Shear dowels in joints in concrete stabs on ground have long been recognised as being essential for construction efficiency by reducing concrete thickness and minimising the potential for random cracking.
Plate dowels have been shown by research to be the most effective type of shear dowel. Plate dowels use 6 mm or 10 mm steel plate, in place of traditional round or square bars, which were usually 16 mm or 20 mm. Diamond Dowels are plate dowels rotated to a diamond configuration, for use in construction joints.
Diamond Dowels were first used in the USA in 1998, and were introduced in New Zealand by Lesa Systems in 1999. They are now successfully used throughout the world, with total sales approaching 10 million.
When first introduced, Diamond Dowels had a two component sleeve system, with a separate sleeve and base. The Mk II version, introduced in the USA last year, combines the sleeve and base into a single moulding, for quicker and easier use. The Diamond Dowel Mk II was first used in New Zealand by Savour Construction of Auckland, and their site manager's response is reported here. Diamond Dowels have proved to be the best and most cost effective dowel system available for almost all installations.
They have three specific advantages:
- The wide steel plate reduces the critical steel/concrete bearing stresses and allows wider dowel spacing and fewer dowels; the tapered shape allows full freedom of movement in the horizontal plane and reduces concrete stresses; the thin plastic sleeve, combined with the wide plate, minimises compression within the plastic sleeve, reducing slab deflection and concrete stresses at joints.
- Diamond Dowels have been fully tested and the test results were completely consistent with the initial mathematical analysis, which highlighted the importance of wide dowels. The test results also showed that thicker plastic sleeves on narrow dowels substantially reduce load transfer and will increase concrete stresses.
- Diamond Dowels are a one size fits all dowel system. You just change the centre spacing to suit the slab thickness. There are no more different length & diameters to confuse contractors from. This also means if you over order, you can use them on the next job.
The Diamond Dowel Mk II is another step forward in the efficient construction of concrete slabs on ground, introduced in New Zealand by Lesa Systems Ltd and also available through Markham Distributing.
THE NEW DIAMOND DOWEL -A CONTRACTOR'S VIEW
The two piece Diamond Dowel was good, but the one piece unit is much more effective as it is quicker to install, and doesn't separate from the timber or itself. The two piece product would separate from time to time which could cause problems.
Eighteen months ago, I started the Yamaha NZ Warehouse and Offices project and I said to Carl Wickins of Lesa Systems, "Why don't you have a one piece system for Diamond Dowels?"
Carl said, "Give me a month to fly the new one in from the United States. You will be the first to use the new type in New Zealand", and we were.
The one piece Diamond Dowel was excellent. Well done, Lesa Systems.
Now we are building Panasonic New Zealand's new warehouse, and we are trialling the New Zealand made one piece dowel. This is also excellent.
All of the above shows me that Lesa Systems have an excellent working relationship with their clients and are interested in improving their products to keep us happy and to retain their market share.
At Savory Construction we are looking to shorten contract time, but not to sacrifice quality at any expense.
Sandy McIntosh, Site Manager.
DIAMOND DOWELS TESTED
The performance of Diamond Dowels has recently been tested for load capacity in comparison with other concrete floor joint dowel systems. The test was carried out by the Queensland University of Technology in conjunction with Danley Construction Products Pty Ltd of Brisbane.
Concrete samples 150 mm thick were cast with pairs of dowels passing through a joint across each sample. The samples were load tested to a pre-determined deflection of 1.25 mm. This deflection dimension was calculated by consulting engineers Naismith & McGovern Pty Ltd (Vic) from finite element analyses.
The test results, which show the loads resisted by the pairs of dowels, are shown in Table 1 below.
Table 1 Summary of Final Full-Scale Testing of Different Dowel Systems
- The highest load was carried by the Diamond Dowels. This was closely followed by the load carried by 10 mm Load Plate dowels.
- The tests give a warning in relation to the use of dowels in thick walled plastic sleeves. The types of round dowels tested were:
"Without sleeve" - the Australian equivalent to the Lesa Roundowel. These show a high load capacity. "With Sleeve" - the Australian equivalent to Lesa Speed Sleeves. These show a 50 % load capacity reduction.
Clearly, the capacity of dowels in thick walled plastic sleeves is significantly inferior. This category includes Lesa Speed Sleeves and almost certainly Speed Dowels, which have even thicker walls, and may he worse again in performance. Such products should only be used at quite close spacings. The Lesa dowel spacing chart is being modified in accordance with the findings of the QUT.
Instructions For Use
System consists of
1 Sleeve, including base and nails
2. Steel Plate
Mark the formwork for mid depth of the slab and dowel spacing (eg 600crs fro 150 slab)
Nail Diamond Dowel sleeve to formwork using nails provided
1. Mark dowel locations on
formwork to identify position
during concrete placement
2. Ensure adequate vibration
of concrete adjacent to
formwork, but avoid striking
Diamond Dowel sleeve
After formwork is stripped, insert steel plate through protective tape into slot in concrete.