render ratios turned on it's head!

[trowelmonkey1]

Hello chaps!

Been reading a book on the construction of houses lately, a text book used in universities apparently.

Anyways, in the rendering chapter it talks of the importance of ratios, now i've always believed it's 4:1 for scratch coat, 5:1 top coat, but the book say 3:1 is what it should be, the reason it says this is that in well graded sand there will 25% air, the aim is to replace this air with the binding agent, whether it be cement, lime or a combination of both.

so, 75% sand to 25% cement say, or 3:1, the idea being that to remove the risk of cracking you need to do 2 things: a) makes ure there isn't too much cement in the mix and b) make sure the mix doesn't need too much water causing cracking when it dries/shrinks.

makes perfect sense to me and i understand the science BUT we know a 3:1 is strong and unsuitable for some backgrounds, in this situation it then says you need to replace some of the cement with lime but always keeping the 3:1 ratio, so for example you mix up 1:1:6 (lime:cement:sand) meaning it's still a 3:1 ratio of binder and sand but now a weaker mix, or 1:2:9.

so nowhere does it talk of the ratios used as rule of thumb today and wondered the the pros on here thought if this info?

 

[1john]

just a slight observation, reason A is not to have too much cement in the mix, in which case a 4:1 mix applys more to that statment that their 3:1 advisory.

 

[trowelmonkey1]

indeed but there is obviously a threshold/tipping point and i suspect stronger than 3:1 is it based on what i've read, i does say that each subsequent coat should be weaker than its substrate so that goes with conventional knowledge.