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Unread 07-19-2014, 06:06 PM   #1
D123
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Deck Mud Ratios

Please excuse the old topic but I think this clarification is probably new.

Background: I'm installing my first scratch shower project and ran into some the traditional deck mud issues as well as what I now believe are oversights posted many places on the web.

1) My first pre-slope failed (so learning experience) because I substituted concrete mix for topping-mix as HD does not carry topping-mix. Another site suggested this would be ok. Bad choice as the aggregate is a mess to work with and the results are very poor. Had to start over.

2) My second pre-slope did much better but contained many weak sections. Yes, a bit was related to incomplete mixing and some to drying but definitely not all. I had used the JB on-line calculator which stated 12 lbs cement + 60 lbs sand. I did that, but it was a mistake.

On more research, it seems "the web" quotes 4:1 and 5:1 ratios but very few specify the units -- weight or volume. It matters. One web posting actually said they did it by volume because they don't remember to bring scales -- as if they were interchangeable units. I discovered it is supposed to be by volume, and that is reference on JB in a few places -- but the JB calculator specifies the ratio by weight, which is what I used. IMHO, the JB calculator has a flaw. (If so, I suspect the fix would be easy.)

Cement has about 2x the density of sand so a 5:1 ratio by weight results in a 10:1 ratio by volume -- which is why my 2nd pre-slope didn't hold together as well as it should. The other JB on-line calc option was topping-mix + extra sand which suggests about a 5.5:1 ratio by volume if one assumes a 3:1 (by volume) ratio for the topping mix -- so that option is much closer than the 10:1 cement:sand. So, IMHO, there is a clear conflict between the two calculated solutions.

While finding the right amount of water requires some trial, error and experience, that is much less of an issue than working with a 10:1 ratio which you think is actually 5:1. Density matters as do the units.

If I've not missed something in the numbers here, it might be good to clarify these details on the site for other first-timers. The info here is, otherwise, very useful to those of use tackling new tile projects so thanks to all.
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Last edited by D123; 07-19-2014 at 06:15 PM.
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Unread 07-19-2014, 07:01 PM   #2
jadnashua
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Mix it by volume. It must be mixed well. WHen adding the water, when you've got it right, if you grab a handful and squeeze, it should hold together when you release it and not drip water in the process. If you do not put something on the floor (plastic or roofing felt), the floor will suck too much moisture out of the mix before it can properly cure, creating a weak material.
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Unread 07-19-2014, 08:16 PM   #3
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Dave, welcome to the forum!

I feel like you're over-thinking this a little and my hunch is that you need to add a little more water to your mix. I'm not sure what area you are in but it might be worth it to check at some of the tile-specific suppliers and see if they have a 4:1 or 5:1 that is already premixed. Then you don't have to worry about volume vs weight.

Mapei makes a 4:1 that is pretty nice. Quikcrete also makes one that's 5:1. When I use these I'm always at the max of whatever the instructions say for water allowed.

I don't know if that's helpful or not.
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Unread 07-19-2014, 10:00 PM   #4
Davy
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Yeah, I would go by volume, not weight. Dry sand is much lighter than wet sand. We get it both ways. I've seen it so wet that we barely had to add water to it.
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Unread 07-19-2014, 10:50 PM   #5
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Welcome, Dave.

I agree with Jim that some overthinking may be afoot here. Not sure where you're getting your weight data, but in all my years working with concrete we've usually presumed dry sand to weigh about 100 pounds per cubic foot and Portland cement to weigh 94 pounds per cubic foot.

While all the batching done in the concrete industry is done by weight, the ratios of weight and volume of the components used in making deck mud are such that it can be measured with sufficient accuracy using either weight or volume.

And since it'll come out close enough for our Space Shuttle work using either method, most of us elect to use the volumetric method on site since it's a whole lot easier and faster.

Hint: If your sand weighs as much as 120 pounds per cubic foot, you've already got too much water to make good deck mud. Don't add any.

My opinion; worth price charged.
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Unread 07-20-2014, 12:07 AM   #6
D123
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Mud Ratios

Thanks to all. To clarify, yes I'm using and referencing to DRY sand -- bagged from HD -- 50 lb bag listed as 0.5 cf. Yes, I'm a semi-retired engineer so I'll admit to paying attention to the details.

However, the JB calculator gives two results (topping-mix and portland+sand) which are presented as equal mixes -- but because cement has about double the density of sand (1.95) a 5:1 by weight (as the JB calc results show) cannot be equal to 5:1 by volume.

My preslope 'experiment' produced a much softer mix (when cured) than any of the descriptions and so I believe this 'error' is why -- the 12lb cement + 60lbs sand is really a 10:1 ratio by volume. So, in this case, using the JB calc, it is not a case of just looking too closely. 10:1 really is much too sandy and all those with experience who mix 5:1 by volume are, I believe, doing it right.

I'm just proposing that the JB calc, and other references, would best serve the less-experienced if the numbers and units were accurate.

Thanks for the nice feedback.
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Unread 07-20-2014, 07:24 AM   #7
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Again, Dave, the bags of sand at your local Homers would weigh 100 pounds per cubic foot (your bag specifications) A one cubic foot bag of Portland from the same place will weigh 94 pounds (or the newer labeling of 92.6 pounds?). Still not sure where you're finding the 2:1 density discrepancy.
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Unread 07-20-2014, 09:07 AM   #8
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Does this explain how he's getting his density info?

Why is portland cement 94 #/cf alone (SG=1.51), but is 197 #/cf (SG=3.15) when used in a mix design?

It is rated at 94#/cf because that is the "bulk" density due to the fineness. The actual particles have a specific gravity of 3.15.

When you are doing a concrete mix design everything is based on the actual weight of the material to maintain a measureable standard.

It is very similar to the situation with aggregate, where the actual specific gravity of the particles is 2.7 +or- while the bulk density many 95 to 110 pcf. Aggregate also has the property of bulking depending on the moisture content and whether it is surface dry or saturated. When the concrete is batched, everything should be converted back to dry weights to control the amount of actual materials, proportions and the w/c of the completed mix.

One exception to using the volume of materials for good proportioning is the preparation of mortar, where all proportions are based on the bulk volume of the materials (cement, lime, sand). This is because of the vast history or performance of mortar and the site mixing process. Trying to use weights would be impossible and would no provide the desired workability, which is the most important property of mortar (ASTM C270 Appendix).

http://www.eng-tips.com/viewthread.cfm?qid=211019
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Unread 07-20-2014, 12:26 PM   #9
D123
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Clear as mud

Well, all of this is the reason for caution when reading on the web. I looked for consensus among many sites, tried that, ripped it out and then I finally did the experiment I should have done in the beginning.

Using a 100cc container, the HD Baselite II-V bagged cement weighs ~120g and the HD bagged, dry 'play sand' weighs 170g which suggests the 'aggregate' (right term?) density of cement is actually ~71% that of sand. (It's always better to actually make the measurement ...) (Yes, the cement was reasonably 'packed' in the container -- not just powdery fluff.) Adding just 50cc of water produced what appears to be a fairly dry mix and 100cc seemed wet. This suggests, by volume, 10 parts dry sand, 2 parts cement and 1 part water then 'season to taste'. I have a witness sample curing.

So, density numbers, relative to a 5:1 mix, are less obvious than I'd thought. What surprises me is that now assuming a lower density for bagged cement, mixing 5:1 by weight instead of 5:1 by volume should have made a 'richer' mixture yet the result appeared too sandy and weak. There's more to this than meets the eye.

I had pre-mixed dry (thoroughly I'd thought) and mixed well (I'd thought) with water then spread, pounded, screed and then wood-floated but with less than ideal results. I now think I'm missing some important details of one or more of the steps -- tricks of the trade -- that likely seem obvious to the experienced. However, I'm still believing in a 5:1 mix by volume. I've done a lot of remodeling and some rough concrete work but this mix seems to need its own magic.

Thanks for all the feedback.
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