Concrete Slab Technology

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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
• 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

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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

• Stress is maximum in the centre of the slab
• Crack usually occurs in centre of slab first

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SUBGRADE FRICTION AND CRACK DEVELOPMENT
Sawcuts
48m
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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

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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
• 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
• Concretes tensile strength
• Diameter of steel reinforcement
• Yield stress of steel reinforcement

STEEL MESH DESIGN IS COMPLICATED!
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PROBLEMS CAUSED BY STEEL MESH

• Restricts cracks opening
• Limits cracks 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
• 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

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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
• 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

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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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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