Presentation held at the ASCE Structures Congress 2011

1. Shear capacity of Reinforced Concrete Slabs
experimental study
Eva Lantsoght
17-5-2011
Delft
University of
Technology
Challenge the future

2. Overview
• Background
• Project description
• Current practice
• Experiments
• Results and discussion
• Loading history
• Distance to support
• Concrete compressive strength
• Preliminary conclusions
Shear in reinforced concrete slabs – wheel loads close to support 2

3. Key message
Slabs under wheel loads behave
differently in shear than beams
Shear in reinforced concrete slabs – wheel loads close to support 3

4. Background
Project description (1)
• Capacity of existing bridges in NL
• TU Delft
• Concrete Structures
• Structural Mechanics
• TNO
• RWS
• 3715 relevant structures
• 2020 built before 1976
• Study: bridge categories and specific
details
Highways in the Netherlands
Shear in reinforced concrete slabs – wheel loads close to support 4

5. Background
Project description (2)
• Concrete Structures
• Long-term tensile strength
• Beam shear – sustained loads
• Continuous girders – shear
• Prestressed slabs – punching + CMA
• Slab bridges - shear/punching
Concrete bridges
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6. Background
Project description (3)
Shear failure of the de la Concorde bridge, Laval
The Netherlands: 60% of bridges built before 1975
Traffic volume and loads have increased
Shear in reinforced concrete slabs – wheel loads close to support 6

7. Background
Project description (4)
• Wheel loads: tire contact
area + loading
• Tandem loads for local
verification
• Eurocode Tire contact area:
400mm x 400mm
• Larger than physical contact
area
Tandem loads, EC2
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8. Background
Project description (5)
• Wheel loads: tire contact
area + loading
• Values for load model 1
Load model 1
Shear in reinforced concrete slabs – wheel loads close to support 8

9. Background
Project description (6)
Load spreading towards the support Shear span to depth ratio
Influence of the support
Shear in reinforced concrete slabs – wheel loads close to support 9

10. Background
Current practice
Beam shear, one-way shear Punching shear, two-way shear
• Design: shear capacity of slabs
• Flexural failure before shear failure
• Punching shear formulas
• Beam shear formulas over effective width
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11. Goals
• Assess shear capacity of slabs
under concentrated loads
• Determine effective width in
shear
Shear in reinforced concrete slabs – wheel loads close to support 11

12. Experiments
Test setup
Size: 5m x 2,5m x 0,3m
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13. Experiments
Test setup
Continuous support, Line supports
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14. Experiments
Test setup
Load: vary a/d and position along width
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15. Results and discussion
Loading history
• Experiments:
• Lower bound for cracked bridges
• Loading in vicinity of failure
• Influence of local failure
• Connecting existing cracks
• Opening existing cracks
• +/- 84% of peak load undamaged
S2T1 cracks
specimen
Slabs with large cracks:
still 84% of uncracked load is carried!
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16. Results and discussion
Distance to support (1)
slabs: decrease due to smaller effective width beams: influence of arching
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17. Results and discussion
Distance to support (1)
slabs: decrease due to smaller effective width beams: influence of arching
Lower bound: 2d
Influence of the distance to the support on the shear capacity of slabs?
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18. Results and discussion
Distance to support (2)
Influence of distance to support on measured peak load
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19. Results and discussion
Distance to support (3)
• Smaller increase than
expected from EC2
• Reasons:
• Cracking behavior
• Possible paths for strut
• Larger effective a/d ratio
Different behavior for slabs and beams!
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20. Results and discussion
Concrete compressive strength (1)
VRd ,c = C Rd ,c k (100 ρ l f ck ) + k1σ cp  bw d ≥ (vmin + k1σ cp )bw d Eurocode
1/ 3
 
1
Vc = λ f c' bw d ACI code
6
• fc’ as parameter in code formulas
• Shear strength related to tensile strength
Test slabs with normal strength and high strength
concrete
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21. Results and discussion
Concrete compressive strength (2)
1600
1400
1200
1000
Pu (kN)
800
600
S3
400
S4
S7
200
S8
0 S2
0 10 20 30 40 50 60 70 80 90
fc' (MPa)
Influence of concrete compressive strength on measured ultimate load
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22. Preliminary conclusions
• Locally failed decks
• 84% of peak load
• Redistribution capacity of slabs
• Distance to support
• Smaller influence than for
beams
• Suggest different behavior
• Concrete compressive strength S4T2 Dominant shear crack
• No measured influence
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23. Key message
Slabs under wheel loads behave
differently in shear than beams
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24. Contact:
Eva Lantsoght
E.O.L.Lantsoght@tudelft.nl
+31(0)152787449
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