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Failure of Adhesion on Bonded Floors
As stated in the name, this is when flooring experiences horizontal or vertical movement. It may be more appropriate to call this problem failure of bonded floors, as the problem is not always caused by poor adhesion.
Failure of adhesion can come about in many ways, each of which is avoidable if good practice is followed. The failure of adhesive bonded floors can be categorised in four basic ways.
Failure due to:
Abnormally high expansion of the wood flooring due to excessive environmental moisture or RH, or moisture content of flooring is too dry for the environment in question.
Excessively high levels of shrinkage of the wood flooring due to excessive environmental heat or dryness or even when moisture content of flooring is too high for the environmental conditions.
Abnormal vertical pressure caused by normal expansion where inadequate expansion space has been provided
Poor adhesion of the wood to adhesive, adhesive to substrate, or failure of adhesion of layers within the substrate, or cohesion within a mineral base.
Because bonded floors can fail due to exposure of wood to excessive moisture without any other contributing problem, when trying to identify the cause of failure of bonded floors the problem of excessive moisture should first be investigated.
You must check if the individual flooring elements cupped or warped in any way as these show potential excess of environmental moisture.
Where individual flooring elements have cupped there will invariably be a problem with moisture. Flooring elements can be uplifted and moisture readings taken to check for abnormally high results, or abnormal variation between the upper and under sides of the product. Moisture readings of the sub-floor itself can also be checked.
Where no moisture problem is found, adhesive bonded floors may still fail because inadequate or complete lack of space for expansion has been provided in the installation. Factors which affect the amount of expansion a floor will include timber species, size of the floor and the moisture content of wood / RH of environment.
Where floors expand and meet an obstruction such as a wall, structural support, or hearth, horizontal pressure builds up on the obstruction and horizontal and vertical pressure builds up on the floor. Eventually the vertical pressure exceeds the adhesion strength resulting in the failure of adhesion, the floors lifting or buckling, or flooring elements simply becoming loose.
Does the floor still have space for expansion where it meets obstructions such as walls, door frames or structural supports for example.
Large floors may require greater expansion space than can be provided where floors meet obstructions.
Failure can come about where the moisture content of the flooring and the ambient conditions (temperature / Humidity) are incorrectly matched. BS8201: 1987 recommends a 15-19% mc for wood flooring used in unheated environments and 10-14% moisture content for intermittently heated environments, 9-11% for continuous heating, (e.g. Typical domestic)
What are the ambient conditions of the installation? (below 30% or above 70% RH)
What was the moisture content of the flooring materials at the time of the installation?
Were the materials acclimatised?
Where excessive environmental moisture or inadequate provision for expansion are not at fault for contributing to this problem, poor adhesion will be found somewhere within the floor
In order to identify the problem more specifically, a loose flooring element can be taken up and the point of the failure identified. It is important to note that any of the following points of failure can occur in combination with the previously mentioned causes. It is therefore necessary to assess these factors first before identifying poor adhesion as a cause.
Points of adhesion
Wood is not attached to adhesive
Unevenness of the sub-floor caused poor, or no contact between flooring and adhesive.
Late placement of the flooring element causing poor adhesion.
Finish on the underside of flooring has come away from wood
Adhesive has not completely cured, due to poor mixing of the adhesive components.
Contaminant on the underside of wood creates a barrier to good adhesion.
Moisture and alkaline condition of the sub-floor causing SBR adhesive to re-emulsify
Wood and adhesive are attached, but the adhesive is not attached to the surface of the substrate
Failure to create a key or prime surface of the substrate where necessary.
Contaminant on the surface of the substrate act as a barrier to good adhesion.
Failure of adhesion has occurred between layers in the substrate (e.g. smoothing compound to DPM, or DPM to mineral base etc.)
Poor adhesion the between layers, due to a lack of key, or surface contaminant. The chosen sub-floor type may not suitable for direct adhesion.
Wrong type of adhesive for the product type (e.g. inflexible adhesive on very wide plank solids). Wrong type of adhesive for conditions of use.
The flooring element is attached to the substrate, but cohesion within the mineral base has failed
Poor cohesion of the mineral base caused by a lack of hydration or accelerated drying for instance.
Wrong type of adhesive for the product type.
Wrong type of adhesive for conditions of use.
Points of Adhesion
A. Flooring element which may be bonded direct to mineral base in some cases)
B. Finish on the underside of the flooring element
D. Surface epoxy DPM
E. Smoothing / levelling underlayment
F. Existing mineral based sub-floor
Cohesion of sub-floors
Concrete hardens not by drying but by a process known as hydration. During the chemical reaction, calcium silicates form two new compounds. The principle new compound is Calcium Silicate Hydrate Gel (C-S-H). This compound results in the engineering properties of concrete including its strength. The cohesion of concrete and sand-cement sub-floors develops over a period of time whilst simultaneously moisture-curing. Once the RH is below 80%, the hydration process of the floor virtually stops and strength development stops with it. Trying to accelerate the drying of the mineral base by heat and / or dehumidification can lead to a reduction in the development of cohesion and can contribute to floor failure, especially in thin screeds. Inadequate cement used in making screeds can also result in poor cohesion
Where direct bonding to mineral bases, a flexible adhesive may reduce the transfer of stress to the sub-floor from movement of wood floor covering.
Shows how the strength development of concrete depends on the length of moisture-curing period.
Provide additional expansion spaces where necessary.