite sheet, in

GPa, given in Table 2, ; tf is the thickness of CFRP, composite sheet, in mm; εf,i+1 , εf,i are the strains recorded on CFRP sheet along

the composite element, corresponding to two

consecutive strain gauges; ∆L is the distance between the same strain gauges, in

mm. The average slip values, (s, in mm) are calculated as the

addition of the CFRP extensions using the following

relationship [16]:

, 1 ,

12

f i f i

i is s Lε ε

+

+

−= + ∆ (2)

The bond-slip curve obtained with the values determined

with equations (1) and (2) is presented in Fig.15. As it

can be seen it leads to a peak bond stress equal to about

7.3 MPa corresponding to a slip value equal to 0.27 mm.

The curve has an ascending branch up to peak bond

shear stress and a descending one in the range of

measurements.

Fig. 15 Load vs. strain corresponding to concrete sample

plated with CFRP sheets Sika Carbodur M1014

The failure of the all specimens occurred suddenly,

and after the concrete cracking in the central area. An

illustrative example of failure is shown in Fig. 16 and an

enlarged detail is given in Fig. 17.

Fig. 16 Bond failure and CFRP sheet debonding

Fig. 17 Detail of CFRP sheet debonding from the

concrete prism


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4 Conclusion In this study hybrid concrete prism samples externally

reinforced with CFRP sheets have been tested using

pull-pull procedure, so that an interfacial debonding due

to shear occurred.

Debonding of CFRP sheets from surface of concrete

prisms represents the most common failure mode. The

failure at the interface occurred in concrete and had a

brittle manner, typical for concrete specimens with

laterally attached CFRP bonded sheets. Therefore, it can

be seen that the strength capacity of the concrete

substrate is the critical characteristics of the interface

region.

All failure modes whose typical example is illustrated

in Fig.17 have been characterized through shear concrete

failure leading to CFRP plate separation.

A perfect alignment of the samples, of the steel

device and of the whole assembly is essential in

conducting the test. If not, premature failure might occur

as a consequence of the load application eccentricities.

Acknowledgement

This work was supported by CNCSIS - UEFISCSU,

project number 737, PNII - IDEI code 369/2008 on

hybrid structures made of polymeric composites and

traditional building materials.

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ISSN: 1792-4294 284 ISBN: 978-960-474-203-5

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