Grading Of Sand And Pebbles

Both fine and coarse aggregates must be well graded.

Voids, Or Air Spaces In Sand Or Pebbles

Any volume of sand or pebbles contains voids, or air spaces, but if the material is graded by being composed of particles of different sizes, the lower will be the volume of these voids, or air spaces. In other words, voids, or air spaces, are greatly reduced in a given mass of sand or pebbles by proper grading of the particles. The same holds true of a given volume of broken stone. In this grading it is desirable that the coarser particles be in excess of fine particles. Just how this grading is accomplished by variation in the size of grains or particles can be illustrated by drawing a series of circles to represent pebbles. It will be noticed that if we lay out a series of circles of uniform diameter, all touching one another, there will be spaces between them in which smaller circles may be drawn, also touching the others. After these have been drawn it will be possible to draw still smaller ones and so on. Imagining these circles to represent pebbles, one can see how the grading of particles in a mass of sand, pebbles or broken stone reduces the voids or air spaces, and will also see that the larger particles form the bulk of the mass. Sand thus graded, for example, presents a smaller area of surfaces than the same volume of fine sand, hence will require less cement to fill the few remaining voids and bond or bind the particles together. This point is brought out to show that it is true economy to go to some trouble to secure a graded sand, because of the saving in cement. This applies strictly to those uses of concrete associated with the usual building construction. There is an exception to the foregoing statement, which will be referred to again, and this exception applies more particularly to some of the concrete work to be described later. What has been said about grading of sand may be repeated as to the grading of pebbles or broken stone, that is, the coarse aggregate, because the requirements of grading for sand apply literally to the grading of pebbles or broken stone.

A bench, some flower vases, and a well curb and platform. These well illustrate the range of use of concrete for such objects.

Bank-Run Material

In any volume of natural run of bank material in a so-called gravel pit, there will be voids, or air spaces, averaging somewhere around 45 per cent of the volume. Sometimes the volume of voids may be less, sometimes it may be more, but it is fair to consider 45 per cent the average that will be found in a series of tests for all kinds of bank-run material from all sources.