CAUSES OF FAILURES OF ASPHALT PAVEMENTS. Asphalt pavements have frequently failed; but when it is remembered that the industry is new and has been rapidly developed, and that there was no precedent, and that therefore the proportions and the methods of mixing and laying had to be determined by actual expe rience, it is not astonishing that some pavements should fail. But the permissible variation in the various ingredients and in the different details of the work is so small, that if good results are expected, the utmost care must always be exercised. Asphalt pavements have some advantages not possessed by any other forms of pavement, and will doubtless always be laid to a considerable extent. There fore the engineer who is responsible for such work should be well informed as to the various steps in the construction of this class of pavements.
Unfortunately the custom has been to contract with asphalt paving companies to lay asphalt pavements and to guarantee them for a term of years, and consequently the municipalities have as a rule made no analysis of either the asphalt or the flux used, and have not examined the and or limestone dust nor paid any atten tion to the methods employed in mixing and laying the materials. The result is that there are no public records showing the history of the pavement, and therefore it is often impossible to determine the cause of either failure or success. City officials should carefully analyze the ingredients and examine the method of mixing and lay ing the materials, not only to secure the best possible pavement, but also to obtain data to serve as a guide for similar work in the future. Owing .to differences in materials, climate, and traffic, a considerable part of such data must be obtained for each particular city.
On account of the lack of such data, it is often impossible to determine certainly the cause of the failure in any particular case. The following are some of the principal causes of the failure of asphalt pavements: Unsuitable Material.The service demanded of a pave ment is quite severe, and to attain a reasonable success each of the three components—the asphalt, the flux, and the sand—must be carefully selected.
Asphalt.The asphalt may have been so changed by nat ural causes as to possess little or no cementing power (see § 649); or it may have contained a soluble salt which was subsequently dissolved by rain water, thus leaving the pavement porous and subjecting it to the disintegrating effect of the acids and oxygen in the rain water as well as to the effect of the freezing of the water in the pores of the pavement.
If the asphalt is deficient in cementing power or is unduly dis integrated by the action of acids, oxygen, etc., this fact will gener ally first be indicated by a premature tendency of the pavement to crack, particularly during cold weather (see § 654).
Flux.The fluxing agent may not have been a solvent of both of the constituent parts of the bitumen of the asphalt, and may have formed a mechanical mixture instead of a chemical union (see § 604). Or the flux may have contained volatile oils which finally evaporated from the pavement and left it porous and devoid of cementing power (see § 604). The use of an improper fluxing agent will produce much the same effect as an asphalt deficient in binding power (§ 586).
Sand.The sand may have been too coarse, or too fine, or have contained too much clay or vegetable matter (see § 621-24). If the sand is too coarse, or is dirty, the pavement will have a ten dency to crack. If the sand is too fine, or is deficient in sharpness, the pavement will have a tendency to roll or push out of place— particularly under a heavy traffic,—and the surface may be marked by the traffic in hot weather.