Aggregates-Sand And Pebbles Or Broken Stone

The strength of any structure cannot exceed that of the materials of which composed. A wood building is weakened if the timbers of which it is built are not of uniform strength throughout. Wherever a weak stick is used for a stud or post, that portion of the structure has less strength than where the material is uniformly of a higher grade. Nothing is more certain to produce unsatisfactory results in concrete work than aggregates which are not suited for use in concrete construction.

Aggregates Defined And Described-Sand

Sand and pebbles or broken stone used in concrete mixtures are referred to as aggregates. The sand is known as fine aggregate. Pebbles and broken stone are called coarse aggregate. It is necessary to distinguish between sand and pebbles or broken stone by fixing some limits of size for these materials. It will be noticed that the term "pebbles" has been used instead of "gravel." Gravel, as ordinarily understood, means material taken from a natural bank or deposit composed of particles of all sizes ranging from dust to even small boulders. If we examine several such natural deposits, we will notice that there is no uniformity among them as to the quantities of fine and coarse materials contained. One pit or bank will be nearly all fine material-fine sand-while another may contain a large quantity of sand and pebbles mixed, and some stones several inches in diameter.

Sand is defined as that fine material free from clay, loam, vegetable, or other foreign matter, in which the particles range in size from 1/4 inch downward to the smallest sized particles permissible, exclusive of dust. The maximum dimension of 1/4 inch is fixed merely because it is convenient to start with that dimension for defining coarse aggregate, which is pebbles or broken stone ranging in size from 1/4 inch upward. A maximum size is fixed for coarse aggregate because it is not convenient nor desirable in most concrete work to use pebbles or broken stone exceeding certain maximum dimensions.

Several illustrations showing how concrete helps to ornament the home and park grounds.

Usually pebbles and broken stone are limited to 1 1/4 or 1 1/2 inches in greatest dimension, but for most small objects 1/2 or not to exceed 3/4 inch is the limit.