“JOINTING AND CRACK CONTROL
FOR CONCRETE SLABS ON GROUND
IN WAREHOUSES AND INDUSTRIAL BUILDINGS”
CONCRETE SLAB TECHNOLOGY
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CONCRETE SLABS ON GROUND
CRACKS
1. Concrete shrinkage restraint
2. Use of steel reinforcement for crack width control – how
effective is it?
JOINTS
3. New problems for joints caused by use of:– Laser screeds
– Hard-wheeled forklifts
ALTERNATIVE SOLUTION
- CRACK CONTROL
- JOINT OPENING CONTROL
4. Mechanical cracking of sawcut joints
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CONCRETE SHRINKAGE RESTRAINT
• All concrete shrinks as it cures (even with admixtures)
• To avoid cracking, must avoid restraining concrete shrinkage
Restrained Shrinkage
Minimising Restraint
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SOURCES OF RESTRAINT
Slab Tied to Walls
Slab Penetrations
Subgrade Friction
Set-downs
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DETAILING FOR STRESS RELIEF
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SUBGRADE RESTRAINT
Stress due to subgrade restraint depends on
• Subgrade material
• Length of pour
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SUBGRADE RESTRAINT
Tensile Stres
• Stress is maximum in the centre of the slab
• Crack usually occurs in centre of slab first
0
6
12
18
24
30
36
42
48
Length from Edge of Pavement (m)
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SUBGRADE FRICTION AND CRACK DEVELOPMENT
Sawcuts
Tensile Stres
48m
0
6
12
18
24
30
36
42
48
Length from Edge of Pavement (m)
48m Concrete Section
24m Concrete Section
12m Concrete Section
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CONCRETE SLABS ON GROUND
CRACKS
1. Concrete shrinkage restraint
2. Use of steel reinforcement for crack width control –
how effective is it?
JOINTS
3. New problems for joints caused by :– Laser screeds
– Hard-wheeled forklifts
ALETERNATIVE SOLUTION FOR CRACK CONTROL
4. Mechanical cracking of sawcut joints
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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
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USE OF STEEL MESH
Continuously Reinforced Concrete Pavements
• 0.6% - 0.7% steel
– Tight cracks at close centres
– Works fairly well
– Expensive
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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
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USE OF STEEL MESH
DESIGN CONSIDERATION
Steel quantity required depends on:• Length of pavement with continuous
reinforcement
STEEL
MESH DESIGN
• Subgrade
friction IS COMPLICATED!
• 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
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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
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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
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PROBLEMS CAUSED BY STEEL MESH
Can create plastic settlement cracks
• Grid of cracks mirroring bars in mesh below
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PROBLEMS CAUSED BY STEEL MESH
• Difficult to place and compact concrete
– Poor compaction reduces concrete strength
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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
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CONCRETE SLABS ON GROUND
CRACKS
1. Concrete shrinkage restraint
2. 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
3. New problems for joints caused by :– Hard-wheeled forklifts
– Laser screeds
ALETERNATIVE SOLUTION FOR CRACK CONTROL
4. Mechanical cracking of sawcut joints
www.getcracking.com.au
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
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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
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CONCRETE SLABS ON GROUND
CRACKS
1. Concrete shrinkage restraint
2. Use of steel reinforcement for crack width control –
how effective is it?
JOINTS
3. New problems for joints caused by :– Laser screeds
– Hard-wheeled forklifts
ALETERNATIVE SOLUTION FOR CRACK CONTROL
4. Mechanical cracking of sawcut joints
www.getcracking.com.au
Tensile Stres
CONVENTIONAL METHOD - SAWCUT JOINTS CRACKING OVER TIME
0
6
12
18
24
30
36
42
Length from Edge of Pavement (m)
48m Concrete Section
24m Concrete Section
12m Concrete Section
Tensile Stress
NEW METHOD -MECHANICALLY CRACKED JOINTS
0
•Sawcut joints cracked early.
•Tensile stress never accumulates.
•Stresses insufficient to cause unplanned cracks
6
12
18
24
30
36
42
48
Length from Edge of Pavement (m)
48m Concrete Section
24m Concrete section
12m Concrete Section
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48
MECHANICALLY INDUCING A CRACK
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JOINT CRACKING MACHINE
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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
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CONCLUSION
• For more information visit
www.getcracking.com.au
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