Experimental study of shear capacity of reinforced concrete slabs

17-5-2011

Challenge the future

DelftUniversity ofTechnology

Shear capacity of Reinforced Concrete Slabsexperimental study

Eva Lantsoght

2Shear in reinforced concrete slabs – wheel loads close to support

Overview

• Background• Project description

• Current practice

• Experiments• Results and discussion

• Loading history

• Distance to support

• Concrete compressive strength

• Preliminary conclusions

3Shear in reinforced concrete slabs – wheel loads close to support

Key message

Slabs under wheel loads behave

differently in shear than beams

4Shear in reinforced concrete slabs – wheel loads close to support

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

5Shear in reinforced concrete slabs – wheel loads close to support

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

6Shear in reinforced concrete slabs – wheel loads close to support

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

7Shear in reinforced concrete slabs – wheel loads close to support

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

8Shear in reinforced concrete slabs – wheel loads close to support

Background

Project description (5)

• Wheel loads: tire contact area + loading

• Values for load model 1

Load model 1

9Shear in reinforced concrete slabs – wheel loads close to support

Background

Project description (6)

Shear span to depth ratioLoad spreading towards the support

Influence of the support

10Shear in reinforced concrete slabs – wheel loads close to support

Background

Current practice

• Design: shear capacity of slabs• Flexural failure before shear failure

• Punching shear formulas

• Beam shear formulas over effective width

Beam shear, one-way shear Punching shear, two-way shear

11Shear in reinforced concrete slabs – wheel loads close to support

Goals

• Assess shear capacity of slabs

under concentrated loads

• Determine effective width in

shear

12Shear in reinforced concrete slabs – wheel loads close to support

Experiments

Test setup

Size: 5m x 2,5m x 0,3m

13Shear in reinforced concrete slabs – wheel loads close to support

Experiments

Test setup

Continuous support, Line supports

14Shear in reinforced concrete slabs – wheel loads close to support

Experiments

Test setup

Load: vary a/d and position along width

15Shear in reinforced concrete slabs – wheel loads close to support

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

specimenS2T1 cracks

Slabs with large cracks:

still 84% of uncracked load is carried!

16Shear in reinforced concrete slabs – wheel loads close to support

Results and discussion

Distance to support (1)

beams: influence of archingslabs: decrease due to smaller effective width

17Shear in reinforced concrete slabs – wheel loads close to support

Results and discussion

Distance to support (1)

beams: influence of archingslabs: decrease due to smaller effective width

Lower bound: 2d

Influence of the distance to the support on the shear capacity of slabs?

18Shear in reinforced concrete slabs – wheel loads close to support

Results and discussion

Distance to support (2)

Influence of distance to support on measured peak load

19Shear in reinforced concrete slabs – wheel loads close to support

• Smaller increase than expected from EC2

• Reasons:• Cracking behavior

• Possible paths for strut

• Larger effective a/d ratio

Results and discussion

Distance to support (3)

Different behavior for slabs and beams!

20Shear in reinforced concrete slabs – wheel loads close to support

Results and discussion

Concrete compressive strength (1)

( )1/ 3

, , 1 min 1100 ( )Rd c Rd c l ck cp w cp wV C k f k b d v k b dρ σ σ= + ≥ +

'1

6c c wV f b dλ=

• fc’ as parameter in code formulas

• Shear strength related to tensile strength

Test slabs with normal strength and high strength concrete

Eurocode

ACI code

21Shear in reinforced concrete slabs – wheel loads close to support

0

200

400

600

800

1000

1200

1400

1600

0 10 20 30 40 50 60 70 80 90

fc' (MPa)

Pu

(kN

)

S3

S4

S7

S8

S2

Results and discussion

Concrete compressive strength (2)

Influence of concrete compressive strength on measured ultimate load

22Shear in reinforced concrete slabs – wheel loads close to support

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• No measured influence

S4T2 Dominant shear crack

23Shear in reinforced concrete slabs – wheel loads close to support

Key message

Slabs under wheel loads behave

differently in shear than beams

24Shear in reinforced concrete slabs – wheel loads close to support

Contact:

Eva Lantsoght

E.O.L.Lantsoght@tudelft.nl

+31(0)152787449