Expansion joints and screed cracks
There is still great uncertainty among installers, builders and architects, regarding how to deal with expansion joints and screed cracks. As joints are not visualy appealing, the question often comes up of which joints can be sealed into the substrate or absorbed into the surface covering. The answer to that is as simple as this: Functional joints may never be sealed.
The following explains what this means in practice:
Description of joints
Joints and how to deal with them are described in detail, for instance, in the reference books for flooring and wood flooring installers [1,2]. The comments and information sheets on substrate work [3,4] also describe, sometimes at great length, joints in substrate and give advice for dealing with them. And last but not least, joints are described in the generally recognised rules of engineering under the current German DIN standards. There is a section dedicated to joints in the DIN 18560, part 2. In section “5.5.3 Screed joints”, types of screed are listed and their function described.
The list also contains the crack inducer joint, and here may be the cause for the instabilities described. The crack inducer joint, in spite of having the word “joint” in its name, is actually not a joint at all and requires a completely different method of handling than the other joints listed in practicality.
Generally speaking, joints in construction are gaps or spaces between two components that are intentionally planned or due to tolerance.
In particular, joints are characterised by the fact that their edges are not (!) connected to each other by force-fitting. Their respective position in the structure is predetermined by the planner based on the rules of engineering. They allow movements between components without causing negative effects, such as restraints, between the components. To do this, of course, they have to be measured correctly and professionally implemented. The crack inducer joint is different. They are deliberately weakened cross-sections of a component. This includes the well-known furrow, frequently used in doorways. Crack inducer joints cause tensions, such as those from shrinking during hardening of the mortar at defined points in the component, to be relieved through controlled stress crack propagation due to the weakened cross sections combined with their notch effect. Thus crack inducer joints are predetermined breaking points in a component. They could also be described as notches or grooves before ripping open. After ripping open, they are simply cracks and should be treated as such.
From this description, it can be concluded that, as previously mentioned, a crack inducer joint does not fulfil the function of a joint at all. The compound phrase here is clearly misleading. Unfortunately, this also leads some of those involved in building to consider force-fit joint sealing as a possible method for dealing with joints.
Consider this linguistically similar case: - Many people have been riding horses throughout history. But no one would think of putting a saddle on a seahorse!
Significance for practice
Based on the definition of joints, it necessarily follows that these cannot be sealed by force-fitting. Otherwise they couldn’t fulfil their function - movement without allowing damage - anymore and would no longer be joints at all. On the contrary, it must be systematically assessed how to deal with them. Thus perimeter joints for acoustic insulation can be covered with an appropriate profile or a skirting board. For construction joints, there are a number of joint profiles, or they can be sealed with a resilient joint sealant. It’s always important in doing so to avoid a force-fit connection between the joint faces.
Crack inducer joints, like the furrows mentioned above, are recesses similar to grooves. If they haven’t opened after the component has dried and shrunk, they should be sealed with suitable materials, like repair mortar. If they have opened as intended, they are merely cracks that should be sealed by professional force-fitting, e.g. with repair resins or casting compounds.
Once you’ve internalised the fact that crack inducer joints are not joints in the actual sense, but are instead predetermined breaking points, dealing with joints becomes relatively easy. Joints may not ever (!) be bypassed with force-fitting. If their path runs through the designated floor covering plane, they’ll need to be absorbed into the surface covering by appropriate means - whether the planner likes it or not!
(1) K. Remmert, J. Heller, H. Spang, J. Haferkorn, Fachbuch für den Bodenleger, 2010
(2) K. Remmert, J. Heller, H. Spang, K.Bauer/T. Brehm Fachbuch für den Parkettleger, 2006
(3) BEB Hinweisblatt 8.1, Beurteilen und Vorbereiten von Untergründen im Alt- und Neubau,
(4) Arbeitskreis Bodenbeläge im BEB, Kommentar zur DIN 18365 Bodenbelagsarbeiten, 2010