Concrete Construction: Methods and Costs

The mixer was a No. 5 Chicago Improved Cube Mixer, operated by a gasoline engine; a larger size would have been preferable since a batch required only two-thirds of a bag of cement which had to be measured which required the services of an additional man. The mixer was in operation 194 hours and mixed 7,702 batches (1,217 cu. yds.), or a batch every 87 seconds, or 6.3 cu. yds. per hour. During the last ten days it mixed a batch every 78 seconds while running. The best short record made was 291 batches in five hours, or one batch every 63 seconds, this being at the rate of 58 batches equal to 9.2 cu. yds. of concrete in place per hour, or nearly 1/6 cu. yd. per batch. It took about ½ minute to mix the concrete and about the same length of time to charge and discharge the mixer.

To convey the concrete from the mixer to the culvert walls a 1 cu. yd. drop bottom car was used. This car ran on 30-in. gage tracks carried on a trestle straddling the culvert walls and having its floor high enough to clear the arch. A track ran lengthwise of the trestle over each culvert wall, and a cross track intersecting both with turntables ran to the mixer. Three men handled the car, a round trip to the extreme end of the trestle being made in about 3 minutes. In the meantime the mixer was discharging into a small hopper which unloaded into the car on its return. One only of the three sections, of the culvert was built at a time, both walls being brought up together. After a point had been reached about 2 ft. above the springing on both walls, one track was removed and the other was shifted to the center of the trestle.

Forms.—There was used in the forms 15,000 ft. B. M. of 2-in. dressed lagging for face work, 21,000 ft. B. M. rough lumber for back work, and old car sills for studding. No charge was made for studding except the cost of loading, the cost of the remaining lumber was $16 per M. for dressed and $12.50 per M. for rough. A credit of one-third the cost was allowed for the old material recovered. The total cost of the labor of erecting the material in forms, bins and platforms was $666. The work was done by a bridge crew of white men, the average rate of wages per man, including the bridge foreman's time, being $2.20 per day. In addition a mason at $3.50 per day and a carpenter at $2.25 per day worked with the bridge crew in erecting forms.

Cost.—The cost of the 1,217 cu. yds. of concrete in the culvert was as follows:

CULVERTS FOR NEW CONSTRUCTION, WABASH RY.—The following data relate to culvert work carried out in constructing the Pittsburg extension of the Wabash Ry. in 1903. All the work was done by contract.

Plant I: This plant was located on a hillside with the crushing bins above the loading floor or platform which extended over the top of the mixer, so that the crushed stone could be drawn directly from the chutes of the bins and wheeled to the mixer. The sand was hauled up an incline in one-horse carts and dumped on this floor, and was also wheeled in barrows to the mixer. The proportions used were 4 bags of cement, 4 barrows of sand and stone dust and 7 barrows of crushed stone. A ⅞-cu. yd. mixer was used and it averaged 40 cu. yds. per 10-hour day at the following cost for labor:

Assuming ⅓ ton of coal per day at $3 per ton, we have 2 cts. more per cubic yard for fuel.

Plant II.—At this plant a Smith mixer was used with a loading floor 4 ft. above the ground, this low platform being made possible by having a hole or sump in which the skip receiving the concrete was set. A derrick handled the skips between the sump and the work. The batch was made up of 2 bags of cement, 2 barrows of sand and 4 barrows of stone. The output was 50 cu. yds. per day of 10 hours at the following cost:

The cost of fuel would add about 3 cts. per cubic yard to this amount.

SMALL ARCH CULVERT COSTS, PENNSYLVANIA R. R.—Mr. Alex. R. Holliday gives the following figures of cost of small concrete culvert work carried out under his direction. The culvert section used is shown in Fig. 171. This section gives a slightly larger waterway than a 36-in. cast iron pipe. Eight culverts, having an aggregate length of 306 ft. were built, using a mixture of Portland cement and limestone and screenings. Each culvert had a small spandrel wall at each end.

The work was done by a gang of six men, receiving the following wages:

The materials were hauled about 1 mile from railway to site of work. Cement, including freight and haulage, cost $1.97 per barrel. Limestone and screenings cost 50 cts. per cu. yd. f. o. b. at quarry. No freight charges are included in cost of any of the materials except cement. The cost of the 306 ft. of culvert was as follows:

26-FT SPAN ARCH CULVERT.—The culvert was 62 ft. long and 26-ft. span and was built of 1-8 and 1-10 concrete mixed by hand. The wages paid were: General foreman, 40 cts. per hour; foreman, 25 cts. per hour; carpenters, 22½ to 25 cts. per hour, and laborers, 15 cts. per hour. The cost of the concrete in place, exclusive of excavation but including wing walls and parapet, was as follows:

There were 1,493 cu. yds. of concrete in the work. The excavation cost $463 and the total cost was $7,243.

COST OF RAILWAY CULVERT.—The culvert was for a single track railway and contained 113 cu. yds. of concrete and required 36 cu. yds. of excavation. The figures are given by C. C. Williams as follows:

12-FT. CULVERT, KALAMAZOO, MICH.—A portion 1,080 ft. long of a new channel built in 1902-3 for a small stream flowing through the city of Kalamazoo, Mich., was constructed as an arch culvert of the form shown by Fig. 172. The concrete section is reinforced on the lines indicated by a double layer of woven steel wire fabric. The concrete was approximately a 1 cement, 6 sand and gravel mixture.

The centers were built in sections 12½ ft. long of the form and construction shown by Fig. 173, and a sufficient number was provided to lay twelve sections of invert and six sections of arch. The arch centers were arranged to be uncoupled at the crown; this with the hinges at the quarter points permitted the two halves to be separated and each half to be folded so that it could be carried from the rear of the work through the forms still in place and erected again for new work. When in place the center ribs rested on the side forms which set on the invert concrete and are braced apart by the hinged cross-strut. This cross-strut was the key that bound the whole structure together; the method of removing this key is indicated by Fig. 174. From his experience with these centers the engineer of the work, Mr. Geo. S. Pierson, remarks:

"In work of this kind it is very important to have the centering absolutely rigid so it will not spring when concrete is being tamped against it and thus weaken the cohesion of the concrete. It is also important to have the arrangement such that all the centering can be removed without straining or jarring the fresh concrete. The centers were generally removed in about three or four days after the concrete arch was in place."

The invert concrete was brought to form by means of templates, Fig. 173, and straight edges. The side forms were then placed and braced apart by the struts and concreting continued to the skewback plane indicated in Fig. 173. The arch form was then placed; it rested at the edges on the side forms and was further supported by center posts bearing on boards laid on the bottom of the invert. A template, Fig. 175, was used to get the proper thickness and form of arch ring. Outside forms were used to confine the concrete at the haunches but nearer the crown they were not required.

Much of the work was done when the thermometer, during working hours, ranged from 12° to 25° above zero. When the temperature was below freezing, hot water was used in mixing the concrete and on a few of the coldest days salt was dissolved in the water. In addition each section of the work was covered with oiled canvas as soon as completed, and the conduit was kept closed so far as was practicable to retain the heat. Concreting was never stopped on account of cold weather.

Account was kept of the cost of all work, and the figures obtained are given in the following tables:

The cost per cubic yard was thus $6.26. Wages were $1.75 per day.

METHOD AND COST OF MOLDING CULVERT PIPE, CHICAGO & ILLINOIS WESTERN R. R.—During 1906, the Chicago & Illinois Western R. R., Mr. O. P. Chamberlain, Chief Engineer, built a number of culverts of concrete pipe with an interior diameter of 4 ft., and 6-in. shells. Fig. 176 shows the forms in which the pipe was molded. Both forms are of ordinary wooden tank construction. The inner form has one wedge-shaped loose stave which is withdrawn after the concrete has set for about 20 hours, thus collapsing the inner form and allowing it to be removed. The outer form is built in two pieces with 2×⅝-in. semi-circular iron hoops on the outside, the hoops having loops at the ends. The staves are fastened to the hoops by wood screws 1¾ ins. long driven from the outside of the hoop. When the two sides of the outer form are in position, the loops on one side come into position just above the loops on the other side, and four ¾-in. steel pins are inserted in the loops to hold the two sides together while the form is being filled with concrete and while the concrete is setting. After the inner form has been removed, the two pins in the same vertical line are removed and the form opened horizontally on the hinges formed by the loops and pins on the opposite side. The inner and outer forms are then ready to be set up for building another pipe.

The concrete used in manufacturing these pipes was composed of American Portland cement, limestone screenings and crushed limestone that has passed through a ¾-in. diameter screen after everything that would pass through a ½-in. diameter screen had been removed. The concrete was mixed in the proportions of one part cement to three and one-half parts each of screenings and crushed stone. All work except the building of the forms was performed by common laborers. In his experimental work Mr. Chamberlain used two laborers, one of whom set the forms, and filled them and the other of whom mixed the concrete. The pipes were left in the forms till the morning of the day after molding. The two laborers removed the forms filled the day before, the first thing in the morning, and proceeded to refill them. The average time the concrete was allowed to set before the forms were removed was 16 hours. Mr. Chamberlain believes that with three men and six forms the whole six forms could be removed and refilled daily. Based on the use of only two forms with two laborers removing and refilling them each day, and on the assumption that a single set of forms costing $40 can be used only 50 times before being replaced, Mr. Chamberlain estimates the cost of molding 4-ft. pipes as follows:

This gives a cost of $1.83 per lineal foot of pipe or practically $7 per cu. yd. of concrete. The pipe actually molded cost $2.50 per lin ft., or $9.62 per cu. yd. of concrete, owing to the small scale on which the work was carried on—the laborers were not kept steadily at work.

The pipes were built under a derrick and loaded by means of the derrick upon flat cars for transportation. At the culvert site they were unloaded and put in by an ordinary section gang with no appliances other than skids to remove the pipes from the cars. As each four-foot section of this pipe weighs about two tons, it was not deemed expedient to build sections of a greater length than 4 ft., to be unloaded and placed by hand. On a trunk line, however, where a derrick car is available for unloading and placing the pipes, there is no reason why they should not be built in 6 or 8-ft. sections.

CHAPTER XIX.

METHODS AND COST OF REINFORCED CONCRETE BUILDING CONSTRUCTION.

If we set aside concrete block construction, virtually all concrete used in building construction is reinforced; plain monolithic or mass concrete now, as in the past, is one of the secondary building materials. It is reinforced concrete building construction that is discussed in this chapter. In no class of concrete work is the contractor's responsibility for the successful outcome of the work greater than in reinforced concrete building construction. No degree of excellence in design can make up for incompetent, careless or dishonest work in construction. This is true not merely in the general way that it is true of all engineering construction—it is true in a special way peculiar to the material. Except for the reinforcing steel, the contractor for concrete building work has no guarantee of the quality of any element of his work except his own faithful care in performing every task that combines to produce that element. The quality of his concrete depends upon the care with which he has chosen his cement, sand and stone, and on the perfection with which he has incorporated them into a homogeneous mixture. The quality of his beam or column, then, depends upon the care with which the concrete is placed in position with the reinforcement and with which the supporting forms are maintained until the member is amply strong to do without support. There is no certainty of any detail except the certainty that is had by performing every part of the work as experience has taught that it should be performed if perfect results are to be attained. We have dwelt thus emphatically on the responsibility in concrete building work of the contractor for the reason that in the past it has been upon the contractor that the burden of failure has been generally shifted.

The construction work of buildings is divided into (1) construction, erection and removal of forms; (2) fabrication and placing of reinforcement; (3) mixing, transporting and placing concrete.

CONSTRUCTION, ERECTION AND REMOVAL OF FORMS.

The stereotyped text-book statement that forms must be true to dimensions and shape and rigid enough in construction to maintain this condition under all loads that they have to sustain mentions only one of the factors that the constructing engineer or the contractor has to keep in mind in designing such forms. His design must be made true and rigid at the least possible cost for first construction of lumber and carpenter work; it must be made with the plan in mind of using either the same forms as a whole or the same form material several times in one structure; it must be made with a view to convenience in taking down, carrying and re-erecting the forms the second or third time; and it must be made with the object in sight of securing the greatest salvage value either in forms fit for use again or in form lumber that can be sold or worked up for other purposes.

The general conditions governing the computation and design of economic form work are discussed in Chapter IX.

COLUMN FORMS.—Concrete columns are usually square or rectangular in section, with, commonly, chamfered or beveled corners. The popularity of these sections is due very largely to the simplicity of the forms required. When hooped reinforcement is used, the column section is always circular or polygonal. Hollow sections, T-section and channel sections are rarely employed and then only for wall columns.

Column forms should be made in units which can readily be assembled, taken apart and re-assembled. The number, arrangement and size of the units are determined by the shape and size of the column and the means adopted for handling the forms. For square or rectangular columns there will be usually four units of lagging, one for each side, plus the number of clamps or yokes used to bind the sides together. Yokes or clamps will seldom be spaced over 3 ft. apart unless very heavy lagging is used; 2 ft. spacing for yokes is common. For circular columns two units of lagging are necessary and this is the number commonly used; the yokes or hoops are spaced about as for rectangular columns. Metal forms can be used to good advantage for cylindrical columns. Forms for polygonal columns are difficult to construct in convenient units. Forms built complete a full story high and concreted from the top are essential where wet and sloppy concretes are used. In Europe, where comparatively dry concretes are employed and where the reinforcement is commonly placed a piece at a time as concreting progresses, three sides of a rectangular form are erected full height and the fourth side is built up as the concrete and metal are placed. This construction is now less common, even abroad, than it was, since wetter mixtures are coming to be approved by European engineers to a greater extent now than formerly. It is a time consuming method and with wet mixtures it has nothing to recommend it. For lagging 1¼ and 2-in. plank are commonly used; with yokes spaced 2 ft. apart the lighter plank is amply strong and reduces the weight of the units to be handled as well as the amount of form lumber required.

Column forms should always be constructed with an opening at the bottom by means of which the reinforcement can be adjusted and sawdust, shavings and other material cleaned out.

Rectangular Columns.—The form shown in section by Fig. 177 was used in constructing a factory building at Cincinnati, O. Two 2×4-in. studs at each corner carry the horizontal side lagging boards and are clamped together by yokes composed of four hardwood corner saddles connected around the form by a hooked rod with center turnbuckle on each side. No nails are used in assemblying the parts; the same studding and yokes serve for several sizes of column, the lagging alone being changed. The lumber required for studding is 5½ ft. B. M. per foot of column length. The lumber required for lagging, using 1 in. boards, would be 2⅔ ft. B. M. for a 12-in. column, and ⅔ ft. B. M. would be added for every 2-in. increase in size of the column. About 3½ ft. B. M. is required for each set of four corner saddles. With the studs rabbeted at the mill, the carpenter work is reduced to the simple task of sawing the boards and struts to length. The form is taken down by simply unscrewing the turnbuckles; it can be erected by common labor in charge of one carpenter to attend to the plumbing and truing-up. The form can be used over and over and for columns of different sizes without change except in the length of the lagging boards.

The form shown by Fig. 178 was used in constructing a nine-story warehouse at St. Paul, Minn.; it is a design which has become almost standard with a number of large building contractors. In this construction lagging boards the full length of the column are used and are held without nails by yokes. The yokes consist of two heads of wood held together by threaded rods with nuts; between the rods and the lagging are struts or blocks serving both as spacers and to hold the lagging to plane and surface. The yoke proper is adjustable to the extent of the threaded portions of the tie rods. It is to be noticed that the lagging boards are not connected by battens or cleats, therefore, two or three widths of stock serve for all ordinary changes in size of columns and carpenter work is limited to sawing them to length. Furthermore as the boards are full column length, their salvage value when removed from the forms is high. Common laborers under a carpenter foreman can assemble and erect the form. For a 12-in. column and using 3×4-in. yokes spaced 2 ft. apart and 1¼-in. lagging, this form requires about 12 ft. B. M. of lumber per foot length of column. The column form shown by Fig. 226 for the six-story building described in a succeeding section differs from the one described only in the details of the yoke construction. In place of the struts between the wooden heads of the yoke a cleat is nailed across the projecting ends which has to be pried loose every time the yoke is removed and nailed into place again every time the yoke is put onto another form; these repeated nailings soon destroy the yoke heads. This form as constructed requires about 8¾ ft. B. M. of lumber per foot length of 12-in. column, which is 3¼ ft. B. M. less than is required for the form shown by Fig. 177. The saving comes entirely in the yoke construction.

The form shown by Fig. 238 is of the same general type as are the two just described, the chief difference in detail being in the yoke construction and in the forming of the lagging boards into a panel or unit for each side by means of battens. This panel construction makes a lagging unit which is more convenient to handle, but less convenient to adapt to changes in size of column. The salvage value of the lumber is also reduced by the nailing. Assuming 1¼-in. lagging and a yoke spacing of 2 ft., to permit direct comparison, this form requires 10½ ft. B. M. of lumber per foot length of 12-in. column as compared with 12 ft. B. M. for the form shown by Fig. 177 and 8¾ ft B. M. for the form shown by Fig. 178. As actually constructed with 2-in. lagging the form shown by Fig. 238 requires about 14 ft. B. M. of lumber per foot length of 12-in. column.

The French constructor, Hennebique, uses the column form construction shown by Fig. 179. Three sides of the forms are built full length of vertical plank and the fourth is built up of horizontal lagging nailed on a board at a time as concreting progresses. In place of rectangular yokes, steel clamps of special form are used to hold the lagging in place. To tear down this form requires drawing the nails in the horizontal lagging and the knocking loose of the clamps. The vertical lagging is of necessity connected by battens into panels to make it possible to hold it in place by the form of clamp used. Assuming 2-in. vertical lagging with ⅞×3-in. battens every 3 ft., and ⅞-in. horizontal lagging this form requires about 12 ft. B. M. of lumber for every foot length of 12-in. column. This form seems to offer no particular merits to American eyes: there is practically no saving in lumber over forms with rectangular yokes and the clamp shown, while adjustable, is not nearly so rigid and secure a bond for the lagging as is a good yoke.

The form shown by Fig. 180 is an extreme example of nailed construction throughout, no yokes or clamps being used. It was used in constructing a factory building in New York City. Horizontal lagging nailed to vertical studs was used for all four sides; three sides were built up full height and the fourth side was placed a board at a time as concreting progressed. This form required 7⅓ ft. B. M. of lumber per foot length of 12-in. column, which is probably about as low in lumber as column form construction can be got. The labor of tearing down and re-erecting the form would be high as also would the waste of lumber. Nailed forms of this type are rarely used.

The form shown by Fig. 181 was used for molding T-section wall columns for a power station. It is noteworthy for its section; because of the provision for molding grooves in the two sides to which the curtain walls join, and because of the manner in which three of the eight sides were built up as the concreting progressed. The sides a b c, d e and f g h were erected in full column units and the sides c d, e f and h a were erected in sections 2 ft. high as concreting progressed. The yokes were spaced 2 ft. apart. Using 1¼-in. stuff for yokes and lagging this form as built required about 16 ft. B. M. per foot length of column. Except for the beveling of the mold for the curtain wall recesses, the framing is all plain saw and hammer work.

A corner wall column form is shown by Fig. 182 and as this was an example of hollow column work the section of the concrete within the form is shown. Forms of this shape and of T-section are properly classed as special form work so that the examples given here are helpful merely as indicating general methods that may be followed. This particular form required 15¾ ft-B. M. of ⅞-in. lagging per foot of column length, and, neglecting the special top frame, about 16 ft. B. M. of "staging" per foot to support the lagging. The core forms for molding the hollow spaces in the columns of this particular building are shown in Fig. 183. The cross pieces or keys carried on the ⅝-in. bolts as pivots are revolved a quarter turn to slip clear of the slots and permit the sides to close together and free the core for withdrawal. In many cases the contractor will find it preferable to use thin sheet metal core molds or light wooden cores and leave them in place. In one case known to the authors where hollow wall columns were used as hot air ducts for a heating system the duct was laid up of one row of bricks, encircled by the column form and the annular space concreted around the brick duct as a core. The rare use of irregular columns makes form and core construction for them a special problem requiring special detailed estimates in each case. The channel section wall column form shown by Fig. 230 is a case in point; here the form became practically a portable mold for duplicating columns as many times as was desired.

As an example of form work for very large columns or pillars that shown by Fig. 184 is particularly good; it was used for constructing eight 3-ft. square pillars for a water tank tower. The lagging consists of four panels made by nailing horizontal boards to vertical studs. The panels are clamped together by rectangular yokes spaced 3 ft. apart. There are nearly 27½ ft. B. M. of lumber per foot length of 3-ft. column in this form.

The form shown by Fig. 185 was used by Mr. R. W. Maxton in constructing a large factory building at St. Louis, Mo., and is notable for the means adopted for centering the forms and for reducing their lateral dimensions to fit them for molding the decreasingly smaller columns of the upper floors. To center the forms the short angles A A are molded into the concrete so as to project slightly above the tops of the floor slab. Also the pieces of wood C are molded into the floor slab. The form is set over the angles and lined up truly by nailing the blocks B to the blocks C. It will be noticed also that the column mold bears only at the four corners the lagging being cut away somewhat on each side so as to afford an opening for cleaning. The lagging for the sides of the column mold is battened together to form four units or panels which are held together by iron clamps of the form shown. Lag screws are used everywhere in place of nails. The notable feature, however, is the piecing out of the lagging panels with 1-in. strips, one or more of which can be ripped off on each side to reduce the size of the forms as the columns grow smaller toward the top of the building.

Polygonal Columns.—Forms for polygonal columns require more lumber and more carpenter work and are less susceptible of ready arrangement into units than forms for rectangular columns. There is no approach to a uniform practice in their construction and the few forms shown here are merely specific examples.