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joeyboy 05-30-2007 08:24 PM

some questions on masonry/cement/concrete (general q's)
I love cement, and have been doing lots of little things with it.

I have a couple basic questions though, that I haven't found answers to.

1) is pure cement stronger than concrete? I can't tell if the aggregates in concrete (or stucco, etc) are there for filler (for economical reasons) or for strength increases.

2) if pure cement isn't the strongest, would sand-only aggregate be stronger, or would straight concrete be strongest? (these are all just aggregate/cement mixtures, no admixtures)

3) to make the product really strong, you can use admixtures. I see bonding agents you can add, fibers, etc. Without bringing rebar into this whole discussion, what makes cement/concrete strongest? Fibers + admixtures? Any recommendations on admixtures?

I've got a couple projects in mind when asking these questions, but they're really just more for my knowledge of concrete/cement in general. (oh, one other thing, is it just me, or when mixing a cement product that you added fibers to, and using a drill to mix it, does anyone else find that the drill mixer tends to get so many fibers on it, that you wonder if there's any left in the mixture itself?)

Thanks in anticipation of some input

concretemasonry 05-30-2007 09:48 PM

Concrete is not just a question of strength.

Aggregate is used because it is an extremely strong material that provides many advantages. It makes concrete easier to mix, handle, move into place, level or slope within reason and provide a durable surface. A pure cement/water mixture would be very costly, hard to work with, prohibit building roads, sidewalks and patios that slope for drainage. Pure cement/water mixes would have much more strinkage and tendancy to crack, would have higher absorption and would not be as durable.

A good concrete mix is like a juggling act to find the ideal comination and amount of different aggregates for the best concrete for the job.

Admixtures can be used to give different properties. One of the most common in air entraining agents that provide the correct amount of MICROSCOPIC pores the absorb the freezing expansion of the water absorbed in the concrete. Air entrained concrete is required for exposed concrete cold climates.

Excess water will reduce concrete strength, decrease durability and increase shrinkage cracks. Admixtures allow mixing, placement and finishing of concrete and help concrete to acheive the strengths is it planned to without adding extra water. Bonding agents are meant to be applied to sufaces and not dumped into concrete.

Some high early strength cements will give a higher early strength, but may give you a lower ultimate strength. - It is all part of the juggling process to design the optimum concrete mix for the project and application.

Fibers do nothing to increase compressive strength. They do help to minimize micro-cracks. Fibers should not be added to ready-mix because the amount and type of mixing is critical. They can decrease the strength and durability if mis-used. Your observation of fibers and drills is an example. There are some non-structural applications where where fibers must be added in the field. Over-mixing of fiber concrete is far too common and rsults in lower strength. - You end up with balls of fibers you may think are aggregate.

For admixtures, have the plant use designed mixes with air-entrainment of plasticizers to improve the performance. Do not arbitrarily add your own admixtures unless you can back up the results with past performance.

joeyboy 05-31-2007 09:35 AM

Awesome, thanks for the answers/advice!

Tscarborough 05-31-2007 03:50 PM

A few more things about concrete.

The Ideal concrete will consist of graded aggregates encapsulated by a matrix of cement paste containing just enough water to hydrate the free cement. The gradation of the aggregate is very important. Imagine a jar full of marbles. Note the amount of space, and varying shapes of the spaces beween them. Now imagine the jar with a marble, BB, and sand mixture. The spaces will be smaller and much more uniform (Aggregates should be sharp though, not rounded).

Reinforced concrete is used to address the weakness of pure concrete: Tensile strength. It is simply not very flexible. By a lucky coincidence, the thermal characteristics of steel and concrete are almost exactly the same (at least the ones that matter for this application), so steel can be imbedded into concrete without causing internal stress. Steel is extremely high in tensile strength and concrete is high in compressive, so by pairing steel and concrete, a very versatile building material is produced.

The issue of admixtures is tricky. As a rule, it is best to stay away from everything but acrylic bonding agents for home mixed projects. You have to check what the solids content is of the product you are using, BTW, to determine the proper dosing ratio. It will usually tell you on the label, as well as dosage rate. More is not better in this situation.

fconcrete 05-31-2007 03:54 PM

well it all depends what your using it for. concrete is been around for so long that is everywhere you go.Regarding your questions I am not a scientist but is a combination of agregates and right cement to water ratio that makes it stronger and the proper curing of the concrete itself.Too much water and makes it weak, not enough water and is hard to work with depends alot on temperature and the installer.hopefully this helps any

Tscarborough 05-31-2007 04:33 PM

Also, note that masonry (mortar) is not the same as concrete. They have different applications and different sets of desired characteristics.

FYI a drill mixer is OK for stucco and mortar, but not so good for concrete.

joeyboy 05-31-2007 06:34 PM

Thanks for all the tips, as I said I'm totally into concrete/cement now. It's my hobby, so to speak. I did my first project maybe 2 months ago (just a dumb sculpture thing to practice molding, edging, curing, etc), and have been doing things since. I love concrete (especially the newer decorative concrete industry), and am actually in the middle of several projects, and will be starting more soon. Within the next month or so I'll have done:
- stucco siding (already started, I've got 3 wall sections done, just scratch coat so far. Doing real proper curing, I've got them tarped, and mist them lightly 2 or 3 times daily)
- refinishing a bunch of concrete slabs, likely overlays
- made some quite impressive (if I do say so myself haha) legs to hold a massive glass table, there's actually pics of this project on this board in the photos section)
- will be building a concrete block mailbox
- making edgers for my mulch beds, probably poured in place, wiht the same color mix as my stucco (1 box tan pigment for every 2 bags of white stucco)
- *may* resurface my driveway, unsure on this though; tough choice, as it's looking 80% good, so I don't know if I should waste the time/money/effort to bring it up to 100% by doing an overlay...

I'm sure I'm missing more things, but that's all off the top of my head.

Oh, any more info on the admixtures that will inhibit (or at least reduce) hairline cracking would be awesome.

Couple more q's, too:

1) is a liquid bonding agent a plasticizer?

2) someone said don't add that to the stucco mix directly, just to the wall. I've been doing both, and was directed to do so by the company who makes the product.

3) are there any resources (besides the portland cement association, I've already read everything they have a million times) that would be kind of geared towards understanding concrete in general, and ways to manipulate, work with it, etc? (I've also torn through and as well)

Tscarborough 05-31-2007 09:55 PM

1) Not in a significant way.

2) It depends upon which "bonding agent" is being used. Some do, some don't. Some integral bonding agents, when painted on the wall, will become a bond breaker if it dries. The person who knows the product best is the manufacturer, so always use their specifications.

3) I do not know online, but the PCA sells some excellent books on concrete. They are not cheap, though.

The best cure for hairline cracking is (relatively) dry mix, fibers, and thorough mechanical mixing using graded aggregate.

joeyboy 06-01-2007 09:50 AM


Originally Posted by Tscarborough (Post 47240)

The best cure for hairline cracking is (relatively) dry mix, fibers, and thorough mechanical mixing using graded aggregate.

I'm using a kinda dry mix, not too dry, but I guess it would qualify as 'relatively' dry (although how the hell woudl i know :laughing: ). I am mixing thoroughly, I'm mixing up 2 bags of stucco into a wheelbarrow, adding water first, then 2 times while adding the stucco. I then use a stiff rake to mix it all up, then once it visually appears uniform, I drill mix it for a while after that.

I have tried fibers, but it always seems they end up solely on my drill's mixing attachment. Dunno.. Also, are fibers different? I have a little bag of fibers from home depot, don't even know if there's a variety out there or what, or if it matters

Tscarborough 06-01-2007 01:29 PM

Yes fibers are different. The one you got from the Depot are for concrete, and are longer than ones designed for stucco. If you want to use fiber, ditch the drill and buy a mortar hoe (a regular hoe with 2 holes in it). Dry mix the stucco and fibers, thouroughly, then add water and continue mixing.

joeyboy 06-09-2007 10:24 AM

tscar- I just realized that you're the guy with the brick avatar on, that's awesome! I learn so much from reading your posts there (I don't post, as I'm clearly not able to haha), that's real cool you give advice here too. (just noticed who you were when I was re-reading a thread about mailboxes you were posting in, you prollly know which one, it was funny. I'm actually about to build one today at my place, but first I need to find out specific minimum heights/distances from street/etc., and I'm also gonna drive through the neighborhood to find at least 1 more person who has one, to make sure that I won't be told to take it down or something. btw, if you do read this post this afternoon, plz let me know any tips on this you may have, specifically things like rebar usage, footing size, etc. Otherwise I'm just gonna do it like i always do - overkill!! Prolly 4 rebar poles and a huuuuge footing haha!)

joeyboy 06-09-2007 11:43 AM

(edit - guess I won't be making one today. My neighborhood is huuuge, and I was only able to find 1 person who has a block mailbox structure. They wouldn't answer their door, so I couldn't ask if they pulled a permit to get it or what, but it seems unlikely that they'd be the only ones unless there was a reason for it...)

joeyboy 06-09-2007 02:39 PM

Well, persistence pays off I guess! Went there again, they answered this time, said they built it themselves w/o a permit a few years ago and never had any problems. Now off to home depot!

Tscarborough 06-10-2007 10:05 AM

If you go to the post office they will give you a flyer on making it to the correct dimensions (height and setback). If you build it wrong, it is possible that they will not deliver mail to it. In lieu of that, when you buy the actual box, there should be generalized directions on placement. Another thing to remember when you build a massive structure on the road is that you could be liable for damages and injuries should some one hit it. Don't grout it solid, for example.

joeyboy 06-10-2007 02:16 PM

what do you mean by 'don't grout it solid'?

I was reading all the threads on ctalk that I could about this, and it seemed important that the thing isn't made so solid that it would not fragment (I think 'breakaway and fragmentation' were some key points?).

Yesterday I just built the footer. I used 2' rebar, high strength concrete, and concrete blocks (filled cores around the rebar), but am only going to do solid cores in the blocks for the first couple levels of concrete. After that I'll just be mortaring the blocks together, but no solid cores/rebar. I'm going to use hollow core bricks to make the top arch. I guess I think that the way I'm doing it is good since it'll be a strong, heavy footer, and the structure pretty much gets lighter the higher up it goes, so it'll be solid to the point it won't be a safety hazard or anything lol, but if hit by a car it wouldn't be like hitting a solid wall. Is that pretty much the deal for this?

(I can post some pics later after I've laid some of teh blocks, but it's easy to just imagine - I'm using regular 8X8X16 concrete blocks, side by side, alternating orientation of the blocks at each level. So the resulting column is going to be 16" wide/deep, with a rounded top, then stucco'd to match the house.)

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