Sand And Cement Render

[tandh]

Hi guys, does any one know how long a sand and cement render will last if the walls haven't been primed?
It's only on two sides of a small out building (9 x 10 foot - and the windows and doors take up most of the wall space) and he added some pva to the mixture, but i've just found out today that he put it on concrete block work without doing any sort of primer or scratch coat. Should i be worried? will it start coming off in a year or two?

[KARL]

This is something of an imponderable as there are lots of conceivable variables and much even depends on to what extent the render dries out in the first days. This probably depends mostly on your efforts. It is a common fallacy to think that rapid drying is beneficial, it is not. Due to the nature of the chemical reaction involved, keeping any cementitious mass moist for at least three, if not seven, days after the mix being produced is something that really benefits ultimate strength. In the case of render, final adhesion improves through the same thing. After the render has thoroughly set (18-24 hours would ensure this), gently spray the entire surface with water and keep doing that repeatedly at the slightest sign of the surface drying out.

Unless the work is seriously substandard and/or the render dries out very rapidly, I would not expect the coating to come of as early as within a year or two. Lots of render is applied without a bonding coat being applied first and still provides perfectly acceptable service. Even with a good mix applied before it loses its best promise, the worst combination is application onto a bone dry substrate followed by very rapid drying - the render shrinks (loss of bond), fails to reach full strength, cracks and the coating is compromised.

[KARL]

Divebuddy is absolutely correct. However, more than half of the "ultimate" strength is reached quite early and the rest builds up over far longer time (and usually continues at a very slow rate for years). Except for very exacting/specialised work, construction specifications routinely limit compulsory curing (keeping wet) to 7 days, even in climates/environments much drier than the UK's summer. The OP explains that his/her concern is about render on a structure which seems not to be a very significant one. I have never seen any proper curing carried out on housing concrete works in the UK, although some may have had curing membrane applied - far less have I seen obvious evidence of curing of render. As serious failure of render is relatively rare in the UK, I think on balance that tandh need not worry unduly, especially if he/she carries out some curing for a few days.

[The Builder]

Good tech stuff from Karl and DB there.

The short answer is ........... S&C render, applied properly, will last indefinitely. No priming is needed, except in hot weather, when damping down is recommended before rendering.

Also... there is no need for PVA to be added to the mix except in specialist applications. Not so in your case.

[tandh]

arrr, thanks so much guys, you've really put my mind at rest once gain :-) it was done a month ago, so all your advice about keeping it moist has gone a little to waste as it's too late now. It rained so much after it was done which i think has really helped it. Thank you all - very informative answers :-)