GUIDELINES FOR WINTER WORKING
Whilst the best advice is to wait until the weather poses less of a risk, we understand that might not be viable. When using a lime-based mortar during the winter months, careful precautions should be undertaken to ensure that the mortar cures correctly and with enough freeze-thaw protection before being left to the mercy of the elements.
Which Lime is best?
Hydraulic lime mortars At temperatures of 18-20°C and an ambient relative humidity of between 55 - 80%,these will take approximately 90 days to achieve an appropriate resistance to the freeze-thaw cycle.
Just a 5°C drop in temperature will increase the time to achieve performance values by about 25%. A 10°C average difference will result in a 50% extension of curing time.
Lime putty mortars rely on being repeatedly dried and dampened by changes in ambient relative humidity. From at least November to March the relative humidity is usually a constant 100% so do not use lime putty mortars for exterior work much after early October.
Formulated Limes offer distinct advantages during winter. They are based on NHL limes with additives to improve performance. In order to achieve frost resistance the compressive strength needs to reach 2MPa. After 7 days a formulated lime will reach 3.4MPa, meaning it has achieved frost resistance. (based on 1 formulated lime:2 sand mix)
Freeze-thaw cycles can only occur when materials become saturated to the extent that their pore structure is full of water with less than 9% of the available pore structure available to handle the expansive forces of water as it freezes. Much of this can be alleviated by protection from wind driven rain by covering the outside face of the work and by providing good sound detailing and roof drainage.
If it’s not wet, it can’t freeze!
If works must continue through winter months we suggest a regime whereby the best possible curing conditions can be sensibly employed. While the following points refer to exterior work, the requirements of mortar for interior work are identical but, hopefully, easier to achieve.
- Ensure that any scaffolding is adequate to support protective sheeting (and resulting potential wind loadings – scaffolding sheeting represents a very large sail area!)
- Safeguard against vertical and horizontally driven rain by covering scaffolding with impervious sheeting in a manner to ensure any water will run off well clear of the new work. Beware of any identical splash zones at the wall base or abutting rooflines
- Strong winds can destroy even the best temporary protection – inadequate ties and poor detailing can often be attributed to failures
- Keep the waterproof coverings distanced from the work to allow moisture to evaporate from the mortar
- To ensure adequate hydration of the mortar, keep new applications damp (not wet) for at least 2-4 days
- Although not ideal, heat may be introduced if considered necessary. In this instance we would recommend propane gas heating as it provides a good level of humidity along with a higher level of carbon dioxide in conjunction with heat generated
- Ensure that any induced heating is adequately circulated to avoid any ‘cold zones’, (particularly at the base of the walls) and that venting is available to regulate the temperature and avoid overly rapid drying
- Provide close covering for the new works throughout the application and initial curing stages. Canvas or hessian hung within 100mm of wall face is ideal
- An open texture of mortar – stippled or rubbed – will allow better vapour permeability than a smooth, fine finish. (When cured, it will also have a more weathered, softer appearance) Be careful not to overwork the surface as this will bring fine particles and moisture to the closed surface.
- Keep an eye on the weather forecast for any severe extremes before commencing any work!
- Ensure materials stored on-site are protected from the frost and off the ground. If possible should be stored inside.
This is not intended as a specification for work during the winter. It is based on our knowledge of best-practice.