The relative value of concrete and uncoursed rubble masonry laid in Portland cement mortar is perhaps still an open question, though it is believed that the former will eventually be preferred by engineers who are familiar with both. Concrete will require in general a larger proportion of cement than does the masonry, so that in localities difficult of access, the masonry may for this reason be cheaper. Usually, however, concrete will be the cheaper, and less skilled labor will be required in the building. With the same amount of inspection, concrete of good materials properly proportioned will form at least as impervious a wall as will rubble.
1 Engineering News, Feb. 27, 1904.
The up-stream face of the dam should be made as nearly water-tight as possible, and therefore a rich concrete employed in which the mortar is in excess of the voids in the stone, and the mortar itself contains about two parts sand to one cement. The body of the wall, however, may be made of a poorer mixture, one to three to six usually being sufficient. Bowlders may also be imbedded in the mass to cheapen the concrete without any serious detriment. Such bowlders should, of course, be sound and clean, and well wet before being placed. They should be kept well back from the face of the wall and should be separated one from another by at least six inches, to allow of thoroughly tamping the concrete between them.
In a wall of rubble the contraction and expansion are taken care of by minute cracks between the stone and mortar which frequently are not noticeable. In a concrete wall, unless provision is made for this, these signs of movement may be concentrated in cracks at intervals of thirty to sixty feet; these are always unsightly, and may in exceptional cases be a serious defect. The remedy evidently lies in so building the dam that if these cracks appear, they shall be confined to predetermined planes where they will not do any serious harm. Such contraction cracks will be very much less likely to occur in a dam arched in plan than in a straight dam, since in the former a slight movement of the masonry up or down stream changes the length of the wall and relieves the tension strains.
The joints in a concrete dam should not be unbroken planes for any great distance. That is, the concrete should be so placed that the joints between work of different days are not planes extending through the wall. The wall may well be kept higher on the down-stream side and step down toward the up-stream side. The vertical joints should also be broken by right-angled off-sets, but the wisdom of using a dovetail joint in such work is very questionable. The joining of one day's work to another necessarily forms a plane of weakness, and therefore the work should be carefully planned to the end that these planes shall be, in direction and location, where they will not unnecessarily weaken the structure or render it pervious to water.
A dam at St. Croix, Wis.,1 was built of sandstone masonry of uncoursed rubble in one-to-three mortar, and faced with concrete of one Portland cement to three parts sand to four parts broken stone of 1 to inch size. The concrete was rammed in place between the stonework and the concrete forms. The selection of the uncoursed rubble was probably made on account of the site being five miles from the railway and the consequent difficulty of getting cement. The dam was arched in plan, and in preparing the foundation, several grooves or trenches were cut in the rock in a longitudinal direction, to avoid, as usual, a through course at the bottom, and these trenches were also filled with concrete. Had the concrete for the facing contained five parts of broken stone having maximum size of 2 or 2 1/2 inches, it would have been more nearly in conformity with the best practice.
In the construction of the dam at the forebay of the Massena water Power Company, Massena, N.Y.,2 it was sought to take up the tension stresses due to contraction by imbedding in a longitudinal direction in the concrete, T-rails two feet apart horizontally and four feet apart vertically.
The dam built in connection with the Butte, Montana, water system is 120 feet high, 350 feet long, 10 feet wide at the top and 83 feet wide at the 120 foot point. The bed rock was granite, which was first covered with four inches of concrete made with small sized stone. In the body of the dam, granite bowlders were thickly imbedded in the concrete, care being taken that each bowlder was entirely enveloped in concrete and that there were no horizontal or nearly horizontal courses either of concrete or bowlders.
1 Engineering News, June 13, 1901.
2 Engineering News, Feb. 21, 1901.
1 Mr. A. B. Moncrieff, Engineer in Chief, Engineering News, April 7, 1904.
The San Mateo Dam of California, one of the highest dams in existence, is built entirely of concrete, 170 feet high. It is 126 feet thick at the base and is arched upstream with a radius of 637 feet. The dam was constructed in blocks of 200 to 300 cubic yards each, of irregular heights, so as to bond the courses together and have no through joints. Concrete, one, two to six, was delivered in small push cars on a high trestle over the dam, and was dropped through iron pipes 16 inches in diameter to the place of deposition. In some cases this drop was 120 feet, and it is stated that the concrete appeared not to be injured by this method of handling.
The Barossa Dam in South Australia 1 is of a bold arch design. The arch has a radius of 200 feet, and the chord is 370 feet subtending an angle of 135 degrees, 20 minutes, and the length of the arc 472 feet. The height of the dam is 94 feet above the ground line, yet the greatest thickness above the foundation is only 34 feet, with a top width of only 4 1/2 feet.
Special care was taken in selecting the materials and fixing the proportions. The cement was aerated fourteen days before use. Test cubes of concrete two feet on a side were prepared with different proportions of materials and subjected to a hydrostatic pressure of two hundred feet before deciding upon the proportions to use in the concrete. As a result of these tests, the aggregate was made up of one part screenings 1/8 to 1/2 inch, two parts "nuts" 1/2 to 1 1/4 inch, and four and one-half parts "metal" 1 1/4 to 2 inch. This mixture contained about 35 per cent, voids. The mortar was made of one part Portland cement to one and one-half parts sand, and was from seven and one-half to fifteen per cent, in excess of voids in aggregate. Plumbs were used in the dam to within fifteen feet of the top, and above this level iron tram rails were placed in string courses. The success accompanying the use of concrete in structures of this magnitude is sufficient evidence of its value and adaptability.