MAINTENANCE OF ASPHALT PAVEMENTS.
To give good service asphalt pavements must be kept clean. On account of the smooth surface and absence of joints, cleaning may be readily accomplished; and the presence of dirt, especially in wet weather when it is likely to cause the surface to remain damp, is liable to cause the asphalt to rot. More than any other pavement, therefore, the durability and wear of an asphalt surface depends upon its cleanliness. The presence of dirt upon asphalt in damp weather is also important in its effect upon the slipperiness of the pavement.
Small repairs of any breaks that may occur in an asphalt surface may be easily made, and such repairs should be constantly attended to in order to keep the surface in good condition. Small breaks will rapidly extend if they are not repaired at once. In making repairs to the surface of the pavement it is necessary to cut away the surface for a short distance about the imperfect spot, stripping the surface from the founda tion and cutting the layer down square at the edges, after which a new piece of surface may be introduced to fill the hole in the same manner that the original surface was constructed. Such a patch may ordinarily be put on so as to make joints that will join perfectly with the old pavement and not show where it has been placed. When a surface has become so worn that patches would be numerous, the old surface may be stripped off and a new one placed upon the original foundation. When repairs are to be made upon a pavement having a bituminous base it is more difficult to cut out the holes in satisfactory shape, as there is no well defined joint between the base and the surface layers.
The repairs that may be required upon an asphalt pavement depend, of course, upon the solidity of con struction and the nature of the surface material. There is so great variation in the materials employed for the wearing surface that, as would naturally be expected, very considerable difference in wear is shown by dif ferent pavements.
It is common to require contractors for asphalt pavements to guarantee the pavement for a period of years, making all necessary repairs and leaving the work in good condition at the end of the period. This makes it an object for the contractor to do good work, and may sometimes be the most effective way of secur ing it where so many elements of uncertainty enter.
In general, it is not desirable to require contractors to guarantee paving for a long period on account of limiting competition and increasing unnecessarily the cost of the work. With asphalt paving, however, many engineers consider the difficulty of control during construction, under ordinary circumstances, such as to make a guaranty necessary, while the fact that the material is for the most part controlled by a few large companies renders the guaranty less undesirable as restricting competition. This method has, however, been found unsatisfactory in many instances, on account of the difficulty of enforcing the guaranty.
The cost of maintenance of asphalt pavements varies widely in different places, depending upon the character of the construction used and the local, conditions sur rounding the pavement. In Washington, D. C., the average life of the surface before renewal is about 20 years, while the annual cost of maintenance is about 2.5 to 2.8 cents per square yard per annum. In loca tions where the surface is kept continuously damp, particularly if it is not kept clean, the asphalt is apt to deteriorate rapidly and, in some instances, scales off and gradually disintegrates. The resistance of asphalt to water action depends very much upon the density of the surface mixture and the ease with which water may penetrate it. Great care should be used in laying pavements where moisture conditions are not good to secure a dense surface mixture in which the voids are well filled. Where water may continuously run in the gutters, it is usually better to construct the gutters of other material less affected by the action of water.
Injury to asphalt surfaces from illuminating gas escaping from leaking mains has been observed by Mr. A. W. Dow at Washington, D. C. The heavy hydro carbons of the gas are absorbed by the bitumen of asphalt, which is thereby softened and caused to cut and flow under the traffic.
The cost of maintenance depends largely upon the system employed in the maintenance work. In some cities repairs are made only at considerable intervals when the surface is in bad condition, and in such instances the ultimate cost is usually much larger than where small repairs are made as they are needed to keep the surface always in good condition.