Over the last twenty or so years it has become common knowledge in the construction industry that when repairing historic buildings, it is generally best to specify and use building limes rather than cements.
As a result, a variety of lime mortars for building, repairing or plastering now appear on every building conservation project. However, despite this “lime revival” there remains much to understand if the material is to be adequately specified and used properly on site to ensure that the expected results materialise (and failures are avoided).
One of the most critical issues to understand is that specifying lime only confirms the family of materials to be used and on its own does not provide adequate guidance to allow for a successful outcome. The specific lime, the aggregate used in the mortar and good work practices on site should allow for the cured material to:
- Accommodate the natural minor seasonal movement of older buildings
- Be no stronger than the masonry it holds
- Allow for vapour permeability
- Be durable for the location and element of construction where they are employed
- Be economical and easily workable on site
In the UK and Ireland, lime putty mixed with an aggregate (sand) to produce a “rough stuff” was the standard choice at the beginning of the “lime revival” of the late 20th century, sometimes gauged with other materials to improve setting characteristics. However, times have moved on, and it should now be viewed as one option with hydraulic limes, natural cements, specialist pre-mixed lime-based mortars and hot limes available for the specifier and contractor.
Hydraulic lime is a natural hydraulic lime which contains impurities such as silica which help the curing process, in addition to the natural carbonation of the mortar. These limes are classified as NHL’s 2, 3.5 or 5 in order of increasing compressive strength. However, the classifications also provide an insight into the permeability of the final mortar and its suitability for use on different building elements and locations.
Specifying the exact lime to be used is considered critical and this might well extend to the manufacturer of the material, as the specifier and contractor should be aware that there are allowable overlaps on the three strength bands and ultimate strengths achieved with each lime. This is important in the context of the five basic characteristics of the material (recorded above) that you are hoping for and a look at the various test results for the lime that you are proposing is always sensible. Remember also that the type of aggregate and the mix proportions of the mortar play an important part in the characteristics of the final repair which you are aiming for.
All building limes including naturally hydraulic limes, require good specification, good working practices in mixing and applying and good tending of work on completion if they are to be successful. The Naturally Hydraulic Limes provide an economic, convenient material for building conservation projects and have an important place in the family of building limes available for the repair of older buildings in modern Ireland.
This article was written by Chris McCollum, Chartered Building Surveyor, accredited in Building Conservation and appeared in Issue 178 of Keystone Ireland.