Users of floors are becoming increasingly aware of the many issues that need addressing at the time of the design and detailing of a new floor slab on ground. Joints are a major consideration and sometimes the need to minimise or eliminate formed joints may be more important than floor flatness or levelness, for example where floors are highly trafficked by heavily loaded pallet trucks with small solid rubber wheels. To understand the use and positioning of joints it is important to understand the basic factors that cause concrete movements. Movements in concrete which can result in cracking if not controlled can be categorised as follows:
Early Movements Prior to Final Set
Plastic cracking caused through the concrete drying too rapidly. Plastic cracks are discontinuous and random in nature, but can lead to continuous shrinkage cracks at a later age. Plastic cracking is controlled by protection of the slab surface from rapid drying and/or the use of fibres in the concrete.
Movements Commencing 14 Hours After Casting
Differential temperature or differential moisture content through the thickness of a slab can cause the slab to warp (or curl). Heat of hydration effects, which can be significant with slabs over say 200mm thick, will result in a positive temperature gradient from the exposed surface of a slab to the underside causing the slab to bow upwards at the ends. A similar effect will occur as the top surface of a slab starts to dry out after curing has ceased. The heating of pavement surfaces from the sun will cause the ends to bow downwards.
Warping can cause problems by effecting the flatness of the floor and slabs can crack across an unsupported edge under wheel loads. A permeable dry subgrade will reduce moisture warping by allowing drying to take place from the bottom of the slab as well as the top. Conversely pouring on a wet subgrade or on polythene sheet may aggravate warping. Because moisture warping causes upward curling at a free slab edge, the effect of warping is apparent at every movement joint.
Long Term Drying Movements Due to Shrinkage
As the excess water which is used in concrete to give the mix adequate workability evaporates out of the concrete, the concrete shrinks. To this extent, the greater the amount of water in the initial mix, the greater will be the drying shrinkage. The designer can influence this by specifying structural concrete mixes using appropriate compaction methods for low slump concrete (e.g. vibrating screed). By comparison, a pumped concrete mix with a higher slump and sand content could exhibit 50% higher ultimate shrinkage. Vacuum dewatering reduces long-term drying shrinkage further by reworking and compacting of the slab after this excess water has been removed.
Coarse aggregate has a significant role in restraining concrete shrinkage. Maximising the coarse aggregate size allows a lower sand content for a given workability which lowers the water demand. Low shrinkage concrete types are available in most areas utilising high coarse aggregate volumes and natural sands. Where joint openings need to be kept to a minimum, to reduce wear from wheeled traffic for instance, concrete for such slabs should be specified with a maximum 56-day drying shrinkage limit of below 750µm (AS 1012.13:2015 Methods of Testing Concrete: Determination of the Drying Shrinkage of Concrete for Samples Prepared in the Field or in the Laboratory).
Theoretically, shrinkage would not be a problem if there was no restraint existing to prevent the concrete shortening. In a floor slab we have the restraint of the subbase caused by friction with the underside of the slab. If this friction was uniform and limitless, shrinkage would not be a problem either, as the stresses resulting would be transferred straight into the ground and not taken by the concrete. However, neither of these extremes is the real world. The concrete will crack if the stress resulting from the restraint to shrinkage exceeds the tensile strength of the concrete.
Typically, concrete shrinkage is accommodated by allowing the slab to move freely at predetermined movement joints, with allowance between the joints typically using reinforcing steel to avoid uncontrolled cracking from stress buildup due to a frictional restraint. Alternatively, jointless solutions rely on relatively high levels of reinforcement to ensure that cracks resulting from shrinkage restraint are very narrow and at close centres (1 to 2m).
A typical concrete shrinkage of say 7mm in 10m length could be reduced to around 4.5mm due to the restraint of the reinforcing. For a 100 - 150mm thick slab drying outdoors, 50% of this could be expected to take place in the first four months drying and 70 - 90% after twelve months. A wet winter, however, will slow the rate of drying and hence the initial shrinkage rate significantly. Floors indoors are likely to shrink at a faster rate, particularly if the environment is air conditioned. Curing of concrete will not effect the shrinkage potential to any extent, however it will optimise concrete strength gain to resist cracking from shrinkage restraint.
There are a number of design options which cater for concrete movement. The appropriate option will consider the client's brief in conjunction with both construction costs and maintenance costs.
A concrete floor slab has to be subdivided into small areas for two reasons:
- To control tensile stresses due primarily to moisture change and thermal contraction of the slab, and thus to limit random cracking.
- For convenience during construction. The size of the area may be governed by practical considerations arising from the method of construction and resources available.
There are two primary types of joint used: free movement joints and tied joints
1. Free Movement Joints
Isolation Joints: These joints permit horizontal and vertical movement between abutting elements, allowing the elements to behave independently of each other. They should be provided between a pavement panel and fixed parts of the building (such as columns, walls, machinery bases, pits, etc).
Isolation joints should also be provided at the junction when an extension is being made to an existing pavement, and at junctions between internal and external pavements, to prevent the development of stresses that may result from differential movements.
However, provision for load transfer may well be required where such extensions occur and the designer needs to address the detail necessary to achieve this. Isolation joints are generally formed by casting against a compressible, preformed filler material (eg self-expanding cork) over the full depth of the joint to provide a complete separation.
Expansion Joints: Expansion joints are used in pavements to provide for thermal and moisture-induced movement of the slab. However, these joints may also be required in areas or rooms subject to large temperature fluctuations. Designers should satisfy themselves that there is a definite need for expansion joints, thereby minimising their unnecessary installation and the relatively wide gap required between panels.
In many instances expansion joints will not be required because the drying shrinkage is the dominant linear movement. These joints within pavements require the provision of load transfer, usually by the provision of dowels fitted with a cap to accommodate the moving dowel as the joint opens and closes.
Contraction Joints: Contraction joints control the random drying shrinkage cracking of concrete by inducing the slab to crack at the contraction joints. They allow horizontal movement of the slab at right angles to the joint and act to relieve stresses which might otherwise cause random cracking. In order to ensure that shrinkage cracking occurs at a contraction joint, a plane of weakness must be created by forming (using crack-inducing tapes or formers) or cutting a groove to a depth of at least one-quarter of the slab thickness. However, if the cut can be formed early enough, by a suitable grooving tool or early-age saw cutting, some reduction in the groove depth may be warranted.
The spacing of contraction joints in jointed unreinforced pavements should be selected to suit the geometry of the pavement being constructed, but should be such that the joint movement does not mean that load transfer by aggregate interlock is lost. If it is, load transfer has to be maintained by dowels or other devices such as sleeper beams. Otherwise the slab thickness should be designed as a free edge.
Contraction joints are usually constructed either by forming a groove in the top of the freshly-placed concrete (Formed Joint) or by sawing one in the panel after the concrete has hardened but before uncontrolled cracking occurs (Sawn Joint).
Formed Joints are constructed after the concrete has hardened sufficiently that it will not be damaged by the sawing, but before shrinkage cracking can occur. The appropriate time for sawing varies with the many conditions, eg concrete strength and ambient temperature, that influence the hardening of concrete. The initial saw cut should be 3 to 5mm in width. If required, for the installation of a joint sealer, the joint can be widened later.
Sawn Joints are constructed after the concrete has hardened sufficiently that it will not be damaged by the sawing, but before shrinkage cracking can occur. The appropriate time for sawing varies with the many conditions, eg concrete strength and ambient temperature, that influence the hardening of concrete. The initial saw cut should be 3 to 5mm in width. If required, for the installation of a joint sealer, the joint can be widened later.
2. Tied Joints
Tied joints are used to:
- restrict the movements at the joint in unreinforced pavements
- provide relief for warping stresses in reinforced pavements
Keyed longitudinal joints should be held together with deformed tie-bars. However, such tie-bars should not be used in panels with a total width of more than 10m unless dowelled longitudinal contraction joints are also provided at a spacing not exceeding 10m.
The tie bar spacing relates to the overall design of the slab between free joints. Typical bars and centres used are D12 at 300mm for slabs up to 150mm thick, or D16 at 350mm for slabs over 150mm.