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ARCHING ACTION IN CONCRETE BRIDGE DECKS
Research at Queen’s University of Belfast Dr. Su Taylor Dr. Barry Rankin Prof. David Cleland Prof. AE Long
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Introduction Background Previous research Changes to bridge design
Recent laboratory and field tests Comparison with existing standards Future research Conclusions
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Background to research
applied load arching thrust Kr = external lateral restraint stiffness Arching action or Compressive Membrane Action (CMA)
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laterally restrained slabs have inherent strength due to in-plane forces set up as a result of external lateral restraint external restraint occurs due to the slab boundary conditions e.g. beams diaphragms continuity of slab
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Load vs. deflection for laterally restrained concrete slab
Arching capacity Applied load Bending strength first cracking Midspan deflection Load vs. deflection for laterally restrained concrete slab
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Arching action and three-hinged arch analogy (Rankin, 1982)
Previous research external lateral restraint, stiffness = K applied load arching thrust K, Le E, A Load, P Arching action and three-hinged arch analogy (Rankin, 1982)
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Clinghan’s bridge test model (Kirkpatrick et al, 1984)
Collaborative project with the DOE (now DRD) which has continued to present day.
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Model Clinghan’s bridge deck slab
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Model Clinghan’s bridge deck slab failures loads (Kirkpatrick et al, 1984)
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Clinghan’s bridge load test
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Advantages from CMA in bridge design
NI bridge code amendment in reinforcement reduced from 1.7% to 0.6%T&B Improved durability and cost benefits BD81/02 – Highways Agency ‘Use of CMA in bridge decks’ is direct result of research at Queens University Kirkpatrick et al’s punching prediction ( leading to BD 81/02) where includes CMA
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Canadian approach Calgary bridge
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Calgary bridge –reinforcement detail no internal reinforcement
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Developments in UK Majority of bridges RC Advance knowledge of CMA:
High strength concrete and fibres Reinforcement Single layer at mid-depth Fibre Reinforced Polymer (FRP’s) Goal: maintenance free deck slabs
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Beam and slab superstructures
standard deck (normal durability) CMA deck (normal durability ) (enhanced durability) Unit cost Years in service Standard deck more expensive as more reinfrcemnt , repir costs at some stage CMA deck Total unit cost over service life
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Recent Laboratory tests
Series of tests on full-scale slab strips typical of a bridge deck slab Variables were: Concrete compressive strength Reinforcement type and position Boundary conditions Despite the amount of research carried out into the arching phenomenon, there has been very few on laterally restrained slabs with high concrete compressive strengths. To fully ascertain the influence of concrete strength a series of tests on full-scale one-way spanning laterally restrained slabs was conducted. The concrete compressive strength varied from 30N/mm2 to 100N/mm2. These were typical of a section of bridge deck slab. The effectiveness of arching action is also dependent upon the degree of lateral restraint. The second Phase of tests, also on full-scale one-way spanning laterally restrained slabs aimed at establishing the effects of an increase in the lateral restraint in both NSC & HSC slabs. Variations in the type, percentage and position of the reinforcement were also investigated.
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Slab strips test load arrangement
Restraint, K b=475mm h=150mm d=75 to 104mm 1425mm KEY : Fixed End & Longitudinal Restraint = F/E+L/R Simple Support & Longitudinal Restraint = S/S+L/R Simple Support = S/S Slab strips test load arrangement
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Typical test set-up
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Summary of test results
F/E + L/R S/S S/S + L/R Kr=410kN/mm Kr=197kN/mm Failure load (kN) BS5400 (F/E) BS5400 (S/S) Concrete compressive strength (N/mm2) Summary of test results
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HSC - F/E + L/R model post-failure
severe crushing in compression zone topside HSC - F/E + L/R model post-failure
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Comparison Phase 1 results with theory
F/E + L/R (S1-S5) S/S + L/R (S8) proposed method Failure load (kN) BS5400 (F/E) Concrete compressive strength (N/mm2) Comparison Phase 1 results with theory
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Bridge model tests Final series of tests on one-third scale bridge deck models HSC with variables of: lateral restraint stiffness reinforcement (type & amount)
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Typical one-third scale bridge deck model
Applied line load PLAN Applied load, PkN 50mm b = 100, 150, mm b Support beam SECTION Typical one-third scale bridge deck model
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Typical reinforcement details
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Typical test
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Third scale bridge model test results - effect of reinforcement
trend line Failure load (kN) conventional bars T&B conventional bars C unbonded bars C fibres only (1%) Two wheel loads 45 units HB (ULS) % reinforcement Third scale bridge model test results effect of reinforcement
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Third scale bridge model test results – varied restraint to slab
Failure load (kN) Edge beam width (mm) BS5400 shear capacity BS5400 flexural capacity conventional bars T&B in slab QUB capacity
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Corick Bridge
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Deck slab reinforcement
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Test panel arrangement
0.5%C %C %C reinforcement Centre reinforce-ment A1 B1 C1 D1 A2 B2 C2 D2 T & B reinforce- ment F2 E2 = testing order F1 E1 0.6%T&B reinforcement Blue = NSC Green = HSC (not!) Red = HSC + fibres Test panel arrangement
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Typical test arrangement
2000mm 1500mm T1 T2 T3 T4 T5 hydraulic jack 300mm steel plate Typical test arrangement
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300mm diam. Concentrated load equivalent to a wheel load at midspan of test slab
Typical test set-up
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Typical test set-up: deck underside
midspan of test panel centreline and span of test panel T1 T2 T3 T4 Typical test set-up: deck underside
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midspan deflection (mm) - comparison of midspan deflections
applied load (kN) max. wheel load (45units HB) =span/4250 midspan deflection (mm) 2m test panels - comparison of midspan deflections
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- comparison of crack widths
applied load (kN) wheel load (45units HB) crack width (mm) 2m test panels - comparison of crack widths
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CMA in FRP Reinforced Bridges
Series of tests on full-scale slab strips FRP and steel reinforcement compared variables: boundary conditions concrete strength
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Preliminary results on GFRP slabs
In simply supported slabs service behaviour of GFRP poor ultimate strengths similar In laterally restrained slabs GFRP & steel slab behaved similarly in service GFRP slabs higher ultimate capacities
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Test results for full scale laterally restrained slab strips
predicted strength from arching theory Failure load (kN) BS predictions Concrete compressive strength (N/mm2) Test results for full scale laterally restrained slab strips
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Conclusions Degree of external restraint and concrete strength influence capacity deflections up to 45 units HB wheel load were independent of %As crack widths up to 45 units HB wheel load were substantially narrower than BS limits strength of panels with centre reinforcement in excess of ultimate wheel load fcu & Kr govern strength in bridge level of restraint is high therefore independent of %As
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Concluding remarks Structural benefits of CMA well understood
CMA incorporated in Ontario & UK codes Improved strength/serviceability less problems for assessment Arching phenomenon has potential for substantial economies
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