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CONCRETE REINFORCEMENT AND GLASS FIBRE REINFORCED POLYMER
chael Kemp
BEng Civil - General Manager Wagners T
David Blowes
Sales Engineer - Wagners CFT
Abstract
The corrosion of steel reinforcement in concrete
reduces the life
of
stnlctures, causes high repair
costs and can endanger the structural integrity
of
the structure itself. Glass fibre rein f
or
ced polymer
GFRP) offers a number of advantages over steel
especially when used in marine and other salt laden
environments.
G
RP reinforcing bars are gradually
finding wider acceptance as a replacement for
conventional steel reinforcement as it otTers a number
of
advantages.
Technical studies on a number
of
concrete structures,
from five to e ight years old and constructed with GFRP
reinforcement, have shown that there is no degradation
of
the
GFRP
from the alkaline environment.
Introduction
Re inforced concrete is a common building material
for construction of facilities and structures. While
concrete has high compressive strength, it has limited
tensile strength. To overcome these tensile limitations,
reinforcing bars rebar) are used in the tension side of
concrete structures.
Steel rebar has historica lly been use.d as an effective
and cost efficient concrete reinforcement. When not
subjected to chloride ion attack, steel reinforcement
can last for decades without exhibiting any visible
signs
of
deterioration.
However, steel rebar
is
very susceptible to oxidation
rust) when exposed to chlorides. Examples of such
exposure in clude coastal areas, salt contaminated
aggregates used in the concrete mixture and sites
where aggressive chemicals and ground conditions
exist. In cold climates, treating snow with salt is
another cause of accelerated deterioration of concrete
bridge decks. When corrosion of steel rebar occurs,
the resulting corrosion products have a volume 2 to 5
times larger than the original steel reinforcement. As
the concrete cannot physically sustain the high internal
tensile stresses developed from this volume increase,
it eventually may crack and spall causing further
deterioration of the steel Figure 1 . The combination
of ongoing deterioration and loss of reinforcement
properties ultimately requires potentially significant
and high cost repairs and possibly the endangerment of
the structure itself.
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Figure 1 Concrete spalling of a bridge soffit in a
corrosive env ronment
GFRP bars are a competitive reinforcing option in
reinforced concrete members subjected to flexure and
shear. GFRP has compelling physical and mechanical
properties, corrosion resistance and electromagnetic
transparency. The lise of
GFRP
reinforcement is
particularly attractive for structures that operate in
aggressive environments, such as in coastal regions,
or for buildings that host magnetic resonance
imaging MRT) units or other equipment sensitive to
electromagnetic fields.
Brief history
Fibre reinforced polymers FRP) have been used [or
decades in the aeronautical, aerospace, automotive and
other fields. FRP is the generic name and its primary
difference from
GFRP
is that
it
can be composed of
a range of materials whereas the
GFRP
is reinforced
with glass fibres.) Their use in civil engineering
works dates back to the 1950s when GFRP bars were
first investigated for structural use . However, it was
not until the 1970s that FRP was finally considered
for structural engineering applications and its
superior performance over epoxy coated steel was
recognised. The first applications
of
glass fibre FRP
were not successful due to its poor performance within
thermosetting resins cured at high molding pressures
I).
Since their early introduction, many new FRP
materials have been developed with a range
of
different forms such as bars , fabric, 20 grids,
3D
grids
or standard structural shapes Figure 2). The fibre
materials include aramid Kevlar®), polyvinyl, carbon
and improved glass fibres .
Figure 2 Available shapes of FRP products
Manufacturing of FRP
A manufacturing process called Pultrusion
is
the
most common technique used for manufacturing
continuous lengths of FRP bars that are of constant
or nearly constant in profile. Figure 3 below shows
this manufacturing technique. Continuous strands of
reinforcing material are drawn from roving bobbins .
A veil
is
introduced and they pass through a resin
tank, where they are saturated with resin followed by
a number of wiper rings to remove excess resin. The
strands are then led to a pre-former and then formed
to their final shape and cured by the heated die.
The
speed
of
pulling through the die is predetermined by
the curing time needed. To ensure a good bond with
concrete, the surface of the bars
is
usually coated with
sand and then cut to length Figure 4). The application
of the sand coating is an additional process, a layer of
resin is applied but not under heated conditions) and
then the bar
is
coated with a thin layer of sand.
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Figure
3 Pultrusion process
for forming FRP
bars
Figure
4. FRP
bars
with
sand coated finish
SI
Nominal
diameter mm)
I
Tensile modulus
of
I
elasticity GPa)
1
I
Guaranteed tensile
strength MPa)
~ 6
6.35 46.1 788
I
~ 1
9.53 46.2 765
~ 1 3
12.70
46.4
710
I
~ 1 6
15.88 48.2 683
~ 1 9
19.05 47.6 656
I
I> 25
25.40 51.0 611
Figure 5
FRP
bar properties
1 For re ference. the el as tic modulus for steel is 200 GPa
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Similar to steel reinforcement, FRP bars are produced
in different diameters, depending on the manufacturing
process. The surface of the rods can be spiral, straight,
sanded-straight, sanded-braided and deformed. The
bar to concrete bond is equal to or better than the bond
with steel reinforcing bars.
The mechanical properties of FRP reinforcing bars are
given in Figure 5.
Resins
A very important issue in the manufacture of
composites
is
the selection of the optimum matrix
because the physical and thermal properties
of
the
matrix significantly affect the final mechanical
properties as well as the manufacturing process. In
order to be able to exploit the full strength of the fibres,
the matrix should be able to develop a higher ultimate
strain than the fibres (2).
The matrix not only coats the fibres and protects them
from mechanical abrasion and chemical attack, but
also transfers stresses bet Neen the fibres . Other very
important roles of the matrix are the transfer of inter
laminar and in-plane shear within the composite, and
the provision
or
lateral support to the fibres against
buckling when subjected to compressive loads (3).
There are two types of polymeric matrices commonly
used for FRP composites - thermosetting and
thermoplastic. Thermosetting polymers are used more
often than thermoplastic. They are low molecular
weight liquids with very low viscosity (3) and with
their molecules
joined
together by chemical cross
links. Hence, they form a rigid three dimensional
structure that, once set, cannot be reshaped by
applying heat or pressure. Thermosetting polymers are
processed in a liquid state to obtain good wet-out
of
fibres. Some commonly used thermosetting polymers
are polyesters, vinyl esters and epoxies. These
materials have good thermal stability and chemical
resistance and undergo low creep and stress relaxation .
The vinyl ester resin predominately cures during the
pultrusion manufacturing process as the bar
is
drawn
through the heated die. y the time the bar reaches
room temperature it is considered to be fully cured.
Thermosetting polymers have relatively low strain
to failure, resulting in low impact strength. Two
major disadvantages are their short shelf
life and long
manufacturing time. Mechanical properties of some
thermosetting resins are provided in Figure 6.
Resin
Specific gravity
Tensile strength
MPa)
Tensile
modulus GPa)
Cure shrinkage
)
Epoxy
1.20 1
.30 55.0 130.0
2.75 4 .10 1.0 5.0
Polyester 1.10 1.40 34.5 103.5
2.10 3.45
5.0 12.0
Vinyl ester 1.12 1.32 73.0 81.00
3.00 3.35
5.4 10.3
Fi
gure 6 Typical properties of thermosetting resins
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esign standar s for GFRP
The design of reinforced concrete using FRP
reinforcing bars is not currently codified by any
Australian standard, however there is a Canadian
code (14) and a pub,ication by the American Concrete
Institute (4). Both
of
these documents use the limit
state approach in their design.
A design manual \5) has been published by thc
ISIS Canada Research Network
2
which describes the
design process
in
line with the Canadian code.
It
is
patiicularly helpful as it describes the differences in
design and behaviour between steel reinforced and
FRP reinforced structures.
The two main differences in designing reinforced
concrete structures using FRP reinforcement are:
• FRP does not yield in a similar way as steel
• FRP bars have a lower modulus
of
elasticity than
steel. Furthermore , both codes do not allow for
the use of FRP reinforcement as longitudina l
reinforcement
in
columns (due to insufficient
research in that area).
enefits of GFRP
The benefits
ofGFRP
rebar are as follows:
• Corrosion resistance - when bonded in concrete
it does not react to salt, chemical products or the
alkali in concrete. As GFRP is not manufactured
from steel, it does not rust
• Superior
te
nsile strength - GFRP rebar produced
by the pultrusion process offers a tensile strength
up to twice that of normal structural steel (based
on area)
•
Thermal
expansion - GFRP rebar offers a level
of
thermal expansion comparable to that of concrete
due to its 80% silica content
•
Electric and magnetic neutrality
- as GFRP
rebar does not contain any metals, it will not
cause interference with strong magnetic fields or
when operating sensitive electronic equipment or
instruments
• The rm al insulation - GFRP rebar does not create
a thermal bridge within structures
•
Lightweight
- GFRP rebar is a quarter the
weight of steel rebar of equivalent strength.
It
offers significant savings in transportation and
installation.
igure 7 Light
weight bundles
of FRP are easily
moved on site
Utilising these inherent benefits, GFRP rebar has a cost
effective application as a concrete reinforcing bar in
the following markets when analysed on a life-cycle
cost basis:
•
Reinfor
c
ed concrete
exposed to
corrosive
environments
- car parking structures,
bridge decks, parapets, curbs, retaining walls,
foundations, roads and slabs
•
Structures
built in or close
proximity
to sea
water
Figures
8,9) - quays, retaining wall,
piers, jetties, boat ramps, caissons, decks, piles,
bulkheads, floating structures, canals, roads and
buildings, offshore platforms, swimming pools and
aquanums
•
Applications subjected
to ot
her
corrosive agents
- wastewater treatment plants, petrochemical
plants, pulp/paper mills, liquid gas plants,
pipelines/tanks for fossil fuel, cooling towers,
chimneys, mining operations
of
various types,
nuclear power plants
2 ISIS Canada Research Network Intelligent Sensing for Innovative Structures) was established
in
1995
to
provide civil engineers with smarter
ways
to
build, repair and monitor structures using high-strength, rlon-corroding, fibre reinforced polymers FRPs) and fibre optic sensors FOSs).
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• Applications rcquiring low electric conductivity
or electromagnetic neutrality
-
aluminium and
copper smelting plants, manholes for electrical and
telephone communication equipment, bases for
transmission/telecommunication towers, airport
control towers, MRI
in
hospitals, railroad crossing
sites , and specialised military structures
• Mining tunneling boring applications
temporary concrete structures, mining walls,
underground rapid transit structures, rock anchors
and wash down areas
• Weight sensitive structures
-
concrete
construction in areas of poor load bearing soil
conditions, remote geographical locations,
sensitive environmental areas, or active seismic
sites posing special issues that necessitate the use
of lightweight reinforcement
•
h r
mally sensitive applications
-
apartment
patio decks, thermally insulated concrete housing
and basements, thermally heated floors and
conditioning rooms.
Figu re 8 GFRP used on the Anthon Jetty
Wyndham Western Australia
Figure 9 Precast deck slab and GFRP rebar for
the Anthon Jetty
Technical case
st
udy - durability o GFRP
composite rods
J
One of the most pressing durab ility concerns of our
time is the rap
id
corrosion
of
reinforcing steel that
occurs in concrete structures subjected to chloride
rich environments. It s often argued that
if
the steel
reinforcement in such structures could be replaced
by chemically inert reinforcement such as fibre
reinforced polymers, the problem of cOITosion could
be eliminated.
Of
the various options, the most
economical choice
is
GFRP, but
it
has been reported
to be highly vulnerable to the alkaline environment
of
concrete.
A report (6), summarising the results of several
published studies on the alkali resistance
of
GFRP,
categorically concluded that G RP should not
be used in direct contact with concrete Similar
conclusions were drawn by other researchers (7,8,9).
Unfortunately, all of these studies were conducted by
subjecting G RP
to
an idealised, simul ated, high p
fluid environment often involving high temperatures.
Such environments are unduly harsh as they provide
an unlimited supply of hydroxyl ions - a condition
not present in rea l concrete. Also, they provide
full sa turation, which
is
also rarely the case. Field
conditions should therefore be expected to be different
from these idealised laboratory conditions.
3 The bulk o this section comes from a technical report s indicated under reference 5)
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Name of structure
Age
years
Concrete strength
MPa
Seasonal
temperat
ure rang
e
·C
Type of chloride
exposure
Hall
Harbor Wharf
5 45 35 to
35
Marine
Joffre
Bridge
7 45 35 to 35
Deicing
salts
Chatham Bridge 8
35
24
to 30
Deicing salts
Crowchild Trail Bridge 8
35
15t023
Deicing
salts
Waterloo
Creek Bridge
6 35 oto 23 Deicing salts
igure 10. Samples were taken from these five structures
In
2004, a major study by ISIS Canada was launched
to obtain field data with respect to the durability
of
GFRP
in
concrete exposed to natural environments.
Concrete cores containing GFRP were removed
from five exposed structures which were five to
eight years old Figure 10). The GFRP was analysed
for its physical and chemical composition at the
microscopic level. Direct comparisons were carried
out with control samples - GFRP rods preserved under
controlled laboratory conditions.
t
least ten 75ml diameter core samples containing
GFRP were taken from each
of
the five structures.
Three concrete cores from each of five structures
were sent for analysis to three teams
of
material
scientists working independently at various Canadian
universities. The removal ofGFRP samples along
with sUlTounding concrete and the polishing
of
the
samples required special care given that GFRP and
concrete have different hardness values.
After sample preparation, the GFRP reinforcement
and surrounding concrete were analysed using several
analytical methods. The entire surface
of
each sample
was examined and photographs were taken at various
locations.
Scanning electron microscopy SEM) was lIsed for a
detailed examination
of
the glass fibre/matrix interface
and individual glass fibres. The specimens used in
SEM analyses were also analysed by energy dispersive
x-ray EDX) to detect potential chemical changes
in
the matrix and glass fibres due to the ingress
of
alkali
from the concrete pore solution. Chemical changes
in the polymeric matrix of GFRP were characterised
by Fourier transfonn infrared spectroscopy FTIR).
Finally, changes
in
the glass transition temperature Tg
of
the matrix due to exposure to severe environmental
conditions were determined using differential scanning
calorimetry DSC).
Findings - The results obtained by the three research
teams were very similar. A complete account
of
their
findings is available
in
their respective individual
reports 10, I 1,12). The results found that there was
no degradation of the GFRP
in
the samples provided.
The results from this scientific study, based on
samples from actual engineering structures, was not
in
agreement with the results obtained in some simulated
laboratory studies.
The results from SEM and EDX analyses confirmed
that there is no degradation
of
the GFRP in the
concrete structures. The
EDX
analyses also indicated
no alkali ingress
in
the GFRP from the concrete
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pore solution. The matrix
in
all GFRPs was intact
and unaltered from its original state. The results
from the FTIR and DSC analyses supported the
results from the SEM examinations. The FTIR and
DSC results indicated that neither hydrolysis nor
significant changes in the glass transition temperature
of the matrix. After exposure, for 5 to 8 years, to the
combined effects of the alkaline environment in the
concrete and the external natural environment, no
detrimental effects were found.
The
results of this study were used as the basis for
changes to the Canadian Highway Bridge Design
Code (13) allowing the use of GFRP both as primary
reinforcement and prestressing tendons in concrete
components. The proviso was made that the stress
level for the serviceability limit state does not exceed
25% of its ultimate tensile strength. Other refenmces
to the use ofGFRP can be found in (14,16,17).
Summary and onclusion
GFRP
has a very important role to playas
reinforcement
in
concrete structures that will be
exposed to harsh environmental conditions where
traditional steel reinforcement could corrode. It is
the unique physical properties of GFRP that makes
it
suitable for applications where conventional steel
would be unsuitable. Detailed laboratory studies
of
samples taken from reinforced concrete structures,
aged from five to eight years old, have confim1ed that
GFRP has performed extremely well when exposed to
harsh field conditions.
References
I. Parklyn B.
Glass Reinforced Plastics,
Iliffe,
London. 1970
2.
Phillips LN. Design with Advanced Composite
Materials ,
S p r i n g e r ~ V e r l a g
1989
3. ACI Committee 440.
State-olthe-Art Report on
Fiber ReinforcedPlastic FRP) Reinforcement for
Concrete Structures, American Concrete Institute,
92-S61. Nov 1995 www.concrete.org
4.
ACI Committee 440. Guidefor
t e Design and
Construction ofStructural Concrete Reinforced
with FRP bars, American Concrete Institute, ACI
440.1 R-06, 440 I03 . April 2006 www.concrete.
org
5. Mufti A, Banthia N, Benmokr B, Boulfizaane M,
Newhook
J. Durability ofG RP Composite Rods,
Concrete international, Vol 29, Issue
2.
February
2007
6.
Malvar J. Durability ofComposites in Reinforced
Concrete, Durability of Fiber Reinforced Polymer
(FRP) Composite for Construction, Proceedings of
the First International Conference on Durability of
Composites, B. Benmokrane and . Rahman, eds.,
Sherbrooke , QB, Canada. 1998
7.
Uomoto r Durability ofFRP as Reinforcement
for Concrete Structures,
Proceedings
of
the 3rd
International Conference on Advanced Composite
Materials in Bridges and Structures, J. Bumar and
AG. Razaqpur, eds., Canadian Society for Civil
Engineering, Ottawa, ON Canada. 2000
8. Sen Research, Marsical D, Issa M, Shahawy M.
Durahili v
and
Ductility ofAdvanced Composites,
Structural Engineering in Natural Hazards
Mitigation,
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2, AB. S.Ang and R. Villaverde,
eds. , Structures Congress, ASCE, Irvine, CA
1993
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9.
Sen Research , Mullins G, Salem T. Durability
ofE-Glassl Vinylester Reinforcement in Alkaline
Solution, ACI Structural Journal, V. 99, No.3.
May-June 2002
10. Benmokrane B Cousin P. University of
Sherbrooke
GFRP D1Irability Study Report,
ISIS
Canada, University
of
Manitoba, Winnipeg, MB,
Canada. 2005
11. Boulfiza M, Banthia N. University of
Saskatchewan University of British Columbia
Durability Study Report, ISIS Canada, University
of Manitoba, Winnipeg, MB, Canada. 2005
12. Onofrei M. Durability ofG RP Reinforced
Concrete from Field Demonstration Structures,
ISIS Canada, University of Manitoba, Winnipeg,
MB, Canada. 2005
13. CAN/CSA-S6-06, Canadian Highway Bridge
Code. December 2008 http://www.ShopCSA.ca
14. CAN/CSA-S806-02, Constmction
of
Building
Components }vit Fibre-Reinforced Polymers,
Product Number 2012972. 2007 http://www.
ShopCSA.ca
15. Rizkalla S, Mufti A. Manual No.3 -
Reinforcing
Concrete Structures with Fibre Reinforced
Polymers FRPs),
ISIS Canada Research Network.
http:// isiscanada.com
16. Various American Concrete Institute Committee
440 reports http: //www.concrete.org/
COMMITTEES/committeehome.asp?committee_
code=0000440-00
17 . AASHTO LRFD, Bridge Design Guide
Specifications for GFRP-Reinjorced Concrete
Bridge Decks and Traffic Railings,
GFRP l
,
ISBN 1-56051-458-9. 2009 https: /lbookstore.
transportation.org/ltem_details.aspx?id= 1545
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