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Recently Connolly Key Joints Pty Ltd came out and said that Danley Key Joints do not perform as well as their own.
At Danley, we understand minimising Deflection via Flexural Stress is the basis of design for concrete on ground slabs.
We also believe Key Joints are for light traffic only and are not to be relied on long term to carry excessive weight in load transfer. Properly operating Key Joints do assist in the prevention of subgrade deterioration.
When using a tapered Key Joint and once the concrete contracts from the joint, to provide shear load transfer via key, the concrete slab has to deflect to re-establish contact between the adjoining chamfers.
Connolly Key Joints criticises the 4 degree chamfer on our Key Joints.
We did this many years ago because engineers requested we do so. What this does, is at an average 5mm joint opening our Key Joint vertically deflects a maximum of 0.35mm, where as Connolly Key Joint at 5mm has a vertical deflection of 1.67mm (as per Connolly Key Joints technical publication). This 1.67mm deflection is outside most acceptable tolerances for flexural tension of concrete and can result in failure of the concrete slab, especially long term.
In summary, Danley Key Joints - Maximise Load Transfer, Minimise Flexural Cracking.
Load Transfer Criteria:
Concrete, despite being a durable construction material, can carry only a limited load in bending without causing any cracking (flexural over stressing). With the release of AS3600-2001 "Concrete Structures" the rules for flexural crack control of slabs have been reviewed and tightened.
For key joints to be effective in load transfer, the true measure of capacity is the load that can be transferred across a joint at a deflection without causing cracks in the slab. To establish from experiment a limiting deflection which is critical to cause cracking in slabs at key joints is a complex task. Recent studies have identified the sensitivity of load transfer with respects to deflection in slabs on ground. These studies indicate that at an approximate 1.25mm vertical deflection resulted in zero load transfer across a joint.
Comparison of Danley Key Joint and Connolly Key Joint:
- minimisation of concrete deflection is critical for both short and long term performance
- based on load transfer versus deflection studies 1.25mm deflection serves as an acceptable limit for deflection when comparing performance characteristics
- for tapered key ways it is critical to establish the required deflection necessary to engage the tongue and groove
- for key joints load transfer can only occur when the tongue and groove are engaged
| Deflection required for tongue and groove contact at 5mm joint opening |
Danley Key Joint 0.35mm |
Connolly Key Joint 1.67mm (new profile) |
Summary:
When designing a concrete slab many issues are important (eg. traffic load & sub-grade conditions). When designing for load transfer through a Key Joint, controlling the deflection is vital particularly when sub-grade support fails. Danley Key Joints are designed to transfer load at a very small deflection to ensure adjacent slabs "do share load". Studies have proven if load is not transferred before 1.25mm deflection, then it is not effectively working. Danley Construction Products prides itself on the quality and technical superiority of its products and continuing on the spirit of perfection.
Free Edge Design Considerations:
Further examination of critical deflection criteria and the sensitivity of load transfer in ground slabs indicates that ineffective load transfer between adjacent slabs will result in the necessity to design using free edge design criteria. The Cement and Concrete Association of Australia's publication "Industrial Floors and Pavements" states where inadequate shear transfer is provided at a joint it should be treated as a free edge".
Using Free Edge Design results in the requirement of thicker slabs (eg. edge thickenings) to deal with the large induced flexural stress. This edge thickening typically calculates to be between 50mm to 100mm thicker than the required thickness of the internal slab. Due to Danley's unique Key Joint design it is unlikely that an edge thickening should be required whereas other proprietary products with inadequate shear load transfer would require edge thickenings.
| With the release of AS3600-2001 "Concrete Structures" the rules for flexural crack control of slabs have been reviewed and tightened. Further more the Cement and Concrete Association of the UK have limited slab stress reduction with respects to load transfer in their document TR550 (ie. Limited the effectiveness of adjacent slabs to share loads). Locally consultant Davis Naismith and McGovern Pty Ltd (Vic) have conducted Finite Element Analysis which identified the sensitivity of deflection with respects to load transfer and indicates that at an approximate 1.25mm vertical deflection resulted in zero load transfer across a joint. |
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