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Www.getcracking.com.au C ONCRETE S LAB T ECHNOLOGY “JOINTING AND CRACK CONTROL FOR CONCRETE SLABS ON GROUND IN WAREHOUSES AND INDUSTRIAL BUILDINGS”

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Presentation on theme: "Www.getcracking.com.au C ONCRETE S LAB T ECHNOLOGY “JOINTING AND CRACK CONTROL FOR CONCRETE SLABS ON GROUND IN WAREHOUSES AND INDUSTRIAL BUILDINGS”"— Presentation transcript:

1 C ONCRETE S LAB T ECHNOLOGY “JOINTING AND CRACK CONTROL FOR CONCRETE SLABS ON GROUND IN WAREHOUSES AND INDUSTRIAL BUILDINGS”

2 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 CONCRETE SLABS ON GROUND

3 All concrete shrinks as it cures (even with admixtures) To avoid cracking, must avoid restraining concrete shrinkage Restrained ShrinkageMinimising Restraint CONCRETE SHRINKAGE RESTRAINT

4 SOURCES OF RESTRAINT Subgrade Friction Slab Penetrations Set-downs Slab Tied to Walls

5 DETAILING FOR STRESS RELIEF

6 SUBGRADE RESTRAINT Stress due to subgrade restraint depends on Subgrade material Length of pour

7 Stress is maximum in the centre of the slab Crack usually occurs in centre of slab first SUBGRADE RESTRAINT

8 SUBGRADE FRICTION AND CRACK DEVELOPMENT Sawcuts 48m

9 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 CONCRETE SLABS ON GROUND

10 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

11 Continuously Reinforced Concrete Pavements 0.6% - 0.7% steel – Tight cracks at close centres – Works fairly well – Expensive USE OF STEEL MESH

12 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 USE OF STEEL MESH

13 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

14 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

15 Creates residual tensile stress in the slab Adds to other stresses from – Applied loads – Shrinkage stresses to increase risk of cracking PROBLEMS CAUSED BY STEEL MESH

16 Can create plastic settlement cracks Grid of cracks mirroring bars in mesh below PROBLEMS CAUSED BY STEEL MESH

17 Difficult to place and compact concrete – Poor compaction reduces concrete strength PROBLEMS CAUSED BY STEEL MESH

18 Steel manufacture creates greenhouse gas emissions – Mass of CO 2 emissions from steel manufacture and distribution is approx. twice the weight of steel e.g. 1500m 2 of SL92 mesh ≈ 12 tonnes of CO 2 PROBLEMS CAUSED BY STEEL MESH

19 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 CONCRETE SLABS ON GROUND

20 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

21 Increasing use of laser screeds Economies with large pours up to 3000m 2 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

22 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 CONCRETE SLABS ON GROUND

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

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26 JOINT CRACKING MACHINE

27 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

28 For more information visit CONCLUSION


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