It must also be remembered that the variations in moisture may be anywhere between the low for dry to the high for wet condition. In pavement work the great advantage of this method is that as it gives assured strength at a definite time, pavements may be safely opened to traffic in half the curing time usually allowed. A second advantage is that a low water-cement ratio with a puddling consistency gives a concrete strong in resistance to erosion. The water-cement ratio is in satisfactory use by two large railroads and by the city of Cleveland. Duluth has just completed a street in the wholesale district by this method, doing half a width at a time to maintain business and traffic. This pavement through the addition of a little more cement and through a longer mixing time developed a strength of 2000 lbs. compression per sq. in. in 8 days, so the specification allowed the pavement to be put in use at the end of that period when the temperature was over 50 degrees Fahrenheit, or in 11 days when it fell between 40 and 50 degrees Fahrenheit, with the added provision that whenever it developed a strength in excess of 2000 lbs. it could be opened for traffic. Tests of cylinders of this concrete show: 38 crushed at 4 days gave an average strength of 2136 lbs. per sq. in. 7 crushed at 5 days gave an average strength of 2434 lbs. per sq. in. 4 crushed at 6 days gave an average strength of 2448 lbs. per sq. in. Joints, longitudinal, transverse, and construction have only lately received the consideration they deserve. Lack of provision for expansion has resulted in the map-cracked surface of many of our earlier pavements. Concrete expands and contracts and with each expansion there is an accumulation of length, which if not provided for, causes internal stress and subsequent cracking. Researches show that unreinforced, a slab of 25 ft. or less in length gives the fewest cracks; that reinforced, a slab 50 ft. or less gives the fewest cracks. Practice has also shown that pavements divided longitudinally into strips from 9 to 12 ft. wide of the usual depths do not crack. These two considerations have brought about a division of the pavement into traffic lanes and of the slabs into lengths of from 30 ft. to 60 ft. In some cases expansion joints are provided at the ends of the block, and at intermediate distances metal plates to form a tongue and groove joint are inserted. Another method is to provide for contraction by inserting down from the top into the concrete at the intermediate distances 2" tapered wooden strips leaving a gap in the reinforcement. When the strip is pulled a plane of weakness is left, which provides for the breaking of the pavement on contraction. The rough fractured surfaces are so interlocked that they allow the load to be carried across the joint. Longitudinal joints are constructed tongue and groove with or without metal plate, the slabs being dowelled together; or they are plain butt joints which depend on the friction at the edge to help transfer the load. The importance of the transfer of load can be readily appreciated as the corners and edges are the weakest parts of the pavement. With a tongue and grcove joint dowelled, from 1/3 to 2/3 of the load or stress is transferred from one edge to the other. Where dowels are used to transfer the load at transverse joints, one end of the dowel must slip back and forth in the pavement. End clearance of the dowel for expansion of the slab must be provided at time of construction. Steel reinforcement is being used more generally in all work. Increased edge and corner resistance are being provided. An increased area of section of steel along the outside edges of the mesh sheet takes care of the edges. At the end of the slabs a part of a sheet of mesh is superimposed to provide additional resistance to corner weakness. The type of fabric sheet mentioned is standard with all manufacturers of mesh. In city work where street openings are necessary the smaller members of the mesh are more easily cut through than bars are, and they can be replaced by tying in a small section of mesh to the members in the pavement which are bent back when the cut is made. The most important conclusions of research on steel reinforcement are contained in the report of the fact finding survey of C. A. Hogentogler in the "Economic Value of Reinforcement in Concrete Roads" published by the National Research Council. The resurfacing of old pavements with concrete is not wholly a new development, since we have other examples like Toledo, Ohio, where a 2" mesh reinforced resurfacing was done in 1912 and reported to be in good condition in 1925. Of immediate interest, however, is the mile of resurfacing of old concrete with reinforced concrete by New York State in 1925 known as the Syracuse Test Road. In this test the major problems to be solved were: How much of an old pavement can be salvaged? What is the minimum thickness we can build over an old pavement? How much steel is it economical to use? Will the cracks in the old pavement come through and cause disintegration of the new surface? It is too early to draw positive conclusions, but certain indications are apparent with the pavement now 14 months old carrying a traffic of 10,000 vehicles per 24 hours. The resurfacing varied in thickness 2 3/4", 3′′, 3 1/2" and 4". It was reinforced with 42 to 73.9 lbs. per hundred sq. ft. of mesh reinforcement without any bars, and other sections reinforced with bars alone had from 55.4 to 117.6 lbs. per hundred sq. ft. The indications are: That while the ordinary 1 1/2" to 2" slump is suitable for a full depth slab, resurfacing in thin sections requires a 2 1/2′′ to 3′′ slump. That in curing it is necessary to follow closely with burlap and sprinkling if check cracks are to be avoided. That where the old full width pavement is cracked through the center the new work should be divided longitudinally. That with increased thickness there is less tendency to crack. Otherwise the pavement has stood up too well to furnish the data expected of it. Some of these recent developments open the way to further research. There is still much knowledge to be sought and found, but by using what we now have, we can be reasonably certain of building good concrete roads today in both city and country. DISCUSSION PRESIDENT HATTON: Mr. Breed has a very valuable paper, and the Chair will now entertain any discussion of it. MR. CORSON: It is very gratifying to me to hear Mr. Breed's paper, the reason is that in 1908 we laid a mass concrete pavement without reinforcement, but we did use expansion joints for the protection of that pavement in lengths the width of the street. In other words, the distance between curbs was 26 feet and at 26 feet we put the expansion joints. If any gentlemen care to come to Norristown, Pa., I would be pleased to show that old concrete pavement, which was laid before we had the niceties of construction of today. It is 6 inches thick at the curb and 8 inches on the center with no center-line expansion joints. In 1908 concrete street pavements were in their infancy. I have seen state highways adjoining the town where the expansion joint was 50 or 75 or 100 or 200, and in one instance a local engineer had a county concrete highway to build 18 feet wide and he told me the reason he put the expansion joints at the end of the 2,000-feet stretch was to put it all in one place. He said, "It is a slab, and let the expansion come to that point instead of having it scattered along the work." It did not take more than three or four months to show the fallacy of that method. As it was torn up and rebuilt ten years ago. I, myself, think that under the circumstances cited a little while ago, that with the niceties of calculation for construction of concrete road surfaces, the nearer we make the joints equal the width of the pavement the better the result will be. PRESIDENT HATTON: I was impressed with one particular thing in Mr. Breed's paper and that was the question of illuminite cement. During the present year I have had occasion to use some illuminite cement for foundations under railway tracks where the tracks could not be kept out of operation for more than two days. I was very much surprised at the success attained by the use of this illuminite cement; and I was also very much surprised at the cost of it. Until the manufacturers of this illuminite cement can get a product that is within reasonable cost, I do not think there are many engineers who will be willing to take the chance of using it, as it may be used to decrease the interference with traffic within our cities and on country highways. In short, it is up to Mr. Breed and the remainder of us engineers to convince these manufacturers that they have a product that is too expensive. RECENT DEVELOPMENTS IN THE DESIGN AND CONSTRUCTION OF CEMENT CONCRETE PAVEMENTS AND BASES. By A. R. Hirst, Chief Engineer of the American We are supposed to talk about recent developments in the field of concrete pavements and bases. Unfortunately, there are no really striking recent developments except some about which modesty forbids us to speak. We take it that this is not a proper ground for advertising one's own pet hobbies. Therefore, we are going to talk in general terms about the design, construction and inspection of concrete slabs-especially the necessity of competent inspection, than the lack of which there is nothing else. but-in far too many cases. The city of Dothings wishes to pave a certain avenue. The city fathers decide that they will pave it with cement concrete. Promptly the proponents of concrete and of everything possible to be used in connection with it besiege the City Hall with suggestions for the betterment of the job. The City Engineer listens more or less patiently, makes his surveys and plans and draws his specifications. Every point is considered, every doubtful item weighed, and when the plans and specifications are turned over to the city council they call for a very fine piece of work and workmanship. The work is advertised and is, as usual, awarded to the low bidder. This happens to be Ignoramus Workwrecker, the well known hay wire exponent. The alderman from the ward, being one of the boys, is given the political plum of appointing the inspector. He names Donothing Buteat, whose principal occupation in life is gone since those nice buildings on the corner were closed and the rails and tables removed. The specifications are left at the city hall because neither the contractor or the inspector could understand them if they had them. Besides, there is soon such an entente cordiale established between the two that such writings between friends are entirely unnecessary. The engineer is so busy in the office splitting hairs about the next job coming up that he is only able to visit this one a time or two for a few minutes. He finds many things not so good, but he is volubly assured by contractor and inspector that these occurrences are absolutely unprecedented, that up to that very hour the job was fine and that the extraordinary combination of circumstances which caused the observed lapses cannot and will not again happen. The job runs it course, is finished, accepted and paid for. The city of Dothings has bought 40,000 square yards of pavement for |