Lime mixes: sand bulking and batching by weight.

[johnandrew]

Wozzy's post has inspired me to raise the following question - a matter I've been confused about for a while.

Do you worry about sand bulking when mixing lime mortars (or non-lime mortars)?
When using powders (hydrates) do you batch the lime by weight rather then volume?

The Holmes and Wingate book mentions sand bulking and I've read somewhere that powders should always be batched by weight and not volume. Can anyone clear this up for me? Or should I stop worrying and get on with it?

John

 

[robgil]

I always go by volume. What is sand bulking?

 

[johnandrew]

This is what I think sand bulking means.

A given weight of dry sand will occupy less volume than damp sand. Therefore is you measure a volume of dry sand and the same volume of damp sand then you end up with different amounts of sand. It is something to do with surface tension of water resulting in air being trapped in the sand and 'bulking' up the volume (I think). Saturated sand, however, has approximately the same volume as dry sand and this fact forms the basis of a simple test to work out the bulking factor.

I think this effect is only important when you are working across batches, that is to say, when you start a job with one load of sand and get a new delivery which may have a different water content to the previous batch.

 

[middi]

The rule of thumb ( well tried and tested):- ... If unsure ... add a bit more lime.

Middi

 

[MdB]

If using hydrate, I always use 50% more hydrate than I would with putty due to the amount of air that is in it. I therefore usually use a 2:1 sand:hydrate mix. The same goes for NHL.

TheForge's link http://www.buildingconservation.com/articles/mythmix/mythmix.htm suggests that 3:1 sandutty is too weak and 2:1 is better. This suggests 1.5:1 would be better for sand:hydrate. Fortunately, for what I've been using it on, it won't matter much.

(all measures by volume)