of Cement and Sand.If the distance from the plant to the street is long or there is unusual delay, some of the asphaltic cement may work down to the bottom of the load, and when the material is dumped there will be both rich and lean spots —both of which are equally objectionable. The rich spots will have a tendency to roll or crowd toward the gutter; and the lean spots will have a tendency to disintegrate under traffic.
The wearing coat may.have been laid on a dirty or damp foundation, and therefore have been prevented front uniting firmly with the foundation (see § 598). This condition will be revealed by a tendency of the pavement to roll or push out of place while sound and firm on the surface.
The wearing coat may not have received sufficient compression. The surface must be thoroughly compacted—particularly in the gutters—to keep out rain water and the acids and oxygen dissolved in it. The effect of oxidation is gradually to convert the petroline into asphaltine, and to leave the bitumen of the flux as the only binding constituent of the mix ture; and therefore the pavement will have a general tendency to crack and disintegrate.
All materials in nature are undergoing changes due to the action of the elements, and asphalt pavements are no exception. The following are some of the principal causes leading to the gradual deterioration of such pavements.
The pavement may decrease in thickness due to loss of material by the abrasion of hoofs and wheels; hut since the surface is smooth and somewhat elastic the loss by wear is almost imperceptible. In come cases the pavement decreases in thickness• with use, but the decrease is due to consolidation rather than to loss of material.
Allasphalts gradually lose their cement ing power with age by volatilization, evaporation, and oxidation. The pavement is peculiarly exposed to the action of the sun's heat, and to the combined action of rain water, acids, oxygen, and frost. The greater the cementing power of the asphalt originally and the softer the cement, the longer the pavement will resist the influence of volatilization and evaporation; and the more nearly the voids of the sand are filled with cement and the more firmly the pavement is consolidated, the longer it will resist the action of water, acids, oxygen, and frost. The general decay of the asphalt will be indi cated by a tendency of cracks to form during cold weather (see § 654), particularly during a sudden and extreme drop in the tem perature.
Aweak or improperly prepared founda tion by unequal settlement or settlement in spots will cause cracks and depressions in the surface which under traffic will speedily en large and cause the pavement soon to break up.
A porous foundation permits the ground water to rise, by capillary action and possibly also by hydro static pressure, to the underside of the wearing coat, where by freezing it may break the bond between the top layer and the base, and thus permit the wearing coat to be pushed out of place and broken. This effect has been known to occur with a concrete foundation; but it is not likely to occur with good concrete. If a section of pavement disintegrating from this cause be examined, there will be found a layer of perfectly sound and good material at the surface, while the lower side of the wearing coat will show evi dence of being disintegrated by water—that is, the sand will appear clean and white in spots as though there had been an insufficiency of asphalt cement to cover it. The concrete base under the affected spot will generally be found to be damp or even wet. The recur rence of this defect may be prevented by underdraining the soil.
Leaky Joints. Lack of a water-tight joint between the asphalt surface and the curb, the gutter, manhole covers, crossings, street-car rails, etc., may permit the water to enter the lower and less compact part of the wearing coat, where by its solvent action and also by freezing it may do material damage. It is nearly im possible to keep these joints tight, particularly adjacent to the street-car rails. The damage often extends a considerable distance from the place where the water enters.
The disintegrating effect of water depends chiefly, if not wholly, upon the contained oxygen, and the effect upon different asphalts varies with the proportion of soluble salts present. Apparently Trinidad asphalt is acted upon to a greater extent by water than any other asphalt; * but it is claimed t that this deterioration can be greatly reduced by removing the soluble salts in the refining process, at a comparatively small expense. For the results of ex periments showing a great variation in the effect of water and of frost upon different asphalt pavements, see Engineering News, Vol. 44, p. 113-15. In these experiments, two samples of asphalt blocks lost considerably less than any one of the seven samples of sheet asphalt.