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
Featured articles and news
London Build Fire and Security Expo
20-21 Nov and now with new Ambassador Programme..
The Scottish Building Safety Levy
Eight weeks of consultation closing on 18 November.
The grey, the brown and the golden rules of housing
shifting policies from the wild west of housing development.
Future proofing homes that are fit for purpose
Specification challenges and the role of plastic.
Thousands of new homes unlocked for brownfield sites
£68 million to 54 councils for neglected land into new homes.
Roof terraces and higher-risk buildings
Context, review, interpretation, case and guidance..
Sustainable Futures. Redefining Retrofit for Net Zero Living
More speakers confirmed for BSRIA Briefing 2024.
Architecture in Britain and Ireland, 1530 - 1830
Steven Brindle’s book is required reading.
Employment Rights Bill; making work pay
‘Biggest uplift in employment rights law since the 1970s’
Battle for Mill Road Free Library
Years of failures by the county and city councils.
The accolades that demonstrate and recognise outstanding achievement.
What is the Conference of the Parties ?
The who, where, what and when before no 29.
CIOB signs up to Green Skills At COP campaign
In preparation for COP29 on 11 November in Azerbaijan.
2024 ECA Industry Awards evening
Full list of electrical contractors scooping top prizes.
Briefing on the implications of the final Grenfell inquiry report
What it means for Architectural Technology professionals.
CIOB Art of Building photo contest 2024
International showcase for the very best photography of the built environment.