Traditional slabs on grade have control joints to accommodate concrete shrinkage strains. It is a fact of life that concrete shrinks as it matures, and there is nothing in practical terms that can be done to avoid it. Good construction practice, astute mix design and the use of certain additives will help minimise shrinkage, however they will not eliminate it.
Traditionally slabs on grade are detailed with control joints at regular centres with the intent of concentrating all shrinkage strain at the joints. In the perfect world therefore, slab panels would not crack between the joints due to shrinkage. Unfortunately however, we do not live in a perfect world.
Control joints are typically installed at 6-8 metres centres in both directions and slab panels are typically kept as square as possible. The aspect ratio of slab panels is very important if the risk of unintended cracking is to be minimized, and special attention must be paid to any acute angles and re-entrant corners.
The control joints most commonly used in Australia are the Keyed Joint, the Dowelled Joint and the Sawn Joint.
Keyed Control Joints are in effect a tongue and groove joint similar to those found in the common floor board. The main difference is that the tongue in a keyed control joint is noticeably tapered so that it does not grip in the groove and cause failure of the concrete above or below the tongue. As the joint opens up therefore, there is no contact between the tongue and the groove and hence there is no load transfer capability from one side of the joint to the other unless the slab panel on one side or the other settles. This is a major shortcoming.
Another problem with keyed joints in that the concrete on the top side of the groove is fragile and very susceptible to damage from certain wheel loads or impact.
Keyed control joints can be formed in the conventional manner using timber edge forms, or they can be constructed using pre-formed metal edge forms. The latter are very popular as concrete can be cast continuously over the joint and the metal form acts as a bond breaker. When timber forms are used, the formwork must be stripped and a bond breaker applied to the concrete face prior to pouring on.
Dowelled Control Joints comprise steel dowels that connect slabs on both sides of a straight joint. The steels dowels in the past were round bars with a bond breaker applied to one end, and in recent times square dowels, flat bar dowels and plate dowels have been gaining popularity. Square, flat bar and plate dowels have far greater load transfer capability than round bar dowels.
The greatest problem with dowelled joints is that the dowels must be installed in a single plane and in true alignment and parallel if they are to work as intended. Any failing in this respect will usually result in a 'locked' joint and the slab will often crack along one end of the dowels.
Several manufacturers are now providing dowels in pre-fabricated cages to overcome the majority of the problems associated with installation, but this comes at a considerable price. Plate dowels by their very nature overcome the biggest problem (alignment) and they have the greatest efficiency with load transfer.
Sawn Control Joints are probably the most popular control joint, and this is because they are the quickest and cheapest to install. They comprise a saw cut installed after the concrete is placed which penetrates the slab to about 25% of its thickness. To ensure the slab is weakened at the location of the saw cut, the mesh reinforcement passing through the joint is typically weakened by cutting every second bar, and sometimes a crack inducer is installed immediately below the intended position of the saw cut.
To be effective, the saw cuts must be installed very soon after the concrete is placed so that shrinkage cracking will not occur before the saw cut exists. If a crack occurs first, the saw cut will be ineffective and all future shrinkage strain will usually occur at the crack. Special soft-cut saws are used to help achieve this requirement. When crack inducers are used in conjunction with saw cuts it can almost be guaranteed that the slab will crack first.
Combined Joints are sometimes specified by engineers. For example there are proprietary dowelled and keyed joints available on the market. The reality is that the key is rendered ineffective by the dowels and there can be no justifiable reason to have both in the one joint.
In traditional slabs on grade with control joints it is absolutely essential to minimise restraint to shrinkage and to do everything realistically possible to postpone the onset of shrinkage.
Blinding or bedding sand, double membranes, compressible isolation etc are some of the details used to minimise shrinkage resistance and thereby give the joints a chance of working as intended. Unfortunately however, engineers sometimes thicken slabs at control joints, particularly dowelled or keyed joints, and this introduces a restraint which has the potential to prevent the joint operating as intended thereby render it useless.
As the concrete in a slab on grade shrinks, it will crack if there is sufficient resistance to shrinkage movement and if there is inadequate tensile strength. Reinforcement mesh is used to overcome this problem in larger slab panels where there is increased frictional resistance. Effective curing also helps as it reduces early shrinkage and thereby allows the concrete to gain tensile strength to resist the tendency to crack.
Joint Free Slabs, as the name infers, do not have control joints and hence the situation is totally different. Joint free slabs use a crack inducer grid to induce fine cracks in both directions at close centres and thereby relieve the shrinkage stresses that build up in the slab. In joint free slabs the frictional resistance of the subgrade is beneficial and there is no requirement to take steps to minimise it. In fact steps are taken to increase it where possible.
Another major difference is that high early shrinkage is a benefit to joint free slabs. This gives the crack inducers a better chance of kicking in before the concrete gains substantial tensile strength.
Overall, joint fee slabs capitalise on many of the features that are undesirable for traditional slabs on grade and that are so difficult to avoid.
