The principal objective with joint free slabs is to dissipate the effects of drying shrinkage as uniformly as possible in the form of fine cracks throughout the entire area of the slab, and to produce only fine cracks that do not adversely affect applied finishes. Induced cracks in joint free slab
It stands to reason that the more cracks there are on a closely spaced regular grid, the finer they will be.
The crack inducer grid used for joint free slabs is a one metre square grid, and a sufficient number of slabs have been completed to show that this is adequate to achieve the principal objective. The cracks produced by this grid are sufficiently fine to have no detrimental affect on applied finishes that are installed in accordance with the specification, and hence they do need to be treated or reflected through the finishes.
With joint free slabs, therefore, it is not only possible to save a lot of cost and time by the deleting of the control joints and their cover strips, but it is also possible to achieve a continuous appearance of the applied finishes.
Concrete compressive strength should not be over specified, as not only does higher strength concrete cost more, it also has a higher drying shrinkage and this will increase the general crack width.
Within reason, everything possible should be done to achieve a low drying shrinkage. This starts with the specification and use of the best available raw materials, and it finishes with the adoption of best practice in the mixing, placing, compaction and curing.
Additives can also be used to assist with placing and finishing and these can have a positive effect in the reduction of drying shrinkage. It is essential to evaluate the benefit of any additives against the cost, as in some cases the effect is absolutely minimal and very difficult to justify.
The use of super plasticisers is recommended especially when very large areas are to be placed in a single pour, as they make the placing and finishing easier and they should have the added effect of reducing drying shrinkage.
It is extremely beneficial to the joint free slabs system to achieve high early shrinkage of the concrete. With high early shrinkage the crack inducer grid gets a chance to "kick in" before the concrete attains significant tensile strength to resist cracking. Methods adopted to increase the early shrinkage should not however compromise the integrity or durability of the concrete.
THE JOINT FREE SLAB CRACK INDUCER GRID
The crack inducer grid used in New Zealand is a one metre square grid comprising extruded plastic tubes and plastic four-way junctions. The plastic tubes are supplied cut to length for the one metre square grid, and the grid is assembled on site with the tubes fitting snugly onto the junctions.
There are currently two different sizes of crack inducer grid, a 30mm high crack inducer for 90-110 thick slabs and a 38mm high crack inducer for 120-135 thick slabs. New sizes will be added to the range for thicker slabs when there is adequate ongoing demand.
The junctions double as bar chairs to provide support for the reinforcement mesh. The height of the junctions has been set to achieve a minimum 40mm cover in 100 and 125 thick slabs.
The original crack inducer tube was circular in cross section and a fair to high degree of random cracking occurred with this grid. The current crack inducer tube has a tear-drop shaped cross section and it is a far more effective crack inducer.
- Thinner Slabs through simplified detailing
- Full engineering design and support provided
- Substantial savings in construction for all slabs
- No sawn control joints means no filling of joints used to cast tilt panels
- Lighter reinforcement due to nature of Joint Free Slab crack inducer grid
SOME OF THE PROJECTS THAT HAVE SPECIFIED AND USED JOINT FREE SLABS
- Windsor Park Resthome - Gore
- Youth Justice Centre - Christchurch
- Adventure Tourism - Christchurch
- Dunstan Hospital - Alexandra
- New World Supermarket - Matamata, Hamilton, Stanmore Rd Christchurch, Kaikohe, Nelson, Turangi, Morrinsville, New Lynn, Mt Roskill, Tokoroa, Lunn Ave Auckland, Opotiki, Tepuke, Kemeu, Browns Bay, Whangaparaoa, Te Kuiti, Papakura
- Apata Ltd Coolstore - Tauranga
- Keri Keri Primary School - Keri Keri
- Mitre 10 - Greymouth,
- Heinz Watties - Napier
- Leisureplex - Palmerston North
- Super Cheap Auto - Levin
- NZ Police Skid Track - Lower Hutt
- Countdown Supermarket - Levin, Whakatane, Moorehouse Rd Christchurch, Nelson, Katare, Pukekohe, Hornby, Te Awamutu, New Plymouth, Tokoroa, Te Rapa, Rangiora, Rolleston, Kati Kati, Keri Keri, Kelvin Grove, Silverdale, Manurewa, Tawa, Petone, Churchill Drive Wellington, Ferrymead, Amberley, St John, Whitianga, Buretta Park, Orewa, Spitfire Square, Hobsonville, Hamilton East, Rotorua, Dinsdale, Mt Eden, Mosgiel, Beachlands
- Rotorua Events Centre - Rotorua
- Video Ezy - Rotorua
- Huntly Power Station - Hamilton
- Fresh Choice Parklands - Christchurch
- Westfield Shopping Centre - Albany
- ABC Learning Centre - Kaiapoi, Rangiora
- Massey University Vet Clinic - Palmerston North
- Supervalue Supermarket - Geraldine
- Food Stuffs Distribution Centre - Auckland
- Meadow Mushrooms - Christchurch
- Geraldine Preschool - Geraldine
- Dannivirke Transfer Station - Dannivirke
- Chateau on the Park - Christchurch
- Wiri Prison - Auckland
- Marlborough Girls College - Blenheim
- KMart - Nelson, Te Rapa, Petone
- Te Awamutu Library - Waikato
- Village Exchange - Hastings
ARCHIVES NZ — DEPARTMENT OF INTERNAL AFFAIRS, WIGRAM — CHRISTCHURCH