Why Is It Important To Remove Laitance?

Posted in Firwood Hints and Tips by Firwood Paints on February 11st 2016

Before Applying Concrete Floor Paint, Why Is It Important To Remove Laitance?

Laitance is always present in new concrete surfaces, and it is important that you remove it whenever possible. If you fail to remove all traces of laitance when painting floors, then that can lead to a premature failure of the floor's coating, which can ruin the application of even the best concrete floor paint.

What Is Laitance?

All laitance is formed from the mixture of water, cement and the fine particles of the concrete mix that is brought to the surface when placing and trowelling up. As the concrete cures, this mixture dries to form a crust, or a thin layer, on the surface of the cement – this layer is what is known as laitance.

Laitance itself can be a variety of different thicknesses, from the barely measurable to the extreme worst cases of 6mm or more. The only real way to determine the thickness of the laitance is to score the surface with a metallic edge – either a screwdriver or a coin – until the main aggregate in the mixture is reached.

Why Do You Need To Remove Laitance?

Laitance has a poor level of adhesion to the actual aggregate in the mix. It is comparatively weak in strength and will, therefore, delaminate under heavy traffic conditions and impact. It can also just dust away under abrasion from traffic.

Therefore, for both security and practical reasons, the new floor surface must be bonded to the mass of the aggregate in the base upon which it is laid. The heavier the use of the floor, and the greater the temperature fluctuations that the floor itself is subjected to, the more essential this is. Laitance is a major cause of dusty and damaged concrete floors, and if you fail to remove it, it may lead to the comprehensive failure of the surface treatment.

The most common methods of surface preparation for concrete floors include:

• Acid Etching;
• Dust-Free Grit Blasting;
• Mechanical Planing;
• Scabbling;
• Grinding;
• Abrading;


Which Is The Most Effective Method Of Laitance Removal?

Floors and their uses vary enormously, and therefore it is important that you judge each method on its own individual merits. There is no single-best choice, and you need to take the entire working environment into account before deciding on a particular method.

For example, scabbling may be a suitable method of preparing a concrete floor in an unoccupied site, though the resulting dust and noise that this method creates is unsuitable for many situations. Similarly, acid-etching is one of the most effective methods, but is completely unsuitable for those areas where the corrosive fumes may attack bare steel parts in engineering work.

Acid-Etching

This method of laitance removal is extremely effective in those areas where the laitance itself is very thin and the base in question is not too porous. It is extremely rare that thicker levels of laitance are effectively dealt with via chemical etching when you're applying new surfaces. It should be carried out only after other methods are considered and rejected as viable options.

However, if carried out properly, it can be fast and extremely cost-effective.

There are three main reasons that acid etching is used:

• To remove laitance;
• To provide a slightly textured surface for better adhesion (normally in preparation for concrete floor paints);
• To clean the substrate;


The acid itself attacks the cement content on the top surface, breaking it down and helping with its removal. For further information, refer to the application instructions for Firwood 2709 – Concrete Etching Fluid.

Dust-Free Grit Blasting

To date, this is the fastest and most efficient form of old and new floor preparation and laitance removal. The machines used vary in size and are generally operated by specialist concrete preparation contractors or flooring contractors.

Horizontal and slightly inclined surfaces are impacted and abraded by steel abrasive propelled at high velocity by the machine. This is a dry process and floor-laying may often continue whilst preparation is progressing in adjacent areas. In many cases, old coatings and other contamination may be removed by this method. The treated surface will be somewhat coarser than an acid etched finish.

Dust-free grit blasting is far quicker, quieter and cleaner than mechanical planing, scabbling, etc. The few disadvantages are "tracking" and the general inability of the machine to remove exceptionally thick, hard or flexible materials e.g. certain epoxy or polyurethane floor surfaces. "Tracking" is the effect of the machine overlap from one strip of prepared floor to another. It occurs more frequently on floors which are less hard or have thicker laitance than normal. It is seldom a problem when the thickness of the coating to be applied is in excess of 3mm.

Mechanical Planing

Often referred to as "concrete planing", the machines used carry rows of rotating cutters tipped with tungsten. The removal of laitance and other forms of contamination is excellent. The profile left by the machine is dependent on the spacing and type of cutters installed by the operator.

The surface may be grooved or flat and is more roughened than with dust-free grit blasting. This system is more frequently used for preparation prior to the thicker (6mm or more) floor coatings being placed. Greater thickness of the substrate can be removed more quickly and effectively than by grit blasting.

Mechanical planing is slower, noisier and nearly always very dusty. The addition of vacuum suction cleaners to the machines does help to reduce dust but seldom eliminates it. "Tracking" may also occur depending on the depth of cut.

Scabbling

This is a heavy duty method of preparing concrete frequently seen on concrete motorways. Scabbling tools are driven by compressed air and the tool head is tipped with tungsten. The tool works by vibrating and impacting the surface of the concrete, thus shattering the surface as it works.

This system is not suitable for preparation if the floor system being applied is less than 10mm. Scabbling is a slow, very dusty, very noisy and dirty process. On certain surfaces it has the disadvantage that it can damage the concrete or screed to the degree that it weakens the substrate too much. This process is now less frequently used for internal work on floors.

Grinding

Grinding is usually carried out by machines used for polishing terrazzo. It is a useful method of preparing a level floor to remove laitance and expose aggregate in a substrate.

Grinding is slow and laborious. If dust is to be restricted the operation must be wet. The resultant slurry may also be ground into the surface, which unless very thoroughly cleaned or high pressure hosed away and re-etched, will adversely affect adhesion of a coating. Delays may also be expected while the substrate is allowed to dry.

Abrading

There are a number of other methods of abrading by blasting or using abrasive papers, etc. Their uses are limited and are not generally are not generally used for larger areas.

Correct surface preparation is essential to any painting project to ensure that not only is the project is a success but that the results lasts for years to come.

For The Best Concrete Floor Paints, Get In Touch With Firwood's Professional Team Today!

Here at Firwood, we are proud to offer a diverse range of concrete floor paints for warehouses and industrial properties of all kinds. If you require any advice on dealing with any volume of laitance, get in touch with our team of experts today on 01204 525231.

Alternatively, you can email any questions or concerns you might have regarding our high-quality concrete floor paints or any other product that we offer, to websales@firwood.co.uk.

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