Cement FAQ qualification

[john]

Hi

The cement FAQ's

wonderful. Besides being a diyer I know nothing about this so it helped enormously.

I've some questions though, the answer to which will be blindingly obvious to you, so here's hoping.

The FAQ says the optimum w/c ratio is .28. In other areas it says ratios are by volume, so is this by volume too? But my key question is .. what is .28? Firstly, is that more water than cement, or more cement than water? I know that'll become clear when I read the back of the packet, but. Am I right in thinking that translates to just over 3.5 cups/buckets/pocketsful of water/cement to 1 of cement/water?

What I'm actually trying to do is to work out what to repair my wall rendering with after a car crashed into it. I've a patch about two feet around and I wanted to know what I was doing. The house is about 150 years old, but the rendering is obviously younger.

One thing I'm worried about is the business of laying concrete on a vertical surface. Isn't it just going to bulge at the bottom? Can I adjust anything to stop that happening?

I have treated the 'wound' with pva. So from what's on the FAQ, here's what I propose to do:

6 parts of soft sand : 1 opc : 1 slaked lime, all by volume, mixed in that .28 proportion with a liquid that's pva diluted 4:1 with water (that's 4 cups water + 1 cup pva, right?)

But then, I wasn't clear about the soft sand part, because I feel like sharp sand should give more grip, and perhaps it might help stop the stuff sliding down my wall before it dries. Which do you think?

This is all fantastic, there was nothing on rendering in my DIY books.

I look forward to your help.

J

[Chris Bacon]

Sounds rather complicated for most purposes - add water until the mix is stiff enough and no sloppier.

Concrete?

You put it on in layers about 1/2" at a time, generally.

Why??

Chuck down 6 measures of sharp sand (not soft sand), then 1 of cement, then one of lime. Mix it all up, dry. When it's a reasonably consistent colour, sprinkle on water until the mix is the right consistency.

Use washed sharp sand. Soft sand makes the render likely to shrink and crack as it sets.

It's not difficult, but then again it's not the "stain removal/glue something together/repair something" common, or garden, task.

[John Schmitt]

Thanks.

I'll try.

When research is done on cement mixes, it is almost, if not exclusively, by weight (to be pedantic mass). For every kilo of cement, the w/c ratio indicates how much water (in litres, it's close enough) per kilo of cement is added. Achieving .28 is not easy, and for ordinary, boring projects there is not a problem. Any reasonable concrete or render will deal with the problem comfortably.

Be aware that there is little chance of getting a precise colour match. Not only do you know the exact cement ratio, but different sands have different levels of ferric pigmentation.

Use a dryish mix, with plasticiser and PVA in the gauging water as per the webpage. Also prime the wall with 4:1 diluted PVA. You can also use SBR or acrylic adhesion promoters. Theoretically these provide better water resistance, but in the real world PVA would appear to be entirely adequate. The best bonding agent in the world will not make up for poor surface preparation. Wire-brushing should suffice for brickwork.

Ah, missed that on first read. Good move.

Dilution ratio correct, but as has been said the key is consistency of the mortar. It wants to be relatively stiff, so that it stays where it is put. Work upwards, the reduced suction from the brickwork (PVA does this) will still make the mortar stiffen further and support the proceeding work. Looking at the weather forecast, you may want to cover the job with building polythene or wait for a couple of days of fair weather.

Soft sand will give you a smoother finish. The primary way that cement locks into brickwork is the acicular crystals locking into the pores in the brick. PVA helps although the mechanism is subject to conjecture.

[Chris Bacon]

Why?

Why?

Again, why?

Well, there's a load of waffle here, but no mention of the fact that using soft sand can easily lead to excessive shrinkage cracks. This is what happens in the real world!

[John Schmitt]

The mere fact that you ask this question suggests that you will be incapable of understanding the answer. Plasticiser makes the mix more workable at a lower w/c ratio, partly by air entrainment. The PVA not only improves adhesion but the polyvinyl alcohol in the aqueous phase retains water for a more thorough cure. The dryish mix is to avoid slumping.

See the above. Try not to move your lips as you read it.

Because this is the 21st. century, and Fred Flintstone technology is a bit passe. Next thing you'll be burning oyster shells.

Shrinkage cracks occur because of high w/c ratios or too much cement or badly managed large pours. Have you ever looked into the mathematics of particle packing? I did once, but it used rather too much machine time even on the model of spherical sand grains. UK mined sands are usually well graded, the German ones are poorly graded in general but much whiter. There are a few in the UK like Loch Aline, but these are in the minority.

John Schmitt

[Chris Bacon]

Hooray! Spoken as if you're a true wanker. Will you please just answer the question?

I'll ask again - why does he *need PVA* in the mix?

Try and come up with a proper answer, abuse tends to mean you can't. It's not even very good abuse.

What is "fred flintstone technology" in this context. Next thing you won't give a sensible reply to any of this.

Once more a load of waffle. Do you say that shrinkage cracks are as likely using sharp sand in this situation as they are using soft sand? Hmm?

So what? Spout less waffle, give a sensible answer. This bloke wants to patch a "2' around" piece of render on the wall of his house. By your "answers", I doubt you've ever done such a job.

[John Schmitt]

I have done. I can explain it to you, but I cannot understand it for you. Sorry.

Need and a good idea are admittedly different. You do not need a seatbelt in a car, but the statistics suggest that this is true. There is someone who will always spout about if they *had* been wearing a seatbelt, they would have died. Has it ever occurred to you why you never hear from people who bitterly regret *not* wearing a seatbelt because they are now dead or a drooling vegetable? The PVA will increase the adhesion by something of the order of 50 times. Did a research project at a previous company on this.

I can explain it to you, but I cannot understand it for you. My commiserations.

Fred Flintstone is two proper nouns. They should be capitalised. I am referring to the Luddite attitude of HABs.

"We've always done it this way"

It is mostly the size distribution which governs this, rather than the angularity. Perhaps you should look at something like Fry's masterpiece or another book on cement and concrete to expand your knowledge.

I was part of a course in tiling supplied by my company, a building adhesives prime manufacturer, and regularly rendered an area for tiling demonstration. There was also a plastered area which was hacked clean and redone. The plasterboard was a doddle, having made a jig to position the holes so that the screws found the rawlplugs every time. Using appropriate washers meant that we could use it twice. Then it was used for tiling onto tile demonstration.

John Schmitt

[Chris Bacon]

Yes, I'm glad you picked that up - but is PVA always a good idea (see below)?

Just trying to answer instead of going off at a tangent would be better...

So what? If he just bungs it on, without PVA, how long will it last? How long will it last *with* PVA?

What did they do before PVA? How long did the "old men's" render last?

*Can PVA in the mix give rise to any problems at all*?

Why do you continually feel the need to put this sort of stuff in? Perhaps it's an insecurity thing.

If you've no argument, prevaricate....

HAB?

I'm asking *you* for your opinion. You haven't given it. Why? Do you say that shrinkage cracks are as likely using sharp sand in this situation as they are using soft sand? Hmm?

Look at the waffle in *there* - no relevance for someone who wants to patch a small bit of outside render? Do you think he should plasterboard and tile it instead? Please stick to things that are relevant.

[Mr Fuxit]

What's this "polyvinyl alcohol" stuff you're on about?

[Rob Morley]

Is it a favourite avian tipple in certain fetish clubs?

[John Rumm]

Erm, looks like he did - rather well I though... or do you just like arguing for the sake of it?

[Roger R]

I'm a novice at brick work too, but I thought it was a bit thin on the subject of lime mortar.

In particular repairing brickwork made with lime mortar. How to recognise lime mortar brickwork. What mix to use when repairing.

There appear to be differing opinions on the web about the mix, with some suggesting adding cement to the mix, and others stating the cement causes the lime to separate out in some way and ruin the result.

There is also comment that the sand should not be the clean washed stuff widely sold, but needs to contain a fair degree of clay to work with the lime.

my case, I am replacing a couple of bricks cut out to gain access to the cavity at various points in a 1950's house. On one occasion I used the three sand, one cement and one lime. Yesterday, after reading one of the above I used just sand and lime, but I'm not sure which way to lean today!

Please advise.

Roger

[john]

All very fab, thank-you. The test will be in the doing.

I can't for the life of me see how you guys got into an argument, sorry to have caused it. Maybe you have history.

To answer a previous question from your adversary, I primed the wall with PVA partly because I knew I'd be lazy enough not to fix it for a while and rather hoped it would offer at least a little protection against damp, but more importantly, although I wire brushed what was there it still felt loose. I was rather hoping the PVA would help to bind the final bits of dust and sand grains. All done in ignorance besides what I'd read in the FAQ, but those were my reasons.

J

[meow2222]

Now I'm going to complicate things again... What youve read is on the mark for modern properties, but 150 year old houses use very soft bricks and little in the way of foundations, thus move a little. Cement renders cause problems because of both these factors, damaging the bricks and leading to brick failure. They also exacerbate damp in walls.

The solution to these risks is to use lime mortar, 3:1 sand and lime putty. Sure you can do it with 1:1:6, but there are a great many old houses that have sustained damage years later as a result of this. Your call.

And go easy when wire brushing the bricks, hand brushing only. 150 yr old bricks are very soft and once the fireskin is damaged they will, very slowly, deteriorate.

For anyone that hasnt done this yet, PVA wasnt the best move, it makes the surface slippery and reduces damp evaporation.

NT

[Sean Mc]

Head hurts, its all been over complicated.

Get down the builders yard ask for builders sand. Mix 4-1 splash of washing up liquid, mix to a double cream thickness. Have a sponge handy to rub it down about two hours after you las worked it.

Sean

Edit dont forget a scratch coat for large area

-- Sean Mc

[Andy Wade]

I assumed it meant hairy-arsed builder, but that's only a guess.

[meow2222]

You can use just about anything, but very hard mixes like this break bricks.

[Chris Bacon]

Said what the effects of using it are, not why it's

*needed*, which it isn't. Don't you know that?

[Chris Bacon]

Ah, I see. Even (most) HABs would not use soft sand for rendering, though.

[John Rumm]

I would have thought the combinations of better adhesion, plus improved curing performance would put it firmly into the "desirable" category, don't you think?

So if you want to be pedantic, then yes you are right, you don't need it in the sense that the stuff will still fill the hole and set, by why make the job harder than it needs to be, and achieve inferior results for the cost of a few pence worth of PVA?