Chapter 5: Flexure o f Two-way Slabs Strengthened with Prestressed CFRP Sheets
6.6. Test Results and Analysis 1. Load-deflection response
6.6.3. Failure and crack patterns
The com m on failure m ode in all o f the tested slabs w as punching shear, bu t the detailed failure o f each slab w as som ew hat different. T he failure m odes for each tested slab are show n in Fig. 6.7. T he sequence o f the failure is briefly explained as follow s. In the u nstrengthened slab (B 3-S L 1), flexural cracks initiated near the m id-span; then spread tow ards the edges o f the slab. A s lo ad in g increased, large splitting cracks, in the tension side, w ere observed and sudden concrete spalling follow ed. T he tension steel reinforcing bars w ere com pletely exposed, w hich caused severe flexural stress due to a lack o f stress sharing w ith the concrete. A fte r fu rth er concrete splitting w as observed, typical p unching shear failure com m enced. T he rein fo rcin g bars, located n ear the p u n ch in g cone, exhibited excessive elongations and the concrete w as split along the re inforcing bars, due to the concentrated pressure applied by the co lu m n stub, as show n in Fig. 6.7 (a). T he concrete split in the shape o f a diam o n d w hose v ertex es w ere rotated from the vertexes o f the slab at angles o f ap p roxim ately 45 degrees.
The slab strengthened w ith n o n -p restressed C F R P sheets (B 3-S L 2) show ed a different failure m ode w ith resp ect to the u n stren g th en e d slab (B 3-S L 1). T he initial cracks form ed near the edges o f the C FR P sheets due to stress co ncentrations in the contact area betw een 178 Y ail J. K im , P .E ng., Ph.D . T hesis
C h ap ter 6: T w o-w ay S lab -C o lu m n C onnections
the C F R P and the concrete. T he cracks then spread outside o f the strengthened area, but no visual cracks n ea r the m id -sp an (or inside o f the strengthened area) w ere o b served at this loading stage. As the applied load increased, the delam ination o f the C F R P sheets w as in itially o b served in the east side; then the delam ination eventually spread to all directions, except for the w est side. E xtensive splitting o f concrete, in the vicinity o f the strengthened area, w as also asso ciated w ith w ide crack openings, as show n in Fig. 6.7 (b), including a sudden splitting o f the m id -sp an concrete cover as the C F R P sheets delam inated. T his inv estig atio n indicates that the dam age near the m id -sp an w as p ro b a b ly initiated inside o f the concrete cover, although the region looked relativ ely sound until the com plete delam ination o f C FR P sheets occurred.
In the slab strengthened w ith prestressed C FR P sheets (B 3-SL3 and -SL 4), the initial cracks form ed n ea r the edges o f the sheets as in the case o f the non -p restressed C FR P application (B 3-S L 2), b u t m uch tin ie r cracks w ere observed, including a com plex crack pattern in all directions. This m ay have been due to the confining effect o f the prestressed C FR P sheets, w hich contributed to reduced strain localization. T he strain localization is obviously show n in Fig. 6.7 (b), including discrete large cracks. T he finer crack d istributions are com m only o b served in the application o f prestressed C FR P sheets (El- H acha et al. 2004, K im et al. 2006b). A s loaded further, the fin er cracks connected together; then the delam ination o f C F R P sheets follow ed, as show n in Fig. 6.7 (c). T he m echanical anchors effectively p rev en ted the entire delam ination o f the sheets in contrast to the n o n -an ch o red application (B 3-S L 2). A s the load-carrying capacity o f the strengthened slab ap p roached its ultim ate level, sm all portions o f concrete spalled n ear m id-span. T he bond b etw een the concrete and C FR P sheets w as strong enough to cause 179 Yail J. K im , P .E ng., P h.D . T hesis
anchorage failure w as o b served during the test.
Typical cross-sectional pu n ch in g shear cracks are show n in Fig. 6.8. T he pu n ch in g shear angle o f strengthened and u n stren g th en e d slabs w as alm ost the sam e (i.e., a = 20-25°) above the steel re b ar level; h o w ever, the inclined crack p attern b elo w the re b ar w as different. In the unstren g th en ed slab, the pu n ch in g cracks w ere very w ide near the supports, Fig. 6.8 (a); on the o th er hand, the inclined cracks w ere b ridged by the C FR P sheets in the case o f the stren g th en ed slabs, Fig. 6.8 (b) (L ongw orth 2004). T he reason is explained by the fact that the stress concentration n ear the intersection betw een the C FR P sheets and the concrete induced strain localizations th at caused cracking o f the concrete at the intersection, as show n in Fig. 6.7 (c). T he p ropagated pu n ch in g sh ear cracks connected w ith the intersection cracks; thus, the applied energy w as d issipated through this cracking path.
T ypical crack p ropagations pred icted by the F E A m odels are show n in Fig. 6.9. Initially, cracks form ed n ea r m id-span, and additional cracks started at the an chorage locations in the case o f the slab w ith p re stressed C FR P sheets (B 3-P R E ), Fig. 6.9 (c). T he cracks propagated tow ards the co m ers o f the slabs, form ing a cross, and fu rth er spread as show n in Fig. 6.9. T he crosses eventually form ed yield lines, w here significant am ounts o f energy d issipation occurred. A s w as o b served in the experim ental control slab, the so- called d iam ond-shaped concrete splitting p attern o f B 3-C O N T n ea r m id-span, in addition to the cross, w as clearly n o ticeable in Fig. 6.9 (a) at load levels o f 51 kN or higher. T he boundary o f this crack p attern m ay indicate the bottom side o f a pu n ch in g cone, as show n 180 Y ail J. K im , P .E ng., Ph.D . T hesis
C hapter 6: T w o-w ay S lab -C o lu m n C onnections
in Fig. 6.7 (a). In the slab w ith non-prestressed C FR P sheets (B 3-N P R ), no delam in atio n failure w as o b served due to the p erfect bond assum ption. N o ap parent d ifferences b etw een B 3-C O N T (u nstrengthened) and B 3-N P R (non-prestressed) w ere found in the com puted cracks. O n the o ther hand, B 3-P R E (prestressed) indicated evenly d istributed cracks th at also con trib u ted to red u ced strain localizations as com p ared to the laboratory results (i.e., m ore w id ely spaced cracks at a load o f 90 kN as show n in Fig. 6.9, for exam ple). F ew er cracks w ere observed at sim ilar load levels w ith resp ect to the other slabs; n evertheless, the sam e crack p attern in the fo rm ation o f yield lines w as observed.