11-07-2012

Challenge the future

DelftUniversity ofTechnology

Residual capacity from aggregate interlockCase: cracked concrete slab bridge

Eva Lantsoght, Cor van der Veen, Joost Walraven

2Residual capacity from aggregate interlock of cracked concrete slab bridge

Introduction (1)

• 50-year-old concrete slab bridge withtraffic restrictions

• Extensive cracking in southernconcrete approach bridge

• Result of settlement• Flexural reinforcement yielded at

crack

• Cores: C33/45• Reinforcement QR 240: fyd =209 MPa; εsu = 19% – 38%

3Residual capacity from aggregate interlock of cracked concrete slab bridge

Introduction (2)

d = 413mm (side) to 493mm (mid)φbottom 14mm – 200mmφtop 25mm – 100mm

flexural through crack

4Residual capacity from aggregate interlock of cracked concrete slab bridge

Aggregate interlock

• Aggregates stronger than cement paste• Particles interlock with opposite face + resist shear displacement

• Contribution to shear capacity: 33% - 90%• Slab bridge, 1% rebar: aggregate interlock is main shear carrying

mechanism

• Fundamental model by Walraven• Shear + axial stress: σ & τ, ∆ & w

• Unreinforced sections: crack-opening• Reinforced sections: capacity

through crack

5Residual capacity from aggregate interlock of cracked concrete slab bridge

Calculations (1)Shear & Aggregate interlock• Shear capacity (inclined cracking load) • VVBC = 273 kN/m (side) and 325 kN/m (mid)

• Aggregate interlock – no tension on cross-section• Based on shear stress capacity τ of reinforced crack• Plain reinforcement => 0.5ρl• Vagg = 1575 kN/m (side) and 1679 kN/m (mid)

• Large resistance provided by aggregate interlock action

• Rusted bearings => deformation due to ∆T is restrained• Conservative assumption: full concrete cross-section in tension

, ,clamp s bottom s top y ctk iF A A f f d b

6Residual capacity from aggregate interlock of cracked concrete slab bridge

Calculations (2)Maximum crack width (1)

• Relation between w and aggregate interlock capacity• Expressions for unreinforced section• Based on graph (Walraven, 1981): Δ = 1.25w

7Residual capacity from aggregate interlock of cracked concrete slab bridge

Calculations (3)Maximum crack width (2)

• Find: crack width Vu_unr < VVBC or Fax < Fclamp

wmax ≈ 1 mm

8Residual capacity from aggregate interlock of cracked concrete slab bridge

Calculations (4)Axial force equilibrium

• wmax ~ rebar, tension in concrete cross-section (vary % Ftc)• Requirement: Vagg ≥ 2VVBC• Find associated ∆• Find Nagg(wmax,∆) (clamping effect)• Remaining capacity of top reinforcement to resist tension:

Ntension = As,topfy – Nagg

• Compare to Ftc => Equilibrium?

• Result: maximum 71% of restraint

9Residual capacity from aggregate interlock of cracked concrete slab bridge

Proposed actions + Conclusions

• Replace rusted steel bearings by elastomeric bearings• Open bridge for all traffic

• Quantify amount of restraint through measurements at support• Measurement points for cracks every 3m (lane width)

• Special cases: use aggregate interlock to check cracked cross-sections in shear

• Quantifies residual bearing capacity• Shear and axial compression

10Residual capacity from aggregate interlock of cracked concrete slab bridge

Contact:

Eva Lantsoght

E.O.L.Lantsoght@tudelft.nl

+31(0)152787449