CHAPTER XXIV

COMPARISON OF ROADS AND PAVEMENTS

The determination of the most economical and efficacious method of construction and maintenance to be employed on highways of various classes constitutes one of the most interesting subjects which the highway engineer has to consider. The solution of the problem depends upon many variable factors, all of which must be given due consideration and the proper value attached to each. The great variety of materials and methods of construction and maintenance used, together with the absence of such essential information as traffic censi, cost data, etc., makes it a difficult matter sometimes to reduce all of the different types of roads to a comparable basis for a given location. In many localities where engineers have adopted some one general method ' for construction or maintenance it may be found that such methods, although they may be successful as far as use is concerned, are not the most economical types which would be equally efficacious.

ESSENTIALS OF AN IDEAL Road or PavemENT. An ideal road or pavement should be durable, noiseless, suitable for road users, easily cleaned and made dustless, non-slippery for horses and all classes of vehicles under varying climatic conditions, easily maintained, should yield neither dust nor mud, have a low tractive resistance, low first cost, low annual cost, low maintenance charge, and an aesthetic and impervious surface.

A consideration of these important characteristics reveals the lack of accurate data which are available relative to each. However, certain information is at hand which materially aids in a comparison of roads and pavements from different standpoints.

Durability. The life of a road or pavement is expressed in (a) the number of years during which it can be maintained in a satisfactory condition at a cost which has not reached an amount

so that reconstruction is economical, and (b) the total number of tons per yard or foot of width that the road or pavement is subJected to during its life. As durability is a function of a number of variables among which the amount of traffic is of primary importance, method (b) conveys more information in many cases than the first method.

In 1911 George W. Tillson, M. Am. Soc. C. E., assigned eighteen years as the average life for sheet asphalt pavements, twenty-five years for granite block pavements, "assuming that only the heavy traffic or streets with steep grades are paved with granite," and twenty to twenty-five years for brick pavements. Rather than give a definite estimate he states that "creosoted blocks have not been in use long enough to enable one to determine from experience how long they will last or what will be the cost of repairs. The first pavement of this character laid in the East was on Tremont Street, Boston, and has been there since 1900. It is now in good condition and has had very little repair done upon it."

The other method is employed by John A. Brodie, M. Inst. C. E., in a comparison of the tonnage life of different kinds of stone block and ordinary macadam.

"Taking heavy-traffic streets first, experience shows that accurately dressed setts-6 inches deep by 6 inches to 8 inches long by 4 inches wide, laid on a sound concrete foundation at least 8 inches deep, with a small sand bed between the underside of the sett and the concrete, the joints being thoroughly racked with hard shingle and afterwards grouted with a permanent pitch mixture, which prevents any movement in the stones, and renders the whole surface perfectly impervious to weathergive a life equal to at least 71⁄2 million tons per yard width, and under these conditions this life has often been exceeded.

"It is at present impossible to say whether an equal tonnagelife can be obtained from the similar shallower 4-inch class of construction in streets taking, say, 60,000 tons per yard of width, as this would mean a life exceeding 100 years; but experience shows that the total average wear of the setts has been exceedingly small, and it is a fact that many such streets exist in

Liverpool today, having a life of from twenty-five to thirty years without requiring any important repairs during that time due to wear of the material.

"When the same quality of stone is used in ordinary macadam laid 6 inches to 7 inches deep on a sound, hand-packed foundation, experience shows that the tonnage-life of the surface before requiring to be recoated is enormously decreased, and instead of 71⁄2 millions, a figure about 100,000 tons only, or about 75 times less than the result for setts previously given, has been obtained on a street of moderate traffic-a somewhat startling difference."

Sanitary Qualities. The sanitariness of a road or pavement is based on its imperviousness, smoothness, and freedom from dust and mud. From the standpoint of the public health noiselessness might properly be considered under this head, as it affects the health of nervous people. L. Mazerolle, Engineer of Bridges and Roads of France, compares certain pavements from this standpoint in the following words:

"From the point of view of hygiene the best road surfacing material is the one which, having no joints whatever, does not absorb the liquids falling on the roadway. Compressed asphalt is theoretically the best road surfacing from this standpoint. Stone block pavements seem subject to criticism because of the apparent infiltration of water through the sand joints. This infiltration is frequently found very slight where old pavements are removed. Wood pavements are often accused of absorbing filthy liquids, but experiments carried out at Paris refute this theory. Altogether it appears that public health is less affected by the nature of the material constituting the road surface than by the quantity of dust produced and by the extent to which cleaning operations are carried out.”

Noiselessness.

George W. Tillson*, M. Am. Soc. C. E., has presented the following admirable discussion on this property of roads and pavements.

"A quiet pavement has become particularly important at this With the strenuous life of the modern cities the nerves of

time.

*See Engineering and Contracting, October 18, 1911.