Concrete Construction Part 61

In constructing the 72-ft. span-ribbed arch bridge over Deer Park Gorge, near La Salle, Ill., a hand railing of the design shown by Fig. 290, was used. In constructing this railing, the posts were molded in place, but the open work panels between posts and the hand rail proper were molded separately and set in place between the posts as indicated. For molding the panels a number of boxes constructed as shown by Fig. 291, were used. These were simple rectangular boxes on the bottom boards of which were nailed blocks of the proper shape and in the proper position to form the openings in the railing. The bottom of the form was first plastered with mortar, then the concrete was filled in and plastered on top. As soon as the concrete had begun to set the blocks were removed so that final setting could take place without danger of cracking. When the concrete had set so that the panel could be safely handled, it was removed from the form and stored until wanted. The hand rail for each side was molded in two pieces in forms constructed as shown by Fig. 292.

The total cost of the railing in place was about $2 per lineal foot.

The concrete was a 1-2-4 mixture of screenings and 7/8-in. broken stone.

~Iron Molds.~--Iron molds have the same disadvantages as wooden molds in the use of wet mixtures. They can be made to mold more intricate ornaments, and in the matter of durability, are, of course, far superior to wood. Iron molds can be ordered cast to pattern in any well equipped foundry. Many firms making block machines also make standard column, bal.u.s.ter, ball and base, cornice, and base molds of various sizes and patterns. These molds are made in two, three or more sections which can be quickly locked together and taken apart. A column mold, for example, will consist of a mold for the base, another for the shaft, and a third for the capitol, each in collapsible sections. Where the pattern of the shaft changes in its height, two shaft molds are commonly used, one for each pattern. Prices of iron molds are subject to variation, but the following are representative figures: Plain bal.u.s.ter molds 14 to 18 ins.

high, $7.50 to $10 each; fluted square bal.u.s.ters, 14 to 18 ins. high, $10, each; ball and base, 10 to 18-in. b.a.l.l.s, $15 to $25 each; fluted Grecian column, base, capitol and one shaft molds, $30; Renaissance column, base, capitol and two shaft molds, $45.

~Sand Molding.~--Molding concrete ornaments in sand is in all respects like molding iron castings in a foundry. Sand molding gives perhaps the handsomest ornament of any kind of molding process, the surface texture and detail of the block being especially fine. It is, however, a more expensive process than molding in wooden or iron molds, since a separate mold must be made for each piece molded. The process was first employed and patented in 1899, by Mr. C. W. Stevens, of Harvey, Ill., and for this reason it is often called the Stevens process. Sand molded ornaments and blocks are made by a number of firms to order to any pattern. The process as employed at the works of the Roman Stone Co., of Toronto, Ont., is as follows: The stone employed for aggregate, is a hard, coa.r.s.e, crystalline limestone of a light grey color, being practically 97 per cent. calcium carbonate, with a small percentage of iron, aluminia and magnesia. Nothing but carefully selected quarry clippings are used and these are crushed and ground at the factory and carefully screened into three sizes, the largest about the size of a kernel of corn. Daily granulometric tests are made of the crusher output to regulate the amount of each size got from the machines. It has been found that next in importance to properly graded aggregates is the gaging of the amount of water used in the mixture. This is done by an automatically filled tank into which lead both hot and cold water and in which is fixed a thermometer to properly regulate the temperature. In gaging the mix about 20% of water is used, but of course when the cast is made the surplus is immediately drawn off into the sand, where it is retained and serves as a wet blanket to protect the cast and supply it with the proper amount of water during crystallization. Experiments seem to indicate that about 15% by weight gives the greatest amount of strength of mortar at the age of six months, while, giving less strength at shorter time tests than mortar gaged with a smaller percentage of water.

The method of handling the mix and casting is quite simple and almost identical with the practice in iron foundries. The mixture is made in a batch mixer to about the same consistency as mola.s.ses, from which it is poured into a mechanical agitator and carried about the foundry by a traveling crane. This agitator is so constructed that it keeps the materials in motion constantly and prevents their segregation. In each cast is inserted the proper reinforcing rods, lifting hooks and tie rods, and the casts are allowed to remain for a proper period in the wet sand after they are poured; they are then taken to the seasoning room which is kept at as constant a temperature as it is practical to maintain. Each cast is marked with the number which determines its location in the building and the date it was cast, and it is then kept in the storage shed a fixed time before s.h.i.+pping.

Records are kept of each cast made and the company is able to get, as in mills rolling structural steel, the exact number and location of all casts made from the same mix. Careful records are always kept of the tests of cement and material, and test cubes are made from each consignment of cement so tested; in this way all danger of defective stone through inferior cement is eliminated. The patterns used in making the molds and the method of molding are quite similar to ordinary iron foundry practice except that the sand used is of special nature.

The finish of the stone is generally tooled finish molded in the sand, the different textures of natural stone being produced by the veneering of the pattern with thin strips of wood which are run through a machine producing the different finishes. Each stone is provided with setting hooks cast in the blocks which take the place of the ordinary lewis holes used in cut stone.

~Plaster Molds.~--Plaster of Paris molds are made from clay, gelatin or other patterns in the usual manner adopted by sculptors. They are particularly adapted to fine line and under cut ornaments. The concrete is poured into the plaster mold and after the cement has become hard, the plaster is broken or chiseled away, leaving the concrete exposed.

Two examples of excellent work in intricate concrete ornaments are furnished by the power house for the Sanitary District of Chicago, and by the State Normal School building, at Kearney, Neb. In the power house, the ornamental work consisted of molded courses, cornice work; and particularly of heavy capitals for pilasters. These capitals were very heavy, being 7 ft. long and of the Ionic design. These were made from plaster molds; made so as to be taken apart or knocked down and to release in this way, perfectly. There were also scrolls, keystones and arches in curved design over all of the 40 windows. None of this ornament was true under cut work. In building the Normal School building, Corinthian capitals, in quarters, halves, corners and full rounds were made in plaster molds. There were some 30 of these capitols.

They were made in solid plaster molds; the molds having been cast in gelatine molds, one for each capitol. Into these, the concrete was tamped, made very wet, and after the concrete had hardened, the plaster cast was chiseled away. This was very easily accomplished. These capitols were true Corinthians, having all the floriation and under-cut usually seen in such capitols.

~ORNAMENTS MOLDED IN PLACE.~--Molding ornaments in place is usually, and generally should be, confined to belt courses, cornices, copings and plain panels. Relief work, like keystones, scrolls or rosettes, can be molded in place if desired, by setting plaster molds in the wooden forms at the proper points. This method is often advantageous in bridge work, where comparatively few ornaments are required, such as keystones.

[Ill.u.s.tration: Fig. 293.--Spandrel Wall Mold for Arch Bridge.]

The construction of forms for ornamental work in place is best described by taking specific examples. Figure 293, shows the face form for the arch ring, spandrel wall and cornice or coping course of the Big Muddy River Bridge on the Illinois Central R. R. The section is taken near the crown of the arch. The lagging only is shown; this was, of course, backed with studding. The point to be noted in this form is the avoidance of any approach to under cut work; there are, in fact, very few straight cut details. This brings up a point that must be carefully watched if trouble is to be avoided, namely, the construction of the form work in sections which can be removed without fracturing the ornament. To ill.u.s.trate by an a.s.sumed example, supposing it is required to mold the wall and cornice shown by Fig. 294. It is clear that if the backing studs are in single pieces, notched as shown, the forms cannot be removed without fracturing at least the corner A. If the studs and lagging be constructed in two parts, separated along the line a b, the form is possible of removal if great care is used without damage to the concrete. The construction shown by this sketch does not greatly exaggerate matters. Figure 295 shows a wall form that has been given several times as a presumably good example in which, as will be seen it is impossible to remove the board a, without breaking the concrete even if the narrow face were not broken by the swelling of the lumber before ever it became time to take down the forms.

[Ill.u.s.tration: Fig. 294.--Diagram Ill.u.s.trating Details of Mold Construction.]

[Ill.u.s.tration: Fig. 295.--Example of Poor Wall Form Construction.]

This matter of making provision for the swelling of the forms is another point to be watched. Referring again to Fig. 294 it will be seen that the swelling of the lagging, even if the cornice instead of being under cut at A were straight cut on the line c d, is liable so to crowd the lagging into the corner A and B that the concrete is cracked along the lines e f or g h. A suggested remedy for this danger is shown by Fig. 296. At a distance of every 3 or 4 ft. insert a narrow piece of lagging a and behind these lagging strips cut notches b in the studs. When the concrete has got its initial set pull back the lagging strip a into the notches b, leaving an open joint to provide for expansion due to swelling.

[Ill.u.s.tration: Fig. 296.--Notched Studding for Removal of Lagging Board to Permit Swelling.]

[Ill.u.s.tration: Fig. 297.--Form for Concrete Facade Shown by Fig. 298.]

[Ill.u.s.tration: Fig. 298.--Concrete Facade for Plate Girder Bridge.]

[Ill.u.s.tration: Fig. 299.--Forms for Curved Concrete Abutments.]

[Ill.u.s.tration: Fig. 300.--Cornice Form.]

[Ill.u.s.tration: Fig. 301.--Method of Supporting Cornice Form Shown by Fig. 300.]

In constructing a concrete facade for a plate girder bridge at St.

Louis, Mo., the form shown by Fig. 297 was used. The completed facade is shown by Fig. 298. The ceiling slab was first built and allowed to set and then the forms were erected for the frieze and coping. After these were molded the forms were continued upward as shown for the base of the railing. Above this point the several parts were separately molded as shown by Fig. 285 previously described. Molded in this manner the ceiling cost 25 cts. per sq. ft.; the frieze and coping cost $2 per lin.

ft., and the railing base cost 45 cts. per lin. ft. In constructing the concrete abutments of this same structure use was made of the forms shown by Fig. 299. These abutments had curved wing walls and for molding these girts cut to the radii of the curves were fastened to the studs and vertical lagging was nailed to the girts. All the lagging was tongue and groove stuff.

[Ill.u.s.tration: Fig. 302.--Cornice and Bal.u.s.trade for Arch Bridge.]

In constructing an open spandrel arch bridge at St. Paul, Minn., the cornice form shown by Fig. 300, supported as shown by Fig. 301, was used. The particular feature of this form was the use of a lath and plaster lining to the lagging. This lining was used for all exposed surfaces of the bridge. So called patent lath consisting of boards with parallel dovetail grooves and ridges was used. This was plastered with cement mortar and the concrete was deposited directly against the plaster after smearing the plaster surface with boiled linseed oil. This lining is stated to have given an excellent surface finish to the concrete. It cost 55 cts. per sq. ft. for materials and labor. A section of the bal.u.s.trade and cornice is shown by Fig. 302. The posts, bal.u.s.ters and railing were molded separately. The bal.u.s.ters were molded in zinc molds. At first some trouble was had in getting good casts on account of air pockets. This was largely done away with by filling the mold as compactly as possible and then driving a -in. iron rod through the center vertically; this rod crowded the concrete into all parts of the mold and also served to strengthen the bal.u.s.ter. The bal.u.s.ter molds were made in two parts; this proved a mistake--three parts would have been better.

CHAPTER XXIV.

MISCELLANEOUS DATA ON MATERIALS, MACHINES AND COSTS.

The following cost data comprise such miscellaneous items as do not properly come in the preceding chapters. They are given not as including all the miscellaneous purposes for which concrete is used but as being such items of costs as were secured in collecting the more important data given in preceding sections.

[Ill.u.s.tration: Fig. 303.--Device for Drilling Green Concrete.]

~DRILLING AND BLASTING CONCRETE.~--Concrete is exceedingly troublesome material in which to drill deep holes, and this statement is particularly true if the concrete is green. The following mode of procedure proved successful in drilling 1-in. anchor bolt holes 6 ft.

and over in depth in green concrete. The apparatus used is shown by Fig.

303, re-drawn from a rough sketch made on the work by one of the authors, and only approximately to scale. The drill is hung on a small pile driver frame, occupying exactly the position the hammer would occupy in a pile driver, and is raised and lowered by a hand windla.s.s.

By this arrangement a longer drill could be used than with the ordinary tripod mounting and less changing of drills was necessary. A wide flare bit was used, permitting a small copper pipe to be carried into the hole with the drill; through this pipe water was forced under pressure, carrying off the chips so rapidly that no wedging was possible. By this device drilling which had previously cost over 25 cts. a hole was done at a cost of less than 5 cts. a hole.

In removing an old cable railway track in St. Louis, Mo., holes 8 ins.

deep were drilled in the concrete with a No. 2 Little j.a.p drill, using a 1-in. bit and air at 90 lbs. pressure. A dry hole was drilled, the exhaust air from the hollow drill blowing the dust from the hole keeping it clean. The concrete was about 18 years old and very hard. Two holes across track were drilled, one 10 ins. inside each rail; lengthwise of the track the holes were s.p.a.ced 24 ins. apart, or four pairs of holes between each pair of yokes.

Common labor was used to run the drills and very little mechanical trouble was experienced. Three cars were fitted up, one for each gang, each car being equipped with a motor-driven air compressor, water for cooling the compressors being obtained from the fire plugs along the route. The air compressors were taken temporarily from those in use in the repair shops, no special machines being bought for the purpose.

Electricity for operating the air compressor motors was taken from the trolley wire over the tracks. The car was moved along as the holes were drilled, air being conveyed from the car to the drills through a flexible hose. Two drills were operated normally from each car. One of the air compressors was exceptionally large and at times operated four drills. The total number of holes drilled in the reconstruction of the track was 31,000. The total feet of hole drilled was 20,700 ft.

With the best one of the plants operating two to three drills 30 8-in.

holes, or 20.3 ft. of hole, were drilled per hour per drill at a labor cost of 2.7 cts. per foot.

For blasting, a 0.1-lb. charge of 40 per cent. dynamite was used in each hole. A fulminating cap was used to explode the charge, and 12 holes were shot at one time by an electric firing machine. The dynamite was furnished from the factory in 0.1-lb. packages, and all the preparation necessary on the work was to insert the fulminating cap in the dynamite, tamp the charge into the hole and connect the wires to the firing machine. In order to prevent any damage being done by flying rocks at the time of the explosion, each blasting gang was supplied with a cover car, which was merely a flat car with a heavy bottom and side boards.

When a charge was to be fired, this car was run over the 12 holes and the side boards let down, so that the charge was entirely covered. This work was remarkably free from accidents. There were no personal accident claims whatever, and the total amount paid out for property damages for the whole six miles of construction was $685. Most of this was for gla.s.s broken by the shock of explosion. There was no gla.s.s broken by flying particles. The men doing this work, few of whom had ever done blasting before, soon became very skillful in handling the dynamite, and the work advanced rapidly. The report made by the firing of the 12 holes was no greater than that made by giant fire-crackers.

For the drilling and blasting the old rail had been left in place to carry the air compressor car and the cover car. After the blasting, this rail was removed and the concrete, excavated to the required depth. In most cases the cable yokes had been broken by the force of the blast.

Where these yokes had not been broken, they were knocked out by blows from pieces of rail. The efficacy of the blasting depended largely upon the proper location of the hole. Where the holes had been drilled close to the middle of the concrete block, so that the dynamite charge was exploded a little below the center of gravity of the section, the concrete was well shattered and could be picked out in large pieces.

Where the hole had been located too close to either side of the concrete block, however, the charge would blow out at one side and a large ma.s.s of solid concrete would be left intact on the other side. The total estimated quant.i.ty of concrete blasted was 6,558 cu. yds., or 0.2 cu.

yd. of concrete per lineal foot of track. The cost of the dynamite delivered in 0.1 lb. packages was 13 cts. per pound. The exploders cost $0.0255 each.

The cost of drilling and blasting was as follows:

Item. Per mile. Per lin. ft. Per cu. yd.

Labor, drilling $ 89.76 $0.017 $0.085 Blasting labor and materials. 285.12 0.054 0.268 ------- ------ ------ Total drilling and blasting. $374.88 $0.071 $0.353

[Ill.u.s.tration: Fig. 304.--Bench Monument, Chicago, Ill.]

The cost of blasting with labor and materials, separately itemized, was as follows, per cubic yard:

Dynamite and exploders $0.192 Labor 0.076 ------ Total $0.268

Two cubic yards of concrete were blasted per pound of dynamite.

~BENCH MONUMENTS, CHICAGO, ILL.~--The standard bench monuments, Fig. 304, used in Chicago, Ill., are mostly placed in the gra.s.s plot between the curb and the lot line, so that the top of the iron cover comes just level with the street grade or flush with the surface of the cement walk. The monument consists of a pyramidal base 6 ft. high and 42 ins.

square at the bottom, with a -in.2-ft. copper rod embedded, and of a cast iron top and cover constructed as shown by the drawing. Mr. W. H.