ADVICE on the correct preparation for calcium sulphate screeds was given in my last article in CFJ (September 2011). This month I will focus on the best way to deal with another difficult and frequent substrate problem, laitance.
Laitance is a major cause of failure in flooring installations. It takes the form of a weak, friable layer on the surface of concrete and sand cement screeds which appears after curing.
It is made from cement and fine aggregates that rise to the surface when too much water is added. It may also be due to rain damage during placing, or from over trowelling. Subsequent poor curing of the surface will also contribute towards the formation of laitance.
I Identifying the problem: Surface preparation is the most important step of any flooring installation. Removing surface contaminants, such as laitance, is therefore paramount to a long- term successful installation.
Laitance is always present on new concrete and must be removed. However, sur face laitance is not to be confused with a poor quality concrete or screed that needs addressing in another way.
Laitance comes in varying degrees of thickness, from a fine dust to several millimeters or more, depending on contributing factors.
To determine the thickness of the laitance, flooring contractors should score the sur face of the substrate with a steel edge until reaching the main aggregate.
Proprietary scratch testing equipment is available which can be used as a guide to determining the depth of laitance.
If laitance is left untreated, the application of subsequent materials, such as smoothing underlayments, will have a high risk of failing.
Although it may not be immediately evident, trafficking impact and the stress imposed on the bond line through repeated fluctuations in temperature will eventually cause the laitance inter face layer to delaminate, resulting in the flooring installation failing and costly remedial work.
I Methods of removal: Removing any existing laitance is vital to ensure that subsequent treatments can successfully adhere to the concrete substrate. There are several ways to remove laitance and the method chosen should depend on the working environment and purpose of the floor.
Shot blasting is the fastest and most efficient form of laitance removal, especially in large areas. Shot blasting machines are available in varying sizes making them ideal for use on most surfaces, no matter the size. Using a shot blaster will allow up to 1000sq m of flooring to be prepared in just one day and, as shot blasting is a dry process, floorlaying may continue in other areas of the room while the surface preparation process is taking place.
Mechanical planing is a method often used to remove greater thicknesses of laitance. Also referred to as concrete planing, the machines used carry rows of rotating cutters tipped with tungsten to provide an excellent removal of laitance.
Scrabbling, grinding and abrading are also recommended for removing laitance. Handheld grinding machines, designed for precision, control and safe operation are recommended for use in smaller areas and edge detail.
I Successful preparation: Unless it has been removed by previous sur face preparation techniques, laitance may still be present on old concrete and screeded floors.
Removal is vital to allow a successful subfloor installation and avoid any costly and timely corrective work in the future. For more information and advice on how to remove laitance, contact F Ball and Co or your local concrete preparation centre.
In CFJ January 2012 issue, I will advise on correct surface preparation for wooden floors.
Neil Sanders is technical manager at F Ball and Co.