salvage the old material for replacement purposes on other streets. and grade out and install a concrete base. A newly laid block pavement will not give as secure a foundation as it would if left undisturbed and where it had been pounded down by heavy traffic for generations.

Another point to consider is the contour of the old pavement as compared with the contour or cross-section of the new. If this difference is great, it has to be made up of new material. In the case of standard two-course sheet asphalt, the difference is made up by the use of extra binder so as to bring the bottom of the wearing course an inch and a half under and parallel with the finished surface. If of one-course bituminous pavement, such as Bessonite or asphaltic concrete, the difference is made up with the use of the same material, all of which is, of course, an added expense. In citing this, it is the purpose of the speaker to endeavor to show that a minute and careful inspection of the old pavement is necessary before the improvement is started and both before and after utilities. are installed. Complications of this sort are bound to arise and it has frequently been necessary to change resurfacing projects to standard construction because these matters were not carefully looked into beforehand. Assuming that the old pavements are in fair condition, resurfacing is a paying proposition and old pavements which have given good and practical service to the public for years-such as stone block, cobble, brick, macadam and gravel-will serve as an excellent base for bituminous pavements.

No matter what the character of the old material to be used as a base, it should be swept clean of all dust and foreign matter. If of stone block or brick it should be well washed, using a hose if necessary, and then well broomed. If the joints are of sand the tops will be blown out to some extent, but this will give the bituminous material an excellent bond. Care should be used not to blow out the joints too deep. It has been the speaker's recent experience that where blown out too deep, unequal compression results, which shows up in the surface of the bituminous material.

If the old material is of macadam or gravel, it should also be broomed clean of all dust and foreign matter. Light sprinkling is sometimes desirable in this case. It is well to cut test holes beforehand so as to obtain the exact thickness of the old roadbed. Not only is this necessary in order to obtain first-hand information regarding thickness of base, but very frequently the crown of old macadam roadbeds between curbs is too high, and in attempting to reduce the crown by cutting off some of the old material the base is sometimes rendered too thin and its usefulness impaired. If contour of old roadbed is too irregular, scarifying and slight resurfacing may be necessary, in which case the surface should be then thoroughly compressed and put under traffic several weeks before beginning resurfacing operations. In case of small depressions in the old sur

face, the cushion coat is desirable that is to say, these depressions should first be filled up with bituminous material and thoroughly tamped or rolled; frequently the old material is in such shape that this method will bring it approximately to the proper contour for wearing surface.

With reference to the economical thickness of new bituminous material for the wearing surface, where one-course material is used there is a great diversity of opinion on this subject. The speaker has resurfaced streets of very light traffic, where the original pavement was of well-laid vitrified block, with a minimum thickness of wearing surface of three-quarters of an inch. Here, however, one has the advantage of a well-laid original pavement of good surface and contour. This could not be done with safety where the old pavement is of rough granite or spall. It is safer, however, to establish a minimum thickness of an inch and a quarter. The maximum thickness depends entirely on conditions and what is necessary to bring the pavement to the proper contour. If over three inches in thickness, it should be laid in two layers, both of them thoroughly compressed. Excessive thickness is to be avoided; if too thick, lack of compression is liable to result as well as excessive cost. Factors of safety in this respect for light traffic are approximately as follows: One and one-quarter inches for minimum thickness and three inches for maximum thickness, or an average of approximately two inches over the entire area. On heavy traffic streets, however, the minimum. thickness should be increased to one and one-half inches, and the maximum to whatever depth that is necessary to bring the pavement to a good contour, avoiding excessive depth as far as possible, principally on account of cost.

If the old pavement is of well-laid brick, in fair condition, it presents a much simpler proposition. Usually on such pavements a uniform thickness of one and one-half inches throughout can be maintained. If the old brick surface is rough or in pot-holes, the method prescribed for rough granite block can be used. While we are dealing somewhat with methods of construction, the question of gutters of some hard material very naturally arises. We all know that bituminous material does not wear well against the curb, particularly on grades under 2 per cent; dampness is almost bound to exist, because all the debris, leaves, etc., finally land next to the curb. It is best to install gutters on all bituminous streets, regardless of grades and conditions. It is true, of course, that the matter of cost arises. Where the matter of parking machines and automobiles does not arise, a narrow gutter of concrete or brick is all that is necessary, say twelve inches in width; this keeps the bituminous material away from the curb, protects it from dampness and at the same time we all know how difficult it is to roll bituminous material directly against the curb and maintain a good grade. The existence of gutters, no matter how narrow, enables us to maintain a much better cross-section. Where the parking of machines is to be con

sidered, a wider strip, say from eight to ten feet in width, of either concrete or brick, is desirable.

A thoroughfare with a wide bituminous strip through the center, with a strip eight or ten feet wide on each side, between the bituminous material and the curb, of some hard material such as concrete or brick, can be maintained in a most serviceable condition. It will be found that this section can often be put into use under resurfacing conditions by allowing the old pavement to remain in place under the bituminous strip.

WOODEN DECK BRIDGES WITH REINFORCED ASPHALTIC CONCRETE.

By D. T. Black, City Engineer, Galt (Ontario), Canada.

Many wooden deck bridges having trusses and floor joists of sufficient strength to carry the loads of present-day traffic are being condemned by reason of the excessive vibration arising from the worn and uneven surface as well as the excessive cost in maintenance and renewals of the plank surface.

Many of these bridges can be saved and given years of extended life by using a reinforced asphaltic concrete surface.

A series of experiments were undertaken by the writer four years. ago to obtain, if possible, something to hold the asphalt together regardless of cracks which will sooner or later develop and have caused failures in the past.

Many ideas were considered, such as canvas of various strengths and weights, all of which were discarded. Asphalt of various percentages of bitumen met with the same fate.

At last wire netting was considered in different sizes of mesh and wire gauge.

Test blocks were made 18 inches square having a 12-inch close binder for the base and asphaltic concrete top 1% inches thick with one layer of reinforcing netting between the two courses. Both courses were laid hot and firmly tamped.

After the slabs had been cooled to 40 degrees Fahrenheit, they were placed on thin boards blocked up and offering a very springy undersurface, and struck severe blows with a heavy hammer until cracks and failure resulted.

A second test was made by suspending the slab from one end, and struck on both sides with the hammer.

Exceptional and highly encouraging results for adhesion were obtained from the slabs reinforced with 2-inch mesh chicken netting, No. 19 gauge. The others gave fair results but failed more quickly. The small mesh gave less adhesion and the larger mesh did not hold the small sections where the cracks developed close together.

The City of Galt has three bridges (truss) crossing the Grand River, having a 4-inch oak plank floor laid diagonally, which after four years' use became so much worn and uneven to be dangerous, and with a very severe vibration.

The cost to replace these floors in oak was $4,192.23, and these tests were carried out to find a substance whereby asphaltic concrete could be adopted for a wearing surface. The cost for the 12-inch close binder, 2-inch mesh netting and 12-inch asphaltic concrete top was $2,778.00 for the three bridges, and was adopted.

Each bridge was taken care of in rotation and was applied to the old worn and uneven top as follows:

The bridge was swept and the surface sprinkled with asphaltic oil, the binder was applied to fill up the low parts and give an even depth over the high parts of 11⁄2 inches and rolled as quickly as possible. The 2-inch mesh netting in 6-foot widths, the length of the width of the bridge, was spiked under a 2-inch by 4-inch wooden curb at one end. Men were stretching the netting and spiking under another 2-inch by 4-inch wooden curb at the other side, while others were tying the joints with tie wire and spiking the netting through the old bridge deck every three feet each way.

This work proceeded rapidly to keep as much heat in the binder as possible, and the asphaltic concrete top, 11⁄2 inches in thickness, having 92 per cent bitumen, was quickly applied to obtain a perfect adhesion between the two courses.

The rolling and finishing was carried out as quickly as the roller could work on the hot surface to complete by compression the two courses as one piece.

The bridges averaged 350 feet in length and were reopened to traffic within 24 hours.

The bridges have now given three years service, subjected to heavy traffic, and show a long lifetime. The wear is very slight, very few cracks, no vibration and a smooth surface, which allows of fast traffic instead of the slow traffic formerly enforced.

DISCUSSION

PRESIDENT HATTON: The discussion on this is by Mr. R. H. Simpson, City Engineer, Columbus, Ohio.

MR. SIMPSON: From the standpoint of economics, I know of no question now being discussed by municipal and highway engineers more important than the salvaging of old pavements. We may differ on pavement types; on construction details and methods, but I believe we will all agree that when it becomes necessary to reconstruct a pavement, it is a sound economie policy to utilize whatever value there is in the old one.

You are no doubt all familiar with the practice of redressing and resetting granite or other kinds of stone block as well as that of

relaying brick and other types of block pavement, but I believe that. the practice of leaving the old pavement undisturbed and covering it with some type of new surface is more generally employed.

The covering of old pavement surfaces with some type of asphalt is not a new idea. It has been practiced in some localities for some years past. There is in Columbus, Ohio, an asphalt surface laid over an old boulder pavement in 1888. It is only in recent years, however, that the practice has become more general. Some of this work has consisted in the application of a thin layer of sheet asphalt directly on the old surface, but a more satisfactory and more enduring job can be obtained, for a relatively small additional cost, by the use of a binder or black base course over the old surface. If a pavement is so rough and uneven as to require a new surface, it is very difficult to correct the condition by the application of a thin layer. It must be leveled up to some uniform contour before laying the new wearing surface; otherwise, the finished work will be uneven, or the asphalt coating will vary so much in thickness as to make it impractical to secure uniform compression under the roller. In many cases the uneven surface of the old pavement is such that it is difficult to form a proper contour by attempting to fill in slight depressions, and the practical thing to do is to cover the entire roadway. If underground structures are installed or renewed the repairs to the cuts contribute to the uneven condition of the old surface, and the best way to secure a high-class pavement is to build up the old surface by the application of a binder course. The binder also serves to strengthen the old base.

Where the old surface is not too uneven a course of asphaltic concrete, stone-filled sheet asphalt, or other similar types may be used with success. My experience indicates that under heavy traffic, a thickness much less than two inches will not prove to be satisfactory. I have in mind a one-course wearing surface laid over a brick surface on a roadway which contained a double track street railway. A one-foot strip along the rail was relaid with brick high enough to provide a shoulder one inch in depth, against which to lay the asphalt. After one year's service the asphalt broke up at a few points adjacent to this shoulder, under heavy traffic.

I wish to emphasize a point brought out by the speaker, and that is the importance of determining whether or not the old pavement has a salvage value. If necessary, excavations should be made to determine definitely the sub-base conditions. If it is found that the curbing or the old paving material has become displaced by frost action or because of some lack of proper drainage, we may be sure that there is an unsatisfactory sub-base condition or a weakness in the foundation, or both, which if extended over a considerable area would make the use of the old pavement as a base for a new surface impractical. Such conditions can generally be observed when the frost is coming out of the ground or after a prolonged rainy season, and the ground is saturated with water. There is in Columbus a brick