During 1924 and 1925 about 100,000 square yards of 411⁄2" "Black Base" with 11" sheet top was laid on native clay and loam subgrade in Quincy, Illinois, under our supervision. The base mixture on this work was extremely dense, by virtue of using a well graded crushed rock coarse aggregate, from 21" down, in size. In every instance this base was laid in one course directly upon the natural subgrade and in some cases the nature of the soil made construction difficult in wet weather, but perhaps no more so than would have been the case with any other type of base, except possibly in the Aldo Boulevard district, which had formerly been a nursery and the soil was "made" for this purpose. It was so sponge-like that some hardship was experienced in two blocks of pavement owing to the difficulty of rolling the base and top on the quaky soil. In this particular case the unstable subgrade should have been covered with a layer of gravel or broken stone, but under the Illinois law governing Special Assessment work, it is impossible to introduce an extra of this kind without endangering the legality of the project. Once the pavement was laid, however, no further trouble developed.

In connection with one of the pavements laid in 1924, viz., East Broadway, "Black Base" was chosen in preference to a rigid base because a sewer had just been laid down the center of the street, and since the soil was clay, it was almost certain some settlement would occur. The City Engineer (W. P. Bushnell) felt that if such settlement did take place, repairs to a "Black Base" pavement would be much simpler than with Portland cement concrete, as the bituminous concrete would eventually follow the subgrade, and a surface patch in the topping would restore the uniformity of contour. A few such areas did subside very slightly, as shown in the photograph which was taken two years after construction. These Quincy pavements, including grading, cost from $2.25 to $2.55 per square yard, whereas, Portland cement concrete pavements laid by the same contractor the same year, cost $2.62.

Another example of deliberately taking advantage of this same virtue of flexible base was in the case of the approach to a bridge on Nelson Park Road (East Wood Street) at Decatur, Illinois, in 1923. "Black Base" was selected for paving the bridge approach over a heavy fill (which was known to be still settling) although the remainder of the project leading to the approach employed a Portland cement concrete base. Heavy rains in the fall and spring following construction caused under-cutting at the edges and the pavement followed the subgrade down. These depressions were removed by simply applying new mixture to the surface in the usual way.

Thus it will be seen that contrary to first thought, the fact that "Black Base" does maintain contact with the subgrade may well be one of its strongest features. If the usual rigid base had been used in these cases, the entire depth of slab, as well as the surfacing. would have required replacements on account of certain rupture of base due to settlement of subgrade.

During the same year, 1923, the Brush College Road, just outside the City of Decatur, Illinois, was paved with 12" sheet on 51%" "Black Base." Since this project has been previously described in detail, only mention is made now, calling attention to photographs taken this summer after three years of service. Not a single defect of even the most trivial nature can be found. Notwithstanding the fact that at least one well known highway engineer said at the time that this experiment on Illinois black soil would fail, the pavement bears every possible evidence of a long and economical service period ahead. Indeed, observation of this particular piece of road has lead the Decatur Park Board to select "Black Base" for paving the park drives.

"Black Base," or better, bituminous concrete foundation for pavements, has established itself in the minds of many engineers and public officials. As soon as the average individual sees how it can be applied to meet the widely varying conditions as to subgrade, climate and rehabilitation of old worn-out pavements, he can readily appreciate its value. No other form of base can be so readily adapted to almost any conceivable local condition. It is the best known medium for resurfacing old brick and block pavements, likewise old worn-out gravel and macadam roadways, to say nothing of the constantly increasing yardage of worn-out Portland cement concrete pavements. Indeed, many engineers design Portland cement concrete pavements with curb heights suitable for the application of an asphalt top when the concrete shall have become cracked and uneven, as it surely will, so that it no longer can be used as a traffic floor.

"Black Base" began as one of the very earliest types of foundation for asphalt surfaces. Portland cement concrete came upon the market as a cheaper material and the attention of engineers throughout the country was directed to the development of this type of rigid base and pavement, for the moment forgetting its predecessor, "Black Base." Then asphalt became cheaper through the great development of the petroleum industry, and simultaneously the cost of Portland cement went up, so that today, in most localities, equal thickness of pavement of the "Black Base" type, regardless of the type of surfacing, will cost no more, and usually less, than Portland cement concrete. This is by no means an idle statement but is the result of very careful observation and inquiry among contractors in the paving business throughout the country.

"Black Base" construction is not experimental as is the case with rigid base. It is not doomed to certain inherent defects that to date are practically impossible of remedy, as is the case with Portland cement concrete, due to moisture and temperature variations. Its development has been sound and logical and when reasonably good engineering judgment is used in design and equally good construction methods are provided by the contractors certain success is bound to follow. This does not mean it is entirely "fool-proof." It is not, but

its use is attended with less hazard and provides greater economy than any other known type of pavement foundation to date.

Its use is not confined to bituminous surfacing. All forms of brick and block may be used with greater assurance of success than attends these many types laid on rigid base. The accompanying photograph of East Clinton Street, N. J., shows brick laid on "Black Base," and examination of this street will show nothing to be desired in the way of a fine brick pavement.

Frequent reference has been made to "Black Base" laid in Washington, D. C., about half a century ago and still in service. Omaha, Nebraska, laid 41⁄2" "Black Base" with 11⁄2" top in the years between 1889 and 1893 to the amount of more than 100,000 square yards. In 1924 these pavements were still in highly serviceable condition, only 2,143 square yards having received a new top during a life of 31 to 35 years. Thus the longevity of "Black Base" is thoroughly established. And when we add to this highly important feature its low maintenance cost, low first cost and versatility of application we have included all of the major considerations affecting the selection of a desirable pavement structure. And thus will be seen the many sound reasons why "Black Base" has become firmly established as a standard type of pavement foundation.

"BLACK BASE AND ITS PLACE IN STANDARD

SPECIFICATIONS"
DISCUSSION

By Harry F. Harris, Mercer County Engineer, Trenton, N. J. The principal advantages resulting from the adoption of bituminous concrete foundations as a satisfactory substitute for Portland cement concrete foundations has been so thoroughly covered in the excellent paper presented by Mr. Skidmore than there remains very little which can be added by way of discussion.

The paper, however, emphasizes two points very clearly, which doubtless account for the rapid increase in the use of this type of base: 1. Its adaptability to both rural and city highway work under a variety of conditions.

2. The great possibilities it has in speeding up construction, thereby lessening the inconvenience to both those who reside along our roads and to those who use the roads for either commercial or for touring purposes.

While the speaker's experience with this type of construction has been limited to the construction of less than 50,000 square yards, distributed over a period of the last four years, it has been sufficient, he believes, to thoroughly endorse the claims made both as to its adaptability for a variety of conditions and the economic savings which are made possible by greatly shortening the period of construction.

This latter possibility was well exemplified in the construction of a section of Hamilton Avenue in Mercer County, New Jersey, which avenue extended from the Trenton city line easterly, a distance of about two miles. The road is one of the principal arteries leading from points in the vicinity of the New York metropolitan district to Philadelphia, and is in direct line to the only free bridge across the Delaware River at Trenton. The street, therefore, carries a considerable volume of interstate motor car traffic, much of which is commercial.

Base. Laid 1923.

Slide No. 7-Sanhican Drive. Sheet Asphalt on Black Base. Laid 1924.

(1)

Slide-Hamilton Avenue Cross Section.

(2) Colored Slide--Completed Section of Hamilton Avenue.

After making a decision to try the black base construction principally on the theory of speeding up construction, an estimate was made as to the proper time allowance which should be specified for completing the work.

As time was one of the principal elements entering into the performance of the contract, it was decided to provide a bonus and penalty clause of ten dollars ($10.00) per day. In order to be fair both to the contractors and to the public, the time allotted for completion had to be given careful consideration. If too long a time was specified, it would mean the payment of a large bonus, and on the other hand, if the time was under-estimated, the contractor would be severely penalized. Finally, 121 days was agreed upon as a reasonable and conservative period for performing the entire work, which consisted of not only paving work but extensive drainage and widening operations. Much to the surprise of everyone, the job was completed in just 71 days, which enabled the contractor to earn a bonus of 50 days, at the rate of ten dollars ($10.00) per day. The county gladly paid the five hundred dollars ($500.00), as it was felt that it was fully worth this much to the taxpayers to have the use of the road for this unexpected period.

Another example of how real service was rendered to owners of a large manufacturing plant along the line of a similar road improvement might be of interest.

These owners controlled plants on both sides of E. Clinton Avenue and each day they had to haul considerable quantities of their products by trucks to a nearby freight station, and in addition the street carried quite a volume of cross-town traffic.

(3) Slide Shows picture of E. Clinton Avenue.

The ordinary procedure for a road of this character would have. been to build a concrete foundation six or more inches in depth, and place thereon a block type of pavement or a bituminous pavement of sheet asphalt or bituminous concrete. But in any case it would have nearly put the plant out of business, so far as shipments were concerned, for at least a week or ten days, under the most favorable conditions, while the curing process was under way.

After some thought, we decided that our "best bet" was to introduce black base with a sheet asphalt top as the most desirable type to adopt to meet the conditions confronting us.

(4) Slide-Cross-section of E. Clinton Avenue.

As a result of this decision, the plant never missed a daily shipment during the entire period of construction. It is needless to say that the plant owners expected the worst before the work was started, but were more than pleased with the results of the efforts put forth to assist them.

(5) Slide-Whitehead's Road, brick approach to P. R. R. bridge. How well the black base can be adapted to other forms of surface paving and to heavy grades is illustrated in the view of Whitehead's