Concrete Construction: Methods and Costs

The mold or form for the coping course was designed to build the coping in successive sections, and was built up around the bracket blocks, and supported from the centers as shown by the drawings. To form the expansion joints in the coping course there were inserted across the mold at proper intervals a short iron plate ¼ in. thick, cut to fit. The cutting of this plate was found to be a slow operation.

The forms for the base of the railing (Section D) consisted of 1¾-in. stock for the sides, and ¾-in. stock for the slopes. They extended across the arch, and were held together by a very simple though very efficient clamp. This consisted of two 2×3×33-in. pieces nailed to a 2×3×17-in. piece by means of galvanized iron strips. About half-way down the long pieces, a ½-in. rod was run through, and secured up against blocks, h, placed about 56 ins. apart. These blocks were removed as the concrete was put in place. It will be noticed from the cross-section of the railing that the balusters are set into sockets formed in the top of the base course. These sockets were formed by means of the mold shown at W and Z.

In casting the balusters, Section (E), a ⅜-in. cast iron mold, consisting of four iron sides and an iron top, was used. Originally there were two end plates of iron, but it was found more convenient to have the bottom one of wood and allow the cast spindle to stand and set. The mold was held together by ½-in. bolts. It would have been more practical to have had the side casting composed of two parts.

The form for the railing is built up around the tops of the spindles. The bottom piece is 1×9 ins., to which 4¼-in. ogee molding is nailed. The sides are of 1-in. stock, and are clamped together. The top is finished off with a trowel.

The mold for the posts is made in four parts, which fit together at the top and bottom by a bevel joint, as shown in the one-fourth section. The broad sides rest against the narrow ones, and are held against the same by means of ½-in. rods running through 2×3-in. stock: 2-in. projections of the broad sides facilitate the removal of the form from the completed post.

In constructing a concrete facade for a plate girder bridge at St. Louis. Mo., the railing above the base was constructed of separately molded blocks as follows: The balusters were cast in plaster molds. To make these molds a box square in plan and the height of the baluster was constructed of wood and cut vertically into three sections. The inside lateral dimensions of this box were made 6 ins. greater than the largest dimension of the baluster. A full size wooden pattern of the baluster was set up and the three sections of the box were set around it. Sheets of thin galvanized metal, with their inner edges cut to conform to the curves of the baluster, were inserted in the joints of the assembled box so as to divide the vacant space between the pattern and the box into vertical sections.

A mixture of 1 part Portland cement and 1 part plaster of Paris, made wet, was then poured around the pattern until the box was filled. When this mixture had become hard, the box was taken down, leaving a plaster and cement casing separated into three parts by the sheets of galvanized metal. This casing was separated from the pattern and given a coat of shellac on the inside. Four or five molds of this description were cast. To cast a baluster, the sections were assembled and a ½-in. corrugated bar was set vertically in the center. A mixture of 1 part Portland cement and 3 parts sand was then poured into the mold and allowed to harden. The molds for the urns on the railing post and the balls on the end posts were made in exactly the same manner as the baluster molds. The construction of the railing posts is shown by the drawings of Fig. 289. Referring first to the end posts, it will be seen that they were molded in place in seven sections marked A, B, C, D, E, F and G. The construction of the mold for each section is shown by the correspondingly lettered detail. The intermediate posts were built up of the separately molded pieces I, K and H. The costs of molding the several parts were: Balusters, 60 cts. each; hand rail, 40 cts. per lin. ft. The six intermediate posts cost $12 each, and the four end or newel posts cost $75 each.

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 ⅞-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, baluster, 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 baluster molds 14 to 18 ins. high, $7.50 to $10 each; fluted square balusters, 14 to 18 ins. high, $10, each; ball and base, 10 to 18-in. balls, $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, coarse, 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 molasses, 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 shipping.

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 7½ 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.

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 illustrate by an assumed 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.

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.

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.

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 balustrade and cornice is shown by Fig. 302. The posts, balusters and railing were molded separately. The balusters 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 ¾-in. iron rod through the center vertically; this rod crowded the concrete into all parts of the mold and also served to strengthen the baluster. The baluster 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.

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 1½-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 windlass. 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 Jap drill, using a 1¼-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 spaced 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 glass broken by the shock of explosion. There was no glass 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 mass of solid concrete would be left intact on the other side. The total estimated quantity 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:

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

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 grass 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 ¼-in.×2-ft. copper rod embedded, and of a cast iron top and cover constructed as shown by the drawing. Mr. W. H. Hedges, Bench and Street Grade Engineer, Department of Public Works, Chicago, Ill., gives the following data regarding quantities and cost. The materials required for each monument are: 1.78 cu. yd. crushed stone, 0.6 cu. yd. torpedo sand, 1½ bbls. cement, 60 ft. B. M. lumber, one ¼×24-in. copper rod, one top and cover. A gang consisting of 1 foreman, 4 laborers and 2 teams construct from one to three monuments per day, the average number being two per 8-hour day. In 1906 the average cost of the monuments was $24.12 each, based on above material and labor charges.

POLE BASE.—Figure 305 shows a concrete base for transmission line poles invented by Mr. M. H. Murray, of Bakersfield, Cal., and used by the Power Transit & Light Co. of that city. These bases are molded and shipped to the work ready for placing. They weigh about 420 lbs. each. One base requires 37½ lbs. of 2×¼-in. steel bar, 40 lbs. of Portland cement, 3 cu. ft. of broken stone or gravel and enough sand to fill the form or mold, which is 10×10 ins. by 4½ ft. Unskilled labor is employed in the molding and two men can mold ten bases per 8-hour day. The cost of molding is as follows per base:

Two men at $2 per day each set five bases in eight hours, making the cost of setting 80 cts. per base. The bases were sunk to a depth of 3 ft. 3 ins. In many cases they were placed under poles without interrupting service by sawing off the pole, dropping it into the ground, placing the new base and setting the sawed-off pole on it and bolting up the straps.

MILE POST, CHICAGO & EASTERN ILLINOIS R. R.—The dimensions of the post are shown by Fig. 306. Each post weighs 498 lbs. They are made when other concrete work is being done. The form is laid flat, with the molds for the letters on the bottom, and bottom and sides are plastered with mortar, which is backed up with a 1-1-2 stone concrete. The cost of the post is given as follows:

BONDING NEW CONCRETE TO OLD.—Concrete which has set hard has a surface skin or glaze to which fresh concrete will not adhere strongly unless special effort is made to perfect the bond. Various ways of doing this are practiced. The most common is to clean the hardened surface from all loose material and give it a thorough wash of cement grout against which the fresh concrete is deposited and rammed before the grout has had time to set. Washing the old surface with a hose or scrubbing it with a brush and water improves the bond, as does also the hard tamping of the concrete immediately over the joint. Mortar may be used in place of grout. The thorough cleansing of the surface is, however, quite as essential as the bonding coat, in fact in the opinion of the authors it is more essential. As a rule, a good enough joint for ordinary purposes can be got by tamping the fresh concrete directly against the old concrete, without grout or mortar coating, if the surface of the latter is thoroughly cleaned by scrubbing and flushing. The secret of securing a good bond between fresh concrete and concrete that has set lies largely in getting rid of the glaze skin and the slime and dust which forms on it. Washing will go far toward doing this. The glaze skin can be removed entirely by acid solutions, but the acid wash must be flushed free from the surface before placing the fresh concrete. Ransomite, made by the Ransome Concrete Machinery Co., Dunellen, N. J., is a prepared acid wash which to the authors' knowledge has given excellent success in a number of cases. The glaze coat can also be removed by picking the hardened surface, but the picking should be followed by washing to remove all loose chips and dust.

DIMENSIONS AND CAPACITIES OF MIXERS.—In planning plant lay-outs it is often desirable to know the sizes, capacities, etc., of various mixers in order to make preliminary estimates. Tables XXII to XXXIII give these data for a number of the more commonly employed machines. The Eureka, the Advanced and the Scheiffler mixers are continuous mixers and the others are batch mixers.

Table XXII—Sizes, Capacities and Weights of Advanced Mixers. Cement Machinery Co., Jackson, Mich.

Table XXIII—Sizes, Capacities and Weights of Scheiffler Proportioning Mixers. The Hartwick Machinery Co., Jackson, Mich.

Table XXIV—Sizes, Capacities and Weights of Eureka Mixers. Eureka Machine Co., Lansing, Mich.

Table XXV—Sizes, Capacities and Weights of Snell Mixers.

R. Z. Snell Mfg. Co., South Bend, Ind.

Table XXVI—Sizes, Capacities and Horsepower of Ransome Mixers.

Ransome Concrete Machinery Co., Dunellen, N. J.

Table XXVII—Sizes, Capacities and Horsepowers of Chicago Improved Cube Mixers. Municipal Engineering and Contracting Co., Chicago, Ill.