Loading of existing structures above design limits: how far can we go?

1. Loading of existing structures above design limits: how far can we go? workshop "limiting the cracks or cracking the limits" J.D. Bakker Utrecht, 5 april 2004 J.D. Bakker

2. Loading of existing structures above design limits: how far can we go? Why this workshop? Project “ZSM” needs some answers NOW… • ZSM in English: ASAP; “As Soon As Possible” • Political aim: significant reduction of traffic congestion by the end of 2005 • Means: Use of existing infrastructure • Emergency lanes will be used as extra traffic lanes during rush hours

3. Loading of existing structures above design limits: how far can we go? Critical issue in ZSM: structures • Most structures (mostly constructed in the sixties or seventies) are not designed for full loading of emergency lanes • Examples: • Bridges • Viaducts • Culverts

4. Loading of existing structures above design limits: how far can we go? ZSM-Taskforce 1 (2003): Fast evaluation of structures • Focus on safety (Ultimate Limit State, ULS) • Limited budget • Limited time • Serviceability Limit State (SLS) of secondary importance Some structures are evaluated based on original calculation, others are recalculated, most structures are qualitatively analysed, based on analogy with other structures

5. Loading of existing structures above design limits: how far can we go? Result taskforce 1: • Some bridges need to be replaced • Some bridges need to be strengthened • For some structures design drawings and design calculations are missing • A yet unknown number of (concrete) structures do not meet the SLS requirements on crack width for critical components (bridge decks)

6. Loading of existing structures above design limits: how far can we go? New taskforce project: Maintenance strategy • Determine how to deal with structures that do not meet SLS requirements • Define a maintenance strategy • Define an inspection strategy

7. Loading of existing structures above design limits: how far can we go? Overview of results of structural analysis regarding requirements SLS Total number of considered structures: 480 - qualitative analysis: 88 - yet known/overall view: 199 Re-calculated concrete bridges: 187 Test ULS Meeting USL requirements: 132 Not meeting ULS requirements: 55 ? Conducted tests SLS: 84 ? ? Within the scope of our regular maintenance scheme Test SLS Meeting SLS requirements: 36 Not meeting SLS requirements: 48 Within the scope of the new ‘Maintenance-strategy’

8. Loading of existing structures above design limits: how far can we go? Analysis report (January 2004) • Not meeting the SLS-requirements according to the building codes (NEN 6720) does not necessarily imply the structure will show the calculated crack width in practice • The cracking criterion adopted in building codes is based on experience with static cracks. This may not be applicable for dynamic cracks • There is insufficient knowledge of the effects of dynamic cracks in concrete on risks of concrete corrosion

9. Loading of existing structures above design limits: how far can we go? Definition of “dynamic cracks” • Cracks that vary in crack width due to dynamic loading and exceed the maximum crack width in the SLS according to the national building codes • Maximum allowable crack width in SLS according to NEN6720: • 0,3 mm in humid environment (outside) • 0,2 mm. in aggressive environment (outside & salt) • 0,1 mm in aggressive environment in case of prestressed concrete

10. Dynamic cracks Loading of existing structures above design limits: how far can we go? Cracks of concrete due to bending moments 4 Moment / deformation diagram 3 • Up to Mcr concrete will not crack and behave elastically. • The cracking pattern develops to a full crack pattern. Tensile forces are transposed to the reinforcement steel. • Cracks open due to elastic deformation of the reinforcement steel (reversible). • Cracks develop due to yielding of the steel rebars (irreversible)

11. Loading of existing structures above design limits: how far can we go? Difference between dynamic and static cracks exceeding the SLS • Static cracks are open all the time. Dynamic cracks vary in crack width, but will only exceed the SLS incidentally • Static cracks show “self healing properties” to a certain extend. This may not be the case for dynamic cracks • Due to self healing, crack width is the leading factor for durability issues on static cracking. For dynamic cracking the crack distance may be an important factor

12. Loading of existing structures above design limits: how far can we go? Parameters in risk evaluation • Using better calculation methods can prove that the expected cracking will not exceed the cracking limits for the SLS • Not all elements can be visually inspected • Preventive maintenance is costly and may slow down the ZSM-project.

13. Structure does not meet SLS requirements More detailed crack evaluation (Nowakovski) Cracks in SLS are within crack limits? Can the elements be inspectedvisually? Cracks in SLS within limits? N N N Y Y Y No action (general inspection procedures) Specific visual inspection procedure Specific condition monitoring procedure (2) Loading of existing structures above design limits: how far can we go? Proposed decision flowchart Scope workshop Preventive maintenance

14. Loading of existing structures above design limits: how far can we go? Specific visual inspection procedure • Specific visual inspection procedure for those areas that will crack according to the calculations on: • Visual cracking- Debounding of concrete- Corrosion products

15. Concrete core Concrete cores Theoretic chloride profile based on two core slices Loading of existing structures above design limits: how far can we go? Specific condition monitoring procedure: inspection validation model • Uncertainty prediction due to: • Spatial variability • Measurement uncertainty • Time dependant uncertainty

16. Loading of existing structures above design limits: how far can we go? Inspection validation average chloride content Expected decay uncracked concrete Expected decay based on concrete cores Uncertainty of prediction in time

17. Loading of existing structures above design limits: how far can we go? Inspection validation local chloride content Expected decay uncracked concrete Expected local decay based on concrete cores Uncertainty of prediction in time

18. Loading of existing structures above design limits: how far can we go? Options preventive maintenance • Strengthening • Application of waterproof membranes • Preventive catholic protection All these measures will cause considerable project delay and extra costs. Therefore these should only be applied if no other option is available

19. Challenge for the workshop: Can we determine these boundary conditions??? No action (general inspection procedures) Specific inspection procedure or specific condition monitoring procedure Preventive maintenance Loading of existing structures above design limits: how far can we go? Cracking the limits?