Illuminating Gas. Ordinary illuminating gas, escaping from leaky pipes under the pavement, is absorbed by the pavement, and causes the disintegration of the asphalt.$ It has been deter mined experimentally that "one volume of asphalt cement will absorb forty-five volumes of illuminating gas in something over a month; and it has been demonstrated practically that pavements do actually absorb illuminating gas from leaky mains, in one in stance 1,000 c.c. of pavement giving off 500 c.c. of gas which it had absorbed. It has been shown that asphalt is much softened by absorbing gas, the ordinary asphalt becoming as soft as maltha after being in an atmosphere of illuminating gas for several months. There is but one way to stop the disintegration of a pavement from this cause, and that is to stop the leak of gas. "* Pavements affected by illuminating gas first give signs of their disintegration by a slight depression over the affected spots, later fine cracks appear parallel to the line of the street, and finally the surface coat begins to crowd.
Cracks.Long irregular cracks in the wearing surface frequently occur during cold weather. They usually start at the gutter or man-hole frame, and gradually extend across the street. They are often found at the joint between an old and a new pave ment or at the joint made between one day's work and another. These cracks are due to the contraction of the wearing surface, and should not be confounded with cracks due to the failure of the foundation (§ 650). Usually these cracks do not occur until the pavement is two or three years old; at least they are most likely to occur in an old pavement—one in which the asphalt has lost part of its cementing power by age. These cracks appear sooner and in crease more rapidly on a street having only a light traffic. When the pavement is subjected to a continuous traffic, the asphalt sur face, which is more or less plastic at all temperatures, is kept from cracking by the constant kneading action of the traffic. Again, when an asphalt surface has but little or no traffic, it becomes more porous owing to expansion and contraction from heat and cold without the compression due to traffic, and as a consequence is materially weakened. If cracks occur on a street having a fair amount of traffic, it is evident that the paving mixture used is at fault; either there was not enough bitumen or the asphalt cement was too hard.
Some engineers leave expansion joints, i. e., cut the wearing coat through, at intervals to prevent these irregular contraction cracks. Such a procedure is of doubtful propriety, since the pave
ment if properly constructed will not crack in several years under the most adverse conditions, and then only at long intervals and generally at some old joint; and if the pavement is improperly made, the expansion joint will have only a slight tendency to pre vent these irregular cracks. The principle of the expansion joint is not applicable to materials with no structural strength, like asphalt mixtures. These joints are not only useless, but really detrimental to a pavement; they are only another form of the defect they are intended to remedy, for they are crevices which retain mud and water which tend to rot the asphalt, and the edges of the joints are easily broken down by traffic which also widens the crack.
It is particularly unfortunate that an asphalt pavement is likely to crack, since not only do the edges of the cracks disintegrate, but the cracks permit water to reach the interior of the pavement where it has a deteriorating effect.
Shifting under Traffic. The surface coat sometimes flows under traffic, i. e., pushes lengthwise of the street into waves or crowds toward the gutter. This defect occurs in pavements having too soft a wearing surface, or where there is a defective bond either between the base and the binder, or between the binder and the wearing surface. This is a defect that is impossible to guard against entirely on a street having very heavy traffic, and especially where the traffic is confined to a narrow section of the street; but this de fect is inexcusable on streets having only moderately heavy traffic. This flowing is commonly caused by the surface of the hydraulic concrete base under the pavement being too smooth, which is the case where gravel concrete is used or where a stone-and-gravel con crete is so rich that its surface is covered with mortar that was brought to the top by ramming. Unless the binder and the surface mixtures are made very hard, a condition which makes the pave ment likely to crack, the wearing coat will slide on such a base if there is much traffic. Pavements often roll from a defect in the •inder—either because it was too rich in asphaltic cement, or be cause it was dirty when the wearing surface was laid.
Damage by Bonfires. Another cause of damage to asphalt pavements is the building of fires upon them. Of course this ought never to occur, but even in the best regulated municipalities it does happen.