Porównawcze zachowanie mostów z betonowych elementów prefabrykowanych ze zbrojeniem z CFRP i stali

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Porównawcze zachowanie mostów

z betonowych elementów prefabrykowanych ze zbrojeniem z CFRP i stali

COMPARATIVE PERFORMANCE OF PRESTRESSED CONCRETE BOX-BEAM BRIDGES WITH CFRP AND STEEL REINFORCEMENTS

Streszczenie

Niniejszy referat przedstawia zachowanie trzech identycznych modeli mostów skła- danych z elementów sprężonych i zbrojonych stosujących dwa różne rodzaje prętów:

z polimeru zbrojonego włóknem węglowym (CFRP) oraz konwencjonalne pręty stalowe.

Każdy model mostu składał się z dwóch prefabrykowanych elementów umieszczonych obok siebie i połączonych poprzecznymi prętami techniką post-tensioning, umieszczonych w czterech poprzecznych membranach, a także odlanej na miejscu płyty mostu.

Pierwszy model mostu (BBD) oraz drugi model mostu (BBC) zostały zbrojone i sprężone odpowiednio przy pomocy ścięgien CFRP-DCI oraz prętów kompozytowych z włókna węglowego (CFCC). Trzeci model mostu (BBS) został zbrojony i sprężony przy pomocy prętów stalowych. Wszystkie modele mostów zostały zaprojektowane jako niedozbro- jone, o podobnych współczynnikach zbrojenia oraz poziomu sprężenia. Owe modele mostów wykazały kontrolowany spadek napięcia w wyniku uszkodzenia spodu zbrojenia z CFRP/stali, po którym nastąpiło powolne wykruszanie betonu. Reakcje polegające na odkształceniu pod obciążeniem w przypadku modeli BBD i BBC były bilinearne, przy czym wytrzymywały większe obciążenia końcowe niż model BBS, wykazujący czteroli- niową reakcję odkształcenia. Plastyczność wykazana przez modele BBD i BBC wynosiła ok. dwie trzecie plastyczności modelu BBS. W przeciwieństwie do modelu BBD, model mostu BBC charakteryzował się postępującym fenomenem reakcji po osiągnięciu szczy- towych wartości, zapewniając wystarczający czas ostrzegawczy i wykazując poważne pęknięcia, podobnie jak w przypadku modelu BBS przed zawaleniem. Zawalenie modelu mostu BBS również miało charakter progresywny, dzięki odkształceniu prętów, po którym nastąpiło kruszenie betonu.

Nabil Grace

dr. Nabil Grace – Professor and Chairman, Civil Engineering Department, Lawrence Technological Uni- versity, Southfield, Michigan, USA

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Comparative Performance of Prestressed Concrete Box-Beam Bridges ...

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Abstract

This paper presents the performance of three identical box-beam bridge models, prestressed and reinforced using two different types of carbon fiber reinforced polymer (CFRP) strands, and conventional steel strands. Each bridge model consisted of two precast box-beams placed side-by-side, joined by transverse post-tensioning strands, contained in four transverse diaphragms, and a cast-in-place deck slab. The first bridge model (BBD), and second bridge model (BBC) were reinforced and prestressed with CFRP-DCI tendons and carbon fiber composite cable (CFCC) strands, respectively. The third bridge model (BBS) was reinforced and prestressed using steel reinforcement. All the bridge models were designed to be under-reinforced having similar reinforcement ratios and level of prestress. These bridge models demonstrated tension controlled failure initiated by rupture/yielding of the bottom CFRP/steel reinforcement and followed by delayed crushing of the concrete. Load-deflection responses of bridge models BBD and BBC were bi-linear, experiencing higher ultimate loads than that of BBS with quadra-linear load- deflection response. The ductility exhibited by bridge models BBD and BBC was approximately two-third of that exhibited by bridge model BBS. Unlike bridge model BBD, bridge model BBC experienced a progressive post-peak phenomenon, giving sufficient warning, and extensive cracking similar to bridge model BBS prior to the failure. Failure of bridge model BBS was also progressive owing to yielding of the prestressing strands followed by crushing of concrete.

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Comparative Performance of Prestressed Concrete Box-Beam Bridges ...

1. Introduction

Most existing bridges reinforced and prestressed using conventional steel reinforcement experience deterioration problems because of weathering effects. The presence of a com- bination of deicing slats, high moisture, temperature variation, and chlorides reduce the alkalinity of the concrete inducing corrosion of the reinforcing and prestressing steel. It is reported that 110,000 out of 470,000 existing bridges in USA are structurally deficient [3]. The number of bridges requiring urgent repair increased to 120,000 in 2004 [1]. Due to the high maintenance cost and inability to protect the steel from corrosion, a corrosion- free alternative for the steel reinforcement has been the use of fiber reinforced polymers (FRP); which while fulfilling all the structural requirements gives many advantages such as higher tensile strength and lighter weight than conventional steel reinforcement. The various types of FRPs available, their manufacturing process, and structural properties can be found in the comprehensive report [2]. Carbon fiber reinforced polymer (CFRP) is more preferable to be used as a reinforcement over the other types of FRPs due to its superior characteristics such as the high ultimate tensile strength; no relaxation with time;

ultimate elongation varying from 0.3 to 2.5%; and high modulus of elasticity. A design approach was developed for CFRP prestressed concrete bridge beams [4], duly considering the nonlinearity of the materials; later validated by conducting laboratory experiments on scaled models [7-9]. The use of box-beam bridges in USA increased gradually due to the advantages of box-beams such as low span to depth ratio; more suitability for bridges with minimum section depth; covering long spans; providing torsional stiffness [11]. The first CFRP bridge, Bridge Street Bridge, constructed in the United States [6], marked the beginning of implementation of this modern technology in real-life projects, a need con- sidered vital to transfer this technology on to field in constructing bridges, after testifying the suitability of FRPs through laboratory experiments [12].

Except a few studies on the flexural performance of box-beam bridges reinforced and prestressed with FRP tendons/strands, no specific study has addressed the relative flexural performance of box-beam bridges reinforced and prestressed with CFRP tendons/strands to those reinforced and prestressed conventionally with steel. Prior to use reliably in structural engineering applications, the FRP materials should satisfy the ductility requirements. The experimental results showed that the change in reinforcing system could lead to various ductility changes [10]; therefore FRP materials cannot be used reliably in structural engineering applications unless they satisfy the ductility requirements.Hence, further investigation is essential to study the comparative performance of FRP reinforcement over the conventional steel rebars for prestressing, from viewpoint of ductility and overall flexural behavior.

The objective of the present investigation is to develop, construct, instrument, and test three identical pairs of precast prestressed concrete box-beam bridge models, two amongst them reinforced with two different CFRP tendons, and one using steel strands.

The main objective of the present study is to examine the relative flexural performance of box-beam bridge models reinforced and prestressed with CFRP to those traditionally reinforced and prestressed with steel reinforcement.

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Nabil Grace Comparative Performance of Prestressed Concrete Box-Beam Bridges ...

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2. Construction details

Three box-beam bridge models with same geometry and different reinforcements designated as BBD, BBC, and BBS were constructed in the laboratory. Each bridge model was composed of two precast box-beams placed side-by-side to each other prior to construction of a cast- in-place deck slab, connecting them together. The typical bridge model and the individual box-beam dimensions were as shown in Figure 1. Each box-beam was provided with seven top non-prestressing and four bottom non-prestressing reinforcing bars, and there were seven pre-tensioning reinforcing bars placed at the same level of the bottom non-prestressing.

Figure 1. Cross-sectional details and longitudinal section of bridge models Table 1. Properties of reinforcements

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The bridge model BBD was reinforced and prestressed with CFRP-DCI tendons (Di- versified Composites Inc., USA), while MIC-C bar stirrups were utilized to avoid shear failure. CFCC1x7 strands (Tokyo Rope Inc., Japan) were used to reinforce and prestress bridge model BBC, whereas CFCC1x7 stirrups were used as shear reinforcement. For bridge model BBS, conventionally used #3 deformed steel rebars and stirrups were used as non-prestressing and shear reinforcement while uncoated low-relaxation 1/2 in.-seven- wire steel strands were used as prestressing reinforcement. Refer to Table 1 for detailed properties of reinforcements. For all these bridge models, the spacing between stirrups was 102 mm (4 in.).

2.1. Construction steps

For all bridge models, the top and bottom non-prestressing reinforcements were joined tightly to the vertical stirrups using commercially available zip-ties, to form reinforcement cages. Commercially available Styrofoam sheets were prepared having dimensions of 1.778 m (70 in.) length, 305 mm (12 in.) width, and 102 mm (4 in.) depth. These Styrofoam sheets were used to form a hollow portion inside the individual box-beams, and to develop four solid transverse diaphragms to accommodate the transverse post-tensioning tendons;

two diaphragms located at both edges of the beams, and the remaining two diaphragms located symmetrically about the mid-span.

After inserting the Styrofoam inside the cages, the two cages were placed and cente- red inside the formworks and subsequently four PVC conduits were passed through the four diaphragms to avoid any bonding between the post-tensioning reinforcement and the surrounding concrete. Four bulkheads were aligned and attached at each end of the formworks; the bulkheads were comprised of seven holes at the level of pre-tensioning reinforcement to facilitate passing of the pre-tensioning reinforcement. Different pre- tensioning systems were used for the three models, BBD, BBC, and BBS.

The average prestressing force per strand for all bridge models was about 44.5 kN (10 kip). CFRP-DCI tendon, CFCC strand, and steel strand were prestressed to an average force up to 46.5, 44.5, and 47.6 kN (10.45, 10, and 10.7 kip), respectively. In order to maintain the prestressing forces i.e. to maintain the elongation developed in the strands due to pre-tensioning, spacers were inserted between the bulkhead and the anchor heads of DCI tendons and CFCC strands.

After pre-tensioning, a ready-mixed high strength concrete (HSC) with designed strength of 48.3 MPa (7000 psi) was placed to cast box-beams. Note that the shear stirrups were projected beyond the concrete surface to act as shear connectors to the deck slab.

The 28-day concrete compressive strengths of the precast beams for bridge models BBD, BBC, and BBS were 54.5, 56.54, and 54.29 MPa (7.9, 8.2, and 7.9 ksi), respectively. After casting, these precast box-beams were wet-cured using soaked burlap for 7 days; and then prestressing forces were transferred to the beams by cutting the prestressing tendons from both ends at the same time.

Both the box-beams were then moved to the testing area under the actuator and aligned on two simple supports; the gap between them was filled using Five Star Structural Concrete^® 300. Prior to casting the deck slab, 50% out of 44.5 kN (10 kip) was applied on each of the four transverse post-tensioning strands to prevent any relative movements between the two precast beams. A 203x203 mm (8 x 8 in.) deck slab reinforcement grid was attached to the protruded stirrups to resist concrete shrinkage. The same concrete mix, which was used in construction of the precast beams, was placed on the top of the

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7 precast beams to cast the deck slab. The 28-day concrete compressive strengths of the deck

slab of bridge models BBD, BBC, and BBS were 54.5, 54.5, and 58.15 MPa (7.9, 7.9, and 8.4 ksi), respectively. Finally, the remaining percentage of the transverse post-tensioning force was applied.

2.2. Instrumentation

In order to measure the strain produced in different elements of bridge models, CEA strain gages were used. Linear motion transducers, commercially called as string pots were installed to measure the deflection during transfer of the prestress and loading. To record the force and/or the change of force inside the pre-tensioning and post-tensioning reinforcements, load cells were mounted on the reinforcing bars; and another load cell was attached to a loading actuator to monitor the applied load throughout the testing.

2.3. Test setup

All the sensors and transducers installed were connected to a data acquisition system.

The bridge model was simply supported because one of its supports was a hinge while the other was a roller; and furthermore, the effective length of the bridge model was 5.791 m (19 ft). A four-point loading frame was placed on the top of the bridge models at their mid-spans; thus, simulating a standard truck loading and at the same time ensuring flexural failure of all the bridge models. The load was applied monotonically up to cracking, and it was applied in a cyclic fashion after cracking stage up to ultimate test. The load increment was about 89 kN (20 kip). The main purpose of conducting such loading and unloading techniques was to separate the elastic and inelastic energies under the load- deflection curve, since the ductility was evaluated based on energy concept.

3. Results and Discussion

The results of experimental investigation on the three box-beam bridge models BBD, BBC, and BBS are discussed in the following sections.

3.1. Load-deflection response and failure behavior

Bridge models BBD, BBC, and BBS were designed as under-reinforced, therefore initia- tion of failure was anticipated to begin by rupture or yielding of the bottom prestressed reinforcement. From the experimental results, it was observed that all bridge models exhibited flexural tension failure, as shown in Figure 2. The failure was initiated by rupture or yielding of the bottom pre-tensioning tendons/strands followed by crushing of the concrete (Figure 3).

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Figure 2. Failure of bridge models (a) BBD, (b) BBC, and (c) BBS

Figure 3: Mapped cracks for bridge models after the failure

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and BBS, respectively. The change in the slope of the load-deflection curve is attributed to the change of the stiffness of the entire bridge model due to the cracking (cracking loads).

Because of the linear behavior of FRPs up to failure, in general, load-deflection response of both bridge models BBD and BBC is bi-linear, owing to initial cracking phase. On the other hand, the load-deflection response of bridge model BBS is quadra-linear as there are three inflection points on the curve. Yielding of the non-prestressed steel bars took place at about 356 kN (80 kip) while yielding of the pre-tensioned strands was initiated at about 467.3 kN (105 kip). Therefore, it can be noticed that a more progressive failure phenomenon was exhibited by bridge model BBS.

Figure 4. Load-deflection behavior and energy ratio for bridge model BBD

Bridge models BBD, BBC, and BBS achieved ultimate loads of 565.15, 578, and 528.2 kN (127, 130, and 118.7 kip), respectively. The bridge models BBD and BBC both demonstrated higher ultimate load carrying capacities than bridge model BBS, due to the higher ultimate tensile strengths of CFRP-DCI tendons and CFCC strands than that of the steel strands by about 13% and 30%, respectively. The ultimate load carried by bridge model BBC was greater than that of bridge model BBD, since CFCC strands had marginally higher tensile strength and modulus of elasticity than CFRP-DCI tendons. All bridge models investigated here showed post-peak response. In bridge model BBD, after reaching the peak load, a progressive rupture of bottom tendons was observed while load reached up to 85% of the peak load, followed by a sudden drop in the load to about 7% of the peak load. Bridge model BBC exhibited better post-peak behavior as the progressive rupture of bottom strands were more noticeable from a sudden drop in the load to about 9% of the peak load. In bridge model BBS, after reaching the peak load, the load dropped sharply to about 67% of the peak load followed immediately by progressive crushing of the concrete until complete failure.

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Figure 5. Load-deflection behavior and energy ratio for bridge model BBC

Prior to the yielding of pre-tensioned strands, bridge model BBS exhibited approximately 33% lower deflection than both bridge models BBD and BBC. At the peak load, bridge models BBD, BBC, and BBS demonstrated approximately similar deflection, although deflections of bridge models BBD and BBC were 50% and 57% greater than that of bridge model BBS at yielding of the pre-tensioning strands, respectively.

Figure 6. Load-deflection behavior and energy ratio for bridge model BBS

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3.2.Ductility of bridge models

Ductility is defined as the tendency of a structure to sustain inelastic deformation while there is no severe loss in its load carrying capacity^[5]. The ductility is evaluated based on energy (E) concept [5]; by expressing ductility index (energy ratio) as the ratio of the inelastic energies dissipated to the total energies released and dissipated by the system.

Figures 4 through 6 present the load-deflection response for all bridge models, along with the loading and unloading cycles and details of computing the ductility indices. For bridge model BBD, the released elastic energy and dissipated inelastic energy before the peak were 50.638 kN-m (448 kip-in.) and 14.129 kN-m (125 kip-in.), respectively while the dissipated additional inelastic energy after the peak was 30.632 kN-m (271 kip-in.). For bridge model BBC, the released elastic energy and dissipated inelastic energy before the peak were 54.353 kN-m (481 kip-in.) and 16.837 kN-m (149 kip-in.), respectively while the dissipated additional inelastic energy after the peak was 30.632 kN-m (271 kip-in.). For bridge model BBS, the released elastic energy and dissipated inelastic energy before the peak were 28.543 kN-m (253 kip-in.) and 47.659 kN-m (422 kip-in.), respectively while the dissipated additional inelastic energy after the peak was 37.353 kN-m (330 kip-in.).

The ductility indices for bridge models BBD and BBC were about 46.9% and 46.6%, respectively whereas bridge model BBS achieved a ductility index approximately about 75% showing superior ductility. Thus, bridge models BBD and BBC demonstrated ductility indices of about 62.5% and 62.1% of the ductility index of the bridge model BBS, respectively. Moreover, the elastic energies released by the bridge models BBD and BBC had higher contribution in the ductility indices of these bridges. However, the inelastic energy dissipated by bridge model BBS was significantly larger than the elastic energy released by the bridge. The yielding of both non-prestressing and prestressing steel reinforcement caused the larger ductility of bridge model BBS over the bridges reinforced and prestressed with CFRP, because CFRPs have no yielding plateau.

4. Conclusions

From the results of the experimental program carried out, it was observed that the three bridge models with different types of prestressing tendons/strands experienced flexural tension failure. In general, bridge models with CFRP-DCI/ CFCC tendons showed equ- ivalent flexural performance in terms of ultimate load, deflection to the bridge model with conventional steel strands, except exhibiting inferior ductility. The following conclusions can be drawn from the present investigation.

1. The ductility exhibited by bridge models BBD and BBC was approximately two-third of that achieved by bridge model BBS. For both bridge models BBD and BBC, the released elastic energy was higher than that of the absorbed inelastic energy, whereas for bridge model BBS the absorbed inelastic energy was higher than the released elastic energy.

2. The cracking loads of all the bridge models were almost the same. The load-deflection response of bridge models BBD and BBC was bi-linear, whereas that of bridge model BBS was quadra-linear. Deflection experienced by bridge model BBS prior to yielding

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of prestressing strands was lower than those experienced by bridge models BBD and BBC, but it increased rapidly after yielding of the pre-tensioning strands.

3. Although bridge model BBS demonstrated a progressive failure phenomenon, a rapid increase in deflection was observed immediately after yielding of the pre-tensioning strands. In contrast, both of bridge models reinforced and prestressed with CFRP-DCI/

CFCC tendons/strands exhibited approximately a constant change in deflection beyond cracking up to the ultimate load.

4. Although bridge models BBD and BBC behaved similarly, the post-peak performance of bridge model BBC was better than BBD, giving sufficient visual warning prior to complete failure. The yielding of prestressing steel strands in bridge model BBS gave sufficient warning prior to failure. Significant visual warning and cracking prior to failure were experienced by the three bridge models.

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