CONCRETE ROADS Although concrete pavements were laid at Bellefontaine, Ohio, in 1893 and 1894, the type did not attract general attention until fifteen years later. During 1913, over 10,000,000 square yards were laid, eight times as much as in 1911. This rapid development was accompanied by marked differences in methods of construction, which aroused some apprehension that poor results from inferior methods would retard the logical adoption of the type in places for which it was suited. Accordingly road and street engineers and contractors from all parts of the country met in February, 1914, for a three-day discussion of concrete road building. The report of this conference exercised a standardizing influence on methods of construction, as had the somewhat earlier adoption by the American Concrete Institute of standard specifications for concrete roads and pavements. Some of the methods of construction presented features which required detailed investigation, and committees were appointed for such work. In February, 1916, a second national conference was held, at which the reports of these committees were received and discussed. This chapter summarizes the information presented at that conference. Foundation and Subgrade The following opinions regarding foundations and subgrade were adopted by the 1916 conference: When roadways are constructed over fills, extreme care should be observed to insure the use of proper materials in layers of such thickness that they may be thoroughly compacted so that when the fill is completed there will be a minimum of settlement. In general, fills shall be made in thin layers, the depth depending on the character of material to be used in making the fill. The fill should be allowed to stand for as long a time as possible, giving it an opportunity to settle thoroughly before the pavement is placed thereon. Deep fills should be allowed to settle through one winter wherever such procedure is possible. Puddling will be found advantageous in compacting deep fills. Wetting and rolling shall be performed when making a fill in order to secure thorough compactness. Fills should never be made with frozen materials nor with lumps greater than 6 inches in their greatest dimension. The fundamental requirement of the subgrade is that it should be of uniform density so that it will not settle unevenly and cause cracks in the surface of the pavement. No part of the work is more worthy of intelli gent care and painstaking labor than the preparation of the subgrade. The slight additional cost necessary to insure good results is abundantly justifiable. When the pavement is constructed on virgin soil, care should be taken to remove all soft spots so as to insure a uniform density; and if constructed on an old roadbed, even greater care must be taken to secure uniform density, as the subgrade is likely to be more compact in the center than at the sides. An old roadbed should be scarified, reshaped and rolled. The subgrade adjacent to curbs should be hand-tamped. The importance of a uniformly firm support for the concrete slab was not fully appreciated by all roadbuilders. There was an opinion among some that the concrete would act as a beam and distribute the loads coming on it over such a wide area that inequalities in the sustaining power of the earth would prove unimportant. The number of cracks in concrete pavements attributable to unwarranted confidence in this beam action is beyond proof, but today the opinion is generally held that the money spent in securing a firm foundation is a wise outlay to insure low maintenance charges. This is particularly the case where an old road is used for a foundation. It is unlikely that a concrete pavement will be laid on it until the old road needs repairs. If the surface is then merely leveled by a thin course of stone or gravel, it is possible that there will be weak places, particularly along the sides of the road. Drainage The 1916 conference adopted the following statement of the principles which should govern the drainage of the roadbed supporting a concrete slab: The drainage of the roadbed is of vital importance. If the subgrade is not well drained there is danger of unequal settlement and of frost action, which will cause cracks. The method of drainage to be used will depend on local conditions. For streets, as well as roads, tile drains may be used which should be laid on each side of the roadway, or on one side only, with cross-drains leading thereto at a suitable depth, depending on the width of the pavement. Drainage trenches, if placed under the subgrade, should be completed before final rolling. There is an objection to the use of cross-drains under thin concrete roads which is not serious in most cases but may be under some conditions. It is practically impossible to compact the material over a blind drain as thoroughly as that of the main portion of the subgrade, and each blind drain is likely to be a weak place in the foundation. An alternative for them which some engineers have advised is the construction of a blind drain just outside each edge of the concrete slab and extending 8 or 10 inches below the subgrade. These drains are connected every 50 feet with the side ditches by blind drains. All drains under the roadway should have a covering of sand or fine gravel on top to prevent the mortar in the wet concrete from passing into the broken stone or gravel of the drain. Cross-section of Roads The following statements were adopted by the 1916 conference regarding the section of the concrete pavement: The thickness of a concrete road or pavement is controlled by many factors, each of which should be given consideration. In view of the increasing use of the heavy motor truck and bus, it seems unwise to build pavements with a thickness of less than six inches at any point. In general, pavements should be thicker at the center than at the sides. Alleys with an inverted crown, and narrow one-slope roads, should have a uniform thickness. Wherever the thickness can be increased without excessive cost, to secure a flat subgrade, or one nearly flat, such increase is advisable. The desirable width for single-track roads is 10 feet. The desirable width of double-track roads is 18 feet. The total width of the roadway should not be less than 20 feet for single-track roads and not less than 26 feet for double-track roads. The crown of roads and pavements should be not less than one onehundredth nor more than one-fiftieth of the total width. Except in unusual cases, one one-hundredth will be sufficient for country roads and onefiftieth will be considered satisfactory for alley pavenents. For city streets an average crown of one-seventy-fifth will generally be found sufficient and should not be reduced, except on grades. Single-track concrete roads are occasionally built on the righthand side of the roadway going in the direction of the heavy traffic. This gives the loaded wagons the better surface. The templet used in determining the section of the road is the same as for a double-track road but only one-half the road is concreted. In Huron and Medina Counties, single-track roads have been built as one-slope roads instead of half the section of double-track roads, and this construction is preferred by some engineers. In Wayne County, Michigan, it has been decided to make concrete roads 18 feet wide wherever possible, and never less than 16 feet. Near Detroit the width will be 20 feet. While this increase in width from 10, 12, 15, and 16 feet will add materially to the first cost, it is expected to prove economical in the long run, because the expense of maintaining heavy broken stone or gravel shoulders will be avoided. This maintenance is a large sum on comparatively narrow roads with heavy traffic. There is a decided difference of opinion regarding shoulders for concrete roads. Some engineers hold that the natural soil should be used, and where satisfactory shoulders cannot be made of it the width of the concrete should be increased. Other engineers recommend gravel or broken stone shoulders after experience with them. The difference of opinion is probably due to differences in the quality of the earth which must be used for earth shoulders and to differences in the character of the traffic, upon which the opinions are based. There is no question, however, that if traffic makes frequent use of shoulders of clay or clayey loam, they are speedily ruined, and if heavy traffic makes frequent use of macadam shoulders, the junction between the concrete and broken stone becomes a long rut. In either case the proper maintenance of the shoulders will be expensive. If two macadam or gravel shoulders wider than 4 feet are considered necessary, it is advisable in every case to consider the alternative of an increased width of concrete. The construction of gravel and macadam shoulders should be postponed, if practicable, until a month after the concrete road has been finished, and they should be left slightly higher than the concrete to facilitate turning out on them. In cuts, where the grades are over 5 per cent, it is usually necessary to pave the ditches and to use gravel or macadam shoulders. The maintenance of these ditches and shoulders is expensive, and it is possible that money will be saved in the end, even on single-track roads, by increasing the width of the concrete and adding a concrete ditch and curb at each side. The ordinary construction will call for a 10-foot concrete road, 8 feet of shoulders and 8 feet of ditches, a total of 26 feet width of cut at the bottom of the excavation. For this can be substituted a concrete roadway 16 feet wide with two integral curbs bringing the total width to 17 feet 8 inches. The curb acts as an abutment for the toe of the slope. On fills over 5 or 6 feet high, where turning out on a soft shoulder may cause a serious accident, it is desirable to widen a singletrack pavement to 16 feet unless the top of the fill is so wide that an overturned car will not roll down the slope. In any such case, the safety of the public requires a more careful study of the dangers on embankments than was given to the subject before automobiles became numerous. Attention is called to the record of deaths and injuries on page 25. Materials Cement is bought for most road work under the standard specifications of the American Society for Testing Materials, which were revised in 1916. They are printed in the next chapter. The following statements regarding fine and coarse aggregate were adopted by the 1916 conference: The selection of proper aggregates for concrete road construction is of utmost importance. Clean, hard, well-graded materials are absolutely essential to success. For this reason samples of the materials proposed for use should be submitted to the engineer for approval before orders are placed. These samples should be carefully inspected; and if possible laboratory tests made to determine their suitability. If laboratory tests on shipments cannot be made, field tests can be used to furnish a general indication of quality. The different aggregates should be kept clean and separate. Aggregates to be used in the wearing course of two-course pavements should never be placed on the subgrade but on planks or some other means provided to keep them free from dirt. When aggregates are placed directly on the subgrade care should be used by the shovelers to avoid getting clay or earth shoveled from the subgrade into the mix. Aggregates should not only be clean when they are delivered on the job, but clean when placed in the mixer. Investigations to determine the usefulness of the rattler test to show the value of different concrete mixtures for road work indicate that it may prove of value. For the present, however, the older practice of relying on tests of the stone is being followed wherever any testing is done. Generally the best clean, hard and tough crushed rock or gravel is used, provided it will give a concrete harder than the mortar used with it. It is desirable to use stone having a French coefficient of wear of at least 8. Experience indicates that cracks occur more often in gravel concrete than in stone concrete. Probably this is largely due to the very fine material on the surface of most gravel pebbles, which must be washed off carefully to make the material fit for road work. Fine aggregate, or "sand," is generally required to pass a 1inch screen. Not more than 25 per cent must pass 50-mesh sieve and not more than 5 per cent a 100-mesh sieve. It must contain no vegetable or other deleterious matter and not over 3 per cent by weight of clay or loam. The sand should be tested frequently in the field by shaking a sample with water in a graduated glass and allowing it to settle for an hour. If there is more than about 5 per cent of very fine material showing on the top of the sand, samples should be sent to the laboratory. Natural sand or screenings from hard, tough, durable rock may be used. Natural sand sometimes contains vegetable acids which reduce its value for good concrete. Their presence is determined by making similar briquettes of the natural sand and of standard Ottawa sand, and no natural sand should be used in road work which does not give a strength at least equal to that obtained with Ottawa sand. The best sand is that in which the coarse particles predominate. Improvements can sometimes be made by mixing two natural sands or a fine sand and screenings. The standard specifications for coarse aggregate, or "stone" call for material passing a 2-inch round opening, with not more |