after that some people in Des Moines thought that if they laid good pavements of wood in the old country they could do the same thing here, and they tried treating the wood blocks with creosote. The first blocks treated had an uncertain quality of oil and an uncertain quality of wood, but the general results were good. This process was followed by treating the blocks with a creosote-resin solution, made of equal parts of creosote oil and resin. There were no requirements in the early specifications as to the character of the oil, and I think the first pavements of that character under general specifications open to the public were laid in Brooklyn in 1903.

A street was paved with wood in 1902, the city prepared the foundation, and the contractor furnished the blocks and laid them for nothing, as an illustration of what the pavement was. The pavement today is in practically as good condition as when it was first laid

We have

In preparing our specifications the question was put up to me, "How do you know our pavement is going to have this treatment?" We did not wish to have an inspector to inspect the blocks, and decided to have an absorption test and a specific gravity test for the block as follows: The block was dried for twenty-four hours at a temperature of 120° F.; it then should not absorb more than 3 per cent. of water after being immersed in water for twenty-four hours, and it was also provided that all blocks should sink when put in water. We let quite a number of contracts under those specifications, with no expansion joints, and have had almost no trouble. some streets that have been down some seven or eight years, and have had absolutely no trouble with them. I stated that the composition used was creosote oil and resin. We did not want to make a specification that would be an absolute monopoly, and although we did not think that more than one man would bid, we made the specification "creosote and resin, or some other suitable material." However, a new contractor turned up as the lowest bidder, and he did not use resin, and my chemist had several weeks wrestling with the analyses and the oils before he could determine if the material was suitable water-proofing material; as near as he could tell it was pitch, but we finally decided it was a suitable water-proofing material and accepted the contract and the blocks were shipped.

Another point is that wooden blocks require a certain amount of

traffic to keep the blocks matted down and to prevent any moisture getting into the blocks and softening them up.

There are quite a number of moot points in the manufacture of wood-block pavement, but the principal question is the character of the wood; then the cushion for the blocks, and the character and quantity of treatment. Now I think there is no question that the best material for the pavement itself is hard pine, long-leaf yellow pine, although that perhaps is not necessary in a residential street. It is hardly possible for a street to wear out, such as we have in Brooklyn, but it must have a certain amount of traffic to keep it down.

The only other material that we have used to any extent is the black gum. The Government is making an elaborate set of experiments to determine not only what is the best wood, but what are some of the cheaper woods that can be used for paving purposes and give satisfactory results, as the hard pine is becoming dearer and dearer, as well as the cost of laying. I think it absolutely safe to use the hard pine.

And now it comes to the question of the cushion: the original pavements laid in Brooklyn had a cushion of cement mortar of 1:4 mixed as dry as it could be. If it was too soft, the heavy blocks would settle in it, and an even surface could not be obtained. We did, however, get an even surface with the wood blocks laid on that cushion. I think the cement cushion is better than the sand cushion.

Then there is also the question of the filling in the joints, which sometimes are filled with sand, sometimes with cement grout, and sometimes with pitch. When there is any traffic in the Brooklyn streets you hardly see any joint at all. We want fine sand so that the joints will be filled, and it seems to me that with this pavement. you get a satisfactory pavement. I do not know but the benefit of not bulging in wet weather is due to having the sand joint. Some people of the West are using the bituminous filler, but, as often happens, the blocks bleed, and the pitch in the joints simply adds to the trouble and makes the nuisance greater. If a bituminous filler is used, it acts as an expansion joint, and there should not be so much bulging with the bituminous filler joint as with the other fillers.

The treatment of the block itself is an undecided question so far as experience is concerned. The question is, shall the block be treated with a plain creosote oil, oil obtained from water-gas tar, or a creosote oil to which enough pitch has been added to give it a

specific gravity of 1.10. The specific gravity of ordinary creosote oil will run from 1.03 to 1.06. The requirements are to preserve the block from decay and prevent its deformation on the street, which if fulfilled should give a pavement that will last as long as the filler that is put into it. In addition to preserving the blocks from decay they must be kept in place and must not absorb water enough to expand in wet weather, or lose moisture enough in dry weather to become loose. The only reason that I prefer the creosote oil is that it will preserve its characteristics longer than the light gravity oils, and it will prevent the water from swelling or shrinking in hot, wet, or dry weather. I prefer a 1.10 oil. The oil is introduced under a

vacuum.

I think the wood-block pavement is more slippery than any, and practically I think that is the only thing you can say against it. Of course, in dry weather, if it is absolutely dry, there is no slipping on it, but if there is a little dirt on it when the pavement is damp, a slimy, slippery surface is produced.

The cost of that paving is $3.20 to $3.50-almost the same as granite laid with the same kind of base and under a five-year guar

antee.

Most of the European blocks are made of soft wood. I have seen blocks which were originally 6 inches worn down to 4 inches. When a truck runs over that it will crush the gravel into the soft blocks, which it might not do with hard pine.

TABLE SHOWING PROPERTIES OF A PERFECT PAVEMENT AND PERCENTAGES ASSIGNED TO THE DIFFERENT STANDARD PAVEMENTS.

DISCUSSION

SOLOMON SWAAB: Is it not fair to assume that if you eliminate the horses and use the motor the figures will change entirely?

A. Yes, the whole problem changes then.

In Brooklyn we have kept accurate costs from 1902, and we have a ten-year curve showing a very uniform cost. Asphalt is not a positive substance that you can tell how it is going to act. Paving fifteen years old cost 9 cents per yard per year. If you strike an average, there would not be so much difference between the Buffalo pavement and the cost of the Brooklyn pavement. In Buffalo there are records of repairs up to twenty years out of guarantee. They say there, however, that they have not had enough money to keep their pavements in proper repair, and that if they had a little more money, it would have cost perhaps 10 to 15 per cent. more than is shown.

A diagram received from the city of Washington shows a remarkable condition. The cost runs up to 2 per cent. for the first year under guarantee, being practically the same as Buffalo and Brooklyn; it then runs along up to 3, 4, and 5 until it gets to the nineteenth year under guarantee, and when the pavement is about twenty-four years old, to about 64 cents, which is the highest cost; from the nineteenth year on it begins to drop. The pavements out of guarantee twentynine years cost 2 cents a yard for repairs. It is a remarkable condition, but then Washington is a remarkable city-abnormal, I may say, in that respect. The streets are wide, the traffic is light, and the traffic is also fairly equable, and there is enough money to keep the streets in repair.

MR. SWAAB: Where you have 6, 7, 8, and 9 cents for maintenance, does that mean that you have maintained that pavement in its original form? Does it not depend upon what standard you use?

A. Absolutely; but in Brooklyn we have kept these pavements in absolutely good condition. In Buffalo they make the explanation that they might have spent 10 to 15 per cent. more. In Washington the pavements have been kept in good condition.

I have a scheme for determining at what time it was proper to relay an asphalt paving:

Let N = Life of proposed paving.

C=Cost per square yard.

I= Rate of interest.

R = Estimated cost of repairing if distributed over entire life.

A = Sinking fund to be paid each year to equal C at the end of N years. X+CI+R/N = the annual expense of new paving.

If this annual expense is more than it is costing to keep the pavement in repair, it is not policy to relay it. Perhaps, by spending 15 cents a yard on a pavement this year, it might be put in such condition that next year it would be necessary to spend only five cents.

It seems to me that this is a scientific way to determine when to relay pavements; that is, for a sheet pavement. Granite cannot be determined in the same

way.

Karri wood is used a little in New York, on Twenty-second Street near Fifth Avenue. It did not give satisfaction. Hard-wood pavements require almost no repairs in the first five years as compared with the soft woods.

WM. EASBY, JR.-Are there any standard surface mixtures in use; that is, standards with reference to the grading of sand, amount of filler and bitumen used? Also, has it been possible to adhere to such standards?

A. We have been figuring on that to quite an extent this past winter. The real thing in asphalt is to have the right kind of bitumen; that is the thing which is absolutely necessary; then the grading of the sand is important, and also the filler, as you suggest. There is much questioning among engineers as to the grading of the sand. There are two schools-one believes in the fine mixture, and the other, in the coarser. I believe in the finer sand myself, and the more bitumen you can carry, the better your pavement will be and the longer it will last. For a heavy traffic street I think you should have from 11 to 11.5 per cent. of bitumen. I have forgotten exactly what the grading of the sand is, but I favor the use of that which will all pass a 10-mesh sieve. It is almost impossible to find a sand that will give a perfect grading, and the contractors, as a rule, use the best they can get in any locality. They mix it to get it right.

The construction of an asphalt street is a very difficult problem, and it is a strange fact that when the asphalt companies keep an accurate record, mechanically and chemically, of how they lay all their pavements, and then duplicate the best as nearly as possible, they do not get the same results.

The first asphalt pavement I had charge of was in Omaha, Nebraska, in 1882 and 1883. Omaha is about as difficult a place in which to maintain an asphalt pavement in as any in the country, as the natural sand grading there is bad. Omaha's summer temperature is from 140° in the sun to about 110° in the shade, and in winter I have seen it 30° below. However, that pavement referred to above was down for twenty-five years; it was laid by rule-of-thumb, and it was a better pavement than many laid today by scientific principles.

A few rules were used in Omaha thirty years ago. They are doing better now; but what I mean is that it is very hard to get the mixture just right. Asphalt is an artificial mixture and sometimes gets too hot; if the sand or the asphalt gets too hot, nobody knows anything about it, because the man in charge of the work knows enough, usually, to keep quiet. If the asphalt is good, and is laid with a good grade of sand under good conditions, it should not fail.

Q. Is there not less chance of burning the asphalt now with the steam coil? A. Yes; and if you use the oil fuel, you can easily maintain the heat as desired. W. H. FULWEILER.-With reference to the treatment of the wood block: you spoke of the proper kind of oil to be used. Is there any accelerated test by which you could determine to a reasonable degree the uses to which this oil could be put; that is, as to the complexity of the oil itself. Would you have to wait twenty years to be able to tell whether the oil was suitable or not?