The compressive resistivity of the usual concretes is considerable. However, in certain bridge construction in New York City, a need was felt for a concrete which should have a very high compressive resistance. And so experiments were made with a concrete formed by substituting wire nails for the crushed stone. About 60 tests were made with concrete following the formula 1:2:2⅔. The resulting material was quite heavy. A cubic foot weighed 196 pounds as compared with 130 to 160 pounds for ordinary concretes. Eighty-eight pounds of nails were used in one cubic foot bringing the cost to about $2.30. This was certainly very expensive material. But where extraordinary qualities are desired, we have to spend money. Cubes were cast measuring 6 inches on a side. These were tested to destruction at different stages of maturity. After the lapse of one week, the lowest crushing resistance obtained was 2,770 pounds per square inch and the highest 3,330 pounds. After one month, the minimum crushing strength was 3,050 pounds, the maximum 8,340 pounds, while the average was 5,645 pounds. When a year had gone by, it was found that four cubes gave an average
of 10,410 pounds. However, the average resistance of 17,235 pounds was obtained in the case of cubes 15 months old.
Since concrete is but little affected by water and by fluctuations between wet and dry conditions, it is not at all remarkable that it has been employed for sewer and water tower construction. In the United States a high standpipe has been constructed at Attleboro, Massachusetts. This is 118 feet high and has an internal diameter of 50 feet. The wall varies from 18 inches in thickness at the bottom to 8 inches at the top. The concrete was made according to the formula 1:2:4. There is another tower 110 feet high and having an external diameter of about 35 feet. At Anaheim, California, a large tank together with its substructure has been constructed entirely of reinforced concrete. The floor of the tank is about 60 feet above the surface. The tank itself is 38 feet in height and 30 feet in diameter and has a wall varying in thickness from 5 to 3 inches. The reinforcement employed was the twisted steel bar.
In order to prevent corrosion of the reinforcement, it is thought necessary to guard against water entering and dissolving away the caustic lime contacting with the steel. One way would be to give the concrete itself a very dense character. Another is to fill the external pores with a bituminous or oleo-resinous paint. Or, an insoluble substance suited to fill the pores may be one of the ingredients when the concrete is mixed. Finally a flexible waterproof coating may be employed where conditions permit. As to the steel itself—it is desirable to have it uniform, as then reliance may be placed upon calculations. For this reason, one of the great concrete construction companies recommends mild steel as opposed to high carbon steel.
One of the great recommendations of concrete is that it permits wonderful rapidity of construction. We had an
example of this in the case of the Geo. N. Pierce automobile factory. Another was in connection with the construction of junction caissons for certain subsurface tubes of the tunnel of the Hudson Companies. These caissons were three in number and were located on the Jersey shore opposite New York City. These structures were quite large, being about 100 feet in length and having a width of about 45 feet. These caissons, one or two of which were put under air pressure, were constructed of concrete with steel reinforcement. The use of concrete in the tunnel system and in the Terminal Building has been very extensive. To complete the concrete construction, about half a million barrels of Portland cement, so it is thought, must be consumed. The Gatun Locks at Panama will require only about four times this amount. The twisted steel bars of the reinforcement have been used in large quantity.
The work on the water front at Baltimore to which reference has already been made involved a considerable variety of reinforced concrete construction. For retaining walls sheet piles were employed. These ordinarily had a face of 18 inches and a thickness of 12 inches and a length of 27 feet. As it was not necessary to retain the soil by an impervious bulkhead, these piles did not interlock. However, they had to resist a horizontal thrust, and so wales were strung along the outside at the top. These wales were themselves of concrete reinforced by means of imbedded lattice girders of steel. In position, the girders lay flat and thus gave their chief strength to the horizontal thrust. The wales were supported, in part, by concrete piers. These were placed by means of steel caissons. These cofferdams were of sheet steel 27 feet deep and were sunk by open air methods. When in place, the concrete was put in and the pier thus formed. An upward surface of the pier provides a means of absorbing the horizontal thrust of the wales. The piers themselves are, some of them, mutually tied together across the dock;
others are tied to reinforced concrete piles sunk in the body of the dock. The ties are themselves of reinforced concrete. The steel of the caissons served only as a mold. It is now a matter apparently of but little importance how soon it corrodes. The extensive concrete work at Baltimore was done by the Raymond Concrete Pile Company.
While the question of the teredo seems to have been a factor at Baltimore because of the probability of its presence in the harbor when certain sewerage improvements are carried out, this matter was really an insistent thing in connection with a wharf constructed by the United Fruit Company at Bocas del Toro in the Republic of Panama. This wharf is itself of reinforced concrete. But the bearing piles are what interest us. The native wooden piling, so it seems, would at this general location become seriously damaged by the teredo within a year. Some kinds of timber might be expected to have a longer life. The service of creosoted piles has been estimated as about 15 years. Besides, piles 70 feet in length were desired. This requirement put the ordinary reinforced concrete piles out of consideration. What was actually done was to use an untreated timber pile and then to encase it where it passed through the water in a reinforced concrete shell. This shell was made of such size as to allow a space between it and the enclosed wooden pile. A rich concrete was put in this space at the bottom and thus excluded the external water. Upon pumping out the retained water, the major portion of the space was filled with a lean concrete and a top layer of rich concrete then added in which the column reinforcement was placed. The steel used for reinforcement was in the main round bars of mild steel. The piles averaged 58 feet in length; the shells, 18.4 feet. The cost of these shells was $1.78 per linear foot. It is said that the cost of the untreated wooden pile together with its protective coating was not greater than what would have been the expense for a creosoted pile.
At both the Baltimore docks and the wharf in the tropics, concrete is exposed to the action of sea water. But there is no violence in this action. However, a very large application of concrete construction has been recently carried out in a very much exposed maritime situation off the coast of Florida. It is 156 miles from the mainland to the island of Key West. Scattered along this interval are a number of islands, so that in reality the total linear amount of intervening land is about one-half the distance. Some of the water passages are only a few hundred feet in width; one is about 2½ miles wide. The greater portion of the aqueous route is of a shallow depth. But for about 6 miles the water reached depths up to 30 feet; and this in connection with an exposed situation. Reinforced concrete viaducts have been built to accommodate trains and resist the storms. A quarter million barrels of cement and about 5,700 tons of steel went into these works.