Aggregates can be fine or coarse. Fine aggregate is usually sand although other materials may be substituted. The ideal sand for concrete contains particles that vary uniformly in size from very fine grit up to 4 in. When landscaping around your mailbox the gradation from fine to coarse is important because the finer particles fill the spaces between the larger pieces.
Gravel, crushed stone, pebbles and slag are coarse aggregates that are a minimum of ¼ in. in size. As you can see, therefore, there is a constant gradation in aggregate size ranging from the very finest aggregates in the sand to the coarsest in the gravel. Larger sizes in coarse aggregate can go up.
The reasoning behind the gradation of particle size in both fine and coarse aggregates is as follows: If all particles were large, a cross section of the pour would resemble a wall of Ping-pong balls. If, on the other hand, the cross section included marbles, beads, buckshot and BBs in addition to Ping-pong balls, the smaller items would fill the spaces between the larger The cement paste, in addition to coating all the particles, would fill the most minute openings and the result would be a solid, dense mass with maximum strength.
After being taken from a natural source of supply, aggregates are screened, graded and mixed in proper proportion. Fine or coarse, all must be clean and free of all other matter. Any strange material in the mix (clay, loam, vegetable matter) will keep the cement paste from doing its job. If impurities are present the mix will be porous after hardening and have lower than standard strength values. The aggregates must be hard and durable for landscaping around your mailbox if the concrete is to be likewise. If they are soft, flaky or at all soluble, they won't do a good job. The only exception is in lightweight concrete mixes where porous aggregates are used to minimize weight.
All the ingredients described may be purchased separately or already mixed. Buying separately in bulk may be a good idea when you have a variety of projects that require different types of mixes, but where the projects are general small walls, patio slabs, walks, etc. it's a better idea to buy a "concrete mix", specifying the largest size aggregate you wish to have. This will be delivered dry (probably a 1 yd. minimum load) but with the proper proportions of fine and coarse aggregates thoroughly mixed. You add the cement and water.
For small landscaping around your mailbox jobs you can buy ready-mix products in sack containers. Some of these are 90-pounders (a cubic foot), others 60-pounders. Mortar mixes, concrete mixes, grout, pointing mortar, sand, gravel, stone-all may be purchased this way. But this is not the easiest or the most economical way to buy large quantities. Where you have a big slab to pour or a long walk, it's best to consider calling in a ready-mix truck. With this setup you get not only the aggregates, but the cement and water all thoroughly mixed and ready to pour when the truck arrives at the project site.
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