Resistance recovery of corrosion
– damaged reinforced concrete
through FRP jacketing
Souzana Tastani, MSc Civil Eng., PhD candidate
Dr. Stavroula J. Pantazopoulou, Professor
Demokritus
University of
Thrace
Composites in
Construction
Lyon 2005
Non conforming structures
designed prior to ’80
Low ductility
systems
lack of stiffness
(soft storeys)
Old type
detailing
Low strength materials
(fc=20MPa / fs=400MPa)
sparse stirrups with
insufficient anchorage
 poor confinement
 buckling of bars
cracking of cover
DUTh
Compounded
Corrosion damage
bar section loss
embrittlement of bars
reduced bond resistance
cover spalling
Excessive displacement brings
out all the potential problems
Base shear
Inadequate construction:
Consistent with new codes
Inadequate
Repaired
DUTh
ductility
qtarg qnew
Requirement in repair
strengthening schemes:
to target for reduced displacement demand
( qtarg < qnew )
addition of stiffness
stiffening the individual members
managing old detailing & corrosion:
Rehabilitation with FRP jackets
 efficient as a confining device in repair / strengthening
 curbing of iron depletion under of continued post-repair exposure
Impermeable to corrosion agents (Ο2, Η2Ο, Cl)
Strength assessment of corroded r.c. members

Vu ,lim  min Vshear ,Vanchor ,Vi ,flex

D
V
 in assessment of dependable capacity
sm
Mcr
Ls
 in dimensioning the upgrading scheme
My
 Corrosion: expansive phenomenon
X=DDb/Db : depth of corrosion penetration (cr = concrete strain at cracking )
ur ,o  0.5  cr Db ( 0.25   rs  1 X 2  X  /  cr  0.5 )
 pitting corrosion: embrittlement of steel
Rcr
fs
X
rust
sc(X)
c: cover
 sy

cor
 su
 su

cor
 su
  sy   su   sy 1  rpit
rpit  a pit a pit ,max  1

DUTh
Recovery of strength through FRP jacketing
DUTh
local intervention for seismic upgrading: increase of strength
and deformation indices of an individual corroded member
without however controlling global demands
Points under consideration
 no influence on lateral stiffness
 by reducing shear cracking in the plastic hinge regions all deformation
occurs within few flexural cracks thereby promoting large strain demands in
the embedded longitudinal reinforcement. This may lead to bar fracture
unless the rehabilitation framework includes measures for stiffening the
affected structure.
 susceptibility to rupture at points of localized deformation demand
 postponement of compression reinforcement buckling to higher levels of
deformation but FRP jacketing cannot altogether prevent buckling, particularly
if stirrups have been wasted away due to corrosion
Assessment of shear strength of corroded member
Vshear
Vshear
res (q)
2
res
( q )  λ Vs  Vc 
Vshear
λ  1.15  0 .075 q

0.7Vshear
6
q
DUTh
; 0 .7  λ  1

 0 .25 ft' K ( 1.2  40 ρs1 )  0 .15 P / Ag  b d  EC2

Vc  
0 .5 fc' bd  ACI 318
cor
Vs  σ lat
, st b d : affected by corrosion
Stretching of stirrup leg by
splitting cracks
Pst,y
Astcor= Ast(1-X)2
cor
s lat
, st
fyred
, st
cor
Ast

fyred
, st
sb
cor
cor
fy , st  E s  st
,

  sy
st


cor
0 ,  st   sy
wcr
Pst,x
cor
 st

w cr
  Db  rs  1 X

 Lb , st
 Lb , st
Recovery of shear strength with FRPs
In redesigning FRP-jacketed r.c. members with corroded
stirrups the objective is to recover the initial shear strength
and to secure sufficient displacement ductility that would
exceed the design demands.
DUTh
res
Vshear enh (q)  λ (Vshear
 Vw,f )
q: behavior index
λ  1.15  0.075 q ; 1  q  3.5 ; 0.7  λ  1
Vw,f  σ lat,f b h
σ lat ,f 
kfv  1
(=1 if adequately closed)
2 kfv  nf t f  Ef   f ,eff
b
f,eff = 0.004 for U – type jacket
= 0.5fu for closed jacket
nf : number of layers
Assessment of bond of corroded anchorages
Based on frictional bond model: fb =  ·sn
DUTh
Bond degradation due to corrosion: loss of frictional resistance and loss
of confining pressure by the cover due to cracking.
fbcor


cor red
 ( X ) Cc  Rcr ' Cc  Rcr
1 Ast fy , st 


ft 
s shr 
  Db
3 Db Nb s 
C
 








 cov er
shrinkage
stirrups


 max ( X )
: Friction coefficient
rmax = 1.5 - 2.0
Rcr 
; sm
ur ,o Cc
max
ur ,o   cr Cc  Rb 
Cc  Rcr
Db
= 0.3 - 0.5 ; sm
res
= 0.1
sn
snres
ur,o

rmax
smmax
: Radius of crack front
Xu=hr /Rb
: Un-cracked cover
sshr=3ft’ : shrinkage
stress
res
Vanchor

 DbLb fbcor Nb jd  P ( d  0.5 h )
Ls
Recovery of anchorage / lap splice strength with FRPs
DUTh
 Inevitable flattening of the ribs  the coefficient of friction cannot be
recovered
 Replacement of cracked cover with new grout in combination with FRP
jacket 
increase of rehabilitation effectiveness of corroded bar
anchorages.
fbenh
cor red
 ( X ) Cc  Rcr ' Cc  Rcr
1 Ast fy , st 2 nf t f Ef  f ,eff

(
ft 
s shr 

)

D
C
3 Db Nb s
D Nb

b 
  

 b


shrinkage
cov er
stirrups
FRP jacket
f,eff = tensile strain of the FRP when the bar develops its bond strength
(surface strain orthogonal to the bar axis, in the order of 0.002)
 f ,eff 

C
Db ( 1  2
)
Db
;   2 ur ,o
;   0.1mm
Slip of the bar
Assessment of residual flexural strength
b
Fci
Fs2 y
x
d
h
DUTh
c
s2
Fsi
Fs1 s1
cor
N res   fci Ac ,i   fsi Asi
cor
h / 2  y si 
M res   fci Ac ,i h / 2  y i    fsi Asi
res / L
; Vires
,flex  M
s
cu= 0.0035 for concrete
Asicor= Ast (1-X)2
&
fsi = g (su ) for reinforcement
cor
 su
  sy   su   sy 1  rpit 
fs
 sy
cor
 su
 su
rpit  a pit a pit ,max  1
Externally bonded longitudinal FRP reinforcement
for flexural strength recovery
DUTh
 externally-bonded laminates
 near-surface mounted (NSM) reinforcement
The required additional reinforcement area to achieve flexural strength
recovery is estimated from the moment reduction owing to primary
reinforcement section loss at the critical section:
DMcor=MRd (2-X)X ; MRd the uncorroded flexural strength
feff = 0.004 : FRP allowable strain
cu = 0.0035 : concrete strain
c
depth of compressive zone: x = c / d
h
xlow = 0.0035·(1+d’/d) / (0.004+0.0035)
xupper = xbalance =0.64
c=0.0035
b
d
d’
feff=0.004
Enhancement of flexural ductility and rotation capacity
through transverse FRP jacketing
DUTh
Transverse FRP affects only indirectly the flexural resistance of concrete
members (Vi,flexenh) through confinement of concrete in the compression zone
Enhanced axial strength & corresponding strain:
'
fcc
confinement

 effectiveness
 conf cor red
'
conf
eff 
 fc  1.5  k st  sv fy , st  k f  fv Ef  f  f,eff = 0.5fu



 
for closed jacket
 

FRP jacket
stirrups


volumetric ratios
Enhanced axial strain
conf cor red
conf
eff
k

f

k

E

st
sv
y
,
st
f
fv
f
f
capacity (at 0.8fcc’ ): εcc  0 .002  0.015
'
fc
u   y ,flex   y , slip   p , slip  Rotation capacity (u) of the upgraded member:
corr
2

Db 1.2 E h (  su   y , s ) 
1
1 2 Db fy
 y , s  ( Ls 



)

d(1  x ) 
3
8 fb
4
fb


fb=fbenh : new cover + FRP
; fb=fbcor : inaccessibility of anchorage
3) Example application:
double bending column with corroded reinforcement
DUTh
Materials:
Stirrups: F8/100, fy=400MPa ; Long. reinf.: 10F20, fy=S500
Lb=0.5m
Ls=1.5m
Concrete: fc’=20MPa  fcc’=22MPa
State
Vi,flex
(kN)
Vshear
(kN)
Vanchor
(kN)
fb
(MPa)
Qu
(%)
failure
Uncorroded
159
248
223
6.7
2.7
Flex.
X=5%
Corroded
- Desucor=18% - rpit=0.2
136
105
66.5
2.0
CFRP: ffu=3500MPa / tf=0.13mm / Ef=230GPa
Recovery
(DMcor=
27kNm)
156
(Vw,f=
153kN)
248
Layers nf
1.6
(long.)
1.6
(trans.)
212
0.46 Splice
- q = 2.5
6.7
8 (5, if new
cover)
2.16
Flexsplice
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