Sim ple tension tests w ere carried out on five specim ens (tw o 650 m m x 100 m m x 12 m m steel anchor plates p e r specim en) to investigate the bond strength o f the adhesive, effectiveness o f slip-prevention w rapping, and d ifferent bonding schem es, as show n in 235 Y ail J. K im , P .E ng., Ph.D . T hesis
100 m m . T he epoxy adhesive had a yield strength o f 54 M Pa, a yield strain o f 2.5 %, an elastic m odulus o f 3.03 G Pa, an u ltim ate strength o f 55.2 M Pa, and a ru pture strain o f 2.5 % (Wabo® M B race Saturant).
T he steel plates w ere p rep ared and th e surfaces w ere ground to im prove bo n d to the sheets (in the field application, the p lates w ere sand-blasted). T he epoxy adhesive w as m ixed and 3 layers o f C FR P sheets w ere bon d ed onto the steel an ch o r plates except for the gripping area (/ = 150 m m ), as show n in Fig. 8.5 (a). A t the ends o f the plates, tapered 3-layer ru b b er pads w ere p laced to p re v en t stress concentrations, as show n in Fig. 8.5 (b) to rep resen t the contribution o f the bevel p late u sed in the site application. A fter five days o f epoxy curing, strain gauges w ere attached at the locations show n in Fig. 8.5 (c). T w o groups o f specim ens w ere tested: G roup A consisted o f longitudinal sheets and w ere tested after 7, 10, and 14 day curing tim es; G roup B consisted o f longitudinal sheets w ith additional w rapping sheets and w ere tested after 7 days o f curing. T he tensile load w as applied up to 80 kN (approxim ately 800 M P a in the sheets), the sam e prestress level as in the repair design, and sustained fo r 15 m inutes to sim ulate the bonding tim e on site (i.e., a tim e interval w ould be required after the prestressing operation to bond the C FR P sheets to the substrate o f the girder). T hen, the load w as increased until com plete failure o f the anchor system occurred.
8.4.1.2. E x p e r im e n ta l resu lts
The failure m odes o f all the specim ens w ere prem atu re debonding failures at the interface region except for case B -2 w hich show ed a C FR P rupture failure. T he initial b reak o f the 236 Y ail J. K im , P .E ng., Ph.D . T hesis
C hapter 8: R ep air o f Im pact-dam aged B ridge w ith P restressed C F R P Sheets
C FR P sheets started n ear the ends o f the steel p late (i.e., opposite sides o f the gripping ends) all across the specim ens due to stress concentrations, as show n in Fig. 8.6 (a). In this situation, the shear stress o f the adhesive on the plate governs the ultim ate strength rath er than the norm al stress w hich is com m on in a beam application. T he failure loads and m odes are presen ted in T able 8.4.
In group A, the increases in the failure loads w ere 1.8 % and 9.4 % for the 10 day and 14 day curing specim ens, respectively, in com p ariso n to the 7 day curing specim en. Thus, a curing tim e lo n g er than one w e ek did no t have a significant effect on the bo n d strength.
B ecause o f stress concentrations at the edge o f the C FR P sheets, ru pture o f the carbon fibres w as observed, Fig. 8.6 (a). A typical debonding failure during the test is show n in Fig. 8.6 (b). T he debonding failure regions show ed tw o typical pattern s on both o f the anchor plates: com plete debon d in g o n one plate and partial debon d in g on the o th er p late o f the specim en as show n in Fig. 8.6. (c) and (d). T hese p h en o m en a m ay be explained by the fact that the bond thickness and applications w ere not p erfectly the sam e on b o th sides.
A fter rem oving the C FR P sheets on the partial debonding side o f the specim en as show n in Fig. 8.6 (e), the lengths o f the partial debonding zone w ere visually observed to be 150 m m to 200 m m (30 % to 40 % o f the entire bond length), w hich is con sisten t w ith the strain data show n in Fig. 8.7, at the failure o f the specim en. T his m eans that the debonding p rocess o f the epoxy adhesive on the steel plate p ropagates g radually and this debonding length should be called an effective length. B eyond the effective length, the bonded zone is free from bond stresses induced by the applied tensile load.
237 Yail J. K im , P .E ng., P h.D . T hesis
b o n d in g s c h e m e s , B - l a n d B - 2 in F ig . 8 .5 . I n s p e c im e n B - l , th e in n e r la y e r o n th e s id e o f th e a n c h o r p la te w ith 2 la y e r s w a s c u t s h o r te r th a n th e o u te r la y e r . T h u s , e a c h la y e r w a s b o n d e d d ir e c tly to th e s te e l a n c h o r p la te . I n s p e c im e n B -2 , a ll th e la y e r s w e r e c u t to th e s a m e le n g th a n d h e n c e th e o u t e r la y e r w a s b o n d e d d ir e c tly to th e in n e r la y e r a n d w a s n o t in d ir e c t c o n t a c t w ith th e s te e l a n c h o r p la te . F o r b o th s p e c im e n s B - l a n d B - 2 , o n e a d d itio n a l la y e r o f C F R P w a s w r a p p e d a r o u n d th e b o n d e d r e g io n a s s h o w n in F ig . 8 .5 (d ) a n d (e ). T h e s li p - p r e v e n tio n s h e e ts tu r n e d o u t to b e e f f e c tiv e in d e l a y in g th e d e b o n d i n g f a ilu re . A n 11.1 % in c r e a s e in lo a d w a s a c h ie v e d w h e n c o m p a r e d to th e s p e c im e n w ith o u t th e w r a p p in g s h e e t. M o r e s tu d y is r e q u i r e d in th is a r e a b e c a u s e s p e c im e n B - 2 d id n o t s h o w th e s a m e f a ilu r e m o d e a s th e o t h e r s p e c im e n s (i.e ., B - 2 s h o w e d C F R P r u p tu re ; o th e rs s h o w e d d e b o n d i n g f a il u r e ) a s s h o w n in F ig . 8 .6 (f). T h e f a ilu r e m o d e o f th e s p e c im e n B - l w a s s li g h tly d if f e r e n t f r o m th e o th e r c a s e s : th e d e b o n d i n g z o n e n e a r th e g r ip p in g e n d w a s v i s u a lly d i s t in g u is h a b le , b u t th e z o n e a t th e o th e r e n d lo o k e d in ta c t.
D e b o n d in g lik e ly o c c u r r e d , b u t it w a s h id d e n u n d e r th e w r a p p in g s h e e t.
T h e a v e r a g e s tr e s s in th e C F R P s h e e ts a t f a il u r e w a s f o u n d to b e 1 ,1 2 0 M P a ( o r 2 9 .5 % o f th e u lti m a t e f ib r e s tr a in ) . T h e s tr a i n p r o f il e s , o n th e j a c k i n g p la te f o r s p e c im e n A - l , c o r r e s p o n d i n g to th e in c r e a s i n g lo a d s a r e s h o w n in F ig . 8 .7 . A s w a s o b s e r v e d in th e v is u a l in s p e c tio n , th e d e b o n d i n g f a il u r e h a p p e n e d g r a d u a lly . O n e c o n s p ic u o u s p h e n o m e n o n is th e s tr e s s c o n c e n t r a tio n a t th e tip o f th e p la te w h e r e th e s tr a in s w e r e s li g h tly h ig h e r th a n th o s e a t m id - s p a n . T h e fir s t n o t i c e a b l e d e b o n d i n g o c c u r r e d a t 7 1 .4 k N ( 6 7 .8 % o f th e f a ilu re lo a d ) a s e v i d e n c e d b y th e in c r e a s e d s tr a i n in th e b o n d e d r e g io n , a s s h o w n in F ig . 8.7 . T h is in c r e a s e d s tr a i n v a l u e w a s m a i n ta in e d u p to th e f a ilu r e . T h e d e b o n d e d le n g th 2 3 8 Y a il J. K im , P .E n g ., P h .D . T h e s is
C hapter 8: R epair o f Im pact-dam aged B ridge w ith P restressed C F R P Sheets
k e p t m o v in g to w a r d s th e g r ip p in g e n d a s th e lo a d in c r e a s e d a n d th e n th e s p e c im e n e v e n tu a lly f a ile d . T h e s tr e s s - s tr a in r e l a tio n s h i p a t th e m id - s p a n , a s e x p e c t e d , f o llo w e d a lin e a r t r e n d a s s h o w n in F ig . 8.8.