Composites in Construction Lyon 2005

1. Resistance recovery of corrosion – damaged reinforced concrete through FRP jacketing Souzana Tastani, MSc Civil Eng., PhD candidate Dr. Stavroula J. Pantazopoulou, Professor Composites in Construction Lyon 2005 Demokritus University of Thrace

2. DUTh Low ductility systems Old type detailing Compounded Corrosion damage • lack of stiffness • (soft storeys) • bar section loss • embrittlement of bars • reduced bond resistance • cover spalling • Low strength materials • (fc=20MPa / fs=400MPa) • sparse stirrups with • insufficient anchorage • poor confinement • buckling of bars • cracking of cover Non conforming structures designed prior to ’80

3. Consistent with new codes Inadequate construction: Excessive displacement brings out all the potential problems Inadequate DUTh Repaired Base shear ductility qtarg qnew Requirement in repair strengthening schemes: addition of stiffness stiffening the individual members to target for reduced displacement demand ( qtarg < qnew ) 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-)

4. DUTh • in assessment of dependable capacity • in dimensioning the upgrading scheme D V Rcr X Mcr rust sm Ls c: cover sc(X) • Corrosion: expansive phenomenon X=DDb/Db : depth of corrosion penetration (ecr= concrete strain at cracking ) My • pitting corrosion: embrittlement of steel Strength assessment of corroded r.c. members

5. DUTh Recovery of strength through FRP jacketing 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

6. DUTh Vshearres(q) Vshear 0.7Vshear q 2 6 Pst,y wcr Stretching of stirrup leg by splitting cracks : affected by corrosion Astcor= Ast(1-X)2 Pst,x • Assessment of shear strength of corroded member

7. DUTh • 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. q: behavior index kfv  1 (=1 if adequately closed) ef,eff = 0.004 for U – type jacket = 0.5efu for closed jacket nf : number of layers

8. Assessment of bond of corroded anchorages Based on frictional bond model: fb = m ·sn Bond degradation due to corrosion: loss of frictional resistance and loss of confining pressure by the cover due to cracking. DUTh sn snres ur,o : Friction coefficient m ; msmres= 0.1 ; msmmax= 0.3 - 0.5 mrmax= 1.5 - 2.0 mrmax : Radius of crack front msmmax : Un-cracked cover Xu=hr /Rb sshr=3ft’ : shrinkage stress

9. DUTh ef,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) Slip of the bar • Recovery of anchorage / lap splice strength with FRPs • 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.

10. DUTh b ec Fci y es2 Fs2 x d Fsi h es1 Fs1 • Assessment of residual flexural strength • ecu= 0.0035for concrete • Asicor= Ast (1-X)2 & fsi = g (esu )for reinforcement

11. DUTh b c d h d’ efeff= 0.004 : FRP allowable strain ecu = 0.0035 : concrete strain ec=0.0035 depth of compressive zone: x = c / d xlow = 0.0035·(1+d’/d) / (0.004+0.0035) xupper = xbalance =0.64 efeff=0.004 • Externally bonded longitudinal FRP reinforcement • for flexural strength recovery • 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

12. DUTh confinement effectiveness ef,eff = 0.5efu for closed jacket Rotation capacity (qu) of the upgraded member: fb=fbenh: new cover + FRP ; fb=fbcor : inaccessibility of anchorage • Enhancement of flexural ductility and rotation capacity through transverse FRP jacketing 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: volumetric ratios Enhanced axial strain capacity (at 0.8fcc’):

13. DUTh Ls=1.5m Lb=0.5m 3) Example application: double bending column with corroded reinforcement Materials: Concrete: fc’=20MPa  fcc’=22MPa Stirrups: F8/100, fy=400MPa ; Long. reinf.: 10F20, fy=S500