Concrete Reinforcement and Glass Fibre Reinforced Polymer

7/25/2019 Concrete Reinforcement and Glass Fibre Reinforced Polymer

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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.

QUEENSLAND ROADS Edition No II September 2 11

4



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).

QUEENSLAND ROADS Edition o II September lOll

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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)

QUEENS

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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

QUEENS L ND ROADS Edition No II Septelilber 2011

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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,

V.

2, AB. S.Ang and R. Villaverde,

eds. , Structures Congress, ASCE, Irvine, CA

1993

QUEENSLAND ROADS Edition No

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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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