Part 37

CONCRETE AND STEEL. 322

BY J. F. SPRINGER.

[New York author of articles in Applied Science.]

The life of properly made concrete is not known. However, specimens from the times of the Romans are yet in good condition. This material has very considerable ability to resist compression; it is practically fireproof and teredo proof; when properly protected it is probably but little deteriorated by weather changes; and, if properly made, it is probably inappreciably subject to chemical disintegration when submerged. But there is one palpable fault--it is weak when subjected to tensile stresses. On the other hand--steel has great tensile resistivity and is strong under transverse stress. These two materials--concrete and steel--supplement each other in valuable qualities. The possibility of using them in combination depends largely upon the fact that their co-efficients of expansion are practically the same for moderate thermal fluctuations. Steel is easily corroded. Nor is it strictly fireproof, as temperatures which are not excessive will induce bending and buckling. When it is surrounded by concrete, steel is protected against both fire and corrosion.

In many situations, steel would not alone supply the best material of construction. And the same remark applies to concrete. A striking instance is the case of the six new docks in Baltimore. Three of these had already been constructed of wood and stone, when it became evident that the building of the remaining three and of the long bulkhead which was part of the scheme along the same lines would entail a larger expense than the use of reinforced concrete construction. Steel by itself would have been impossible of consideration because of its susceptibility to corrosion. Concrete alone could not be used because 323 of the excessive cost of the increased amount of Portland cement. It is said, that a retaining wall of all concrete would have cost about $600 per linear foot. Reinforced concrete costs about $58 per foot.

Steel is used, not merely as a reinforcement, but as the material of forms. Used thus, it may, at times, not only retain the concrete in position but also prevent the action of the surrounding soil or water. The possibility of using steel for forms depends largely upon the fact that many applications of concrete are becoming standardized rapidly. Thus is permitted a re-use of the steel form that justifies the expense. But the employment of steel forms sometimes involves the use of steel in the handling of them. A further use, although perhaps more remote, is in connection with the arrangements for the handling of the mixed concrete and of the raw materials. Still more remote, but still a necessary application, is the use of steel and iron in the crushing mills and the like. When we look at the question and inform ourselves of the ramifications, it is not difficult to see that concrete and steel are materials whose engineering applications are mutually involved. Concrete is certainly replacing steel in some applications. But, notwithstanding this, these two are to be regarded as unopposed to each other on the whole.

When concrete is cast about steel, an adhesive bond ensues. But this is scarcely to be regarded as sufficient to enable the two to act as one under tensile stresses. A mechanical bond should be employed. This is the explanation of the somewhat complicated forms of standard reinforcement bars.

Concrete properly reinforced is an admirable material for factory construction. It permits of rapid erection, is fireproof, has a long life, is adapted to weather conditions, and is economical. The floors of concrete buildings are easily cleaned and do not develop splinters.

One of the large automobile factories--that of the Geo. N. Pierce 324 Company at Buffalo, N. Y.--is a good instance of the rapidity with which reinforced concrete buildings may be erected. Within seven months of the date of signing the contract with the Trussed Concrete Steel Company, Detroit, Mich., which employs the Kahn system of reinforcement, certain large structures were ready for use. The floor space here is 325,000 square feet. It was necessary to provide a number of large areas unbroken by supports. It was found possible to use girders having spans of 55 and 61 feet. When subjected to a load, a girder develops compressive strains above and tensile ones below. The concrete is well adapted to withstand the one, but not the other. In an ordinary bridge truss, there may be diagonals that are also under tensile stress. In the Kahn system of reinforcement, a horizontal bar from which rigidly attached diagonals extend upward and outward is provided with a view of enabling the girder to withstand the tensile stress. In accordance with this design the long girders were constructed. Girders providing runways for 3-ton cranes were also constructed. A load of 14 tons placed upon one of the reinforced concrete girders having a span of 25 feet induced a deflection of only 1/16 inch. This girder is 12 inches wide and 22 inches deep and its reinforcement consists of three 1 x 3 in. Kahn bars. Hollow tile was largely employed here in connection with the concrete.

What is known in the trade as the corrugated bar, supplied by the Corrugated Steel Bar Company is a steel reinforcing rod which provides shoulders by means of which the concrete is mechanically engaged. This general type of reinforcement is, however, not confined to this concern. By means of this style of bar, the engineer is able to secure the desired mechanical interlock. As the concrete and steel expand and contract they do so together--unless the temperature change is excessive--and so the relation between the two is maintained. Such standard types of reinforcing bars are applicable to multitudes of 325 construction. An interesting example is the railroad bridge over the Vermilion River near Danville, Illinois. There are three arches, the central one of which has a span of 100 feet. About 130 tons of corrugated bars were employed in the construction of this beautiful bridge.

Another good example of bridge construction is the bridge over the Maumee River near Waterville, Ohio. This structure follows the designs of the National Bridge Company. It has a width of 16 feet between copings and crosses the river at a point where it is 1,000 feet wide. It is said that this reinforced bridge will carry a load of 5 tons per linear foot. The arches are 12 in number, the longest having a span of 90 feet, and the shortest, one of 75 feet. The loading of a bridge arch produces a lateral thrust upon the piers. If the next arch is not loaded, then this thrust is unbalanced and must be cared for. This was done in this case by employing part of the 100 tons of reinforcement in a vertical position. This bridge having a very long expectation of life was built at a cost of $77,000. The total amount of concrete was about 9,200 cubic yards.

The city of Philadelphia has gone into the construction of city bridges of concrete in rather an extensive way. Among a total of 30 or more is the reinforced bridge across Poquessing Creek, having a span of 71 feet. This bridge is rather flat, having a rise of but 9½ feet. The reinforcement employed here consisted in part of angle bars placed in pairs to form a kind of T-bar. The principal reinforcement here was the arch ribs. These were each composed of two of the T-bars arranged one above the other in such manner that their points of nearest approach were at the crown. These were latticed together. Such ribs were placed 4 feet apart. Transversely disposed steel rods held the whole together. The mechanical interlock here depended upon was due, no doubt, to the mutual disposition of the various rods, etc.

A railway viaduct, one-half mile or more in length is another example 326 of the Kahn methods. This structure belongs to the Richmond & Chesapeake Railway and is located at Richmond, Va. There is a span of 70 feet which has girders nearly 6 feet deep. At another span the girders, probably of about the same depth, sagged but ⅛ inch upon removal of the falsework.

A style of reinforcement much used consists of a net-like fabric of metal. This is employed largely in floors to bind the whole mass together. In the manufacture of this netting, a Canadian company has found it desirable to repair the inevitable breakages of strands in manufacture by the use of the Davis-Bournonville Company’s oxyacetylene torch. It is said that welds can be made on the average of one in two minutes in the case of an ordinary weight of the fabric. This netting is made by expanding sheets of perforated metal from a narrow to a considerable width. It is during this expansion that the strands sometimes break.

Another style of floor reinforcement is the fabric made from wire. That floors properly reinforced are quite substantial may be judged from the case of the United States Fidelity & Guarantee Company. Their building in Baltimore was exposed to intense heat in the great fire of 1904. In fact, a considerable part of the side walls and the front fell, leaving floors of concrete. A load of brick giving a pressure of 300 pounds per square foot was arranged on one of the floors to a distance of 5 feet to each side of one of the girders. The deflection amounted to ⅛ inch. This was about 1/20 of 1 per cent of the span. This is an example of Hennibique construction.

Reference has already been made to the Kahn truss reinforcement. With the same general object in view, the Hennibique truss has been designed. There are two horizontal bars, one above the other. The upper is, however, not perfectly horizontal except near the center. Towards either side, this bar rises as it recedes from the center. 327 These two bars are enveloped by loose stirrups arranged vertically and at intervals. These are open at the top and closed below.

There are two varieties of piles--the bearing pile and the sheet pile. Their duties are quite different. One sustains a compressive load, the other withstands a transverse thrust. But concrete has been used for both kinds. In the case of the bearing pile, its own intrinsic qualities are eminently suitable. It has good compressive resistance; it is teredo proof, and has the prospect of long life whether conditions are wet, dry or a mixture of the two. Wood makes an admirable bearing pile, if constantly submerged, but it is a prey to the teredo. The necessity for constant submergence limits the usefulness of the wood pile. It must be cut off below the hydraulic level, and this necessitates carrying the foundation footings to a lower level than would otherwise ordinarily be the case. With the concrete bearing pile, on the contrary, the footings may be constructed at any level desired as the pile itself may be partly submerged and partly in the dry. However, the concrete pile may be subjected to other than compressive stresses, especially during its placement. And so, some reinforce it. Some, no doubt, have in view a possible buckling when in the ground, particularly if the surrounding soil is yielding. Reinforcement both longitudinal and transverse is employed. Longitudinal bars are arranged at intervals around and within the periphery. These may be bound together by separate hoops disposed along the length or by wire wound about the longitudinals in spirals. In the case of concrete sheet piling, the concrete supplies a surface and forms a protective covering to the imbedded reinforcement which is here a vital matter and consequently indispensable. In the dock improvements at Baltimore, to which reference has already been made, reinforced concrete sheet piling was largely used. The steel sheet pile could not well have been used here because of its 328 susceptibility to corrosion. The concrete slabs, 12 × 18 inches in cross-section perform the duty of retaining masses of earth in place both above and below the water line. There were certain other concrete constructional elements of an auxiliary character. The total reinforcement amounted to about 1,200 tons.

With regard to its fireproof qualities, an eloquent testimonial arises from the fact that the immense Marlborough-Blenheim Hotel at Atlantic City, a concrete and tile structure, is said to enjoy a saving of $18,000 per year in fire insurance premiums. The insurance is based on $600,000. This structure is 560 feet in length and has a width varying from 60 to 200 feet.

Reference has been made to the close identity of the co-efficients of expansion for steel and concrete for moderate intervals of temperature. While this is so, if the thermal range is considerable, the concrete and the steel cannot be expected to expand and contract together. In most engineering construction, the range is small, say 150 degrees F., but there are exceptions. One of these relates to the material used in tall chimneys. The hot gases which pass up these give rise to rather high temperatures. In fact, it is well-recognized practice to build a large part of such chimneys double, one shell enveloping another, with an air space between. Some four or five years ago what is, perhaps, the very tallest concrete chimney in the United States was built for the Colusa Parrot Mining & Smelting Company, Butte, Montana. It is 352½ feet high and has a flue 18 feet in diameter. A solid wall 1½ feet thick constitutes the base of 21 feet in height. Above this level, an air space 4 inches wide radially is arranged between two shells of 5 and 9 inches thick. The inner one is the thinner. The steel reinforcements used were T-bars. The footing is of reinforced concrete and rests upon a fill 18 feet deep. A further important factor which has to be considered is the serious effect 329 of repeated stresses. Partly because of this, it is recommended that a large factor of safety be adopted. Further, the best practice would seem to be in the direction of a complete divorce between the inner and outer shells all the way up and of a uniformity in wall thickness from bottom to top. Vertical cracks have been noted in some chimneys. This would indicate the advisability of strong circular reinforcement. It is thought that a tone concrete following the formula 1:2:2 is better for the outer shell than a cement mortar. It is said to be stronger, denser and more impervious to water than a mortar following the formula 1:3. In order to secure adhesion between layers, the fresh concrete should be applied wet and the old should perhaps be resurfaced by tooling.

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 330 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 331 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 332 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 333 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.