Project Site: Bridge in East Lothian
Project Scope: Historic Mortar Analysis
This is a typical example of a historic mortar analysis. The mortar sample was collected by the Scottish Lime Centre Trust’s Buildings Advisory Team from a bridge in East Lothian. The sample is a construction mortar collected from deep within a mortar joint.
The sample showed large white lime inclusions and coal / burnt fragments indicating that this mortar was prepared as a ‘hot lime’ mortar by slaking quicklime and sand together in one operation. The coal / burnt fragments are most likely the remains of the fuel used for burning the limestone to create the quicklime. The mortar was tested for its strength by trying to break it with finger pressure, as strong resistance was shown this was considered to be an ‘eminently hydraulic’ lime (approximately corresponding to an NHL 5). The aggregate was assessed to check that it is not based on calcium carbonate. It was not and therefore the sample was suitable for our ‘Standard Mortar Analysis’ process by acid dissolution.
A sub-sample of the mortar was crushed and put into a beaker to which 10% hydrochloric acid was added. The acid reacted with the lime binder to leave just the aggregate remaining. Once the reaction had fully ended the acid was filtered off and the aggregate was dried, weighed and graded.
By knowing the dry weight of the dry sub-sample and weight of the dry aggregate we calculated the weight of the binder and therefore the mix proportions in their ‘as received’ state which in this case were:
1 part lime to 1 part aggregate
By using information on relative bulk densities and the weight changes during carbonation we calculated that the original mix proportions of this mortar were:
1 part eminently hydraulic quicklime to 1.2 parts aggregate (by weight) or
1 part eminently hydraulic quicklime to 0.51 parts aggregate (by volume)
The mix proportions that come from a mortar analysis should not be used directly as a specification. This needs to take account of prevailing site conditions, including stone type and condition, location and function of the new mortar, building details, exposure, seasonal working etc. Also commonly, as in this example it was a quicklime that was used and historically mortars used to be far more binder-rich than we would specify for modern slaked lime.
The aggregate separated from the sample was graded using British Standard size sieves and each proportion retained was weighed. This information was used to produce one of our unique ‘Aggregate Graphs’ utilising the aggregate grains to give a full image of the properties of the aggregate separated from the mortar. This was then compared to our extensive ‘SLCT Sands & Aggregates Database’ where we were able to identify currently available aggregates that would be suitable matches for the original.
The Buildings Advisory Team went on to create mortar specifications for the repairs to be carried out on the bridge based on the mortar analysis and the assessment made during the site visit.