Durability of FRP in Concrete

Procedures for Reduced Alkalinity Exposures

Sotiris DemisProfessor Kyrpos Pilakoutas

Dr Ewan Byars

Department of Civil & Structural EngineeringThe University of Sheffield, UK

Definition of the Problem

What is the main aggressive agent for FRP ?

Is deterioration affected by the pH level ?

Can cement replacements address the issue?

Can carbonation address the issue?

What is the rate of deterioration ?

Pilot Studies

Programme

FRP type: GFRPConcrete mix : OPC 40

Exposure Environnent

Air, 20 C (lab temp.)

Testing Technique

GFRP Tensile Testing GFRP Flexural (3-point bending)Direct Cube pull-out

Programme

FRP type: GFRP

Concrete mixes:OPC 40OPC/PFA 40/30OPC/GGBS 40/70

Exposure Environment

CO2: 15%, RH: 55 %

Test ages: 0, 1, 6, 12, 24 months

Standards

BS EN ISO 14125, 1998BS EN 2746, 1998

BS 2782-10 (1005), 1977BS EN ISO 178, 2003

BS EN 2562, 1997ACI 549 R, 1997

ACI 440.3 R, 2004

Standards

fib T.G. 9.3, 2006 BS EN 13295, 2004

Programme

FRP type: GFRP

Exposure Environnent

Air (control) 20 C Alkali: pH 9, 20 C

pH 12, 20 C

CO2 : 15 %, 23.94 C

Test ages: 0, 1, 6, 12 months

Standards

BS EN ISO 527-4, 1997 BS EN 2561, 1995BS EN 2747, 1998ACI 549 R:, 1997

ACI 440.3 R:, 2004

GFRP Tensile Testing

Concrete strength Carbonation development

Programme

Concrete mixes:OPC 40OPC/PFA 40/30OPC/GGBS 40/70

Exposure Environment

WaterCO2: 15%, RH: 55 %

Test ages:0, 1, 6, 12, 24 months

Standards

BS 12, 1989BS 1881 (125), 1986BS 1881 (112), 1983

BS 146, 1996BS 6588, 1996BS 5328, 1997

ACI 232.2 R, 2003ACI 233 R, 2003

Pull-out Testing

Experimental Programme

Results Samples in Solution

Reductions in the tensile capacity of the FRP bars up to 41.6 %.

Caused by alkali ingress to fibre/matrix interface

fibresmatrix

Accelerated with a non-perfect quality of FRP bars tested.

Results Samples in Concrete

0

2

4

6

8

10

12

1 6 11 16 21 26

Time of Exposure (months)

Mea

n B

ond

Stre

ngth

(MPa

)

OPC (control)OPCOPC/GGBSOPC/PFA

Initial bond strength value of control samples

Embedment length: 150 mmBar size: 8 mm squareConcrete Strength: 40 MPa

Results Samples in Concrete

0

2

4

6

8

10

12

1 6 11 16 21 26

Time of exposure (months)

Nor

mal

ised

Bon

d St

reng

th (M

Pa)

OPC (control) OPC OPC/GGBS OPC/PFA

Initial bond strength value of control samples

-20

-15

-10

-5

0

5

1 6 11 16 21 26

Time of exposure (months)

Loss

of b

ond

(%)

OPC OPC/GGBS OPC/PFA

Carbonated OPC bond reduced by 12.9 %

PFA bigger increase in bond after 1 year

Considerable changes in bond after 6 months

Results Samples in Concrete

Position of the FRP bar at the centre of the 150 mm cube

70.4 mm

Edge of the 100 x 100 x 500 mm concrete prism

Carbonation Test on 100 x 100 x 500 mm prisms

Carbonation Test on 150 mm pull/out cubes

Car

bona

tion

Dep

th (m

m)

0

20

40

60

80

100

120

Time of Exposure (months)0 5 10 15 20 25 30

Car

bona

tion

Dep

th (m

m)

0

20

40

60

80

100

OPCOPC/GGBSOPC/PFA

Carbon Dioxide

Phenolphthalein

WaxedSurfaces

Uncarbonated Area

CarbonatedArea

Carbon Dioxide

100100

500

exposed concrete

freshly brokenand then re-waxed

waxed surfaces

0.90

0.95

1.00

1.05

1.10

1.15

1.20

0 20 40 60 80 100 120

Carbonation Depth (mm)

Rat

io o

f Con

cret

e C

ompr

essi

ve

stre

ngth

to th

e co

ntro

ls O

PC

com

pres

sive

stre

ngth

OPC OPC/GGBS OPC/PFA

0

2

4

6

8

10

12

0 10 20 30 40 50 60 70

Cube compressive strength (MPa)

Bon

d St

reng

th (M

Pa)

OPC (control) OPC

OPC/GGBS OPC/PFA

Results Samples in Concrete

Results Samples in Concrete

Bon

d S

treng

th (M

Pa)

5

6

7

8

9

10

11 OPC (control)OPCOPC/GGBS

OPC/PFA

Nor

amlis

edBo

nd

Stre

ngth

with

resp

ect t

o co

ncre

te c

ompr

essi

ve

stre

ngth

(MPa

)

6

7

8

9

10

Concrete Compressive Strength (MPa)

30 35 40 45 50 55 60Nor

mal

ised

Bond

Stre

ngth

with

re

spec

t to

revi

sed

conc

rete

co

mpr

essi

ve s

treng

th

(MP

a)

5

6

7

8

9

10

11

Mea

n B

ond

(MPa

)

5

6

7

8

9

10

11

opc control opcopc/ggbsopc/pfa

Cla

ssic

al N

orm

alis

edB

ond

(MPa

)

5

6

7

8

9

10

11

Time of Exposure (months)0 5 10 15 20 25 30

Nor

mal

ised

Bon

d w

ith

resp

ect t

o fc

u' (M

Pa)

5

6

7

8

9

10

11

Results Samples in ConcreteBefore carbonation reached

the barAfter carbonation reached

the barBefore and after

carbonationSignificant statistical

differenceNo significant statistical

differenceNo significant statistical

difference

Model to predict the penetration of the carbonation front

Model to predict the effect of carbonation

on bond strength

Chemical Deterioration takes place once the

carbonation front reaches the FRP bar

2 models required to predict this behaviour

Key finding

Results Samples in Concrete

Rt

ty(years)

28 days

5

10

20

30

50

100

(%)

100

4.6

81.6

78.6

76.9

74.6

71.6

fcm(ty)

(MPa)

35.7

51.1

51.6

51.9

52.1

52.2

52.4

(MPa)

6.81

9.30

9.35

9.39

9.41

9.73

9.45

(MPa)

6.81

7.87

7.63

7.38

7.23

7.04

6.77

1

1.24

1.20

1.11

1.08

1.04

1.00

D

(mm)

0

8.32

11.8

16.6

20.4

26.3

37.2

Rt

Rttt tRt

: Relative bond retention

fcm(ty)

t

Rtt

tRt

: OPC concrete strength as enhanced by time and carbonation

fcm(t) = cc(t) fcm

cc(t) =0.528exp 1s

t

The strength is calculated according to Eurocode 2 as,

and is increased by 15% to account for the effect of

carbonation

: expected bond strength calculated according to Eurocode 2, taking into account the gain in compressive strength.

: expected bond strength due to the effect of carbonation

: total bond strength retention in time

The above table assumes that concrete had zero cover and carbonation took place at 28 days

Results Samples in Concrete

the negative effect of carbonation is counteracted by the gain in strength with time and as such does not reduce the initial bond strength for length of exposure up to 100 years.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

0 20 40 60 80 100 120

Time of Exposure (years)

Tota

l Bon

d R

eten

tion

no cover10 mm cover

20 mm cover30 mm cover

Conclusions pH 9 and pH 12 solutions had detrimental effect on the GFRP tensile strength

Although pH has impact on FRP in solutions, its impact may be assignificant as the impact of moisture on FRP.

Alkalinity does not have a major impact on FRP bond

As concrete strength increases due to time and carbonation, bond also increases

Carbonation operates as a switch on deterioration of FRP

Expected bond deterioration due to carbonation is more than compensated by the increase in strength of concrete due to time and carbonation

There is no need to take into account the carbonation effect on the stress reduction factors used for durability design

Blended cements reduced alkalinity but did not improve behaviour