www.getcracking.com.au c oncrete s lab t echnology “jointing and crack control for concrete slabs...
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CONCRETE SLAB TECHNOLOGY
“JOINTING AND CRACK CONTROL FOR CONCRETE SLABS ON GROUND IN WAREHOUSES AND INDUSTRIAL BUILDINGS”
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CRACKS1. Concrete shrinkage restraint2. Use of steel reinforcement for crack width control – how
effective is it?
JOINTS3. 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
CONCRETE SLABS ON GROUND
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• All concrete shrinks as it cures (even with admixtures)• To avoid cracking, must avoid restraining concrete shrinkage
Restrained Shrinkage Minimising Restraint
CONCRETE SHRINKAGE RESTRAINT
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SOURCES OF RESTRAINT
Subgrade Friction
Slab Penetrations
Set-downs
Slab Tied to Walls
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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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• Stress is maximum in the centre of the slab• Crack usually occurs in centre of slab first
SUBGRADE RESTRAINT
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SUBGRADE FRICTION AND CRACK DEVELOPMENT
Sawcuts
48m
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CRACKS1. Concrete shrinkage restraint2. Use of steel reinforcement for crack width control –
how effective is it?
JOINTS3. New problems for joints caused by :-– Laser screeds– Hard-wheeled forklifts
ALETERNATIVE SOLUTION FOR CRACK CONTROL4. Mechanical cracking of sawcut joints
CONCRETE SLABS ON GROUND
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Steel mesh in concrete slabs on ground:-• Does not prevent cracks• Does not increase load carrying capacity• Steel is used to control crack width
USE OF STEEL MESH
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Continuously Reinforced Concrete Pavements• 0.6% - 0.7% steel – Tight cracks at close centres– Works fairly well– Expensive
USE OF STEEL MESH
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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
USE OF STEEL MESH
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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
DESIGN CONSIDERATION
STEEL MESH DESIGN IS COMPLICATED!
USE OF STEEL MESH
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• 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
PROBLEMS CAUSED BY STEEL MESH
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Creates residual tensile stress in the slab• Adds to other stresses from– Applied loads– Shrinkage stressesto increase risk of cracking
PROBLEMS CAUSED BY STEEL MESH
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Can create plastic settlement cracks• Grid of cracks mirroring bars in mesh below
PROBLEMS CAUSED BY STEEL MESH
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• Difficult to place and compact concrete– Poor compaction reduces concrete strength
PROBLEMS CAUSED BY STEEL MESH
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Steel manufacture creates greenhouse gas emissions– Mass of CO2 emissions from steel manufacture and
distribution is approx. twice the weight of steele.g. 1500m2 of SL92 mesh ≈ 12 tonnes of CO2
PROBLEMS CAUSED BY STEEL MESH
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CRACKS1. Concrete shrinkage restraint2. 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
JOINTS3. New problems for joints caused by :-
– Hard-wheeled forklifts– Laser screeds
ALETERNATIVE SOLUTION FOR CRACK CONTROL4. Mechanical cracking of sawcut joints
CONCRETE SLABS ON GROUND
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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
PROBLEMS CAUSED BY HARD-WHEELED FORKLIFTS
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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
PROBLEMS CAUSED BY LASER SCREEDS
11mm wide sawcut, 3 months after concrete placement
Easily damaged by hard wheeled forklifts
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CRACKS1. Concrete shrinkage restraint2. Use of steel reinforcement for crack width control –
how effective is it?
JOINTS3. New problems for joints caused by :-– Laser screeds– Hard-wheeled forklifts
ALETERNATIVE SOLUTION FOR CRACK CONTROL4. Mechanical cracking of sawcut joints
CONCRETE SLABS ON GROUND
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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
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MECHANICALLY INDUCING A CRACK
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JOINT CRACKING MACHINE
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• 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
MECHANICAL CRACKING OF SAWCUT JOINTS
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• For more information visit www.getcracking.com.au
CONCLUSION
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