Laitance

Laitance is a weak friable layer of fine particles that can form on the surface of concrete after it has cured and is a common cause of floor failure. It is comprised of hydrated cement and fine aggregates which rise to the surface with the addition of too much water.

It is formed when an abundance of water in the mix creates an upward movement through the concrete. It can also be a result of rain damage during the placing process, or of over-trowelling and excessive tamping, or vibration of the concrete.

The thickness of laitance can be determined by scratching the surface with a screwdriver or other metallic edge until reaching the main aggregate. Thicknesses can vary but are seldom more than 5 or 6 mm. Since laitance has poor adhesion to the actual aggregate in the concrete mix, heavy traffic and impact will delaminate it or cause it to dust away under abrasion.

The removal of laitance is important in order that subsequent materials applied to the floor will properly adhere to it.

Methods for removal include:

• Shot blasting: A fast and efficient method that involves firing abrasive elements on the area at high velocity.
• Mechanical planning: Used for thicker laitance and involving a machine with rows of rotating tungsten-tipped cutters.
• Scabbling: A heavy-duty method that involves compressed air tools which vibrate and impact the concrete surface.
• Grinding: Handheld tools are used for smaller areas and edges.
• Acid etching: The acid attacks and breaks down the the top surface. This should only be used if other options have been ruled out.

NB Short Guide, Lime Mortars in Traditional Buildings, published on 1 March 2013 by Historic Scotland, defines laitance as: ‘The thin layer of fine lime particles that migrate to the surface of lime mortars. Laitance reduces the breathability of the mortar due to its very fine grained and less permeable texture. Laitance is made worse by overworking lime but can be removed by ‘beating back’ with a hard bristle brush, or gently rubbing with an abrasive pad.’

[edit] Related articles on Designing Buildings

• Alkali-silica reaction (ASR).
• Concrete.
• Concrete repair mortars.
• Concreting plant.
• Construction dust.
• Construction tools.
• Curing.
• Efflorescence.
• Flooring defects.
• Lime.
• Lime run-off.
• Mould growth in buildings.
• Scabbling.
• Stain.
• Stratification of concrete.
• The properties of concrete.