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That is an interesting question. I remember the floors in our old barns, one them is still in-tact, and there were larger round stone in the mix it seems. It defintitely appears that in old buildings, things were done different, I wonder what the mix looked in in the Hoover Dam, in regards to aggregate size. I just dug up around the base of my foundation here, and it the footings were not trowelled, so you can see the round stone in the mix, it was poured in 1977. When in college, we studied strenghts and statistics of materials and we did a lot with concrete, creating our own mix design, testing same in cylinders and beams. Most of what we did in the design was out of ASTM C 104 - 106 or somewhere around there, from analyzing sieve sizes and specific gravity of the aggregates to acutally making mixes. I used to think it was fun, because most of these materials I 've hauled in a dump truck, but I never thought I'd be analyzing them. From what I can understand and based on all the things that comprised formulating a mix design to meet a certain compressive strength, it seems ASTM ( American Society of Testing Materials ) that one thing was important, the uniformity of the aggregate, of all the composite sizes from a source. It's all combined like a recipe, so much of each sieve size aggregate, in measured by weight increments, besides the cementitious material ( portland ) and water as well as admixtures used today. I believe this is what gives it it's compressive strength, uniformly, whereas if you were to place large aggregate into the mix, it could concentrate in an area, or a variety of things, including voids, hard to really say, but could jeopardize the strength in areas of the pour, creating weak spots. That is just a theory though, on a large pour like a bridge pier or column, or maybe not so, but if your using 6" aggregate on an 8" foundation wall, maybe it's an issue. Another thing that comes to mind, I see pea gravel and round aggregate still used in bag mixes, I don't see it in mixes made by the suppliers and delivered in trucks. I think that the majority of suppliers use a crushed stone aggregate, it has a rougher surface and I would think makes a better bond, vs. round aggregate. I'm sure when the footings were poured here, that the top had a layer of fines on it, and was gray, no stone showing, now after disturbing it, seems like that layer spalls a little easier, exposing the round aggregate inside. This would appear to also be even more prevalent if exposed or in areas where the concrete was around harsh elements, freezing, thawing, chemicals, salt like on roads. Concrete has capillaries and things can be absorbed into it, I would think the round stone may lose it's bond easier, but who is to day if that is correct, just an opinion here. ACI , is another reference standard for concrete, ( American Concrete Institute ) I recall using some of those journals for research papers I did as part of my class work, did one on the abrasiveness of concrete as used in highway applications, got an A on it, was always imaginative, no one in the class had any far fetched topics like this one I did, went out on a limb I recall, but those journals were more suited to engineers, not written so much in laymans terms if I recall. It seems that they test sections of slabs and also insert or put test sections on highways and analyze same for wear. In the lab there is a giant arm with a tire and all the pieces are a trapezoid shape, creating a circle, the wheel goes around in circles to simulate years of wear. From what I recall about that, is the harder the aggregate, and the stronger the mix design, the higher the abrasion resistance. It's a vast subject no doubt.
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