Joints vs Cracks

How to identify and deal with joints and cracks is a common issue faced by flooring installers, whether or not to cover them, can lead confusion when a customer requests an aesthetically pleasing floor without any unsightly breaks in the surface of the intended floorcovering. Cementitious floors of a certain size will have joints designed and incorporated into them by an Architect or Structural Engineer.  Incorporating joints makes for easier placing and allows space for the natural expansion and contraction of concrete floors.  There are various different types of joints to consider; Construction, Expansion and Stress induced joints are some of the most common joints a Flooring Installer will have to address.  In essence, these planned joints are to prevent, or minimize cracks in subfloors due their natural expansion and contraction.   


Slabs in long narrow strips or large bays are separated by Construction Joints.  They provide suitable bays in which the material can be placed in any one given day; they are also referred to as day joints. On large scale industrial floors, Construction Joints may have a tooled angled edge and are finished with appropriate semi-rigid filler, dependent on the end users’ requirements.  In domestic locations slabs are often poured in one day negating the need for construction joints.  If present however,  they may be located in doorways which are considered a weak point due to the effects of drying shrinkage where they double up as a Contraction Joint. 


Expansion Joints allow for thermal movement in concrete and other building materials, typically they run through the entirety of a building structure, they are generally wider than Construction Joints and are filled with a compressible material to allow for natural expansion and contraction due to temperature changes.


To prevent random cracking, Stress Induced Joints or Control Joints as they are sometimes referred to, are cut into fresh concrete. They are usually machine cut to a depth of approximately 25% of the slab thickness.  Control Joints are planned cracks; they create a weak point in the slab in which a crack forms underneath the cut as a result of natural drying shrinkage.  As opposed to random cracks, planned cracks can be neatly and efficiently repaired.


Like joints, cracks come in various different guises. Cracks can form in slabs, whether it is in a plastic (still wet) or hardened state.  Slabs experiencing high evaporation rates can produce Plastic Shrinkage Cracks, these occur when the slab is exposed to extreme conditions at its surface such as high temperatures.  When a slab is in its drying phase it shrinks due to the loss of water, if this movement is restricted in any way a crack will form at this point, for example on re-entrant corners. Unlike Plastic Shrinkage where the cracks usually stop short of all edges, Drying Shrinkage cracks run right across the width of the slab and also right through the depth due to the tensile stresses created at these points.  A Structural crack will run through the whole depth of the slab, this can happen as described above at re-entrant corners, or if a newly poured slab experiences stress from above in terms of excessive weight being applied too early when the slab is still weak.  

Most floors in commercial buildings are left as a wearing surface, Construction and Expansion joints here do not pose much of an issue as they rarely have a covering installed on them.  If a floorcovering is to be installed, then joints like these must be brought through to the surface of the floorcovering material.  Once you place a rigid material like a smoothing compound over any of these joints then a crack will form and mirror through onto the surface of that material.  This occurs due to the rate of expansion and contraction, or the Thermal Coefficient, of concrete.  Unlike Drying Shrinkage this physical behaviour is constant as individual slabs can move independent of one another due to ambient temperature fluctuations.  

In domestic situations cracks in subfloors tend to be the most common complaint Flooring Installers are faced with, and here at Uzin we have many different systems to address such problems.  Whether repairing a Structural crack with a resin system, or bridging a Construction Joint in a doorway with a renovation fleece, Installers can rest assured that Uzin has a system to overcome these challenging issues and will help to provide a successful installation. Find more information here.                                 

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