Design Recommendations for Bridge Deck Slabs … Recommendations for Bridge Deck Slabs Reinforced by...

13
Design Recommendations for Bridge Deck Slabs Reinforced by FRP T. Hassan, S. Rizkalla, A. Abdelrahman, G.Tadros Synopsis: The linear characteristics of fiber reinforced polymers (FRP) up to failure and their relatively low elastic modulus and strain at ultimate has raised concerns with structural engineers regarding their use as reinforcement for flexural members. Based on a non- linear finite element analysis and testing of a full-scale model at the University of Manitoba, Canada, design guidelines on the use of glass and carbon fiber reinforced polymers (GFRP and CFRP) as reinforcement for bridge deck slabs are proposed. The accuracy of the non-linear finite element model is demonstrated by comparing the predicted behavior to test results of two models. The influence of the degree of edge restraint, percentage of reinforcement of CFRP and GFRP, type of reinforcement and presence of top reinforcement on the structural behavior and mode of failure of continuous concrete bridge decks is discussed. Based on serviceability and ultimate capacity requirements, reinforcement ratios of CFRP and GFRP for typical bridge deck slabs are recommended.

Transcript of Design Recommendations for Bridge Deck Slabs … Recommendations for Bridge Deck Slabs Reinforced by...

Page 1: Design Recommendations for Bridge Deck Slabs … Recommendations for Bridge Deck Slabs Reinforced by FRP T. Hassan, S. Rizkalla, A. Abdelrahman, G.Tadros Synopsis: The linear characteristics

Design Recommendations for Bridge Deck Slabs Reinforced by FRP

T. Hassan, S. Rizkalla, A. Abdelrahman, G.Tadros

Synopsis:

The linear characteristics of fiber reinforced polymers (FRP) up to failure and their relatively low elastic modulus and strain at ultimate has raised concerns with structural engineers regarding their use as reinforcement for flexural members. Based on a non­linear finite element analysis and testing of a full-scale model at the University of Manitoba, Canada, design guidelines on the use of glass and carbon fiber reinforced polymers (GFRP and CFRP) as reinforcement for bridge deck slabs are proposed. The accuracy of the non-linear finite element model is demonstrated by comparing the predicted behavior to test results of two models. The influence of the degree of edge restraint, percentage of reinforcement of CFRP and GFRP, type of reinforcement and presence of top reinforcement on the structural behavior and mode of failure of continuous concrete bridge decks is discussed. Based on serviceability and ultimate capacity requirements, reinforcement ratios of CFRP and GFRP for typical bridge deck slabs are recommended.

Page 2: Design Recommendations for Bridge Deck Slabs … Recommendations for Bridge Deck Slabs Reinforced by FRP T. Hassan, S. Rizkalla, A. Abdelrahman, G.Tadros Synopsis: The linear characteristics

Keywords: bridges; deflection; fiber; reinforcement; concrete; punching; slabs; arch action; shear; codes.

Page 3: Design Recommendations for Bridge Deck Slabs … Recommendations for Bridge Deck Slabs Reinforced by FRP T. Hassan, S. Rizkalla, A. Abdelrahman, G.Tadros Synopsis: The linear characteristics

Tarek Hassan received his B.Sc. in 1995 from Ain Shams University, Cairo, Egypt, and his M.Sc. in 1999 from the University of Manitoba, Winnipeg, Manitoba, Canada. He is currently a Ph.D. student at the University of Manitoba.

Sami Rizkalla is an ACI Fellow. He is currently the President of the Canadian Network of Centres of Excellence on Intelligent Sensing for Innovative Structures (ISIS Canada), and Professor in the Department of Civil Engineering, University of Manitoba, Winnipeg, Manitoba,Canada. He is the chairman of ACI Committee 440, FRP Reinforcement for Concrete Structures, and Chief Editor ofthe FRP International newsletter.

Amr Abdelrahman is a member of ACI Committee 440. He received his B.Sc. and M.Sc. from Ain Shams University in 1986 and 1989, and his Ph.D. from the University of Manitoba in 1995. He is presently Assistant Professor at Ain Shams University, Cairo, Egypt.

Gamil Tadros is an ACI member. He graduated from Cairo University, Cairo, Egypt, with a B.Sc. (honours) in 1962 and received his Ph.D. in 1970 from the University of Calgary, Alberta, Canada. He is a structural engineering consultant involved primarily with bridge design and construction. He has won numerous awards and is a member of CSCE, PCI, ASCE and IABSE.

INTRODUCTION

Fiber reinforced polymers (FRP) offer an excellent solution for overcoming the problem related to corrosion of steel reinforcement, which is considered to be one of the main causes of deterioration of reinforced concrete members. This paper describes the behavior of a full-scale model of a bridge deck slab reinforced by CFRP. The paper also presents a non-linear finite element model calibrated with the tested model and used to evaluate the influence of various parameters, thereby reducing significantly the high cost associated with the experimental work.

FULL-SCALE TEST MODEL

A full-scale model of a continuous bridge deck slab with double cantilever was tested at the University of Manitoba, Canada (1), to examine the behavior and the ultimate capacity of typical bridge deck slabs reinforced with CFRP. The model consisted of three continuous spans of 1.8 meters each and two cantilevers, with overall dimensions of 7.2 x 3.0 m and a thickness of 200 mm, as shown in Fig. 1. Each of the spans and the two cantilevers were tested independently using a single concentrated load applied over a

Page 4: Design Recommendations for Bridge Deck Slabs … Recommendations for Bridge Deck Slabs Reinforced by FRP T. Hassan, S. Rizkalla, A. Abdelrahman, G.Tadros Synopsis: The linear characteristics

contact area equivalent to the area of a wheel, as specified by AASHTO (2) for HSS 25 (MSS 22.5) truck. The test of the mid-span was performed in the presence of steel straps connected to the two ends of the supporting beams to restrain rotation and lateral movement up to a load of 600 kN. The slab was reloaded up to failure without the presence of the end restraints. The steel straps were used to simulate the effect of typical cross-girders used in bridges.

The slab was reinforced with Leadline CFRP bars produced by Mitsubishi Chemicals Corporation, Japan (elastic modulus of 147 GPa, ultimate strength of 2250 MPa). The bottom reinforcement consisted of two 10 mm diameter Leadline bars spaced at 125 mm in the main direction, providing a reinforcement ratio of 0.57 percent, and one 10 mm diameter bar at 125 mm in the secondary direction, which is equivalent to a reinforcement ratio of 0.29 percent. The top reinforcement consisted of one 10 mm diameter Leadline bar at 125 mm in each direction. The average concrete compressive strength and elastic modulus were 59 MPa and 36 GPa, respectively. The slab was instrumented using linear variable differential transducers (L VDT) to measure the deflection of the slab and the rotation of the supporting beams, as well as PI gauges to measure the concrete strain at different locations. The strain of the CFRP reinforcement was monitored using 64 electrical strain gauges and eight Bragg grating fiber optic sensors.

TEST RESULTS

A typical load-deflection relationship of the deck slab with and without end restraints is shown in Fig. 2. The initial load-deflection was linear up to cracking followed by non­linear behavior after cracking with reduced stiffness. The non-linearity of the load­deflection was highly pronounced for the test without end restraints due to the presence of extensive cracks at the bottom surface of the mid-span of the slab and at the top surface close to the supporting beams.

The first crack was observed at the bottom surface of the mid-span at a load of 100 kN. The cracks on the top surface occurred at a load level of 240 kN. Flexural cracks were more pronounced in the second test without the end restraints than in the first test with the end restraints. The slab failed due to punching shear at a load level of 1000 kN. The failure load is more than seven times the service load recommended by the AASHTO code, 1996 (2). The top surface of the failure zone had an elliptical shape with a perimeter 25 percent larger than the perimeter of the loaded area shown Fig. 3. The maximum compressive concrete strain at the face of the loaded area was 0.0029. The strains measured by the electric resistance gauges were in agreement with the measurements of the fiber optic sensors.

Page 5: Design Recommendations for Bridge Deck Slabs … Recommendations for Bridge Deck Slabs Reinforced by FRP T. Hassan, S. Rizkalla, A. Abdelrahman, G.Tadros Synopsis: The linear characteristics

ANALYTICAL MODEL

The analytical model is based on the "Anatech Concrete Analysis Program" (ANACAP), version 2.1, 1997 (3). Verification of the ANACAP program was evaluated using a two­way slab model tested at Ghent University, Belgium (4). The predicted load-deflection behavior of the slab compared very well with the measured values, as shown in Fig. 4. The predicted punching shear failure load was only 1.2 percent higher than the measured value.

Based on the confidence established in the analytical model, the analysis was extended to model the behavior of the full-scale bridge deck slab tested at the University of Manitoba. One quarter of the slab was modelled using 20-node brick elements, as shown in Fig. 5. To focus on the slab behavior and remain with realistic computer execution time, the cantilever was not included for this particular loading case. The slab thickness was divided into three layers. The spacing between layers was selected to produce a finer mesh in the compression zone near the top surface of the slab. The steel straps were modelled using a spring element. The slab was loaded up to 600 kN in the presence of the spring element and unloaded. The spring element was removed and the slab was reloaded up to failure to simulate the case without end restraints. The predicted load-deflection behavior of the slab with and without end restraint compared well with the experimental values, as shown in Fig. 2. It is observed from the load-compressive strain behavior, shown in Fig. 6, that at a load of 1039 kN there is a change in behavior leading to a significant increase in the compressive strain with only a slight increase in the applied load. This phenomenon was used to identify the failure load criterion. The corresponding compressive strain at the face of the loaded area at failure was 0.0027, which is in agreement with the measured value of 0.0029 under the location of the load.

PARAMETRIC STUDY

End Restraint

To demonstrate the effect of the boundary conditions on behavior, three different cases were studied. In the first case no strap was used, while in the other two cases, steel straps of dimensions 90 x 12 mm and 150 x 12 mm were used to simulate the effect of cross­girders in restraining the rotation and lateral movement of the supporting girders. For all cases, the slab was reinforced with CFRP Leadline bars as bottom reinforcement with a reinforcement ratio of 0.4 percent in the main direction. The top reinforcement in the main direction and the top and bottom reinforcement in the secondary direction were set to 0.3 percent. The load-deflection behavior ofthe three cases studied is shown in Fig. 7. The analysis indicates that increasing the stiffuess of the steel straps decreases the mid­span deflection. Increasing the strap dimensions from 90 x 12 mm to 150 x 12 mm

Page 6: Design Recommendations for Bridge Deck Slabs … Recommendations for Bridge Deck Slabs Reinforced by FRP T. Hassan, S. Rizkalla, A. Abdelrahman, G.Tadros Synopsis: The linear characteristics

caused a slight decrease in the deflection. The failure loads increased by 14 percent and 21 percent by using 90 x 12 mm and 150 x 12 mm steel straps respectively. This behavior is due to the increase in the membrane forces induced by increasing the strap stiffuess which, consequently, increases the punching shear load capacity ofthe deck slab.

Reinforcement Ratio

Six different reinforcement ratios of CFRP and GFRP were studied for the model under consideration. In the case of CFRP, the bottom reinforcement ratio in the main direction was varied from 0.3 percent to 0.8 percent. The top reinforcement ratio in the main direction and the top and bottom reinforcement ratio in the secondary directions were set to 0.3 percent.

The load-deflection behavior using different reinforcement ratios of CFRP is given in Fig. 8. Before cracking, linear behavior was observed and the deflection was almost identical, regardless of the reinforcement ratio used. After cracking, the deflection decreased with increasing reinforcement ratio. The reinforcement ratio of 0.4 percent for CFRP was selected to provide an equivalent steel ratio of 0.3 percent (5). The behavior indicated that the failure load increased by 8 percent and 25 percent as the reinforcement ratio was increased to 0.4 percent and 0.8 percent, respectively. Similar behavior was observed for GFRP bars (6). For all cases, the failure was due to crushing of the concrete, reSUlting in punching shear failure.

Type of Reinforcement

The load-deflection behavior for the continuous deck slab reinforced by 0.3 percent steel and that reinforced by 0.4 percent CFRP and 1.46 percent GFRP as main bottom reinforcement is shown in Fig. 9. The top reinforcement in the main direction, as well as the top and bottom reinforcement in the secondary direction, were set to 0.3 percent in all cases. The load-deflection indicates similar behavior up to the load corresponding to initiation of the first crack. Upon yielding of steel reinforcement, deflections increased significantly with a small increase in the applied load for the slab with steel reinforcement. In the case of CFRP, the calculated failure load was 17 percent higher than the corresponding value for the deck slab reinforced with steel. The higher deflection observed for the case of GFRP is due to the lower elastic modulus and the low reinforcement ratio of 0.3 percent used in the analysis for the top reinforcement, as well as the top and bottom reinforcement in the secondary direction.

Page 7: Design Recommendations for Bridge Deck Slabs … Recommendations for Bridge Deck Slabs Reinforced by FRP T. Hassan, S. Rizkalla, A. Abdelrahman, G.Tadros Synopsis: The linear characteristics

Top Reinforcement

Three different cases were studied to investigate the effect of the top reinforcement on the behavior of continuous bridge decks. The bottom reinforcement was kept constant at 1.2 percent GFRP in both directions. In the first case, no top reinforcement was used in either direction. In the second and third cases, top reinforcement ratios of 0.6 percent and 1.2 percent GFRP were used in both directions, respectively. The calculated failure load for the deck slab without top reinforcement was 677 kN. Doubling the top reinforcement ratio increased the failure load by 4 percent. Therefore, it was concluded that the use of top reinforcement does not affect the ultimate capacity ofthe deck slab.

DESIGN RECOMMENDATIONS

Based on the reasonable agreement between the analytical model and the full-scale test results, from a more detailed parametric study (6), and applying appropriate factors to ensure safety and serviceability, the following guidelines are recommended for design:

1. Use of a minimum CFRP reinforcement ratio of 0.3 percent for top reinforcement and also for bottom reinforcement in each direction is recommended.

2. When GFRP is used, the following minimum reinforcement ratios are recommended: 1.2 percent for bottom reinforcement in the main direction, 0.6 percent for top reinforcement in the main direction, and 0.6 percent for each of the top and bottom reinforcement in the secondary direction.

3. AASHTO code, (1996) (2), requirements for deflection and for concrete compressive stress limitations may be safely applied to CFRP and GFRP reinforced bridge decks.

The parametric study included evaluating deflections, compressive stresses in the concrete and tensile stresses in the reinforcement under these service load conditions. The results yielded concrete compressive stresses of0.36.fc' and 0.28.fc' for the deck slabs reinforced with CFRP and GFRP, respectively. The maximum tensile stresses in the bottom GFRP reinforcement were only 20 percent of the ultimate tensile strength and, consequently, the alkalinity problem associated with GFRP is not a concern. The predicted strengths of the deck slab reinforced with the recommended reinforcement ratios were 1.8 and 1.6 times greater for CFRP and GFRP, respectively, than the strengths required by the code.

CONCLUSIONS

Based on the findings of this investigation, the following conclusions can be drawn:

Page 8: Design Recommendations for Bridge Deck Slabs … Recommendations for Bridge Deck Slabs Reinforced by FRP T. Hassan, S. Rizkalla, A. Abdelrahman, G.Tadros Synopsis: The linear characteristics

1. The failure load of a continuous full-scale bridge deck slab model is more than seven times the service load recommended by the AASHTO code (2) and is due to punching shear.

2. The analytical model is capable of predicting the behavior, ultimate load carrying capacity and mode of failure of continuous bridge deck slabs reinforced with different types ofFRP.

3. Restraining the girders of concrete bridge decks from outward movement increases the punching shear capacity of the deck. The maximum concrete compressive strain at failure ranges between 0.0027 and 0.0035.

4. Presence of top reinforcement in continuous bridge deck slabs has a negligible effect on the punching shear capacity.

5. To satisfy serviceability and ultimate capacity requirements for span-to-depth ratios ranging between 9 and 15, the use 0.3 percent CFRP as top and bottom reinforcement in each direction is recommended. For GFRP reinforcements, use of 1.2 percent for the bottom reinforcement and 0.6 percent for the top reinforcement in the main direction, as well as 0.6 percent as top and bottom reinforcement ratios in the secondary direction, achieves the code requirements.

REFERENCES

1. Abdelrahman, A., Hassan, T., Tadros, G., and Rizkalla, S., (1998), "Behavior of Concrete Bridge Deck Model Reinforced by Carbon FRP", Proceedings of the Canadian SOCiety for Civil Engineering Annual Conference, Halifax, Nova Scotia, Vol.IIIb, pp. 521-526.

2. AASHTO, "Standard Specifications for Highway Bridges 1996", American Asso­ciation of State Highway and Transportation Officials, Washington D.C., 450 p.

3. Rashid, Y. R. (1968), "Ultimate Strength Analysis of Prestressed Concrete Pressure Vessels", Nuc. Eng & Design, 7, pp. 334-344.

4. Matthys, S. and Taerwe, L., "Behavior of Concrete Slabs Reinforced with FRP Grids Under Service and Ultimate Loading", Proceedings of the First International Conference on Fiber Composites in Infrastructure, Tucson, Arizona, USA, pp. 359-373.

5. OHBDC, (1991), "Ontario Highway Bridge Design Code", Ministry of Transportation of Ontario, Downsview, Ontario, 370 p.

6. Hassan, T., (1999), "Behaviour of Concrete Bridge Decks Reinforced with FRP", MSc. Thesis, Department of Civil and Geological Engineering, University of Manitoba, Winnipeg, Manitoba, Canada.

Page 9: Design Recommendations for Bridge Deck Slabs … Recommendations for Bridge Deck Slabs Reinforced by FRP T. Hassan, S. Rizkalla, A. Abdelrahman, G.Tadros Synopsis: The linear characteristics

Fig. 1

Fig. 2

Fig. 3

Fig. 4

Fig. 5

Fig. 6

Fig. 7

Fig. 8

Fig. 9

Fig. 10

LIST OF FIGURES

Schematic of the deck slab

Load-deflection behaviour of the deck slab

Punching failure of the deck slab

Load-deflection behaviour ofthe two-way slab (Ghent University)

Mesh dimensions of the analytical model

Load-compressive strain behaviour of the deck slab

Load-deflection behaviour using different strap dimensions

Load-deflection behaviour using different reinforcement ratios of CFRP

Load-deflection behaviour using different types of reinforcement

Serviceability requirements

Page 10: Design Recommendations for Bridge Deck Slabs … Recommendations for Bridge Deck Slabs Reinforced by FRP T. Hassan, S. Rizkalla, A. Abdelrahman, G.Tadros Synopsis: The linear characteristics

I 900 1800 1800 1800

Plan

beam

Cross section

Fig. 1 Schematic of the deck slab

Page 11: Design Recommendations for Bridge Deck Slabs … Recommendations for Bridge Deck Slabs Reinforced by FRP T. Hassan, S. Rizkalla, A. Abdelrahman, G.Tadros Synopsis: The linear characteristics

1200r I

1000r ,-...,

~ 800 "-' '"0 ro 0

600 ~ '"0

<l) .-- 400 0..

< 200

0

0

3001 -t

2501 ,-...,

! ~ "-' 2001 '"0 I ro 0

....:l 150 '"0

<l) .--0.. 100 < 50

Unrestrained

__ Experimental

- Analytical

1 2 3 4 5 6 7 8 9 10 11 Deflection (mm)

Fig. 2 Load-deflection behaviour of the deck slab

Fig. 3 Punching failure of the deck slab

----

__ Experimental

-- Analytical

g s

4 5 6 7 8 9 10 Deflection (mm)

Fig. 4 Load-deflection behaviour of the two-way slab (Ghent University)

Page 12: Design Recommendations for Bridge Deck Slabs … Recommendations for Bridge Deck Slabs Reinforced by FRP T. Hassan, S. Rizkalla, A. Abdelrahman, G.Tadros Synopsis: The linear characteristics

-------~~- -----

~I Spring Supports

l()

~.t.

:::+ 0 0 l() ......

C'l

~~

Fig. 5 Mesh dimensions of the analytical model

1200

1000 ,.-.,

g 800

~ j 600 '"0

OJ

~ 400 .<

200

o -

__ Experimental

-- Analytical Unrestrained

l039kN

o 0.0005 0.001 0.0015 0.002 0.0025 0.003 0.0035 0.004 Compressive Strain

Fig. 6 Load--compressive strain behaviour ofthe deck slab

12001--­f I ~_ No-st-r-ap----I

10001 I b- Strap 90x12 mm

~ 800 I i c-Strap 150x12 mm I

~ I --=-

~ 6001

~ 400\

'"' I 2001

3 4 5 6 Deflection (mm)

c

7 8 9 10

Fig. 7 Load-deflection behaviour using different strap dimensions

Page 13: Design Recommendations for Bridge Deck Slabs … Recommendations for Bridge Deck Slabs Reinforced by FRP T. Hassan, S. Rizkalla, A. Abdelrahman, G.Tadros Synopsis: The linear characteristics

1200r

~ 10001

~ f :;- 800 i

~ 600 I

!400 I

200

a- P = 0.3% CFRP b- P = 0.4% CFRP

l_~_= P = 0.8% CFRP

1 2 3 4 5 6

Deflection (mm)

1002 kN

7 8 9 10

Fig. 8 Load--deflection behaviour using different reinforcement ratios of CFRP

1000r a- p = 1.46% GFRP b I

8001 b- p = 0.40% CFRP

~ c-P = 0.30% Steel ~ '-" "0 600 C':S 0

.....:l "0 (!) 400 .--0.. 0.. <

200

1 2 3 4 5 6 7 8 9 10 Deflection (mm)

Fig. 9 Load--deflection behaviour using different types of reinforcement

100

~800 '-"

~ 600 0 .....:l "0 (!)

;.::: 400 0.. 0.. <

200

§ lr)

t'--(")

II

Span-to-depth ratio= 15

Recommended reinforcement 526 kN ratio of CFRP

Recommended reinforcement ratio of GFRP

." """""". ". r"""""""s'~~i'~'~"i~'~d:ii7'i~N"(MSHT'6;96)"'"''''''''''''''''''".

2 4 6 8 10 12 14 16 18 20 22 24 26 Deflection (mm)

Fig. 10 Serviceability requirements