The Building Conservation Directory 2022

BUI LDING CONTRACTORS 2 53 C AT H E D R A L C O MM U N I C AT I O N S T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 2 2 most lime binders. It therefore takes a lesser number of particles and smaller volume of OPC to reach 25kg than an NHL 2 lime, which takes more particles (because of the smaller surface area) and therefore more volume to reach 25kg. The bag of hydraulic lime is therefore bigger. The differences in the volumes of binder for the same weight are known as relative bulk densities. Each binder will be different even though they may be in the same British Standard classification (BS-EN 459). Saint-Astier NHL 2 for example, will have a different relative bulk density than Otterbein NHL 2, because they are all made from naturally occurring limestones. CALCULATING THE WEIGHT OF BINDER TO BE USED FOR A GIVEN VOLUME If we know the relative bulk density (RBD) of a binder we can work out the weight of any given volume. Often bags of binder will have the RBD printed on them in kilograms per litre, but many don’t. Some even include workings out for full bag mixes, which is useful for producing larger amounts of mortar. If not, check with the person who supplied it: any specialist suppliers worth their salt will be able to advise on the relative bulk density of the binders they stock. So how do we convert a mix ratio that is quoted nominally by volume to a weight? The simplest way is to calculate the volume of binder required in litres and use the RBD to convert this to weight. For example, an RBD of 0.63 means that one litre of this lime binder weighs 0.63kg. In practice, however, it is more useful to start by working out how much mortar you are going to need for the day’s work. This is equivalent to the volume of sand you are going to use because, as mentioned earlier, the binder only fills the voids or spaces between the sand grains. For example, if 150 litres of NHL 2 mortar are expected to be used on site in the day, the sand can be measured in buckets – that’s ten buckets, 15 litres each. The next question is the amount of lime. If a simple void test suggests a 1:2 ratio is required for this particular sand, the equivalent of 75 litres of the chosen lime will be needed for 150 litres of sand, but how much does this weigh? If the RBD of the chosen lime is 0.5 then this means that one litre of lime weighs 0.5kg, so 75 litres of this lime weighs 37.5kg (75 x 0.5kg). At the Scottish Lime Centre we recommend using a spring balance (like fishermen use) to weigh out your lime binders on site. Remember to ‘zero’ your scales to take account of the weight of your bucket or container used to measure the lime, as a standard builders’ bucket can weigh up to 1kg empty. Have fun measuring and mixing, knowing you are going to produce repeatable and consistent mixes! It will save you time and money in the end… Happy mixing! ROZ ARTIS MRICS IHBC is director of the Scottish Lime Centre Trust (see page 169), one of its primary course tutors and the head of its building advisory service. Roz has written widely on the practical use of lime and other mortars and is an RICS certified historic building professional. ACCESS SPECIALISTS BCD The Building Conservation Directory 2023 30 TH EDITION Will you be in it? Contact Cathedral Communications 01747 871717 THE BUI LDING CONSERVATION DIRECTORY 30 TH EDITION All of these bags of binder weigh 25kg – they just take up lesser or more volume