concrete strength


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Old 11-01-12, 12:30 PM
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concrete strength

Quickcrete has a strength of 4000 psi, Sakrete "professional grade" has a strenght of 5000 psi. If I make my own mix using the standard 3:2:1 ratio of gravel/sand/portland how will the strengh compare ? I've searched online for weeks and can't find an answer. any feedback ?
 
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Old 11-01-12, 03:01 PM
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It would be very similar to the 4000 psi BUT gravel? It is rock, sand and portland. Gravel puts in dust/dirt etc that would weaken the bond. Clean rock, good sand and portland would make a nice mix. The question would be what are you using the concrete for? Driveway, to put around posts, sidewalk?
 
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Old 11-01-12, 03:37 PM
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The strength you are looking at (3000, 4000, 5000 psi, etc.) is the strength of concrete cylinders cured in a testing lab for a prescribed time period at ideal temperatures ans humidity.

What you actually have in a project when you mix your own is variable because of the batching/weighing accuracy, mixing and timing. This is compounded by how well you place, finish and consolidate the concrete and how much water you use, followed by how good the curing is.

The guaranteed proportions are much more uniform than you can get, but that is only a major factor if you need a lot of batches.

It is very similar to baking a cake. From a box mix you are O.K. until you add the wrong amount of milk, bake at the wrong temperature or leave it in the oven too long or not long enough.

Dick
 
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Old 11-01-12, 05:50 PM
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I'm making a gravestone for a friend that passed away. Parts of the mold are very detailed and can't have pea gravel in the mix (The name plate). Quickcrete makes a basic mix that doesn't have pea gravel but the freeze/thaw additive i'm using is not compatible with quickcrete (per the mfg.) I was going to use a 3:1 mix of sand and portland for that. Once the name plate is done i'm going to install it into another mold and fill it all up with concrete. Because I don't want ugly pea gravel showing on the surfaces I was going to make my own mix of concrete using the 3:2:1 recipe and use a decorative granite aggregate for the gravel/rock portion of the mix. I was just wondering how my own recipe of concrete would compare in strength to the ready mix products available. The reason i'm looking into mixing my own batch is because of the pea gravel in the ready mix brands.
 
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Old 11-01-12, 06:23 PM
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I think what i'm going to do is make some 6 inch square x 3 inch thick test slabs using professional grade ready mix and my own blend of 3:2:1 mix. After about 2 weeks of curing i'll crush them with a sledge hammer and take note of how well each block held up.
 
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Old 11-01-12, 08:12 PM
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I'd be cautious about using a mortar mix for the nameplate portion of your project, because it simply doesn't have strength and durability comparable to that of concrete. And the reason for that is the fact that there is no rock, which is what contributes to the strength of a typical concrete mix. I've never heard of any air entraining agent (what you're calling a "freeze-thaw additive") not being compatible with pea gravel in general, but maybe I haven't been around the industry long enough. I've always thought that getting a good cure on the finished concrete, along with applying a decent sealer, should take care of most potential freeze-thaw problems. I've batched and placed literally tons of concrete since 1970, never used any air entraining additives, and never had any scaling or deterioration problems typically associated with lack of freeze-thaw resistance. My typical 1:2:3 mix proportions always gave me durable concrete having a nice "ring" to it when struck with a hammer, after full cure.

Your idea to make some samples is a good one. If it were me, I would make a few using conventional concrete, both prepackaged (Quikrete 5000) and batching my own, and forget about the mortar option. Using a clean, coarse aggregate having at least 50% fractured faces, and concrete sand (instead of mason's or play sand) should result in some decent samples. Throw in some silica fume if you want more durability in the nameplate concrete. Crushing your samples with a hammer will not tell you how well they resist freeze-thaw cycles, however; for that, you need to soak them in salt brine and then freeze and thaw them a few times.

I'm confused by your reference to using decorative granite in the mix, implying you intend to expose the aggregate. Doing so is likely to make the nameplate lettering somewhat obscure and difficult to read (assuming the lettering is done in relief by fashioning the letters, and recessing them in the wet concrete face).
 
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Old 11-02-12, 06:13 AM
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Concrete can get pretty heavy. A friend of the family poured a concrete block and inserted the bronze plaque in it for his mother's grave. He had to have help to put it in the trunk of his car and then rode around with it for a long time until he could get someone to go to the cemetary with him to unload and place it.
 
 

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