Concrete Slab Technology

1. “Jointing and Crack Control for Concrete Slabs on Ground in warehouses and industrial buildings” Concrete Slab Technology

2. CONCRETE SLABS ON GROUND Cracks • Concrete shrinkage restraint • Use of steel reinforcement for crack width control – how effective is it? JOINTS • New problems for joints caused by use of:- • Laser screeds • Hard-wheeled forklifts ALTERNATIVE SOLUTION - CRACK CONTROL - JOINT OPENING CONTROL • Mechanical cracking of sawcut joints

3. CONCRETE SHRINKAGE RESTRAINT • All concrete shrinks as it cures (even with admixtures) • To avoid cracking, must avoid restraining concrete shrinkage • Restrained Shrinkage Minimising Restraint

4. SOURCES OF RESTRAINT Slab Tied to Walls Slab Penetrations Subgrade Friction Set-downs

5. DETAILING FOR STRESS RELIEF

6. SUBGRADE RESTRAINT Stress due to subgrade restraint depends on • Subgrade material • Length of pour

7. SUBGRADE RESTRAINT • Stress is maximum in the centre of the slab • Crack usually occurs in centre of slab first

8. SUBGRADE FRICTION AND CRACK DEVELOPMENT Sawcuts 48m

9. CONCRETE SLABS ON GROUND Cracks • Concrete shrinkage restraint • Use of steel reinforcement for crack width control – how effective is it? JOINTS • New problems for joints caused by :- • Laser screeds • Hard-wheeled forklifts ALETERNATIVE SOLUTION FOR CRACK CONTROL • Mechanical cracking of sawcut joints

10. USE OF STEEL MESH Steel mesh in concrete slabs on ground:- • Does not prevent cracks • Does not increase load carrying capacity • Steel is used to control crack width

11. USE OF STEEL MESH Continuously Reinforced Concrete Pavements • 0.6% - 0.7% steel • Tight cracks at close centres • Works fairly well • Expensive

12. USE OF STEEL MESH Industrial Building slabs on ground • Much less steel - 0.1% - 0.15% steel • Poor crack width control for large pours • Sawcut joints open wide at mesh discontinuities • Poor control of sawcut joint widths

13. USE OF STEEL MESH DESIGN CONSIDERATION Steel quantity required depends on:- • Length of pavement with continuous reinforcement • Subgrade friction • Magnitude of thermal shrinkage • Magnitude of drying shrinkage • Required crack width • Required crack spacing • Use of stress concentrators (e.g. sawcuts) • Pavement thickness • Concrete’s tensile strength • Diameter of steel reinforcement • Yield stress of steel reinforcement STEEL MESH DESIGN IS COMPLICATED!

14. PROBLEMS CAUSED BY STEEL MESH • Restricts cracks opening • Limits crack’s ability to accommodate shrinkage Creates more cracks • Concrete pump required for accurate mesh positioning • Increased fines in concrete mix design Leads to increased concrete shrinkage Leads to increased joint widths • Increased cost of construction

15. PROBLEMS CAUSED BY STEEL MESH Creates residual tensile stress in the slab • Adds to other stresses from • Applied loads • Shrinkage stresses to increase risk of cracking

16. PROBLEMS CAUSED BY STEEL MESH Can create plastic settlement cracks • Grid of cracks mirroring bars in mesh below

17. PROBLEMS CAUSED BY STEEL MESH • Difficult to place and compact concrete • Poor compaction reduces concrete strength

18. PROBLEMS CAUSED BY STEEL MESH Steel manufacture creates greenhouse gas emissions • Mass of CO2 emissions from steel manufacture and distribution is approx. twice the weight of steel e.g. 1500m2 of SL92 mesh ≈ 12 tonnes of CO2

19. CONCRETE SLABS ON GROUND Cracks • Concrete shrinkage restraint • Use of steel reinforcement for crack width control – how effective is it? Use of steel mesh in slabs on ground to control cracking is far from ideal JOINTS • New problems for joints caused by :- • Hard-wheeled forklifts • Laser screeds ALETERNATIVE SOLUTION FOR CRACK CONTROL • Mechanical cracking of sawcut joints

20. PROBLEMS CAUSED BY HARD-WHEELED FORKLIFTS Hard-wheeled forklifts • Are here to stay (improved forklift stability) • Cause damage to unprotected joint edges • Joint armouring of construction joints • Difficult to accurately install • Expensive

21. PROBLEMS CAUSED BY LASER SCREEDS Increasing use of laser screeds • Economies with large pours up to 3000m2 per day. Large pours lead to • Wide openings at construction joints • Dominant sawcut joints at centre of pour Easily damaged by hard wheeled forklifts 11mm wide sawcut, 3 months after concrete placement

22. CONCRETE SLABS ON GROUND Cracks • Concrete shrinkage restraint • Use of steel reinforcement for crack width control – how effective is it? JOINTS • New problems for joints caused by :- • Laser screeds • Hard-wheeled forklifts ALETERNATIVE SOLUTION FOR CRACK CONTROL • Mechanical cracking of sawcut joints

23. CONVENTIONAL METHOD - SAWCUT JOINTS CRACKING OVER TIME NEW METHOD -MECHANICALLY CRACKED JOINTS • Sawcut joints cracked early. • Tensile stress never accumulates. • Stresses insufficient to cause unplanned cracks

24. MECHANICALLY INDUCING A CRACK

26. JOINT CRACKING MACHINE

27. MECHANICAL CRACKING OF SAWCUT JOINTS • Use of steel mesh in slabs on ground to control cracking is far from ideal • By mechanically cracking sawcut joints, soon after concrete placement • Random shrinkage cracks eliminated • Joints open more evenly • With shrinkage cracks controlled to sawcut joints the following can be eliminated • Steel reinforcing mesh and its associated problems • Concrete Pumps • With joints opening relatively evenly • Semi-rigid sealants can be used to protect joints • No need for joint-edge armouring • Without steel reinforcing mesh • Greenhouse gas emissions are greatly reduced

28. CONCLUSION • For more information visit www.getcracking.com.au