B ecause o f increasing live loads and structural d eterioration, appropriate m ethods o f rehabilitatio n are necessary for extending the lifespan o f bridges and buildings. A viable solution using p restressed C FR P sheets has been investigated in this thesis.
C hapter 2 has investigated the flexural b eh a v io u r o f prestressed concrete beam s (3.3 m span) strengthened w ith prestressed C FR P sheets, particularly focusing on cracking and ductility aspects. A 3-D n o n lin ear finite elem ent analysis (FEA ) w as conducted to p redict the experim ental investigations, including a param etric study on prestress levels in the C FR P sheets to evaluate th eir contributions to the flexural b eh a v io u r o f strengthened beam s. T he strengthened beam fully reco v ered the load-carrying capacity o f the dam aged beam to the level o f the undam aged beam . T he param etric study d em onstrated the level o f p restress in the C FR P sheets significantly contributed to the structural ductility o f strengthened beam s. T he suggested level o f effectiv e prestress in the C FR P sheets w as recom m ended to be 20 % o f the ultim ate fibre strain.
314 Yail J. K im , P .E ng., Ph.D . T hesis
C h ap ter 10: C onclusions and R ecom m endations
C hapter 3 has dem o n strated d etailed investigations on a m echanical an ch o r system for p restressing C FR P sheets. A ten sio n test w as conducted and fracture m echanics m odels w ere d eveloped to p redict the failure load o f the p late anchor system bon d ed w ith C FR P sheets. V arious p aram eters affecting the failure o f the system w ere evaluated, and the fracture energy w as found to be the m ost significant factor. C losed-form solutions w ere d eveloped to p red ict the b eh a v io u r o f the m echanical anchorage m ounted on the prestressed concrete beam s. F or d esign pu rp o ses, short-term prestress loss o f 10 % in C FR P sheets w as recom m en d ed w ith this type o f anchor system .
C hapter 4 has evaluated a novel anchoring technique for strengthening reinforced concrete beam s strengthened w ith p re stressed C FR P sheets. T en d o ubly-reinforced concrete beam s w ere tested w ith various types o f non-m etallic anchorages. A v ariety o f different failure m odes, d epending u p o n the ty p e o f anchorage, w ere observed. T he developed non-m etallic anchorage successfully transferred the sustained p restress in the C FR P sheets w ith insignificant losses o f prestress (i.e., typically less than 1 %). A closed- form solution w as d eveloped and analytical m odels w ere proposed to p redict the flexure o f the test beam s. T he m echan ically anchored U -w raps w ere recom m ended in practice.
T he side sheets w ere also re co m m en d ed to im prove the energy absorption n ear the critical anchor region. A dequate an chorage w as critical to p revent prem atu re failure o f the strengthened beam s.
C hapter 5 has looked into the ap p licatio n o f p re stressed or non-prestressed C FR P sheets for strengthening tw o-w ay slabs, fo cusing on the flexural behaviour. A n o n lin ear 3-D F E A w as co nducted to p redict the b eh a v io u r o f tested slabs. T he experim ental d ata w ere 315 Y ail J. K im , P .E ng., Ph.D . T hesis
carrying capacity, load-deflection response, strain distribution, crack p ro g ressio n , and crack opening. T he governing failure m ode in the strengthened slabs w as either delam ination or partial rupture o f the C F R P sheets. The prestressed C FR P sheets provided a load-sharing m echanism , resu ltin g in the increase o f yield load. T he effectiveness o f the prestresed C FR P sheets on controlling the crack m outh opening displacem ents w as insignificant.
C hapter 6 has exam ined the b eh a v io u r o f slab-colum n connections o f tw o-w ay slabs strengthened w ith prestressed or n o n -p restressed C FR P sheets. T he experim ental inform ation w as obtained from L o n g w o rth (2004). A n o nlinear 3-D F E A w as co nducted to p redict the b eh av io u r o f the slab-colum n connection, including pu n ch in g shear failure.
T he effects o f C FR P sheets on the num erical crack grow th, shear stress d istributions at the slab-colum n connection, and critical p erim eters for pu n ch in g shear failure w ere studied. T he C FR P sheets contributed to the increase o f punching shear failure load and cracking load; nevertheless, no noticeab le increase o f yield load w as found. P restressed C FR P sheets w idened the effective strain zone n ear the slab-colum n connection that resulted in a load-sharing effect b etw een the slab and the colum n. A nalytical m ethods have been studied to p redict the p u n ch in g shear capacity o f strengthened slabs, including a com parison to the code. T he p re d ic tio n based on the yield line theory overestim ated the failure load, bu t the predictio n based on the pu n ch in g shear concept pro v id ed good agreem ent com pared to the experim ent.
316 Yail J. K im , P .E ng., Ph.D . T hesis
C h ap ter 10: C onclusions and R ecom m endations
C hapter 7 has evaluated the app licab ility o f a fracture m echanics m odel p ro p o sed by H illerborg (1990). T he prop o sed m odel w as rev iew ed and extended to cover reinforced concrete and prestressed concrete beam s including a p restressed concrete beam strengthened w ith prestressed C F R P sheets. T o assess the proposed m odel, tw o reinforced concrete beam s w ere tested and p re v io u s research d ata w ere used. A size-p aram eter o f £/
= 0.005 w as reco m m en d ed for the p ro p o sed fracture m echanics m odel. A nalytical studies, such as the iterative n o n lin ear strength-based m odel and FEA , w ere co nducted to evaluate the predictio n by the fracture m echanics m odel, in addition to the experim en t results. T he conventional analysis m ethods based on strength-based theory w ere still acceptable, although the fracture m echanics m odel provided b etter predictions.
C hapter 8 has studied a full-scale bridge strengthening project, the M ain S treet B ridge, W innipeg. M B. An innovative strengthening m eth o d w ith prestressed C FR P sheets w as applied to rep air the dam aged ex terio r girder. D etailed design and analysis w ere conducted based on the A m erican A sso ciatio n o f State H ighw ay and T ransportation O fficials L oad R esistance F acto r D esig n (A A S H T O L R FD ) and the C anadian H ighw ay B ridge D esign C ode (C H B D C ). A full-scale F E A w as conducted as w ell. E x perim ental w ork w as carried out to confirm the feasibility o f the anchor system for p restressin g C FR P sheets. A fte r strengthening, the load -carry in g capacity o f the dam aged exterio r gird er w as recovered to the u n d am ag ed state and the serviceability w as also enhanced.
The rep air w ork w as com pleted w ith o u t m ajo r difficulty on site. T he proposed strengthening m ethod w as re com m endable. T he A A S H T O rating factor also supported the strengthening effects. T he rating factor w as conservative up to 25 % com pared to that obtained from the refined analysis.
317 Y ail J. K im , P.E ng., Ph.D . T hesis
on live load distributions, u n d e r various loading configurations. T hree states o f the bridge such as the undam aged, dam aged, and repaired w ere investigated based on the calibrated FEA m odels. T he load-carrying capacity o f the exterior girder w as increased by 25 % after strengthening; n evertheless, no notable change o f stiffness w as found. A ssessm en t on the existing bridge codes has been p erform ed, including a p aram etric study. M ore even live load d istributions on the superstru ctu re w ere observed as m ore trucks w ere loaded.
T he bridge codes such as A A S H T O L R F D and C H B D C overestim ated the live load effect for the exterior girder, bu t the codes u n derestim ated the live load effect fo r the interior girders. T he factored m om ents to be used for design and construction w ere still conservative.
In accordance w ith the p revious investigations, the application o f prestressed C F R P sheets for strengthening concrete structural m em bers is a p ro m isin g solution to the rehabilitation o f deteriorated structures.