secure weatherproofness even at the expense of stability. To us it seems quite clearly a matter of either slightly underfilling the voids to insure satisfactory contact and interlocking between the mineral particles, or possibly slightly overfilling them to insure that moisture will not enter.

There are many points in Dr. Dow's paper with which I am sure the younger generation will take issue, at least as to their relative importance; and, knowing and respecting the knowledge of Dr. Dow, and also knowing and respecting the vigor of the younger generation, as I have said, a good time will surely be had, much to the benefit of the future state of the asphalt paving art.

With respect to that part of Dr. Dow's paper which deals with a comparison of the results as determined by the voids test, using spheres, I can not see that there is anything whatever to be gained by this comparison. In one case, the accidence of mixing and placing in bulk governs; in the other, conscious selection is the factor. In a twin-pug asphalt mill there is, of course, no conscious selection, but accidence only.

DR. Dow: I believe Mr. Mullen has misunderstood the entire trend of the paper. I do not object to cutting down the bitumen or increasing the dust in cases where it is necessary to do this to meet conditions. What I am opposed to is this indiscriminate lowering of the per cent bitumen and increasing the per cent dust for the sole purpose of increasing the stability of the paving mixtures without a proper study of the materials and the conditions to be met. I know of several cases within the past two years where pavements have been laid so poor in bitumen that they have cracked badly within a few months. There are other properties besides stability to be considered in the design of an asphalt paving mixture.

CONSTRUCTION DETAILS ESSENTIAL TO EFFECTIVE

HOT MIX PAVEMENTS

By Francis P. Smith, of Dow & Smith, Consulting Engineers, New York City, N. Y.

The art of laying Hot Mix Pavements has been developed in this country during the past fifty years. Like most arts with a life history. of similar length, it has made great strides, been confronted with changing conditions and has been hampered with bad habits and traditions acquired during the early period of its existence.

For the first forty years it had to meet the problem of producing a pavement which would successfully carry a slow moving iron tired traffic of light to moderate intensity. During the last ten years the density and speed of highway traffic has increased beyond the wildest dreams of human imagination a decade ago and iron tires have been almost entirely replaced by tires made of rubber. This change in the character of the traffic carried by our roads and streets calls for a pavement possessing the same general characteristics as those of the earlier type but with a marked change in their relative value and importance. Methods of manufacture have also been changed and improved and the daily output of single plants has been greatly increased.

Some of the qualities of a hot mix pavement are inherent in the mixture itself and are fundamentally therefore questions of design. These questions will be covered by others at this meeting. Other qualities involve design, manufacture and laying methods, while still others are functions of the last two only.

It is the purpose of this paper to discuss the necessary qualities and the best methods for securing them, except those which are solely a question of design.

The effective life of a modern Hot Mix Pavement is largely dependent upon:

1. The quality of the mixture.

2. The uniformity of the pavement as to:

(a) Composition

(b) Thickness

(c) Density.

3. Contour of the finished pavement.

Of equal, if not greater importance, are the kind and character of the foundation, subsoil and drainage but these have been considered by the speaker as falling outside the limits of this paper.

In the earlier days much less attention was paid to these details than is now necessary. The factor of safety was much greater because much less was required of the pavement and this inevitably led to

more or less careless work and the acquired habit is hard to shake off. Old time superintendent and formen are inclined to jeer at extreme precautions and class them as "old fogyism" utterly failing to realize that they themselves are the "old fogies".

The strain of modern traffic is so terrific that it calls for the exercise of the most skillful workmanship to enable even the best of materials to withstand it and this is true of every type of modern pavement. In most instances it costs no more to lay a pavement in accordance with the best modern methods than it does to lay it carelessly. The importance of these factors from the standpoint of competitive design and maintenance costs, cannot be too strongly stressed.

The quality of the mixture is dependent upon its design, the character and uniformity of the materials entering into its composition and the methods of manufacture employed. Eternal vigilance is necessary to insure uniformity and quality of deliveries of raw materials to the plant. The chief variation is found in sand and stone de liveries. If these vary to any great extent, it is impossible to make a uniform mixture out of them. The product of stone quarries and sand pits is far from uniform. Major variations in mesh composition, even if within specification limits, call for variations in bitumen and dust. contents of the mixture. If the formula remains unchanged, the mixture will vary markedly in physical characteristics and uniformity will be lost even if quality is not impaired. Dirty stone or sand after passage through the drier frequently has fine dust partly baked on the outer surfaces of the larger grains or particles. This seriously interferes with the proper and permanent adherence of the bitumen coating, and materials of this type should be rejected.

Adequate temperature control is essential to prevent injury to the bituminous binding material. This applies not only to the heating of the bituminous material but also to the heating of the mineral aggregate. Stone or sand which is too hot will damage the bitumen more seriously in a short time than hours of moderate overheating in a large. kettle. In order to secure proper bond between the bitumen and the aggregate all moisture must be driven out of the aggregate and it must be heated to the correct temperature to facilitate mixing and insure delivery on the work at such a heat as will be most favorable to laying and compression. Too low a temperature increases the difficulty of properly and uniformly coating the mineral aggregate with bitumen and makes a mixture which is difficult to rake on the street and impossible to compress effectively. Varying percentages of moisture in the aggregate call for variations in drum temperatures which must be carefully watched. Electrical pyrometers, if properly used, not only indicate temperatures, but are of great aid to the fireman and result in increased output as well as necessary uniformity of mixture. Notwithstanding this, the speaker has had their installation bitterly opposed by foremen for the reason that they would seriously curtail the plant output! The logical deduction to be drawn from such. an argument is too obvious to require further comment. Undoubtedly

the future will see the widespread development of automatic thermostatic control.

Mixer teeth in good condition, shafts operating at proper speed and sufficient time given to mixing are extremely important. The mineral aggregate must be evenly, uniformly and completely coated with bitumen. Too often mixer teeth that are half worn down are kept in service and the time of mixing reduced to increase the output. This results in a mixture in which some of the particles are imperfectly coated with bitumen and some parts of the mixture are excessively rich at the expense of other portions which are too lean. Such a mixture is especially liable to wave under heavy traffic.

Thorough adherence of the bitumen to the mineral aggregate is greatly increased by the sustained attrition of the particles for an appreciable period of time. The principle is the same as that involved in thoroughly brushing out the coat of paint applied to a surface for protective purposes. It is absolutely essential that the film of bitumen should be strongly adherent to enable it to successfully resist water action, weathering and the movement of the particles of the mineral aggregate under traffic stresses.

Uniformity is perhaps a greater aid toward retarding wave formation than is generally recognized. A pavement in which soft rich spots occur, surrounded by a notably harder mixture, is much more liable to form objectionable waves and humps than one which is uniformly soft. In the latter case the displacement will be less localized, i. e., will be distributed over a larger area, and will have a tendency under certain conditions to heal under traffic, which will be noticeably absent in the former case.

Uniformity of composition is largely a matter of plant control. It is dependent upon uniformity of raw materials (chiefly mineral aggregate), accurate weighing and thorough mixing. If any of these are slighted a non-uniform pavement will result. Another source of danger is segregation during transit between plant and street. The coarser aggregates are particularly liable to this, more especially if they are poorly graded and lacking in intermediate sizes of stone, particularly 1/4" and 11⁄2" particles. Most specifications are very lax or indefinite in their requirements as to the amount of 1/4" material which the mixture should contain. When mechanical spreading is employed it is difficult to correct segregation. With hand spreading it is possible to correct it by turning the mixture over with shovels before putting it in place and this should always be done in the case of coarse aggregate wearing surface. It is perhaps unnecessary with binder except in extreme cases. The appearance of the average binder course after laying, especially where intermediate sized stone is absent, plainly shows the extent to which segregation takes place in coarse aggregates during transit.

Uniformity of thickness is chiefly dependent upon the grade of the finished base and the care exercised in raking. It used to be almost an axiom in the bituminous paving industry that any errors in the

grade of the base would be remedied by the binder and these in turn by the top and as it was desirable to have the surface of the concrete or other base rough in order to prevent the bituminous mixture from sliding on the base, this precluded the possibility of laying it accurately to grade. This fallacy led to a widespread disregard of the contour and grade of the finished base which is all too prevalent at the present time. It would appear to be axiomatic that a bituminous wearing surface of greatly varying depth is more liable to displacement under heavy traffic than one of uniform thickness, and yet it is very difficult indeed to convince engineers of the necessity of fine grading their concrete foundations. The speaker at times feels like advocating the use of a template or paving gauge on both the foundation and the finished surface. Roughness of surface should be made subordinate to maintaining grade and can be obtained by the use of corrugated rollers, vibrating mats or similar devices.

The raking of the hot mixture is usually done by eye. Here again a gauge would be an improvement. It takes many months to make a good raker out of a man who is fitted for the work and many men never become good rakers. In former days when the plant output was less than at present and horse-drawn wagons delivered the hot mixture on the street, there was sufficient interval between the loads for the head raker to go over the work of the other rakers and correct their errors. Today this is rarely done. There is no time available and in the speaker's judgment there are fewer good rakers, or possibly the demand for them has increased, which amounts to the same thing as far as any one job is concerned. The use of a long handled screed 4 feet to 5 feet in length operated at right angles to the direction of raking after the raking is completed will correct many of the errors of poor raking. Careless methods of spreading or too small a street gang to handle the plant output frequently necessitates standing in the hot mixture. This inevitably causes variations in thickness unless the mixture where trampled upon is "fluffed" up to the same density as the balance of the mixture by the rakers. With a stiff mixture this is hard work and rakers will not do it conscientiously and thoroughly all day. With the plant pushing the street gang to the limit of their capacity to do good work, this will inevitably happen. The remedy is to increase the street gang, but with a narrow roadway this cannot be done beyond a certain point.

The density of a pavement is dependent upon its design and the amount of compression it receives. In order to secure maximum stability it is essential that rolling should be applied and continued in such a manner as to produce as nearly as possible ultimate compaction. Here again tradition hampers, for in former times areas which were not fully compressed when the pavement was laid received their final compaction under traffic and in most cases no great harm resulted. The danger of such a practice is increased one hundred fold today. Inadequate roller equipment is all too common and the fault is partly due to lack of realization of what is necessary to properly handle the