cement mixes

[Stephen]

Hello,

I read this on the paving expert web site:

"There is often a temptation for installers to use either a mortar with a higher cement content, say a 3:1 mix in place of a 6:1 mix, to use PVA as an alleged bonding agent, or to combine a cement-rich mortar with PVA and hope for the best. Adding cement to a mortar does not, in very general terms, improve its adhesive properties, and PVA, being water-soluble, is a sort-term fix that will eventually be washed away."

It goes on to recommend using SBR. My question is, I have always followed the instructions on the cement bag, which says to use 5:1 for general use or 3:1 for a strong mix. The advice above, appears to contradict that, saying a stronger mix will not actually be stronger.

To put this into context, the paragraph is talking about cementing pavers as the treads of garden steps. I did use SBR as the web site recommended, and they do seem rock solid but I was not using a 6:1 mix. Could I have saved cement by doing so? What ratios do you use?

Thanks, Stephen.

[Martin Bonner]

When the instructions say "stronger mix" they mean "higher compressive strength". They *don't* mean that the concrete sticks more firmly to an attached brick. (Which is, at least in part, about tensile strength).

[Tim Watts]

The 2 ways I improve bonding are:

1) Wet the bricks first and use a mix that in itself is right (ie not " a bit wetter to allow for brick suckage");

2) If it's a fiddly brick - top corner for example, the ones that always seem to fall off when "professionals" do them: Wet the brick and paint with a cement/water slurry (honey thickness). I might switch up to a 3:1 mix too for that. My bricks do not come apart, ever... Not that's I've done a lot - few repairs, couple of brickwork gulley drains.

SBR would work too...

[John Rumm]

I think you are confusing a string mix (which indeed, 3:1 would be) with one that has strong adhesion. All he is saying, is making a mix stronger, does not make it any better at sticking slabs down.

You don't need a particularly strong mix for bedding slabs, so 6:1 would be plenty.

[stuart noble]

Disagree. What PVA does (and the only thing it does) is prevent water being sucked out of the mortar before it has had time to cure. Doesn't matter what happens to it after that. Interesting that the end result is the same whether you mix the PVA with the mortar or coat the surface first.

[Andrew Gabriel]

PVA/EVA/SBR all increase the tensile strength of cement based mortars by cross linking with the set cement crystal structure.

PVA is unsuitable for use where moisture will be present, as water dissolves PVA. EVA (usually sold as Exterior PVA although it's not really PVA at all) is more waterproof, but *only* when used as a mortar additive, because the cross-bonds with set cement are insoluble. (Any excess EVA in the mix is still water soluble, which can still cause weakness if it gets wet and there was too much EVA or it wasn't well mixed in.) SBR goes one step further, in that's it's not water soluble at all.

The other use for the bonding agents is where you want to bond cement to something it doesn't stick to too well (including cement which is already well set). Then a slurry made from cement and one of the bonding agents at the join can work very well.

[Tim Watts]

Which makes me wonder why people don't just wet the bricks - always worked for me. I usually just have a bucket of water to dunk a few in to form a working pile.

OK - it's not "pro" and it's not fast, but us DIY folk don;t care too much on either front :)

[stuart noble]

IME unadulterated mortar sticks well to any reasonably solid surface, including a builder's bucket if the plastic isn't flexed. It only ever fails when the underlying surface is porous. Never tried, but I'm sure you could render a jam jar given the inflexibility and zero porosity.

SBR is interesting stuff but I get the impression there is no chemical reaction with the cement, in fact you have to keep stirring it to achieve any kind on integration.

[tabbypurr]

Cement mortar does bond well to glass. But a jamjar would offer nothing to hold it up when first applied.

NT

[The Natural Philosopher]

never seen a wall of glass bricks?

[Jonno]

Tim Watts scribbled

Wet sacks over the brickwork helps a lot too.

Subject to getting any sacks.

[John Rumm]

You are misquoting me....

[stuart noble]

Yes, sorry about that

[stuart noble]

On its side a bit at a time maybe? I'm not about to try it :-)

[stuart noble]

Not rendered, no

[Stephen]

I shall remember that for the future, thanks!

[Stephen]

Thanks. I think you and Tim have solved it for me. I was interpreting the word "strong" in a different (wrong) way.

[Stephen]

Why has the builder down the road covered his pile of bricks with plastic sheeting? Can they get too wet?

[harry]

Yes they can, plus to protect from frost until the cement sets.

[polygonum]

And stops bird crap. And, by reducing wetness, stop algae growing.