Concrete Construction: Methods and Costs

The construction of the girder forms is shown by Fig. 242. This drawing shows one of the four main sections making up a complete form. A full size form of this construction contained about 1,100 ft. B. M. of lumber, and three were built, so that 3,300 ft. B. M. of form lumber were used for molding 20 girders, or 33 ft. B. M. per cubic yard of concrete. A full size column form contained about 225 ft. B. M. of lumber, and eight were constructed, so that 1,800 ft. B. M. of form lumber were used for molding 56 columns, or about 16 ft. B. M. per cubic yard of concrete.

The following was the cost of erecting a full column form including lining, plumbing, bracing and yoking, but excluding lumber and original construction:

This gives a cost of $7.25 per M. ft. B. M. for erecting column forms.

The cost of erecting a full size girder form including lining, plumbing, bracing and setting six bolts was as follows:

This gives a cost of $4.35 per M. ft. B. M. for erecting girder forms.

The reinforcement was erected inside the forms for both columns and girders. The cost of erection for one column was:

This gives a cost of about 0.22 cts. per pound for erecting column reinforcement, including the bending of the horizontal ties or hoops. The girder reinforcement was erected by piece work at a cost of $1.80 per girder—or about 0.08 ct. per pound.

The concrete used was a 1-6 mixture of Portland cement and crusher run stone all passing a ½-in. sieve and 10 per cent. passing a 200 mesh sieve. No trouble was had in handling this fine aggregate. It was mixed in a Ransome mixer, elevated so as to deliver the batches into cars on a standard gage track. This track ran between the base slabs on which the molding was done. Each car held about 3 cu. yds. and discharged through a side gate and spout directly into the forms, the mixture being made so wet that it would flow readily. The company used its own cement and stone for concrete and charged up the cement at $1 per barrel and the stone at 60 cts. per cubic yard. At these prices, and assuming that a cubic yard of concrete of the mixture above described would contain about 1.25 bbl. of cement and 1.5 cu. yd. of stone, we have the following cost of materials per cubic yard of concrete:

The actual cost of mixing the concrete and delivering it to the cars was as follows:

The cost of hauling the concrete from mixer to forms ran about 2.7 cts. per cubic yard, so that we have a cost for concrete in place of:

The cost, then, per column or girder molded, assuming that it was necessary to erect a full form, was about as follows:

These figures give a unit cost of $12.41 per cu. yd. for molded columns, and of $15.07 per cu. yd. for molded girders, The columns were erected by a Browning locomotive crane, which lifted and carried them to the work and up-ended them into place. To facilitate lifting the columns from the molding bed a 1½-in. pipe 8 ins. long was cast into both ends; pins inserted into these sockets provided hitches for the tackle. The column was lifted off the molding bed and blocked up, then iron clamps were attached, one at each end, as shown by Fig. 243. A gang of 1 foreman and 14 men erected from 5 to 7, or an average of 6 columns per 10-hour day. The average wages of the erecting gang were 21 cts. per hour. The cost then of column erection was (14 × $2.10) ÷ 6 = $5.25 per column, or $2.63 per cu. yd. of concrete.

The roof girders had 1-in. eye-bolts 24 ins. long cast into them vertically about 4 ft. from the ends. They were lifted off the molding bed by tackle by the locomotive crane to these eye-bolts and blocked up to permit the adjustment of the sling. This sling is shown by the sketch, Fig. 244, and as will be observed acts as a truss. At first it was used without the vertical, but the cantilever action of the unsupported ends caused cracks. The girders were loaded onto cars by the locomotive crane and taken to the work, where they were hoisted and placed by a gin pole. The girder erecting gang consisted of 1 foreman and 14 men, working a 10-hour day at 21 cts. per hour. This gang erected four girders per day, at a cost of (15 × $2.10) ÷ 4 = $7.87 per girder, or $1.57 per cu. yd. of concrete.

The cost of girders and columns in place was thus about as follows:

In this same building the roof was composed of 12×6¼ ft.×4-in. slabs molded in tiers; a slab was molded and when hard was carpeted with paper and the form moved up and a second slab molded on top of the first. This operation was repeated until a tier of slabs had been molded. By molding each slab with a 3-in. overlap, as shown by Fig. 245, they could be easily separated by lifting on hooks inserted under the overhanging ends. Each slab contained 0.925 cu. yd. of concrete and about 116¾ lbs. of reinforcement. The cost of molding one roof slab, including materials, forms and labor, was as follows:

The roof slabs were raised from the casting beds by means of the locomotive crane and hooks, as shown by Fig. 245, and loaded onto cars; eight slabs made a carload. The cars were run to the work, where the gin poles hoisted the slabs one at a time to cars running on a track built on timbers laid on top of the roof girders. A small derrick on rafters picked the slabs from the hand car and set them in place. A gang of 15 men erected from 18 to 20 slabs per 10-hour day. With average wages at 21 cts. per hour the cost of erection was (15 × $2.10) ÷ 19 = $1.66 per slab, or $1.79 per cu. yd. The total cost of slabs in place was thus:

In studying these cost figures their limitations must be kept in mind. Because of the character of the available data quantities had in several cases to be estimated from the working drawings. The cost of lumber for and of framing column and girder forms is not included, but this is partly balanced at least by the assumption that each form was erected complete for each column and girder, which was not the case, as has been stated. Cost of plant is not included nor is cost of shoring the columns until girders and struts were placed, nor are several minor miscellaneous items.

HOLLOW BLOCK WALL CONSTRUCTION.—Three general processes of molding hollow wall blocks of concrete are employed: (1) A dry mixture is heavily tamped into a mold and the block is immediately released and set aside for curing; (2) a liquid is poured into molds, where the block remains until hard: (3) a medium wet mixture is compressed into a mold by hydraulic presses or other means of securing great pressure. The molds used may be simple wooden boxes with removable sides or mechanical molds of comparative complexity. Generally mechanical molds, or concrete block machines as they are commonly called, will be used. There are a score or more kinds of block machines all differing in construction and mode of operation. None of them will be described here, but those interested may consult "Concrete Block Manufacture" by H. H. Rice or "Manufacture of Concrete Blocks and Their Use in Building Construction" by H. H. Rice, Wm. M. Torrance and others.

Factory Buildings, Grand Rapids, Mich.—The buildings ranged from one to four stories high and altogether occupied some 74,000 sq. ft. of ground. The owners installed a block making plant fully equipped with curing racks, two Ideal machines, two National concrete mixers, 5 h.p. gasoline engine, platens, tools and a Chase industrial railway.

The walls were constructed of 24-in. square pilasters of blocks arranged as shown by Fig. 246, connected by curtain wall belt courses of single blocks. The blocks were 8×8×16 ins., and after molding the faces were bush hammered and the edges tooled. The pilasters, consisting of four blocks laid around an 8×8-in. hollow space, were solidified by pouring the 8×8-in. space and all but the three outside block cavities with wet concrete. The interior of the building was of regulation mill construction, and as the pilasters reached the heights for beam supports cast iron plates with downward flanges were set in the concrete. These plates had a cast pin projecting upward to fasten the beam end.

The materials used for the block were Sandusky Portland cement and ¾-in. bank gravel well balanced from fine to coarse. The blocks were molded with 1-3 mortar faces, the mortar being waterproofed by a mixture of Medusa waterproofing compound. All concrete was machine mixed. The men operating the block machines were paid 1 ct. for each block molded, so that their pay depended upon the energy with which they worked. The men handling materials and engaged in handling and curing the blocks were paid $1.75 per day. The gravel was shoveled from the railway cars onto the screens and from the screen piles to the mixers. The gang was organized as follows:

The average daily run was 1,500 blocks, or 300 blocks per machine.

This output was easily maintained after the gang got broken in; sometimes it ran higher and sometimes lower, but the average was as given. The men operating the block machines thus earned $3 each per day. The labor cost of molding and curing per block was thus 2.87 cts. As the blocks had about 25 per cent. hollow space, each block 8×8×16 ins. contained 0.45 cu. ft. of concrete; a cubic yard of concrete, therefore, made 60 blocks, so that the labor cost of making the blocks was 60 × 2.87 cts. = $1.72 per cubic yard. This cost does not include foreman's time, materials, interest, depreciation or general expenses. It was estimated by the owners that the blocks cost them 9 cts. apiece cured, or about $5.40 per cubic yard of concrete. This 9 cts. evidently includes materials and labor alone.

Upon removal from the molds the blocks were loaded onto cars, taken to a large shed and there unloaded onto shelving arranged to hold five rows of blocks one above the other, two blocks opposite each other on each shelf. The blocks were left in the shed 24 to 48 hours to get the preliminary set, then they were loaded on small cars and taken to the yard, where they were removed from the cars and stacked. They were sprinkled every day for six days, being kept covered meanwhile with oiled cotton cloth. The labor costs given above include molding, sprinkling and handling the blocks up to this point.

To lay the blocks they were again loaded on cars and run to an elevator in a wooden tower outside the building. The elevator lifted the car to the floor on which the blocks were to be used, where it was run off onto a track reaching the full length of the building. The blocks were unloaded directly behind the masons. Where the walls were high enough for scaffolding the blocks were unloaded directly onto the first scaffold and, when necessary, handed up to the scaffolds above. The masons employed were regular stone masons receiving the regular scale of wages of $3.50 per day. The number of blocks laid by each mason was 125 per day in building pilasters and 200 per day in building plain wall. Sometimes 250 blocks per day per man were laid in plain wall work. The cost per block of laying above was thus 2.8 cts. pilasters and 1.75 cts. in plain wall. This cost does not include transporting the blocks from yard or of handling them to the scaffold behind the masons, nor does it include the cost of materials and labor for mixing and delivering mortar.

One of the features of this work was the method of transporting the blocks by cars. A complete system of tracks was provided covering the block plant and yard, the building sites and the several floors of the buildings themselves. All blocks and other materials were transported by cars running on these tracks, both cars and tracks being of the type made by the Chase Foundry & Manufacturing Co. of Columbus, Ohio.

Residence, Quogue, N. Y.—The following record of methods and cost of constructing a concrete block residence is furnished by Mr. Noyes F. Palmer: A mixture of sand and pebbles was had on the site; screening was necessary merely to sort out the odd size stones. A mixture of 1 cement and 5 sand was really a 1-2-3 mixture, the 2 being the finest grades of sand and the 3 being various gravel sizes—none too large, none too small—so that the proportion was 2/5 fine sand and 3/5 gravel.

The concrete was hand mixed, and as the gravel had always just been excavated it contained moisture and did not have to be wetted. The sand and gravel were mixed and turned three or four times and spread out thin, and the cement was carefully spread over them in a uniform layer. The mass was then turned three or four times until the eye could detect no difference in color; that is, each grain large enough for the eye to discern seemed to be coated with cement. After this dry mixing, water was added in a fine spray—not a deluge from a pail—but only enough to moisten the mixture. The mass was then turned three or four times. The mixture was then shoveled into the mold, no special face mixture being used, so as to about half fill it, and was then tamped by two men, one standing on each side of the machine. Altogether three layers of material were so placed and tamped and then a shovelful of sand and cement mixture was spread over the top to permit an even "strike-off."

As each block was molded it was carried on the working plate and set down on skids properly spaced to fit the marks on the plate. This is an important detail and Mr. Palmer comments on it as follows: "The writer saw inexperienced men careless about it and who would break the backs of many blocks by not having the skids properly placed. After the blocks have been at rest for half an hour commence to spray them with a revolving garden sprinkler or by carefully wetting with a sprinkling pot on the center of the block only. The blocks should not be allowed to dry out for at least ten days after removal from the working plate. The removal from the working plate can be done the morning after molding and should never be done before even if the block was made in the morning. In removing the green block from the skids let there be cones of sand between the rows of blocks and up-end each working plate so as to let the end of the block come upon the sand cushion. Don't twist and turn the block, and to remove the working plate pass a stick through the core holes in both block and plate so that the plate will not fall when loosened. A slight rap on the center of the plate will loosen it. As soon as the blocks are up-ended commence the spraying and soak the sand underneath the block. It may seem unnecessary to dwell on these points so long, but barrels of cement and barrels of money have been wasted by neglecting to supply the hardening block with water. Curing is just as important as molding in making concrete blocks."

The block construction had been detailed by the architect from cellar to roof, so that it was known beforehand how many blocks of given size were to be made. The unit of length was 32 ins.; this afforded fractional parts of 8 ins., 16 ins. and 24 ins., therefore all openings were in multiples of 8 ins. Odd sizes were made, by inserting "blanks" in the mold box, to inches or fractions of an inch if desired. This unit length was less mortar joints, while the unit of height was 9 ins., or the same as four ordinary bricks with joints. The floor levels were calculated in multiples of 9 ins., so that the wall could be finished all around where the beams were to be seated. This beam course was made of solid blocks; that is, no cores were used in molding them. With the machine used no change was required to mold these solid blocks except to remove the cores. The core holes in the working plate were simply covered with pieces of tin. The shape of the block was the same and the same materials were used.

The best record in making blocks for this work was 30 blocks, 8×9×32 ins., in one hour, working six men, three mixing and three on the machine, and using one barrel of cement for 16 blocks. This was a record run, however, a fair average being 20 blocks per hour, or 200 per ten hours, which was the day worked. We have then the cost of making blocks as follows:

This gives a cost per block of $50.50 ÷ 200 = 25¼ cts. The displacement in the wall of each block is 1.75 cu. ft., or the same as 30 bricks.

The cost of laying blocks is the most uncertain item in the whole industry. Mr. Palmer states that he has known of instances where it cost only 5 cts. per block and of other instances where, because of the difficulty of getting help and its inexperience, it cost 15 cts. per block. In this particular building one mason and three helpers laid 100 blocks per day. The building had no long walls, but it did have many turns. The cost of laying, then, was as follows:

This gives a cost for laying of 10 cts. per block. We have, then:

This gives a cost of 35¼ cts. per block for making and laying.

The use of a derrick for laying the blocks proved a considerable item of economy in this work. This derrick cost $50 and two men could mount and move it on the floor beams. It had a boom reaching out over the wall and was operated by a windlass. A plug and feather to fit the center 6-in. hole in the block was used for hoisting the blocks. By this means blocks only seven days old were laid without trouble. It may be noted that the walls were kept drenched with water to make sure that the blocks did not dry out until they were at least 28 days old. In laying the blocks a thin lath was used to keep the mortar back about one inch from the face. This precaution will prevent much labor in cleaning the walls from mortar slobber.

Two-Story Building, Albuquerque, N. M.—The following record of cost of making 9×10×32-in. hollow blocks in a Palmer machine and of laying 2,000 of them in two-story building walls is given by Mr. J. M. Ackerman. Sand cost 60 cts. per cu. yd., and cement cost $3 per barrel. Lime cost 30 cts. per bushel. One barrel of cement made 20 blocks, using a 1-4 sand mixture. In making 2,000 blocks about 100 blocks, or 5 per cent., were lost by blocks breaking in hauling from yard to building or by cutting blocks to fit the work. The blocks were molded by piece work for 5 cts per block, all materials, tools and plant being supplied to the molders. Three men with one machine made from 100 to 150 blocks per day. The cost was as follows:

As each block gave 9 × 32 = 288 sq. ins., or 2 sq. ft., of wall surface, the cost of the wall per square foot was 19.5 cts. Assuming 40 per cent. hollow space, each block contained 1 cu. ft. of concrete, which cost 23 cts., or $6.21 per cu. yd., for materials and molding. Blocks in the wall cost $10.55 Per cu. yd. of concrete.

General Cost Data.—The following data are given by Prof. Spencer B. Newberry. The average weights of three sizes of hollow blocks are as follows:

Costs of materials are assumed as follows:

Mixed in batches of 750 lbs., sufficient for six 8-in. or four 12-in. blocks, the cost of materials per batch and per block will be for a 1-4 mixture as follows:

In general a factory producing 600 8-in. blocks per day will require 25 men to operate it. At an average wage of $1.80 per day the following is considered as a fair estimate of cost:

This gives for 8×9×32-in. blocks a cost of about $6.78 per cu. yd. of concrete for materials and molding or of 11.3 cts. per sq. ft. of face.

Mr. L. L. Bingham gives the following as the average cost per square foot of face for 10-in. wall from data collected from a large number of block manufacturers operating in Iowa in 1905:

Assuming one-third hollow space, the cost for materials and molding was $5.05 per cu. yd. of concrete not including interest, depreciation, repairs, superintendence or general expenses.

CHAPTER XXI.

METHODS AND COST OF AQUEDUCT AND SEWER CONSTRUCTION.

Aqueducts and sewers in concrete are of three kinds: (1) Continuous monolithic conduits, (2) conduits laid up with molded concrete blocks, and (3) conduits made up of sections of molded pipe. Block conduits and conduits of molded pipe are rare in America compared with monolithic construction; examples of each are, however, given in succeeding sections, where forms, methods of molding, etc., are described. The following discussion refers to monolithic construction alone.

FORMS AND CENTERS.—Forms and centers for conduit work have to meet several requirements. They have to be rigid enough not only to withstand the actual loads coming on them, but to keep from being warped by the alternate wetting and drying to which they are subjected. They have also to be constructed to give a smooth surface to the conduit. To be economical, they have to be capable of being taken down, moved ahead and re-erected quickly and easily. The carpenter costs run high in constructing conduit forms, so that each form has to be made the most of by repeated use.

Three different constructions of traveling forms are described in the succeeding sections. For small work, such forms appear to offer certain advantages, but for conduits of considerable size their convenience and economy are uncertain. The experience with the large traveling form employed on the Salt River irrigation works in Arizona was, when all is said, rather discouraging. The authors believe that for work of any size where the concrete must be supported for 24 hours or more, forms of sectional construction will prove cheaper and more expeditious than any traveling form so far devised.

No class of concrete work, perhaps, offer so good an opportunity for the use of metal forms as does conduit work. The smooth surface left by metal forms is particularly advantageous, and there is a material reduction in weight and a large increase in durability due, both to the lack of wear and to freedom from warping. Steel forms of the Blaw type shown by Fig. 247, have been used for conduits up to 25 ft. in diameter. The form illustrated, Fig. 247, was for a 12-ft. 3-in. sewer; in this case a roof form alone was used, but full circular and egg-shape forms are made. The Blaw collapsible Steel Centering Co., of Pittsburg, Pa., make and lease steel forms of this type.

Sectional wooden forms for conduits of large diameters are shown by the drawings in several of the succeeding sections. Figures 248 and 249 show such forms for small diameters. The form shown by Fig. 248 is novel in the respect that after being assembled a square timber was passed through it lengthwise, occupying the holes B and having its ends projecting and rounded to form gudgeons. The form was mounted with these gudgeons resting on horses, so that it could be rotated and thus wound with a narrow strip of thin steel plate. Thus sheathed, the form was lowered into the trench and the concrete was placed around it. When the arch had been turned, the wedges A were driven in until the ribs C dropped into the slots a and clear of the steel shell; the arch form was then pulled out and finally the invert form, leaving the steel shell in place to hold the concrete until hard. The strip of steel was then removed by pulling on one end until it unwound like cord from the inside of a ball of twine. Steel strips 6 ins. wide and 1/24 in. thick were used successfully in constructing a 5-ft. egg-shaped sewer in Washington, D. C. The forms were made in sections 16 ft. long, and were taken out as soon as the concrete had been placed.

The form shown by Fig. 249, is an invert form, used in constructing the sewer shown by Fig. 249, built at Medford, Mass., in 1902, by day labor. The concrete was 1-3-6 gravel. The forms for the invert were made collapsible and in 10-ft. lengths. The two halves were held together by iron clamps and hook rods. The morning following the placing of the concrete the hook rods were removed and turnbuckle hooks were put in their places, so that by tightening the turnbuckle the forms were carefully separated from the concrete. The concrete was then allowed to stand 24 hours, when the arch centers were set in place. These centers were made of ⅞×1½-in. lagging on 2-in. plank ribs 2 ft. apart, and stringers on each side. Wooden wedges on the forward end of each section supported the rear end of the adjoining section. The forward end of each section was supported by a screw jack placed under a rib 2 ft. from the front end. To remove the centers, the rear end of a small truck was pushed under the section about 18 ins.; an adjustable roller was fastened by a thumb screw to the forward rib of the center; the screw jack was lowered allowing the roller to drop on a run board on top of the truck; the truck was then pulled back by a tail rope until the adjustable roller ran off the end of the truck; whereupon the truck was pulled forward drawing the center off the supporting wedges of the rear section. Each lineal foot of sewer required 1¼ cu. yds. of excavation which cost 74.2 cts. per foot, and 1 cu. ft. of brick arch which cost $12.07 per cu. yd., or 44.2 cts. per lineal foot of sewer. The invert required 4 cu. ft. of concrete per foot, which cost as follows:

The cost of the invert was thus $1.002 per lin. ft. of sewer.

Collapsible metal forms for manholes and catch basins are made by several firms which make block and pipe molds. A cylindrical wooden form construction is shown by Fig. 250. The outside form consists of three segments of a cylinder made of 2-in. lagging bolted to hoops. Bent lugs on the ends of the hoops, were provided with open top slots and were bolted together through 1×⅜-in. bars which extended the full length of the form between lugs. The assembled form was collapsed by pulling up on the bars, thus lifting the bolts out of the slots. The inner mold is also made in three sections with strap hinges at two of the joints and at the third joint a wedge-shaped stave. The other details are shown by the drawing. To mold the top of the basin two cone-shaped forms are used, an outer form made in one piece and an inner form made in sections. Some 26 catch basins were built in Keney Park, Hartford, Conn., by Mr. H. G. Clark, at a cost of $7 apiece for concrete in place, and there was closely 1 cu. yd. of concrete in each.

CONCRETING.—Except for pipes of small diameter, the concreting is done in sections, each section being a day's work. Continuity of construction has not proved successful, except for pipes of moderate size, in the few cases where it has been tried. Examples of continuous construction methods are given in succeeding sections. Methods of molding and laying cast concrete pipe are also best shown by the specific examples given further on. In concreting large diameters, the work may be done by molding successive full barrel sections, or by molding first the invert and then the roof arch, each in sections. The engineer's specifications generally stipulate which plan is to be followed. Construction joints between sections are molded by bulkhead forms framed to produce the type of joint designed by the engineer; the most common type is the tongue and groove joint.