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The Sand

grains, voids, cement, limestone, proportion, cent and pail

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THE SAND - The asphaltic cement is mixed with inert mineral matter, mainly sand, to form the wearing coat. The min eral matter constitutes about 90 per cent of the wearing coat, and its character and composition has an important effect upon the quantity and durability of the pavement.

The sand should be (1) clean and (2) sharp, and (3) be composed of grains not easily crushed, and (4) have as small a proportion of voids as possible. 1. It should be free from clay, loam or vege table matter, because these substances are devoid of cementing power, are easily reduced to powder, and will prevent the cement from adhering to the sand grains. 2. The sand should have sharp angular grains rather than smooth round ones, since the former afford a better surface for the adhesion of the asphaltic cement, and since sand with angular grains is much less mobile and hence is more easily cemented into a solid mass which will not flow under traffic. 3. The sand grains should not be easily crushed by the traffic, for, if they are broken after the pavement is laid, numerous surfaces will be produced which are not coated with cement, and to that extent the pavement will be weakened. 4. The sand should contain as small proportion of voids as possible, since less asphaltic cement will then be required.

The ideal condition is that each grain shall be coated and all interstices between the grains be filled with asphaltic cement, and hence the smaller the proportion of voids the less the cement re quired. Again, the asphaltic cement is more or less of a liquid having capillary action between the sand grains, and therefore the smaller the interstices between the grains the greater the force of attraction between the liquid cement and the sand. An example of the adhesive force of a limpid liquid is seen in a sand beach when the tide has just left it. The water holds the sand grains so firmly that a wagon can be driven over the sand without leaving a mark; but when the sand dries out, it becomes loose and mobile. This seems to indicate that the finer the sand the better; but fine sand is usually less sharp than coarse, and the finer the sand the greater the surface to be coated, and hence the greater the amount of asphalt required. The asphalt is not only more expensive than the sand,

but it is leas able to resist displacement by pressure; and conse quently the greater the amount of asphalt present the more expen sive the pavement, and the more liable it is to flow under traffic. On the other hand, the smaller the voids, the greater the binding action of the cement; and also the finer the sand the smaller the voids, although the per cent of voids may be greater than with sand having grains of graded sizes.

As far as is known, no experiments have been made to determine the relative value of different sands for asphalt pavements; but apparently the best sand is that in which the coarse and fine grains are so adjusted that the finer grains fill the voids between the coarser ones. To secure this condition, it has been the custom to mix a certain amount of pulverized limestone with the sand. The limestone dust is used to fill the voids between the coarse sand grains, and thus to secure at once a minimum surface to be coated, the smallest interstices, and the least per cent of voids.

The proper proportion of pulverized limestone depends upon the fineness of the limestone and of the sand; and the best proportion, i. e., the one having the smallest per cent of voids, in any particular case can be determined only by trial. To do this, thoroughly mix the sand and the limestone dust in some definite proportion and then determine the per cent of voids in the mixture. To determine the voids, procure a deep and rather narrow vessel, say a tin pail, and find the weight of water required to fill it. Next fill the pail with the mixture of sand and limestone; and then slowly pour in all the water the pail will hold. The weight of water required to fill the pail containing the sand and limestone dust divided by the weight of water required to fill the empty pail is the per cent of voids in the mixture of sand and limestone dust.

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