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