Again, part of the resistance of concrete to crushing is due to the frictional resistance of one piece of aggregate to moving on another; and consequently for this reason broken stone is better than gravel. It is well known that broken stone makes better macadam than gravel, since the rounded pebbles are more easily displaced than the angular fragments of broken stone. Concrete differs from macadam only in the use of a better binding material; and the greater the frictional resistance between the particles the stronger the mass or the less the cement required.
A series of experiments made by the City of Washington, D. C.,* to determine the relative value of broken stone and gravel for con crete, which are summarized in 4 396, gives the following results: Each result is the mean for two 1-foot cubes, except that the values for a year are the means for five cubes.
A series of forty-eight experimentst using four different propor tions of concrete, each having mortar equal to 40, 50 and 67 per cent of the volume of the stone or gravel, and the stone and the gravel be ing the same in all the experiments, gave average results as follows: The gravel had 40 per cent of voids, while the broken stone had 47, which favcred the gravel. The strength of the gravel concrete approaches that of the broken stone as the age increases, which is probably due to the internal friction of the broken stone having a greater relative effect at the earlier ages, i.e., on the weaker con crete.
All of the above results seem to show that broken stone makes a stronger concrete than gravel.
Relative Density. Experience in practice shows that gravel concrete is more easily compacted and has fewer cavities in it than broken-stone concrete; and hence, other things being the same, gravel concrete is denser and more waterproof. The specific gravity of gravel is generally greater than that of broken stone; and hence the gravel concrete is generally the heavier—usually a desirable quality. In general, any rounded material like sand or gravel gives under similar conditions a denser concrete than an angular material, like screenings or broken stone.
However, since gravel is liable to contain so much clay or loam as to materially reduce the strength of the concrete, some engineers prefer broken stone to gravel for this reason alone. Even though only portions of the gravel are naturally dirty, or even though only portions of it are likely to contain an undue amount of stripping, some engineers prefer broken stone to gravel owing to the greater care required in inspection and to the uncertainty of elimina ing all dirty gravel.
Relative Cost. As a rule, the first cost of the gravel is less than that of broken stone, and the former is also considerably easier to handle.
Since gravel is frequently cheaper than broken stone, a mixture of broken stone and gravel may make a more efficient concrete than either alone, i.e., may give greater strength for the same cost, or give less cost for the same strength.