1. Concrete Slab Technology
“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. Stress due to subgrade restraint depends on Subgrade material
Length of pour

7. Stress is maximum in the centre of the slab
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. Steel mesh in concrete slabs on ground:- Does not prevent cracks
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. Continuously Reinforced Concrete Pavements 0.6% - 0.7% steel
USE OF STEEL MESH
Continuously Reinforced Concrete Pavements
0.6% - 0.7% steel
Tight cracks at close centres
Works fairly well
Expensive

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

13. STEEL MESH DESIGN IS COMPLICATED!
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
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. For more information visit www.getcracking.com.au
CONCLUSION
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