THE MATERIALS.
The cement has been fully described in Chapter IV.
The sand is considered in Art. 1 of Chapter V.
When concrete is considered as mortar with pieces of hard material embedded in it, the mortar is called the matrix and the coarse material the aggregate; but sometimes con crete is considered as a mixture of cement, sand, and coarser material, in which case the cement paste is called the matrix, the sand the fine aggregate, and the stone or pebbles the coarse aggregate.
The coarse aggregate may consist of small pieces of any hard material, as pebbles, broken stone, broken brick, shells, slag, cinders, coke, etc. It is added to the mortar to reduce the cost; and within limits the addition of a reasonably strong aggregate also adds to the strength of the concrete. Ordinarily either broken stone or gravel is used. Coke or cinders are used when a light and not strong con crete is desired, as for the foundation of a pavement on a bridge or for the floors of a tall building.
Screened vs. Unscreened Broken Stone. It is sometimes specified that the broken stone to be used in making concrete shall be screened to practically a uniform size; but this is unwise for three reasons, viz.: 1. With graded sizes the smaller pieces fit into the spaces between the larger, and consequently less mortar is required to fill the spaces between the fragments of the stone. Therefore the unscreened broken stone is more economical than screened broken stone. 2. A concrete containing the smaller fragments of broken stone is stronger than though they were replaced with cement and sand. Experiments show that sandstone screenings give a con siderably stronger mortar than natural sand of equal fineness, and that limestone screenings make stronger mortar than sandstone screenings, the latter making a mortar from 10 to 50 per cent stronger than natural sand.* Hence, reasoning by analogy, we may conclude that including the finer particles of broken stone will make a stronger concrete than replacing them with mortar made of natural sand.
Further, experiments show that a concrete containing a considerable proportion of broken stone is stronger than the mortar alone (see the second and third paragraphs of § 294). Since the mortar alone is weaker than the concrete, the less the proportion of mortar the stronger the concrete, provided the voids of the aggregate are filled; and therefore concrete made of broken stone of graded sizes is stronger than that made of practically one size of broken stone. 3. A single size of broken stone has a greater tendency to form arches while being rammed into place, than stone of graded sizes; and consequently does not make as strong or as dense concrete.
Therefore concrete made with screened stone is more expensive, less dense, and weaker than concrete made with unscreened stone. In short, screening the stone to nearly one size is not only a needless expense, but is also a positive detriment.
The dust should be removed, since it has no strength of itself and adds greatly to the surface to be coated, and also prevents•the contact of the cement and the body of the broken stone. Particles of the size of sand grains may be allowed to remain if not too fine or in excess. The small particles of broken stone should be removed, if to do so will reduce the proportion of voids (1 Gravel vs. Broken Stone. Often there is debate as to the relative merits of gravel and broken stone as the aggregate for con crete. The elements to be considered are strength, density, and cost.
Relative Strength. In Chapter VI it was shown that finely crushed stone gave mortars of greater tensile and compressive strengths than equal proportions of sand; and hence, reasoning by analogy, the conclusion is that concrete composed of broken stone is stronger than that containing an equal proportion of gravel. This element of strength is due to the fact that the cement adheres more closely to the rough surfaces of the angular fragments of broken stone than to the smooth surface of the rounded pebbles.