As the name implies joint free slabs feature none of the shrinkage control joints that are typically found in traditional slabs on grade (see Control Joints).
Joint free slabs use a plastic grid (left) to induce a closely spaced network of fine cracks throughout the entire length and breadth of large-area slabs on grade, and thereby eliminate all traditional formed and sawn shrinkage control joints.
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.
It stands to reason that the more cracks there are on a closely spaced regular grid, the finer they will be (right).
The crack inducer grid used for joint free slabs in Australia 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 crack inducer grid used in Australia 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.
Only a light mesh reinforcement is required with joint free slabs, and for most slabs F62 mesh is specified. This is sometimes increased in applications where finishes are not to be applied to the slab surface, and heavier mesh will generally be required for thicker slabs.
The full perimeter of each joint free slab pour is trimmed with 2N12 bars and special attention is paid to the trimming of re-entrant corners and penetrations.
Traditional bar chairs are generally only required around the perimeter of a slab and at penetrations. The one metre spacing of the grid junctions is generally adequate for the support of the broad area of the reinforcement mesh.
The objective with joint free slabs is to produce only fine cracks. When everything works as intended crack widths in the order of 0.3 to 0.5mm are achieved. However, in reality not all crack inducers always kick in and therefore some cracks may be wider than this. Based on observations and experience to date it is reasonable to expect cracks to be generally less than 1mm wide, and with the new profile crack inducer grid the maximum crack width should be closer to 0.5mm.
The maximum pour size is generally only limited by the ability of the concretor to place and finish the concrete and the availability of locally produced ready mixed concrete. Typically a single crew is provided to place and finish the concrete, and a comfortable pour size for a single crew is 1,000 to 2,000 square metres.
With proper planning pours can be programmed for a twenty four hour cycle, and in such circumstances pour sizes closer to 1,000 square metres seem to be preferred.
joint free slabs are particularly suitable for applications that are to have finishes such as vinyl or ceramic tiles applied. Such applications include supermarkets, retail centres, schools, hospitals, sports halls and large residential projects (see Project List). It is with these slabs that the savings in construction cost and construction time is maximised.
The list of suitable applications has grown with the introduction of the current crack inducer tube cross section. With this crack inducer the outcome is far more predictable.
A fair to high degree of random cracking often occurred with the original crack inducer tube profile and although the result was more than satisfactory for floors with applied finishes, it was considered by some to be less than satisfactory for slabs with no applied finishes.
It must be appreciated that the acceptability of the finished appearance is a very subjective issue, especially in circumstances where there are to be no applied finishes. It is recommended that clients be made fully aware of the anticipated outcome before they commit to using the system. The potential benefits of faster construction time, lower construction cost, lower maintenance cost, superior termite resistance and superior aesthetics of applied finishes will normally weigh heavily into the decision process.
The direct savings in construction cost can be substantial. They are maximised when slabs are to have finishes such as vinyl or ceramic tiles applied, and they emanate from all or some of the following:
In Australia the savings vary from job to job and from builder to builder. However, typically they are estimated to be in the range AU$20 to AU$50 per square metre for retail floors. The savings are significantly less for slabs with no applied finishes, however there is a growing popularity in the use of joint free slabs for basement car parks where the significant benefit is reduced construction time.
The savings in construction time are equally as impressive as the savings in construction cost:
Joint Free Slabs Pty Ltd has been advised by a representative of the largest supermarket chain in Australia that they spend more each year on the maintenance of control joints than any other item in their stores. Deletion of the joints and all associated cover strips and flexible fillers completely eliminates this expenditure.
Control joints in industrial floors are usually the first, and often the only, region of failure. They fail under the impact action of solid wheeled fork trucks and pallet movers, and for a variety of other reasons. Once the suitability of joint free slabs for industrial floors is established beyond question, the potential savings in maintenance expenditure will be huge.
The joint free slab system is now well established in Australia, and it has been used in a wide variety of projects since mid 2000.
Within just twelve months of its launch, joint free slabs received industry recognition in the form of an excellence certificate awarded by the Concrete Institute of Australia at its biennial conference in Perth in September 2001.
By October 2003 almost 100,000 square metres of joint free slabs had been placed in Australia and the system is being repetitively used in projects for large national retailers such as Woolworths, Coles, Bi-Lo and Officeworks.Joint Free Slabs Pty Ltd