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CONCRETE SLAB TECHNOLOGY

“JOINTING AND CRACK CONTROLFOR CONCRETE SLABS ON GROUND IN WAREHOUSES AND INDUSTRIAL BUILDINGS”


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


• All concrete shrinks as it cures (even with admixtures)

• To avoid cracking, must avoid restraining concrete shrinkage

Restrained Shrinkage Minimising Restraint

CONCRETE SHRINKAGE RESTRAINT


SOURCES OF RESTRAINT

Subgrade Friction

Slab Penetrations

Set-downs

Slab Tied to Walls


DETAILING FOR STRESS RELIEF


SUBGRADE RESTRAINT

Stress due to subgrade restraint depends on

• Subgrade material

• Length of pour


• Stress is maximum in the centre of the slab

• Crack usually occurs in centre of slab first

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Length from Edge of Pavement (m)

SUBGRADE RESTRAINT


SUBGRADE FRICTION AND CRACK DEVELOPMENT

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48m Concrete Section 24m Concrete Section 12m Concrete Section

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Sawcuts

48m


CRACKS1. Concrete shrinkage restraint2. Use of steel reinforcement for crack width control –

how effective is it?

JOINTS3. New problems for joints caused by :-


ALETERNATIVE SOLUTION FOR CRACK CONTROL4. Mechanical cracking of sawcut joints



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


Continuously Reinforced Concrete Pavements

• 0.6% - 0.7% steel

– Tight cracks at close centres

– Works fairly well

– Expensive

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

USE OF STEEL MESH


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


• Restricts cracks opening– Limits crack’s ability to accommodate

shrinkageCreates 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


Creates residual tensile stress in the slab• Adds to other stresses from

– Applied loads– Shrinkage stressesto increase risk of cracking



Can create plastic settlement cracks

• Grid of cracks mirroring bars in mesh below



• Difficult to place and compact concrete

– Poor compaction reduces concrete strength



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



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


– Hard-wheeled forklifts– Laser screeds




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


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


CRACKS1. Concrete shrinkage restraint2. Use of steel reinforcement for crack width control –

how effective is it?






CONVENTIONAL METHOD - SAWCUT JOINTS CRACKING OVER TIME

NEW METHOD -MECHANICALLY CRACKED JOINTS

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48m Concrete Section 24m Concrete Section

12m Concrete Section

•Sawcut joints cracked early.•Tensile stress never accumulates. •Stresses insufficient to cause unplanned cracks

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48m Concrete Section 24m Concrete section

12m Concrete Section


MECHANICALLY INDUCING A CRACK


JOINT CRACKING MACHINE


• 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


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

CONCLUSION

http://www.getcracking.com.au/

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