COMPARATIVE ECONOMY OF PLAIN AND REINFORCED CONCRETE WALLS.—Prior to the construction of some 2,000 ft. of retaining wall ranging in height from 2 ft. to 38 ft., at Seattle, Wash., calculation was made by the engineers of the Great Northern Ry. to determine the comparative economy of plain concrete and reinforced concrete sections. The sections assumed were those shown by Fig. 98, and comparisons were made at heights of 10, 20, 30 and 40 ft., with the following results:
Height in Plain. Reinforced. Per cent. feet. Cu. yds. per ft. Cu. yds. per ft. Saving. 10 1.63 1.29 20.4 20 4.08 2.59 36.4 30 8.40 4.73 43.3 40 14.70 8.07 45.0
The saving in concrete increased as the height of the wall increased; for a 40-ft. wall reinforced concrete at nearly double the cost per cubic yard in place would be as cheap as plain concrete.
Taking substantially the section of reinforced wall being used on the Chicago track elevation work of the Chicago, Burlington & Quincy R. R., and comparing it with a plain wall as shown by Fig. 99, Mr. F. F. Sinks obtained the following results:
Plain Wall, Cost per Lineal Foot— 4.8 cu. yds. concrete at $4 $19.20 115 ft. B. M. of forms at $31 3.56 ——— Total 4.8 cu. yds. at $4.74 $22.76
Reinforced Wall, Cost per Lineal Foot— 3.46 cu. yds. concrete at $4.10 $14.18 115 ft. B. M. of forms at $31 3.56 109 lbs. reinforcing steel at 3 cts. 3.54 1.34 cu. yds. extra fill at 20 cts. 0.27 0.32 cu. yd. extra excavation at 20 cts. 0.06 ——- Total, 3.46 cu. yds. concrete at $6.25 $21.61
The saving in this case was $1.15 per lineal foot of wall with the unit cost of reinforced concrete in place 24 per cent. greater than the unit cost of plain concrete. It will be noted that there is some 28 per cent. less concrete per lineal foot of wall in the reinforced section and also that this section is so designed that the form work is about as simple for one section as for the other. Another point to be noticed is that there is no saving in excavation by using a reinforced section instead of a gravity section, in fact the excavation runs slightly more for the reinforced section.
FORM CONSTRUCTION.—Retaining wall work often affords an opportunity for constructing the forms in panels and this opportunity should be taken advantage of when possible. Several of the walls described later give examples of form work that may be studied with profit in this respect.
Figure 100 shows a panel form construction employed on the New York Central & Hudson River R. R. The 38-in. studs are erected, care being taken to get them in proper line and to true batter and also to brace them rigidly by diagonal props. Generally the studding is erected for a section of wall 50 ft. long at one time. The lagging, made in panels 2 ft. wide and 10 ft. long, by nailing 2-in. plank to 24-in. cleats, is attached to the studding a panel at a time and beginning at the bottom, by means of the straps, wedges and blocks shown. Five bottom panels making a form 2 ft. high and 50 ft. long are placed first. When the concrete has been brought up nearly to the top of these panels, a second row of panels is placed on top of the first. When it is judged that the concrete is hard enough the lowermost panels are loosened and made free by removing the wedges, blocks and straps and the panels are drawn out endwise from behind the studding and used over again for one of the upper courses. The small size of the panels makes it practicable to lay bare the concrete while it is yet soft enough to work with a float or to finish by scrubbing as described in Chapter VIII. In cases where this object is not sought, panels of much larger size may be used. Working with panels 212 ft. of 2-in. plank it was found that each panel could be used 16 times before becoming unfit for further use, but as, owing to the nicety of molded surface demanded, panels were discarded when showing comparatively small blemishes, this record cannot be taken as a true indication of the life of such forms. These panel forms are used by the railway named for long abutments and piers as well as for retaining walls.
A different type of sectional form construction is illustrated by Figs. 101 and 102. It has been extensively used for retaining wall work by the Chicago, Burlington & Quincy R. R. The studding and waling are framed in units as shown. The lagging is framed in panels for the rear of the wall, for the face of the coping, and for the inclined toe of the wall, and is ordinary sheathing boards for the main face of the wall. The make-up of the several panels is shown by the drawings. The reason for using ordinary sheathing instead of panels for the face of the wall is stated by Mr. L. J. Hotchkiss, Assistant Bridge Engineer, to be that "the sections become battered and warped with use, do not fit closely together, and leave the wall rough when they are removed." The manner of bracing the form and of anchoring it down against the up-thrust of the wet concrete is shown by Fig. 102.
Two other examples of sectional form construction are given in the succeeding descriptions of work for the Grand Central Station terminal in New York City and for the Chicago Drainage Canal. In the former work it is notable that panels 5120 ft. were used, being handled by locomotive crane. The panels used on the drainage canal work and in the forms previously described are of sizes that can be taken down and erected by hand, and the means of handling them should always be given consideration in deciding on the sizes to be adopted for form panels not only in wall construction but in any other class of work where sectional forms may be used. Wet spruce or yellow pine will weigh 4 lbs. per ft. B. M., so that a panel 102 ft. made of 2-in. plank and three 24-in. battens will weigh some 225 lbs. In form work where the panels are removed and re-erected in succession facility in handling is an important matter. When one figures that he may handle both the concrete and the form panels with it a cableway or a locomotive crane becomes a tool well worth considering in heavy wall work.
Three details in retaining wall form work that are often sources of annoyance out of proportion to their magnitude are alignment, coping construction and wall ties. Small variations from line in the face of the wall are seldom noticeable, but a wavy coping shows at a glance. For this reason it is often wise to build the coping after the main body of the wall has been stripped, or if both are built together to provide in the forms some independent means of lining up the coping molds. In the form shown by Fig. 101 the latter is done by bracing the coping panel so as to permit it to be set and lined up independently of the main form. A separate form for molding the coping after the main body of the wall is completed may be constructed as shown by Fig. 103. Bolts at B and C permit the yokes to be collapsed and the form to be shifted ahead as the work advances. This mold provides for beveling the top edges of the coping and also the edge of the overhang, and the beveling or rounding of these edges should never be omitted where a neat appearance is desired. It is not essential, however, that this finishing be done in the molds. By stripping the concrete while it is still pliable the edges can be worked down by the ordinary cement sidewalk edger.
Wall ties are commonly used to hold the face and back forms to proper spacing, but occasionally they are not permitted. In the latter case the bracing must be arranged to hold the forms from tipping inward as well as from being thrust outward. A good arrangement is that shown by Fig. 102. In fastening the forms with ties the choice is usually between long bolts which are removed when the molds are taken down and wire ties which are left embedded in the concrete. The selection to be made depends upon the character of the work. When sectional forms are used like the one shown by Fig. 101, for long stretches of wall of nearly uniform cross-section bolts are generally more economical and always more secure. If the bolts are sleeved with scrap gas pipe having the ends corked with waste the bolts can be removed ordinarily without difficulty. To make the pipe sleeve serve also as a spacer the end next the face may be capped with a wooden washer which is removed and the hole plastered when the forms are taken down. With bolt ties the forms can be filled to a depth of 15 to 20 ft with sloppy concrete. This is hardly safe with wire ties unless more wire and better tieing are employed than is usual. It takes four strands of No. 10 to give the same working stress as a -in. threaded rod and the tieing in of four strands of wire so that they will be without slack and give is a task requiring some skill. Bolts are much more easily placed and made tight. In the matter of cost of metal left in the wall, the question is between the cost of scrap gas pipe and of wire; the pound price of the wire is greater but fewer pounds are used and the metal is in more convenient shape to cut to length and to handle. This convenience in shaping the tie to the work gives the advantage to wire ties for isolated jobs or jobs which involve a continual change in the length and spacing of the ties. In general the contractor will find bolts preferable where sectional forms are used and wire ties preferable when using continuous forms.
One objection urged against the use of wire ties is that the metal is exposed at the face of the work when they are clipped off unless the concrete is chipped and the cavity plastered. To obviate this objection various forms of removable "heads" have been devised. Two such devices are shown by Figs. 104 and 105. In both the bolt is unscrewed, leaving the "heads" embedded. The head shown by Fig. 104 has the advantage that it can be made by any blacksmith, while the head shown by Fig. 105 is a special casting.
MIXING AND PLACING CONCRETE.—Where a long stretch of wall is to be built the system of mixing and handling the concrete must be capable of being shifted along the work. For isolated walls of short length this problem is a simpler one. Where the mixer can be installed on the bank above, wheeling to chutes reaching down to the work is the best solution. As shown in Chapter IV concrete can be successfully and economically chuted to place to a greater extent than most contractors realize. Where the mixer has to be installed at the foot of the wall wheelbarrow inclines, derricks, gallows frames, etc., suggest themselves as means of handling the concrete. It is not this class of work, however, but the long stretches of heavy section walls such as occur in depressed or elevated railway work in cities that call for thought in the arrangement and selection of mixing and handling plant.
In building the many miles of retaining wall in the work of doing away with grade crossings in Chicago, Ill., trains made up of a mixer car and several material cars have been used. The mixer is mounted on a flat car set at the head of the train and is covered by a decking carrying two charging hoppers set above the mixer. The material cars are arranged behind, the sand and stone or gravel being in gondola cars. Portable brackets hooked to the sides of the gondola cars carry runways for wheelbarrows. Sand and stone or gravel are wheeled to the charging hoppers, the work being continuous since one hopper is being filled while the other is being discharged into the mixer. The mixer discharges either into a chute, wheelbarrows or buckets. The foregoing is the general arrangement; it is modified in special instances, as is mentioned further on. The chief objection to the method is the difficulty of loading the wheelbarrows standing on runways level with the tops of the gondola sides. The lift from the bottom of the car is excessive, and as pointed out previously, shoveling stone or gravel by digging into it from the top is a difficult task.
The delivery of the concrete into the forms was accomplished by chute where possible, otherwise by wheelbarrows or cranes, and in one case by belt conveyor. In the last instance the mixer car was equipped with a Drake continuous mixer and was set in front. Behind it came three or four gondola cars of sand and stone, and at the rear end a box car of cement. All material was wheeled on side runways to two charging hoppers over the mixer. The mixer discharged onto a belt conveyor carried by a 25-ft. boom guyed to an A-frame on the car and pivoted at the car end to swing 180 by means of a tag line. The outer end of the conveyor was swung over the forms. A -in. wire rope wrapped eight times around two drums on the mixer car and passing through slots in the floor to anchors placed one 500 ft. in front and one 500 ft. to the rear enabled the train to be moved back and forth along the work. This scheme of self-propulsion saved the hire of a locomotive. In another case the mixer was discharged into buckets which were handled by a crane traveling back and forth along a track laid on two flat cars.
Another type of movable mixer plant used in constructing a sea-wall some 3 miles long at Galveston, Tex., is shown by Figs. 106 and 107. Two of these machines mixed and placed some 127,000 cu. yds. of concrete, in 1 cu. yd. batches. Two 12-HP. engines operated the derricks and one 16-HP. engine operated the Smith mixer; all engines took steam from a 50-HP. boiler. The rated capacity of each machine was 300 to 350 cu. yds. per day. The method of operation is clearly indicated by the drawings.
Placing the concrete in the forms is generally required to be done in layers; with wet mixtures this means little more than distributing the concrete somewhat evenly along the wall and slicing and puddling it to get rid of air and prevent segregation. Where mortar facing is required the face form described in Chapter VIII may be used. A reasonably good surface can be secured without mortar facing by spading the face. With dry concrete, placing and ramming in layers, calls for such care as is necessary in dry concrete work everywhere. Where new concrete has to be placed on concrete placed the day before, good bond may be secured and the chance of efflorescence be reduced by the methods described in Chapter VIII.
WALLS IN TRENCH.—In canal excavation, in subway work in cities, and the like, it is often necessary to dig trenches and build retaining walls in them before excavating the core of earth between the walls. The following examples of such work are taken from personal records:
Example I.—A Smith mixer was used, the concrete being delivered where wanted by a Lambert cableway of 400 ft. span. The broken stone and sand were delivered near the work in hopper-bottom cars which were dumped through a trestle onto a plank floor. Men loaded the material into one-horse dump carts which hauled it 900 ft. to the mixer platform. This platform was 2424 ft. square, and 5 ft. high, with a planked approach 40 ft. long and contained 7,300 ft. B. M. The stone and sand were dumped at the mouth of the mixer and shoveled in by 4 men. Eight men, working in pairs, loaded the broken stone into the carts, and 2 men loaded the sand. Each cart was loaded with about 70 shovelfuls of stone on top of which 35 shovelfuls of sand were thrown. It took 3 to 5 minutes to load on the stone and 1 minute to load the sand. The carts traveled very slowly, about 150 ft. a minute—in fact, all the men on the job, including the cart drivers, were slow. After mixing, the concrete was dumped into iron buckets holding 14 cu. ft. water measure, making about cu. yd. in a batch. The buckets were hooked on to the cableway and conveyed where wanted in the wall. Steam for running the mixer was taken from the same boiler that supplied the cableway engine. The average output of this plant was 100 cu. yds. of concrete per 10-hour day, although on many days the output was 125 cu. yds., or 250 batches. The cost of mixing and placing was as follows, on a basis of 100 cu. yds. per day:
Per day. Per cu. yd.
8 men loading stone into carts $12.00 $ .12 2 men loading sand into carts 3.00 .03 1 cart hauling cement 3.00 .03 8 carts hauling stone and sand 24.00 .24 4 men loading mixer 6.00 .06 1 man dumping mixer 1.50 .01 2 men handling buckets at mixer 3.00 .03 6 men dumping buckets and ramming 9.00 .09 12 men making forms at $2.50 30.00 .30 1 cable engineman 3.00 .03 1 fireman 2.00 .02 1 foreman 6.00 .06 1 waterboy 1.00 .01 1 ton coal for cableway and mixer 4.00 .04 ———- ——- Total $107.50 $1.07
In addition to this cost of $1.07 per cu. yd. there was the cost of moving the whole plant for every 350 ft. of wall. This required 2 days, at a cost of $100, and as there were about 1,000 cu. yds. of concrete in 350 ft. of wall 16 ft. high, the cost of moving the plant was 10 cts. per cu. yd. of concrete, bringing the total cost of mixing and placing up to $1.17 per cu. yd. As above stated, the whole gang was slow.
The labor cost of making the forms was high, for such simple and heavy work, costing $10 per M. of lumber placed each day. The forms were 2-in. sheeting plank held by 46-in. upright studs 2 ft. apart, which were braced against the sides of the trench. The face of the forms was dressed lumber and all cracks were carefully puttied and sandpapered.
The above costs relate only to the massive part of the wall and not the cost of putting in the facing mortar, which was excessively high. The face mortar was 2 ins. thick, and about 3 cu. yds. of it were placed each day with a force of 8 men! Two of these men mixed the mortar, 2 men wheeled it in barrows to the wall, 2 men lowered it in buckets, and 2 men put it in place on the face of the wall. If we distribute this labor cost on the face mortar over the 100 cu. yds. of concrete laid each day, we have another 12 cts. per cu. yd.; but a better way is to regard this work as a separate item, and estimate it as square feet of facing work. In that case these 8 men did 500 sq. ft. of facing work per day at a cost of nearly 2 cts. per sq. ft. for labor.
Example II.—The building of a wall similar to the one just described was done by another gang as follows: The stone and sand were delivered in flat cars provided with side boards. In a stone car 5 men were kept busy shoveling stone into iron dump buckets having a capacity of 20 cu. ft. water measure. Each bucket was filled about two-thirds full of stone, then it was picked up by a derrick and swung over to the next car which contained sand, where two men filled the remaining third of the bucket with sand. The bucket was then lifted and swung by the derrick over to the platform of the mixer where it was dumped and its contents shoveled by four men into the mixer, cement being added by these men. The mixer was dumped by two men, loading iron buckets holding about cu. yd. of concrete each, which was the size of each batch. A second derrick picked up the concrete bucket and swung it over to a platform where it was dumped by one man; then ten men loaded the concrete into wheelbarrows and wheeled it along a runway to the wall. One man assisted each barrow in dumping into a hopper on the top of a sheet-iron pipe which delivered the concrete. The two derricks were stiff-leg derricks with 40-ft. booms, provided with bull-wheels, and operated by double cylinder (710-in.) engines of 18-HP. each. About 1 ton of coal was burned daily under the boiler supplying steam to these two hoisting engines. The output of this plant was 200 batches or 100 cu. yds. of concrete per 10-hr. day, when materials were promptly supplied by the railroad; but delays in delivering cars ran the average output down to 80 cu. yds. per day.
On the basis of 100 cu. yds. daily output, the cost of mixing and placing the concrete was as follows:
Per day. Per cu. yd. 5 men loading stone $ 7.50 $.07 2 men loading sand 3.00 .03 4 men charging mixer 6.00 .06 2 men loading concrete into buckets 3.00 .03 1 man dumping concrete from buckets .50 .01 10 men loading and wheeling concrete .00 .15 1 man dumping wheelbarrows 1.50 .01 3 men spreading and ramming 4.50 .04 2 enginemen 5.00 .05 1 fireman 2.00 .02 1 waterboy 1.00 .01 1 foreman 6.00 .06 10 men making forms 25.00 .25 1 ton coal 4.00 .04 Total 85.00 $.85
In addition there were 8 men engaged in mixing and placing the 2-in. facing of mortar as stated above.
CHICAGO DRAINAGE CANAL.—The method and cost of constructing some 20,000 ft. of concrete wall by contract in building the Chicago Drainage Canal is compiled from records kept by Mr. James W. Beardsley. The work was done on two separate sections, Section 14 and Section 15. In both cases a 1-1-4 natural cement concrete was used with a 3-in. facing and a 3-in. coping of 1-3 Portland cement mortar.
Section 14.—The average height of the wall was 10 ft., and the thickness at base was one-half the height. The stone for the concrete was obtained from the spoil bank of the canal, loaded into wheelbarrows and wheeled about 100 ft. to the crusher; some was hauled in wagons. An Austin jaw crusher was used, and it discharged the stone into bins from which it was fed into a Sooysmith mixer. The crusher and the mixer were mounted on a flat car. Bucket elevators were used to raise the stone, sand and cement from their bins to the mixer; the buckets were made of such size as to give the proper proportions of ingredients, as they all traveled at the same speed. Only two laborers were required to look after the elevators. The sand and cement were hauled by teams and dumped into the receiving bins. There were 23,568 cu. yds. on Section 14 and the cost was as follows:
Typical Wages per Cost per General force: force. 10 hrs. cu. yd. Superintendent 1.0 $5.00 $0.026 Blacksmith 1.1 2.75 0.016 Timekeeper 0.5 2.50 0.007 Watchman 0.6 2.00 0.007 Waterboys 3.9 1.00 0.022
Wall force: Foreman 0.9 2.50 0.013 Laborers 8.6 1.50 0.073 Tampers 2.3 1.75 0.022
Mixer force: Foreman 1.2 2.50 0.017 Enginemen 1.8 2.50 0.025 Laborers 6.7 1.50 0.057 Pump runner 1.0 2.00 0.010 Mixing machines 1.7 1.25 0.012
Timber force: Foreman 0.6 2.50 0.008 Carpenters 4.7 2.50 0.057 Laborers 1.2 1.50 0.010 Helpers 5.3 2.50 0.075
Hauling force: Laborers 2.6 1.75 0.026 Teams 6.3 3.25 0.116
Crushing force: Foreman 0.5 2.50 0.007 Engineman 1.7 2.50 0.023 Laborers 3.5 1.50 0.032 Austin crushers 1.7 1.20 0.011
Loading stone: Foreman 1.7 2.50 0.023 Laborers 32.9 1.50 0.280 ——— Total for crushing, mixing and placing $0.975
The daily costs charged to the mixers and crushers include the cost of coal, at $2 a ton, and the cost of oil.
The gang "loading stone" apparently did a good deal of sledging of large stones, and they also wheeled a large part of it in barrows to the crusher.
The plant cost $9,600, distributed as follows:
2 jaw crushers $3,000 2 mixers 3,000 Track 1,260 Lumber 500 Pipe 840 Sheds 400 Pumps 600 ——- Total $9,600
If this first cost of the plant were distributed over the 23,568 cu. yds. of concrete it would amount to 41 cts. per cu. yd.
The cost of the concrete was as follows:
Per cu. yd.
Utica cement, at $0.65 per bbl. $0.863 Portland cement, at $2.25 per bbl. 0.305 Sand, at $1.35 per cu. yd. 0.465 Stone and labor, as above given 0.975 ——— Total $2.608 First cost of plant $0.407
Section 15.—The conditions on this section were much the same as on Section 14, just described, except that the limestone was quarried from the bed of the canal, and was crushed in a stationary crusher, No. 7 Gates. The stone was hauled 1,000 ft. to the crusher on cars drawn by a cable from a hoisting engine. The output of this crusher averaged 210 cu. yds. per day of 10 hrs. The crushed stone was hauled in dump cars, drawn by a locomotive, to the mixers. Spiral screw mixers mounted on flat cars were used, and they delivered the concrete to belt conveyors which delivered the concrete into the forms.
The forms on Section 15 (and on Section 14 as well) consisted of upright posts set 8 ft. apart and 9 ins. in front of the wall, held at the toe by iron dowels driven into holes in the rock, and held to the rear posts by tie rods. The plank sheeting was made up in panels 2 ft. wide and 16 ft. long, and was held up temporarily by loose rings which passed around the posts which were gripped by the friction of the rings. These panels were brought to proper line and held in place by wooden wedges. After the concrete had set 24 hrs. the wedges were struck, the panels removed and scraped clean ready to be used again.
The cost of quarrying and crushing the stone, and mixing the concrete on Section 15 was as follows:
Typical Wages per Cost per General force— force. 10 hrs. cu. yd. Superintendent 1.0 $5.00 $0.024 Blacksmith 0.9 2.75 0.011 Teams 1.7 3.00 0.025 Waterboy 4.5 1.00 0.022
Wall force— Foreman 1.1 2.50 0.010 Laborers 14.4 1.50 0.105 Tampers 0.1 1.75 0.001
Mixer force— Foreman 2.1 2.50 0.026 Enginemen 2.1 2.50 0.022 Laborers 23.1 1.50 0.180 Mixing machines 2.1 1.25 0.022
Timber force— Carpenters 0.8 3.00 0.013 Laborers 0.7 1.50 0.005 Helpers 10.2 2.50 0.125
Hauling force— Foreman 0.7 2.50 0.009 Enginemen 1.4 2.50 0.019 Fireman 0.4 1.75 0.003 Brakeman 2.2 2.00 0.018 Teams 0.4 3.25 0.007 Laborers 1.5 1.50 0.010 Locomotives 1.4 2.25 0.015
Crushing force— Foreman 1.0 2.50 0.014 Enginemen 1.0 2.50 0.014 Laborers 11.1 1.50 0.081 Firemen 1.0 1.75 0.008 Gyratory crusher 1.0 2.25 0.011
Quarry force— Foreman 1.2 2.50 0.012 Laborers 19.0 1.50 0.140 Drillers 1.8 2.00 0.017 Drill helpers 1.8 1.50 0.013 Machine drills 1.8 1.25 0.011 ——— $0.993
The first cost of the plant for this work on Section 15 was $25,420, distributed as follows:
1 crusher, No. 7 Gates $12,000 Use of locomotive 2,200 Car and track 5,300 3 mixers 3,000 Lumber 1,200 Pipe 720 Small tools 1,000 ———- Total $25,420
This $25,420 distributed over the 44,811 cu. yds. of concrete amounts to 57 cts. per cu. yd.
It will be noted that 2 mixers were kept busy. Their average output was 100 cu. yds. each per day, which is the same as for the mixers on Section 14.
The total cost of concrete on Section 15 was as follows:
Per cu. yd. Labor quarrying, crushing and mixing $0.991 Explosives 0.083 Utica cement, at $0.60 per bbl. 0.930 Portland cement, at $2.25 per bbl. 0.180 Sand, at $1.35 per cu. yd. 0.476 ——— Total $2,660 First cost of plant $0.567
It is not strictly correct to charge the full first cost of the plant to the work as it possessed considerable salvage value at the end.
Comparison.—For the purpose of comparing Sections 14 and 15 the following summary is given of the cost per cubic yard of concrete:
Sec. 14. Sec. 15. General force $0.078 $0.082 Wall force 0.108 0.116 Mixing force 0.121 0.250 Timbering force 0.150 0.140 Hauling force 0.142 0.081 Crushing force 0.073 0.128 Quarry force 0.303 0.275 Cement, natural 0.863 0.930 Cement, Portland 0.305 0.180 Sand 0.465 0.476 Plant (full cost) 0.407 0.567 ——— ———- Total $3.015 $3.225
It should be remembered that on Section 14 there was no drilling and blasting of the rock, but that the "quarry force" not only loaded but hauled the stone to the crusher. The cost of mixing on Section 15 is higher than on Section 14 because the materials were dumped on platforms and shoveled into the mixer, instead of being discharged from bins into the mixer as on Section 14.
GRAND CENTRAL TERMINAL, NEW YORK, N. Y.—In building a retaining wall of the cross-section, shown in Fig. 108, a traveling tower moving on tracks parallel to the wall contained the concrete mixing plant. The construction of the tower is shown in Fig. 109. The tower had two platforms, one at the top carrying two 10-cu. yd. bins for sand and stone and the other directly below carrying 40 cu. ft. (4 cu. ft. cement, 12 cu. ft. sand and 24 cu. ft. stone) Ransome mixer driven by a 30 H.P. motor and a Lidgerwood motor hoist. The elevator tower carried two 40-cu. ft. Ransome dumping buckets traveling in guides and dumping automatically into the bins. These buckets were operated by the Lidgerwood motor hoist on the mixer platform. Sand and broken stone on flat cars were brought alongside the tower. The sand was shoveled direct from the car into the sand bucket, but the broken stone was shoveled into wheelbarrows which were wheeled over a light bridging from car to bucket and dumped. Wheelbarrows were used for handling the stone chiefly because the capacity of the plant was so great that enough men could not be worked in the limited space around the bucket to keep up the supply by shoveling. The wheelbarrow work added materially to the cost. Cement was carried from the cars to the sand bucket, hoisted and stored on the mixer platform which provided storage room for 100 bags. A 1-3-6 mixture was used; the sand and stone were chuted directly from the bins to the charging hopper and the cement was charged by hand. The mixed concrete was delivered to two 1 cu. yd. dump cars running on a 2-ft. gage track laid in sections on the cross pieces connecting the uprights of the forms. The track had no switches, so that one car had to wait for the other. Four men were required to push each car and two more men assisted in dumping the car and kept the track clear. The wall was built in sections 51 ft. long, each containing 250 cu. yds. One of these sections was filled in 8 hours with ease and by a little hustling a section was filled in 6 hours, which is at the rate of 37 cu. yds. of concrete per hour. Working 8 hours per day the cost of mixing, transporting and placing concrete with this mixing plant, with wages for common labor of $1.50 per day, was as follows:
Total. Per cu. yd. 2 men carrying cement $ 3.00 $0.012 6 men shoveling sand 9.00 0.036 17 men shoveling stone 25.00 0.100 11 men wheeling stone 16.00 0.064 2 men at stone and sand bins 3.00 0.012 2 men opening cement bags 3.00 0.012 1 man dumping hopper 1.50 0.006 1 man dumping mixer 1.50 0.006 1 man cleaning chute, mixer, etc. 1.50 0.006 1 motorman or engineer 3.00 0.012 ———- ———- Total labor mixing $66.50 $0.266 8 men pushing 2 cars 12.00 0.048 2 men cleaning track, etc. 3.00 0.012 7 men spading concrete 10.50 0.042 ———- ———- Total labor transporting, placing $ 25.50 $0.102 1 foreman 5.00 0.020 Electricity estimated 7.00 0.028 ———- ———- Total general $ 12.00 $0.048 Grand total $104.00 $0.416
It will be noted that the cost of shoveling and wheeling the broken stone amounts to 16.4 cts. per cu. yd., or nearly 40 per cent. of the total cost of mixing and placing. The cost of spading the concrete is also high for a sloppy mixture, but is probably accounted for by the fact that the concrete had to be spaded so as to have 2 or 3 ins. of clear mortar next the forms. The forms used in constructing the wall are shown by Figs. 110 and 111. They were made in panels 51 ft. long and a locomotive crane was used to shift the panels. This crane worked handling forms only a small part of the time, but a form gang of 10 carpenters was kept busy all of the time moving and reassembling. Assuming the work of the crane to amount to $5 per day and the wages of the carpenter gang to amount to $25, we get a cost of 12 cts. per cubic yard of concrete for shifting forms. It should be noted carefully that the costs given for this work do not include cost of materials, interest on plant, superintendence and other items.
WALL FOR RAILWAY YARD.—For building a retaining wall 7 ft. high, forms were made and placed by a carpenter and helper at $8 per M., wages being 35 cts. and 20 cts. an hour, respectively. Concrete materials were dumped from wagons alongside the mixing board. Ramming was unusually thorough. Foreman expense was high, due to small number in gang; 2 cu. yds. were laid per hour by the gang.
Per day. Per cu. yd. 7 mixers, 15 cts. per hour $10.50 $0.53 2 rammers, 15 cts. per hour 3.00 0.15 1 foreman 30 cts. per hr., 1 waterboy 5 cts. 3.50 0.17 ———- ——— Total labor $17.00 $0.85
The total cost was as follows per cubic yard:
Per cu. yd. 0.8 bbls. Portland cement, at $2 $1.60 Sand 0.30 Gravel 0.70 Labor mixing and placing 0.85 Lumber for forms, at $16 per M. 0.56 Labor on forms, at $8 per M. 0.28 ——— Total, per cubic yard $4.29
The sheathing plank for the forms was 2-in. hemlock.
CONCRETE FOOTING FOR RUBBLE MASONRY RETAINING WALL.—In constructing a footing for a retaining wall at Grand Rapids, Mich., a 1-2-5 natural cement concrete was used. It was found that 1 cu. yd. of concrete was equivalent to 29.8 cu. ft. of material composed of 3.6 cu. ft. or 1.1 bbls. of cement, 8.4 cu. ft. or 2.7 bbls. of sand and 17.8 cu. ft. or 5.5 bbl. of broken stone. The labor cost of 15.5 cu. yds. of concrete was as follows:
Item. Total. Per cu. yd. Foreman, 14 hours at 40 cts. $ 5.60 $0.3613 Foreman, 20 hours at 22.5 cts. 4.50 0.2903 Laborers, 49 hours at 12.5 cts. 6.11 0.3942 Mason, 2 hours at 35 cts. 0.70 0.0451 ——— ———— Total labor $16.91 $1.0909
All material was furnished by the railway company, the contractor furnishing labor only; his contract price for this was $1 per cu. yd.
TRACK ELEVATION, ALLEGHENY, PA.—The wall was 6,100 ft. long and 75 per cent. was on curves. The first wall built had a top width of 2 ft. and a bottom width of 0.4 the height with the back on a smooth batter. Later the back was stepped and last the wall was proportioned as follows: Calling the height from top of foundation to under coping, then width of base was 0.45 (h + 3), the top measuring 2 ft. The back was arranged in steps 24 ins., 30 ins. and 36 ins. high, and the thickness of wall at each step was, calling h equal to height of step from base, 0.45 (h + 3). Several forms of expansion joints were tried. The first was tarred paper extending through the wall every 50 ft.; the second was -in. boards running through the wall every 50 ft.; the third was -in. board extending 2 ft. into the wall, with a -in. cove at the angles, every 25 ft. The third construction gave perfect satisfaction.
A 1-2-5 natural cement and a 1-3-6 Portland cement concrete mixed fairly wet were used. The concrete was laid in 8-in. courses and faced with a 1-2 mortar. The forms were 2-in. white pine faced and jack planed on the edges; upon removal of the forms board marks and other defects were removed and a wash of neat cement was applied. One contractor used hand mixing. The sand and gravel were measured in wheelbarrows and wheeled onto the platform; the sand and cement were spread in thin layers, one over the other, and thoroughly mixed dry; the gravel was then spread over the mixture, the whole was shoveled into barrows or the pit again shoveled into place and rammed. The other contractor used a cubical mixer. A charging box holding 1 cu. yds. and graduated to show the correct proportions of sand and gravel was filled by shoveling; cement was placed on top and the box hoisted and dumped into the mixer. A barrel holding the correct amount of water was emptied into the mixer which was turned 10 or 15 times and discharged into cars. The costs of mixing by hand and by machine were as follows:
Hand mixing. Total. Per cu. yd. foreman at $3 $ 1.50 $0.025 3 men wheeling barrows at $1.50 4.50 0.075 10 men wheeling materials at $1.50 15.00 0.250 3 men mixing sand and gravel at $1.50 4.50 0.075 6 men mixing concrete at $1.50 9.00 0.150 1 man sprinkling at $1.50 1.50 0.025 ——— ——— Total $36.00 $0.600
The output of the hand mixing gang was 60 cu. yds. per day.
Machine mixing. Total. Per cu. yd. 1 foreman at $3.50 $ 3.50 $0.035 1 stationary engineer at $3 3.00 0.030 foreman at $1.75 0.87 0.009 15 men loading charging bucket at $1.50 22.50 0.225 2 men dumping charging bucket at $1.75 3.50 0.035 2 tagmen at $2, time 2.00 0.020 1 man at trap at $2, time 1.00 0.010 ——— ——— Total $36.37 $0.364
The output of the cubical mixer was 100 cu. yds. per day.
The costs of placing concrete in the forms above the foundation by hand below 12 ft., and by cars and derricks any height, were as follows:
By hand (barrows) below 12 ft. Total. Per cu. yd. 4 men loading concrete at $1.50 $ 6.00 $0.100 1 foreman time at $3 1.50 0.025 10 men wheeling at $1.50 15.00 0.250 1 man scraping barrows at $1.50 1.50 0.025 2 men placing concrete at $1.50 3.00 0.050 1 man placing mortar face at $1.50 1.50 0.025 2 men mixing and carrying mortar at $1.50 3.00 0.050 ——— ——— Total $31.50 $0.525
By cars and derricks— 1 horse and driver at $3 $ 3.00 $0.030 2 men dumping concrete time at $1.50 1.50 0.015 1 fireman time at $1.75 0.88 0.009 3 tagmen at $1.50 4.50 0.045 8 men placing and ramming conc. at $1.50 12.00 0.120 2 men mixing mortar at $1.50 3.00 0.030 2 men placing mortar at $1.50 3.00 0.030 2 men carrying mortar at $1.50 3.00 0.030 1 foreman at $3 3.00 0.030 1 stationary engineer at $3 3.00 0.030 2 men attending hook at $1.50 3.00 0.030 ——— ——— Total $39.88 $0.399
The costs of placing concrete in the foundations were as follows:
By hand— Total. Per cu. yd. 1 foreman time at $3 $ 1.50 $0.025 4 men shoveling concrete at $1.50 6.00 0.100 1 man placing concrete at $1.50 1.50 0.025 1 man ramming concrete at $1.50 1.50 0.025 ——— ——— Total $10.50 $0.175
By machine— 1 horse and driver at $3 $ 3.00 $0.030 3 men pushing and unloading car at $1.50 4.50 0.045 5 men placing and ramming at $1.50 7.50 0.075 1 foreman at $3 3.00 0.030 2 men dumping mixer at $1.50 3.00 0.030 ——— ——— Total $21.00 $0.210
COST OF RETAINING WALL.—The following figures of the cost of a concrete retaining wall are given by C. C. Williams:
Cost of Material. Unit Kind and amount of material— Price. Cost. Stone, 441 tons $ .70 $308.70 Sand, 182.5 yds. .55 100.37 Cement, 536 bbls. .85 453.60 ———- Total $862.67
Lumber value $205.33 Wheelbarrows, value, 6 at $3.50 15.75 ———- Total $221.08
Excavation— Labor, 4,002 hours at 15 cts. $600.30 Carts, 800 hours at 12 cts. 100.00 Foreman, 460 hours at 35 cts. 171.00 Waterboy, 240 hours at 10 cts. 24.00 ———- Total $895.30
Concrete— Labor, 2,398 hours at 15 cts. $359.70 Foreman, 224 hours at 35 cts. 77.40 ———- Total $437.10
Handling material— Unloading cars, 380 hours at 15 cts. $ 57.00 Foreman, 40 hours at 35 cts. 14.00 ———-
Total $ 71.00
Forms— Carpenters, 997 hours at 22 cts. $224.33
Work to support bridge— Carpenters, 542 hours at 22 cts. $121.95 Labor, 458 hours at 15 cts. 68.70 ———-
Superintendence and office— Superintendent, 30 hours at 50 cts. $15.00 Office 20.00 ———— Total $35.00 ———— Grand total $2,937.13
Proportional costs— Cost Per Per Cent. Yard of of Total Item. Cost. Concrete. Cost. Concrete materials $ 862.67 $2.02 46.7 Laying concrete 437.10 1.03 23.4 Lumber 205.33 .48 11.3 Building forms 224.33 .53 12.3 Handling material 71.00 .17 03.8 Wheelbarrows 15.75 .04 01.0 Supt., etc. 35.00 .07 01.5 ————- ——- ——— Total $1,851.18 $4.34 100.00 Work on bridge 190.65 Excavation 895.30 ————- $2,937.13
METHODS AND COST OF CONSTRUCTING CONCRETE FOUNDATIONS FOR PAVEMENTS.
Contractor's skill or want of skill in systematizing and managing labor counts as high in street work as in any class of concrete construction. As previously demonstrated, the cost of mixing is a very small portion of the labor cost of concrete in place; the costs of getting the materials to the mixer and the mixed concrete to the work are the big items, and in street work the opportunity for increasing the cost of these items through mismanagement is magnified by the large area of operations involved per cubic yard of concrete placed. One cubic yard of concrete makes 6 sq. yds. of 6-in. pavement foundations and 100 cu. yds. of concrete make a 6-in. foundation for 300 ft. of 30-ft. street, while 4 to 5 cu. yds. will build 100 ft. of ordinary curb and gutter. Thus the haulage per cubic yard is considerable at best, and lack of plan in distributing stock piles and handling the concrete can easily result in such increased haulage expenses as to change a possible profit into a certain loss. A little thought and skill in planning street work pays a good profit.
MIXTURES EMPLOYED.—A comparatively lean concrete will serve for pavement foundations; mixtures of 1-4-8 Portland cement or 1-2-5 natural cement are amply good and it is folly, ordinarily, to employ richer mixtures. Until recently, natural cement has been used almost exclusively; a 1-2-5 natural cement mixture requires about 1.15 bbls. of cement per cubic yard of concrete. A 1-4-8 Portland cement mixture requires about 0.7 bbl. of cement per cubic yard. In the opinion of the authors a considerably leaner mixture of Portland concrete is sufficiently good when it is well mixed in machine mixers—for a 6-in., foundation 0.5 bbl. per cu. yd. The mixtures actually employed are proportioned about as stated and their cost, or that of any other common mixture, may easily be computed from Tables XII and XIII, giving for different mixtures the quantities of cement, sand and stone per cubic yard of concrete; the product of these quantities and the local prices of materials in the stock piles gives the cost. When the concrete is mixed by hand the ordinary labor cost of foundations is 0.4 to 0.5 of a 10-hour day's wages per cubic yard of concrete; occasionally it may be as low as 0.3 of a day's wages where two mixing gangs are worked side by side under different foremen and with an exacting contractor. Data for machine mixing are too few to permit a similar general statement for machine work, but in one case coming under the authors' observation, the cost figured out to a little less than 0.2 of a day's wages per cubic yard.
DISTRIBUTION OF STOCK PILES.—Assuming a 30-ft. street and a 1-3-5 concrete laid 6 ins. thick, the quantities of concrete materials required per lineal foot of street are: Cement 0.60 bbl., sand 0.27 cu. yd., stone 0.44 cu. yd. The stock piles should be so distributed that each supplies enough materials for a section of foundation reaching half way to the next adjacent stock pile on each side, and they should not contain more or less material, otherwise a surplus remains to be cleaned up or a deficiency to be supplied by borrowing from another pile. A little care will ensure the proper distribution and it is well paid for in money saved by not rehandling surplus or borrowed materials. For a given mixture and a given width and thickness of foundation, the sizes of the stock piles are determined by their distance apart and this will depend upon whether hand or machine mixing is employed and upon the means adopted for hauling the raw materials and the mixed concrete. It is worth while always in stock piles of any size, to lay a flooring of plank particularly under the stone pile; if dumped directly on the ground it costs half as much again to handle stone. Current practice warrants everything from a continuous bank, to piles from 1,000 to 1,500 ft. apart, in the spacing of stock piles.
HINTS ON HAND MIXING.—All but a small percentage of the concrete annually laid in street work is hand mixed. The authors are confident that this condition will disappear as contractors learn more of the advantages of machine mixing, but it prevails at present. The general economics of hand mixing are discussed in Chapter II; in street work as before stated, the big items of labor cost are the costs of handling materials and the data in Chapter II on these processes deserve special attention. It is particularly worth noting that it is seldom economical to handle materials in shovels where carrying is necessary; it is a common thing in street work to see an attempt to get the stock piles so close to the mixing board that the material can be handled with shovels, and this is nearly always an economic error. Street work is readily measured; in fact, its progress can be seen at a glance, and advantage can often be taken of this fact to profit by the rivalry of separate gangs. The authors have known of the labor costs being reduced as much as 25 per cent., due to pitting one gang against another where each could see the progress made by the other.
METHODS OF MACHINE MIXING.—Concrete mixers have been slow to replace handwork in laying pavement foundations. In explanation of this fact it is asserted: (1) That frequent shifting of the mixer causes too much lost time, and (2) that the principal item of labor cost in street work is the conveying of materials to and from the mixer, and this item is the same whether hand or machine mixing be employed. The records of machine mixer work given elsewhere in this chapter go far, in the opinion of the authors, toward disproving the accuracy of both assertions. If the machine used and the methods of work employed are adapted to the conditions of street work, machine mixing can be employed to decided advantage.
A continuous and large output is demanded in a mixer for street work; the perfection of the mixing is within limits a minor consideration. This at once admits for consideration types of mixers whose product is classed as unsuitable for reinforced concrete work, and also admits of speeding up the output of the better types to a point beyond that at which they turn out their most perfect product. Keeping these facts in mind either of the following two systems of work may be employed: (1) Traction plants which travel with the work and deposit concrete in place, or so nearly in place that little shoveling is necessary; (2) portable plants which are set up at wide intervals along the work and which discharge the concrete into carts or dump wagons which distribute it to the work.
The secret of economic work with plants of the class cited first is the distribution of the stock piles so as practically to eliminate haulage from stock pile to mixer. The mixer backs away from the work, its discharge end being toward the work and its charging end away from it. Then deposit the materials so as to form a continuous stock pile along the center of the street; the mixer moving backward from the completed foundation keeps close to the materials and if the latter are uniformly distributed in the pile the great bulk of the charging is done by shoveling direct into the charging bucket. The point to be watched here is that the shovelers do not have to carry the materials; separate stock piles within moderate hauling distance by wheelbarrows are a far more economic arrangement than a continuous pile so irregularly distributed that much of the material has to be carried even a few paces in shovels.
Economic work with plants of the second class depends upon efficient and adequate means of hauling the mixed concrete to the work. The plant should not be shifted oftener than once in 1,000 to 2,000 ft., or, say, four city blocks. This does away with the possibility of wheelbarrow haulage; large capacity hand or horse carts must be employed. With 6 cu. ft. hand carts, such as the Ransome cart, a haul of 500 ft. each way from the mixer is possible and with horse carts, such as the Briggs, this economic distance is increased to 1,000 ft. each way from the mixer. The mixer must be close to the stock pile and it will pay to make use of improved charging devices. A 6-in. foundation for 2,000 ft. of 30-ft. street calls for 667 cu. yds. of concrete, and if both sides are curbed at the same time, 100 cu. yds. more are added, or 767 cu. yds. in all; where intersecting streets are to be paved in both directions from the mixer plant these amounts are doubled. A very small saving per cubic yard due to mechanical handling of the materials to the mixer amounts to the interest on a considerable investment in such plant. A point that should not be forgotten is that carts such as those named above spread the concrete in dumping so that little or no shoveling is required.
FOUNDATION FOR STONE BLOCK PAVEMENT, NEW YORK, N. Y.—Mr. G. W. Tillson, in "Street Pavements and Paving Materials," p. 204, gives the following data on the cost of granite block pavement in New York City in 1899. The day was 10 hours long:
Per Per Per Concrete gang— day. sq. yd. cu. yd. 1 foreman $ 3.00 $0.0125 $0.075 8 mixers on two boards, at $1.25 10.00 0.0416 0.250 4 wheeling stone and sand, at $1.25. 5.00 0.0208 0.125 1 carrying cement and supplying water, at $1.25 1.25 0.0051 0.031 1 ramming, at $1.25 1.25 0.0051 0.031 ——— ———- ——— Total, 240 sq. yds. (40 cu. yds.). $20.50 $0.0851 $0.512
The concrete was shoveled direct from the mixing boards to place.
Cost 1-2-4 concrete— Per cu. yd. 1-1/3 bbls. natural cement, at $0.90 $1.20 0.95 cu. yd. stone, at $1.25 1.19 0.37 cu. yd. sand, at $1.00 0.37 Labor 0.51 ——- $3.27
In laying 5,167 sq. yds. of granite block pavement on one job in New York City in 1905, the authors' records show that one laborer mixed and laid 1.3 cu. yds. of concrete per day in a 6-in. foundation; this is a very small output. The work was done by contract and the labor cost was as follows:
Per Per Item. Total. sq. yd. cu. yd. 28 days foreman at $3.50 $ 99.75 $0.0193 $0.118 399 days laborers at $1.75 698.25 0.1351 0.826 ———- ———- ——— $798.00 $0.1544 $0.944
The average day's wages was $1.86, so that the labor cost was about 0.5 of a day's wages per cubic yard of concrete.
FOUNDATION FOR PAVEMENT, NEW ORLEANS. LA.—Mr. Alfred E. Harley states that in laying concrete foundations for street pavement in New Orleans, a day's work, in running three mixing boards, covering the full width of the street, averaged 900 sq. yds., 6 ins. thick, or 150 cu. yds., with a gang of 40 men. With wages assumed to be 15 cts. per hour the labor cost was:
Cts. per cu. yd. 6 men wheeling broken stone 6 3 men wheeling sand 3 1 man wheeling cement 1 2 men opening cement 2 7 men dry mixing 7 8 men taking concrete off 8 3 men tamping 3 3 men grading concrete 3 1 man attending run planks 1 3 water boys 1 2 extra men and 1 foreman 4 — Total labor cost 39 cts.
FOUNDATIONS FOR STREET PAVEMENT, TORONTO, CANADA.—The following cost of a concrete base for pavements at Toronto has been abstracted from a report (1892) of the City Engineer, Mr. Granville C. Cunningham. The concrete was 1-2-7 Portland; 2,430 cu. yds. were laid, the thickness being 6 ins., at the following cost per cubic yard:
0.77 bbl. cement, at $2.78 $2.14 0.76 cu. yd. stone, at $1.91 1.45 0.27 cu. yd. sand and gravel, at $0.80 0.22 Labor (15 cts. per hr) 1.03 ——- Total $4.84
Judging by the low percentage of stone in so lean a mixture as the above, the concrete was not fully 6 ins. thick as assumed by Mr. Cunningham. Note that the labor cost was 1 to 2 times what it would have been under a good contractor.
MISCELLANEOUS EXAMPLES OF PAVEMENT FOUNDATION WORK.—The following records of pavement foundation work are taken from the note and time books of one of the authors:
Case 1.—Laying 6-in. pavement foundation; stone delivered and dumped upon 2-in. plank laid to receive it. Sand and stone were dumped along the street, so that the haul in wheelbarrows to mixing board Was about 40 ft. Two gangs of men worked under separate foremen, and each gang averaged 4.5 cu. yds. concrete per hour. The labor cost was as follows for 45 cu. yds. per gang:
Per day. Per cu. yd. 4 men filling barrows with stone and sand ready for the mixers, wages 15 cts. per hour $6.00 $0.13 10 men, wheeling, mixing and shoveling to place (3 or 4 steps), wages 15 cts. per hour 15.00 0.33
2 men ramming, wages 15 cts. per hour 3.00 0.07 1 foreman at 30 cts. per hour and 1 water boy, 5 cts 3.50 0.08 ——- ——- Total $27.50 $0.61
Case II.—Sometimes it is desirable to know every minute detail cost, for which purpose the following is given:
——Per cu. yd.—— Day's labor. Cost. 3 men loading stones into barrows $0.06 $0.09 1 man loading sand into barrows 0.02 0.03 2 men ramming 0.04 0.06 1 foreman and 1 water boy equivalent to 0.035 0.05 Wheeling sand and cement to mixing board 0.02 0.03 Wheeling stone to mixing board 0.026 0.04 9 men mixing mortar 0.013 0.02 Mixing stone and mortar 0.049 0.07 Placing concrete (walking 15 ft.) 0.072 0.11 ——— ——- Total $0.335 $0.50
In one respect this is not a perfectly fair example (although it represents ordinary practice), for the mortar was only turned over once in mixing instead of three times, and the stone was turned only twice instead of three or four times. Water was used in great abundance, and by its puddling action probably secured a very fair mixture of cement and sand, and in that way secured a better mixture than would be expected from the small amount of labor expended in actual mixing. About 9 cts. more per cu. yd. spent in mixing would have secured a perfect concrete without trusting to the water.
Case III.—Two gangs (34 men) working under separate foremen averaged 600 sq. yds., or 100 cu. yds. of concrete per 10-hour day for a season. This is equivalent to 3 cu. yds. per man per day. The stone and sand were wheeled to the mixing board in barrows, mixed and shoveled to place. Each gang was organized as follows:
Per day. Per cu. yd. 4 men loading barrows $ 6.00 $0.12 9 men mixing and placing 13.50 0.27 2 men tamping 3.00 0.06 1 foreman 2.50 0.05 ——— ——- Total $25.00 $0.50
These men worked with great rapidity. The above cost of 50 cts. per cu. yd. is about as low as any contractor can reasonably expect to mix and place concrete by hand in pavement work.
Case IV.—Two gangs of men, 34 in all, working side by side on separate mixing boards, averaged 720 sq. yds., or 120 cu. yds., per 10-hour day. Each gang was organized as follows:
Per day. Per cu. yd. 6 men loading and wheeling $ 9.00 $0.15 8 men mixing and placing 12.00 0.20 2 men tamping 3.00 0.05 1 foreman 3.00 0.05 ——— ——- Total $27.00 $0.45
Instead of shoveling the concrete from the mixing board into place, the mixers loaded it into barrows and wheeled it to place. The men worked with great rapidity.
Mr. Irving E. Howe gives the cost of a 6-in. foundation of 1-3-5 natural cement at Minneapolis, Minn., in 1897, as $2.80 per cu. yd., or $0.467 per sq. yd. Cement cost 76 cts. per barrel and stone and sand cost delivered $1.15 and 30 cts. respectively. Mixers received $1.75 per day.
Mr. Niles Meriwether gives the cost of materials and labor for an 8-in. foundation constructed by day labor (probably colored) at Memphis, Tenn., in 1893, as follows:
Per sq. yd. Natural cement at $0.74 per bbl $0.195 Sand at $1.25 per cu. yd 0.075 Stone at $1.87 per cu. yd 0.355 Labor mixing and placing 0.155 ——- Total $0.780
Labor was paid $1.25 to $1.50 per 8-hour day and 1.16 bbls. of cement were used per cubic yard of concrete. The cost of materials, as will be noted, was high and the labor seems to have been inefficient.
FOUNDATIONS FOR BRICK PAVEMENT, CHAMPAIGN, ILL.—The concrete foundation for a brick pavement constructed in 1903 was 6 ins. thick; the concrete used was composed of 1 part natural cement, 3 parts of sand and gravel, and 3 parts of broken stone. All the materials were mixed with shovels, and were thrown into place from the board upon which the mixing was done. The material was brought to the steel mixing board in wheelbarrows from piles where it had been placed in the middle of the street, the length of haul being usually from 30 to 60 ft. The foundation was 6 ins. thick and it cost as follows for materials and labor:
Cost per cu. yd. 1.2 bbls. cement, at $0.50 $0.600 0.6 cu. yd. sand and gravel, at $1 0.600 0.6 cu. yd. broken stone, at $1.40 0.840 6 men turning with shovels, at $2 0.080 4 men throwing into place, at $2 0.053 2 men handling cement, at $1.75 0.023 1 man wetting with hose, at $1.75 0.012 2 men tamping, at $1.75 0.023 1 man leveling, at $1.75 0.012 6 men wheeling stone, at $1.75 0.070 4 men wheeling gravel, at $1.75 0.047 1 foreman, at $4 0.027 ——— $2.387
This is practically 40 cts. per sq. yd., or $2.40 per cu. yd. of concrete for materials and labor. It is evident from the above quantities that a cement barrel was assumed to hold about 4.5 cu. ft., hence the cement was measured loose in making the 1-3-3 concrete. The accuracy of the quantities given is open to serious doubt. It will also be noted that the labor cost of making and placing the concrete was only 35 cts. per cu. yd., wages being nearly $1.85 per day. This is so remarkably low that some mistake would seem to have been made in the measurement of the work. The authors do not hesitate to say that no gang of men ever made any considerable amount of concrete by hand at the rate of 5.75 cu. yds. per man per day.
FOUNDATION CONSTRUCTION USING CONTINUOUS MIXERS.—The following are records of two jobs of pavement foundation work using continuous mixers with one-horse concrete carts in one instance and wheelbarrows in the other instance. The mixer used was the Foote mixer, as arranged for the work being described it is shown by Fig. 112. One particular advantage of this and similar mixers for street work is that no proportioning or measuring of the materials is required of the men. The mixers are provided with an automatic measuring device, by means of which any desired proportion of cement, sand and stone is delivered to the mixing trough. The mixer is mounted on trucks, and the hoppers that receive the sand and stone are comparatively low down. The sand can be wheeled in barrows up a run plank and dumped into a hopper on one side of the mixer, and in like manner the gravel or broken stone can be delivered into a hopper on the other side. The cement is delivered in bags or buckets to a man who dumps it into a cement hopper directly over the mixer. All that the operator needs to attend to is to see that the men keep the hoppers comparatively full. The records of work on the two jobs mentioned are as follows:
Job I.—The sand was delivered from the stock pile by a team hitched to a drag scraper, and was dumped alongside the mixer where two men shoveled it into the hopper. On the same job the concrete was hauled away from the mixer in Briggs' concrete carts. With a gang of 30 men and 2 to 4 horses hauling concrete in Briggs' carts, the contractor averaged 1,200 sq. yds., or 200 cu. yds., per day of 10 hours. With wages of laborers at 15 cts. per hour, and a single horse at the same rate, the cost of labor was 26 cts. per cu. yd., or less than 4 cts. per sq. yd. of concrete base 6 ins. thick. The coal was a nominal item, and did not add 1 ct. per cu. yd. to the cost. In this case the mixer was set up on a side street and the concrete was hauled in the carts for a distance of a block each way from the mixer. At first four carts were used, but as the concreting approached the mixer, less hauling was required, and finally only two carts were used. An illustration of a Briggs cart is given by Fig. 113; it is hauled by one horse, which the driver leads, and is dumped by an ingenious device operated from the horse's head. The cart dumps from the bottom and spreads the load in a layer about 8 or 9 ins. thick, so that no greater amount of shoveling is necessary than when barrows are used. It took about 20 seconds for the cart to back up and get its load and about 5 seconds to dump and spread the load.
Job II.—In this job the mixer was charged with wheelbarrows and wheelbarrows were also employed to take the mixed concrete to the work, the mixer being moved forward at frequent intervals. The stock piles were continuous, sand on one side of the street and stone on the other side. A 1-3-6 Portland cement concrete was used, a very rich mixture for a 6-in. foundation. The organization of the working gang was as follows:
Men loading and wheeling gravel 8 Men assisting in loading gravel 2 Man dumping barrows into hopper 1 Men loading and wheeling sand 3 Man dumping barrows into hopper 1 Men wheeling concrete in barrows 7 Men spreading concrete 3 Men tamping concrete 2 Man pouring cement into hopper 1 Man operating mixer 1 Man shoveling spilled concrete 1 Man opening cement bags 1 Engineer 1 — Total men in gang 32
The average day's output of this gang was 150 cu. yds., or 900 sq. yds. in 8 hours; but on the best day's work the output was 200 cu. yds., or 1,200 sq. yds. in 8 hours, which is a remarkable record for 32 men and a mixer working only 8 hours.
The following is the labor cost of 8,896 sq. yds. of 4-in. concrete foundation for an asphalt pavement constructed in New York City in 1904:
Item. Per sq yd. Foreman at $3.75 $0.030 Laborers at $1.50 0.242 Teams at $5 0.040 Steam engine at $3.50 0.028 ——— Total $0.340
The concrete was a 1-3-6 mixture and was mixed in a Foote mixer. These costs are compiled from data collected by the authors.
FOUNDATION CONSTRUCTION FOR STREET RAILWAY TRACK USING CONTINUOUS MIXERS.—The following account of the methods and cost of constructing a concrete foundation for street railway track at St. Louis, Mo., is compiled from information published by Mr. Richard McCulloch. The work was done by day labor by the United Railways Co., in 1906. Figure 114 shows the concrete construction. A 1-2-6 Portland cement, broken stone concrete mixed by machine was used.
The material for the concrete was distributed on the street beside the tracks in advance of the machine, the sand being first deposited, then the crushed rock piled on that, and finally the cement sacks emptied on top of this pile. The materials were shoveled from this pile into the concrete mixing machine without any attempt at hand mixing on the street. Great care was taken in the delivery of materials on the street to have exactly the proper quantity of sand, rock and cement, so that there would be enough for the ballasting of the track to the proper height and that none would be left over. Each car was marked with its capacity in cubic feet, and each receiver was furnished with a table by which he could easily estimate the number of lineal feet of track over which the load should be distributed.
The concrete mixing machines were designed and built in the shops of the United Railways Co. Three machines were used in this work, one for each gang. The machine is composed of a Drake continuous worm mixer, fed by a chain dragging in a cast-iron trough. The trough is 36 ft. long, so that there is room for 14 men to shovel into it. Water is sprayed into the worm after the materials are mixed dry. This water was obtained from the fire plugs along the route. In the first machine built, the Drake mixer was 8 ft. long. In the two newer machines the mixer was 10 ft. long. Both the conveyor and the mixer were motor driven, current being obtained for this purpose from the trolley wire overhead. Two types of machines were used, one in which the conveyor trough was straight and 45 in. above the rail, and the other in which the conveyor trough was lowered back of the mixer, being 25 in. above the rail. The latter type had the advantage of not requiring such a lift in shoveling, but the trough is so low that a motor truck cannot be placed underneath it. In the high machine the mixer is moved forward by a standard motor truck under the conveyor. In the low machine the mixer is moved by a ratchet and gear on the truck underneath the mixer. A crew of 27 men is required to work each machine, and under average conditions concrete for 80 lin. ft. of single track, amounting to 22 cu. yds., can be discharged per hour.
The costs of the concrete materials delivered per cubic yard of concrete were: Cement, per barrel, $1.70; sand, per cu. yd., $0.675, and stone, per cu. yd., $0.425. The cost of the concrete work per cubic yard and per lineal foot of track was as follows:
Item. Per lin. ft. Per cu. yd. Concrete materials $0.791 $2.92 Labor mixing and placing 0.071 0.26 ——— ——- Total labor and materials $0.862 $3.18
FOUNDATION CONSTRUCTION USING BATCH MIXERS AND WAGON HAULAGE, ST. LOUIS, MO.—The following record of the method and cost of laying a concrete foundation for street pavement using machine mixing and wagon haulage is given by Mr. D. A. Fisher. The foundation was 6 ins. thick. The gravel was dumped from wagons into a large hopper, raised by a bucket elevator into bins, and drawn off through gates into receiving hoppers on the charging platform where the cement was added. The receiving hoppers discharged into the mixers, which discharged the mixed concrete into a loading car that dumped into wagons, which delivered it on the street where wanted. The longest haul in wagons was 30 mins., but careful tests showed that the concrete had hardened well. The wagons were patent dump wagons of the drop-bottom type. Mr. Fisher says:
"You may consider the following figures a fair average of the plant referred to, working to its capacity. To these amounts, however, must be added the interest on the investment, the cost of wrecking the plant and the depreciation of the same, superintendence, and the pay roll that must be maintained in wet weather. I am assuming the street as already brought to grade and rolled.
"With labor at $1.75 per day of 10 hours, teams at $4, engineer and foremen at $3, and engine at $5 per day, concrete mixed and put in place by the above method costs:
Per cu. yd. To mix $0.12 to $0.15 To deliver to street 0.10 to 0.14 To spread and tamp in place 0.08 to 0.11 ——————— Total $0.30 to $0.40
"The mixers are No. 2 Smith, sold by the Contractors' Supply Co., Chicago, Ill., and a yd. cube, sold by Municipal Engineering & Contracting Co., Chicago.
"The above figures are on the basis of a batch every 2 minutes, which is easily maintained by using the loading car, as by this means there will be no delay in the operation of the plant owing to the irregularity of the arrival of the teams.
"My experience leads me to believe that a better efficiency can be obtained by using mixers of 1 cu. yd. capacity, and that the batch mixer is the only type of machine where any certainty of the proportion of the mixture is realized."
FOUNDATION CONSTRUCTION USING A TRACTION MIXER.—In laying a 6-in. foundation for an asphalt pavement in Buffalo, N. Y., an average of 100 sq. yds., or 16.6 cu. yds., of concrete in place was made per hour using the traction mixer shown by Fig. 115. This mixer was made by the Municipal Engineering & Contracting Co., of Chicago, Ill., and consisted of one of that company's improved cube mixers operated by a gasoline engine and equipped with the regulation mechanical charging device and also with a swinging conveyor to deliver the mixed concrete to the work. The feature of the apparatus in its application to paving work is the conveyor. This was 25 ft. long and pivoted at the mixer end so as to swing through an arc of 170. The mixer discharged into a skip or bucket traveling on the conveyor frame and discharging over the end spreading its load anywhere within a radius of 25 ft. In operation the mixer traveled along the center of the street, backing away from the finished foundation and toward the stock pile, which was continuous and was deposited along the center of the street. The bulk of the sand and stone was thus shoveled direct into the charging bucket and the remainder was wheeled to the bucket in barrows. As the charging bucket is only 14 ins. high the barrows could be dumped directly into it from the ground. The gang worked was 17 including a foreman and one boy, and with this gang 100 sq. yds. of 6-in. foundation was laid per hour. Assuming an average wage of 20 cts. an hour the cost of mixing and placing the foundation concrete was 3.4 cts. per sq. yd. or 20.4 cts. per cu. yd. for labor alone.
FOUNDATION CONSTRUCTION USING CONTINUOUS MIXER.—The foundation was 6 ins. thick for an asphalt pavement and was laid in Chicago, Ill. The concrete used was exceptionally rich for pavement foundation work, it being a 1-3-6 Lehigh Portland cement, broken stone mixture. The mixing was done by machine, a mixer made by the Buffalo Concrete Mixer Co., Buffalo, N. Y., being used. This mixer was equipped with an elevating charging hopper and was operated as a continuous mixer. The mixer was mounted on wheels and was pulled along the center of the street ahead of the work with its discharge end toward the work. Moves of about 25 to 30 ft. were made, the mixer being pulled ahead for this distance each time that the concrete came up to its discharge end. The stock piles were continuous, sand on one side and stone on the other side of the street. Cement was stored in a pile at each end of the block. All materials were wheeled from stock piles to mixer in wheelbarrows. The men wheeling sand and stone loaded their own barrows, wheeled them to the mixer and discharged them directly into the elevating hopper. No runways were used, the barrows being wheeled directly on the ground. The cement was brought in barrows, two or three bags being a load, and dumped alongside a cement box which was located close to and at one side of the elevating hopper. A man untied the bags and emptied them into the cement box and another man scooped the cement out of the box in bucketfuls and emptied it over the sand and stone in the elevating hopper. The mixer discharged onto a sheet iron shoveling board, and the concrete was carried in shovels from shoveling board to place, the length of carry being a maximum of 25 to 30 ft. Two men were required to pull down the cone of concrete at the discharge end of the mixer and to keep the stone from separating and rolling down the sides. The gang was organized as follows:
No. Men. Loading and wheeling stone 10 Loading and wheeling sand 3 to 4 Loading and wheeling cement 2 Untieing and emptying cement bags 1 Charging cement to hopper 1 Operating mixer and hopper 1 Pulling down and tending discharge 2 Carrying concrete in shovels 8 Spreading concrete 2 Tamping concrete 2 Sweeping concrete 1 General laborers 3 Foreman 1 Watchman 1 Timekeeper 1 — Total gang 40
This gang averaged 1,000 sq. yds. of 6-in. foundation per 10-hour day; a maximum of 1,400 sq. yds. was laid in a day. We have thus an average of 167 cu. yds. and a maximum of 234 cu. yds. of concrete foundation mixed and placed per 10-hour day. At an average wage of $2 per day the average labor cost of mixing and placing concrete was 48 cts. per cu. yd. or 8 cts. per sq. yd. of 6-in. foundation. It was stated that the gang was larger by three men than was ordinarily used owing to certain extra work being done at the time that the above figures were collected. Taking out three extra men and the timekeeper and watchman we get 34 men actually working in mixing and placing concrete. This reduced gang gives us a labor cost for mixing and placing of about 41 cts. per cu. yd. or 6.8 cts. per sq. yd. of 6-in. foundation.
FOUNDATION CONSTRUCTION USING A BATCH MIXER.—The following figures are an average of several jobs using a Ransome -cu. yd. mixer for constructing 6-in. foundations. The mixer was moved 1,000 ft. at a time and the work conducted 500 ft. in each direction from each station. The concrete materials were delivered from stock pile to mixer in wheelbarrows and the mixed concrete was hauled to the work in two-wheeled Ransome carts. Run planks were laid for the carts and one man readily pushed a cart holding 6 cu. ft. The men had to work fast on the long haul but had an easy time when the haul was short. The organization of the gang was as follows, wages being $1.50 per day:
10 men loading and wheeling stone $15.00 4 men loading and wheeling sand 6.00 2 men handling cement 3.00 1 fireman 2.00 1 man dumping mixer 1.50 5 men wheeling carts 7.50 3 men spreading and ramming 4.50 1 foreman 3.50 ——— Total wages per day $43.00
This gang averaged 1,080 sq. yds. of 6-in. foundation or 180 cu. yds. of concrete in place per day which gives a labor cost of 24 cts per cu. yd. or 4 cts. per sq. yd. for mixing and placing.
METHODS AND COST OF CONSTRUCTING SIDEWALKS, PAVEMENTS AND CURB AND GUTTER.
Next to pavement foundations the most extensive use of concrete in street work is for cement walks and concrete curb and gutter. Usually the mixing and placing of the concrete is hand work, practically the only exceptions being where pavement base, curbing and sidewalks are built all at once, using machine mixers. The same objections that have been raised to machine mixers in laying pavement foundation are raised against them for curb and walk construction, and owing to the much smaller yardage per lineal foot of street in walk and curb work these objections carry more force than they do in case of paving work. Another argument against the use of mixers is that both walk and curb and gutter work involve the use of forms and the application of mortar finish, the placing of which are really the limiting factors in the rate of progress permissible, and this rate is too slow to consume an output necessary to make a mixer plant economical as compared with hand mixing where so much transportation is involved. Concrete sidewalk and curb work are essentially hand mixing work; they, therefore, involve a careful study of the economies of hand mixing and wheelbarrow haulage which are fully discussed in Chapter II.
Sidewalk construction consists in molding on a suitably prepared sub-base a concrete slab from 3 to 7 ins. thick, depending on practice, and finishing its top surface with a to 1-in. wearing surface of cement mortar.
GENERAL METHOD OF CONSTRUCTION.—The excavation and preparation of the sub-grade call for little notice beyond the warning that they should never be neglected. The authors have seen many thousands of feet of cement walk laid in the middle West in which the sub-base was placed directly on the natural sod, often covered with grass and weeds a foot high. Such practice is wholly vicious. The sod should always be removed and the surface soil excavated to a depth depending upon the climate and nature of the ground and the foundation bed well tamped. From 4 to 6 ins. depth of excavation will serve where the soil is reasonably hard and there are no heavy frosts; with opposite conditions a 12-in. excavation is none too deep. The thickness of the broken stone, gravel, cinder or sand sub-base should likewise be varied with the character of the soil, the conditions of natural drainage and the prevalence of frost. In well drained sandy soils 6 to 8 ins. of sub-base are sufficient, but in clayey soils with poor natural drainage the sub-base should be from 10 to 12 ins. thick at least; the local conditions will determine the thickness of sub-base necessary and in places it may be desirable to provide by artificial drainage against the accumulation of water under the concrete. Tile drains are better and cheaper than excessively deep foundations. The thorough tamping of the sub-base is essential to avoid settling and subsequent cracking of the concrete slab. This is a part of sidewalk work which is often neglected.
Portland cement concrete, sand and broken stone or gravel mixtures in the proportions of 1-3-5 and 1-3-6 are used for base slabs. For walks up to 7 ft. wide the slab is made 3 ins. thick for residence streets and 4 to 5 ins. thick for business streets; for wider walks the thickness is increased to 7 ins. for 8-ft. width and 7 ins. for 9 to 10-ft. width. Roughly the thickness of the walk in inches (base and top together) is made about equal to its width in feet. The concrete is deposited in a single layer and tamped thoroughly, either in separate blocks behind suitable forms or in a continuous slab which is while fresh cut through to make separate blocks. For walks up to 8 ft. wide the slab is divided by transverse joints spaced about the width of the walk apart, but for the wider walks the safety of this division depends upon the thickness of the base; an 8-ft. walk with a 5-in. base can safely be laid with joints 8 ft. apart, but if the slab is only 4 ins. thick it had better be laid in 44-ft. squares. The mode of procedure in base construction is as follows:
The sub-base being laid, side forms held by stakes are placed as shown by Fig. 116, with the top edges of the boards exactly to the grade of the top surface of the finished walk. The concrete is then deposited between these side forms and tamped until it is brought up to the level marked by the templet A. If the plan is to deposit the base in sections transverse plates of 3/8 to in. steel are set across the walk between the side boards at proper intervals and the concrete tamped behind them; sometimes the concreting is done in alternate blocks. When the steel plate is withdrawn an open joint is left for expansion and contraction. Where the plan is to lay the base in one piece which is afterwards cut into blocks, the cutting is done with a spade or cleaver.
Portland cement mortar mixed 1 to 1 to 1 to 2 is used for the wearing surface, and is laid from in. to 1 ins. thick, depending upon the width of the walk and the thickness of the base. As a rule the mortar is mixed rather stiff; it is placed with trowels in one coat usually, but sometimes in two coats, and less often by tamping. The mortar coat is brought up flush with the top edges of the side forms by means of the templet B, and the top finished by floating and troweling. The wearing coat is next divided into sections corresponding with the sections into which the base is divided, by cutting through it with a trowel guided by a straight edge and then rounding the edges of the cut with a special tool called a jointer and shown by Fig. 117. An edger, Fig. 118, is then run around the outside edges of the block to round them. The laying of the mortar surface must always follow closely the laying of the base so that the two will set together.
BONDING OF WEARING SURFACE AND BASE.—Trouble in securing a perfect bond between the wearing surface and the base usually comes from one or more of the following causes: (1) Applying the surface after the base concrete has set. While several means are available for bonding fresh to old concrete as described in Chapter XXIV, the better practice is not to resort to them except in case of necessity but to follow so close with the surfacing that the base will not have had time to take initial set. (2) Poor mixing and tamping of this base concrete. (3) Use of clayey gravel or an accumulation of dirt on the surface. In tamping clayey gravel the water flushes the clay to the surface and prevents the best bond. (4) Poor troweling, that is failure to press and work the mortar coat into the base concrete. Some contractors advocate tamping the mortar coat to obviate this danger. Conversely, to make the surface coat adhere firmly to the base it must be placed before the base concrete has set; the base concrete must be thoroughly cleaned or kept clean from surface dirt; the surface coat must be tamped or troweled forcibly into the base concrete so as to press out all air and the film of water which collects on top of the concrete base.
PROTECTION OF WORK FROM SUN AND FROST.—Sun and frost cause scaling and hair cracks. For work in freezing weather the water, sand and gravel should be heated or salt used to retard freezing until the walk can be finished; it may then be protected from further action of the frost by covering it first with paper and then with a mattress of sawdust, shavings or sand and covering the whole with a tarpaulin. Methods of heating concrete materials and rules for compounding salt solutions are given in Chapter VII. The danger from sun arises from the too rapid drying out of the surface coating; the task then is to hold the moisture in the work until the mixture has completely hardened. Portable frames composed of tarpaulin stretched over 24-in. strips may be laid over the finished walk to protect it from the direct rays of the sun; these frames can be readily removed to permit sprinkling. Practice varies in the matter of sprinkling, but it is the safe practice in hot weather to sprinkle frequently for several days. Moisture is absolutely necessary to the perfect hardening of cement work and a surplus is always better than a scarcity. In California the common practise is to cover the cement walk, as soon as it has hardened, with earth which is left on for several days.
CAUSE AND PREVENTION OF CRACKS.—Cracks in cement walks are of two kinds, fractures caused by any one of several construction faults and which reach through the surface coating or through both surface and base, and hair cracks which are simply skin fractures. Large cracks are the result of constructive faults and one of the most common of these is poor foundation construction; other causes are poor mixing and tamping of the base, too large blocks for thickness of the work, failure to cut joints through work. Hair cracks are the result of flushing the neat cement to the surface by excessive troweling or the use of too wet a mixture. The prevention of cracks obviously lies in seeing that the construction faults cited do not exist. If expansion joints are not provided, a long stretch of cement walk will expand on a hot day and bulge up at some point of weakness breaking the walk.
COST OF CEMENT WALKS.—The cost of cement walks is commonly estimated in cents per square foot, including the necessary excavation and the cinder or gravel foundation. The excavation usually costs about 13 cts. per cu. yd., and if the earth is loaded into wagons the loading costs another 10 cts. per cu. yd., wages being 15 cts. per hr. The cost of carting depends upon the length of haul, and may be estimated from data given in Chapter III. If the total cost of excavation is 27 cts. per cu. yd., and if the excavation is 12 ins. deep, we have a cost of 1 ct. per sq. ft. for excavation alone. Usually the excavation is not so deep, and often the earth from the excavation can be sold for filling lots.
In estimating the quantity of cement required for walks, it is well to remember that 100 sq. ft. of walk 1 in. thick require practically 0.3 cu. yd. concrete. If the concrete base is 3 ins. thick, we have 0.3 3, or 0.9 cu. yd. per 100 sq. ft. of walk. And by using the tables in Chapter II we can estimate the quantity of cement required for any given mixture. In cement walk work the cement is commonly measured loose, so that a barrel can be assumed to hold 4.5 cu. ft. of cement. If the barrel is assumed to hold 4.5 cu. ft., it will take less than 1 bbl. of cement to make 1 cu. yd. of 1-3-6 concrete; hence it will not require more than 0.9 bbl. cement, 0.9 cu. yd. stone, and 0.45 cu. yd. sand per 100 sq. ft. of 3-in. concrete base. The 1-in. wearing coat made of 1-1 mortar requires about 3 bbls. of cement per cu. yd., if the barrel is assumed to hold 4.5 cu. ft., and since it takes 0.3 cu. yd. per 100 sq. ft., 1 in. thick, we have 0.3 3, or 0.9 bbl. cement per 100 sq. ft. for the top coat. This makes a total of 1.8 bbls. per 100 sq. ft., or 1 bbl. makes 55 sq. ft. of 4-in. walk.
As the average of a number of small jobs, the authors' records show the following costs per sq. ft. of 4-in. walk such as just described:
Cts. per sq. ft.
Excavating 8 ins. deep 0.65 Gravel for 4-in. foundation, at $1.00 per cu. yd. 1.20 0.018 bbl. cement, at $2.00 3.60 0.009 cu. yd. broken stone, at $1.50 1.35 0.006 cu. yd. sand, at $1.00 0.60 Labor making walk 1.60 —— Total cents 9.00
This is 9 cts. per sq. ft. of finished walk. The gangs that built the walk were usually two masons at $2.50 each per 10-hr. day with two laborers at $1.50 each. Such a gang averaged 500 sq. ft. of walk per day.
Cost at Toronto, Ont.—Mr. C. H. Rust, City Engineer, Toronto, Ont., gives the following costs of constructing concrete sidewalks by day labor. The sidewalks have a 4-in. foundation of coarse gravel or soft coal cinders, thoroughly consolidated by tamping or rolling, upon which is placed a 3-in. layer of concrete composed of 1 part Portland cement, 2 parts clean, sharp, coarse sand, and 5 parts of approved furnace slag, broken stone or screened gravel. The wearing surface is 1 in. thick, or 1 part Portland cement, 1 part clean, sharp, coarse sand, and 3 parts screened pea gravel, crushed granite, quartzite or hard limestone. Costs are given of a 6-ft. and a 4-ft. walk as follows:
COST OF 6 FT. SIDEWALK. Per 100 Item. sq. ft. Labor $ 5.59 Cement, 1.66 bbls., at $1.54 2.49 Gravel, 2.7 cu. yds., at $0.80 2.21 Sand, 0.46 cu. yd., at $0.80 0.37 Water 0.05 —— Total $10.71
COST OF 4 FT. SIDEWALK. Per 100 Item. sq. ft. Labor $ 6.73 Cement, 2.04 bbls., at $1.54 3.15 Gravel, 2.06 cu. yds., at $0.80 1.65 Sand, 0.49 cu. yd., at $0.80 0.39 Water 0.07 —— Total $11.99
The rates of wages and the number of men employed were as follows: 1 foreman, at $3.50 per day; 1 finisher, at 30 cts. per hour; 1 helper, at 22 cts. per hour; 15 laborers, at 20 cts. per hour.
Cost at Quincy, Mass.—The following costs are given by Mr. C. M. Saville for constructing 695 sq. yds. of granolithic walk around the top of the Forbes Hill Reservoir embankment at Quincy, Mass. This walk was laid on a broken stone foundation 12 ins. thick; the concrete base was 4 ins. thick at the sides and 5 ins. thick at the center; the granolithic finish was 1 in. thick. The walk was 6 ft. wide and was laid in 6-ft. sections, a steel plate being used to keep adjacent sections entirely separate. The average gang was 6 men and a team on the base and 2 masons and 1 tender on the finish. The average length of walk finished per day was 60 ft. The cost was as follows:
Stone Foundation: Per cu. yd. Per sq. ft.
Broken stone for 12-in. foundation $ 0.40 $0.015 Labor placing at 15 cts. per hour 1.50 0.056 ——- ——— Totals $ 1.90 $0.071
Concrete Base 4 ins. Thick:
1.22 bbls. cement per cu. yd. at $1.53 $ 1.87 $0.026 0.50 cu. yd. sand per cu. yd. at $1.02 0.51 0.007 0.84 cu. yd. stone per cu. yd. at $1.57 1.32 0.019 Labor (6 laborers, 1 team) 3.48 0.050 —— ——- Total for 90 cu. yds. $ 7.18 $0.102
Granolithic Finish 1 in. Thick:
4 bbls. cement per cu. yd. at $1.53 $ 6.12 $0.019 0.8 cu. yd. sand at $1 0.80 0.002 Lampblack 0.29 0.001 Labor (2 masons, 1 helper) 6.36 0.016 —— ——- Totals $13.57 $0.038
The two masons received $2.25 per day each and their helper $1.50 per day, and they averaged 360 sq. ft. per day, which made the cost 1-2/3 cts. per sq. ft. for labor laying granolithic finish. The cost of placing the foundation stone is very high and the cost of concrete base also runs unusually high, the reasons for these high costs are not evident.
Cost at San Francisco.—Mr. George P. Wetmore, of the contracting firm of Cushing & Wetmore, San Francisco, gives the following figures relating to sidewalk work in that city. The foundations of cement walks in the residence district of San Francisco are 2 ins. thick, made of 1-2-6 concrete, the stone not exceeding 1 in. in size. The wearing coat is in. thick, made of 1 part cement to 1 part screened beach gravel. The cement is measured loose, 4.7 cu. ft. per barrel. The foundation is usually laid in sections 10 ft. long; the width of sidewalks is usually 15 ft. The top coat is placed immediately, leveled with a straight edge and gone over with trowels till fairly smooth. After the initial set and first troweling, it is left until quite stiff, when it is troweled again and polished—a process called "hard finishing." The hard finish makes the surface less slippery. The surface is then covered with sand, and watered each day for 8 or 10 days. The contract price is 9 to 10 cts. per sq. ft. for a 3-in. walk; 12 to 14 cts. for a 4-in. walk having a wearing coat to 1-in. thick. A gang of 3 or 4 men averages 150 to 175 sq. ft. per man per day of 9 hrs. Prices and wages are as follows:
Cement, per bbl. $2.50 Crushed rock, per cu. yd. 1.75 Gravel and sand for foundation, per cu. yd. 1.40 Gravel for top finish, per cu. yd. 1.75 Finisher wages, best, per hr. 0.40 Finisher helper, best, per hr. 0.25 Laborer, best, per hr. 0.20
Cost in Iowa.—Mr. L. L. Bingham sent out letters to a large number of sidewalk contractors in Iowa asking for data of cost. The following was the average cost per square foot as given in the replies:
Cts. per sq. ft. Cement, at $2 per bbl. 3.6 Sand and gravel 1.5 Labor, at $2.30 per day (average) 2.2 Incidentals, estimated 0.7 —- Total per sq. ft 8.0
This applies to a walk 4 ins. thick, and includes grading in some cases, while in other cases it does not. Mr. Bingham writes that in this respect the replies were unsatisfactory. He also says that the average wages paid were $2.30 per man per day. It will be noted that a barrel of cement makes 55 sq. ft. of walk, or it takes 1.8 bbls. per 100 sq. ft. The average contract price for a 4-in. walk was 11 cts. per sq. ft.
Concrete pavement is constructed in all essential respects like cement sidewalk. The sub-soil is crowned and rolled hard, then drains are placed under the curbs; if necessary to secure good drainage a sub-base of gravel, cinders or broken stone 4 to 8 ins. thick is laid and compacted by rolling. The foundation being thus prepared a base of concrete 4 to 5 ins. thick is laid and on this a wearing surface 2 to 3 ins. thick. As showing specific practice we give the construction in two cities which have used concrete pavement extensively.
Windsor, Ontario.—The street is first excavated to the proper grade and crown and rolled with a 15-ton roller. Tile drains are then placed directly under the curb line and a 616-in. curb is constructed, vising 1-2-4 concrete faced with 1-2 mortar. Including the 3-in. tile drain this curb costs the city by contract 38 cts. per lin. ft. The pavement is then constructed between finished curbs, as shown by Fig. 119.
The fine profile of the sub-grade is obtained by stretching strings from curb to curb, measuring down the required depth and trimming off the excess material. The concrete base is then laid 4 ins. thick. A 1-3-7 Portland cement concrete is used, the broken stone ranging from in. to 3 ins. in size, and it is well tamped. This concrete is mixed by hand and as each batch is placed the wearing surface is put on and finished. The two layers are placed within 10 minutes of each other, the purpose being to secure a monolithic or one-piece slab. The top layer consists of 2 ins. of 1-2-4 Portland cement and screened gravel, in. to 1 in., concrete. This layer is put on rather wet, floated with a wooden float and troweled with a steel trowel while still wet. Some 20,500 sq. yds. of this construction have been used and cost the city by contract:
Per sq. yd.
Bottom 4-in. layer 1-3-7 concrete $0.57 Top 2-in. layer 1-2-4 concrete 0.32 Excavation 0.10 ——- Total $0.99
This construction was varied on other streets for the purpose of experiment. In one case a 4-in. base of 1-3-7 stone concrete was covered with 2 ins. of 1-2-2 gravel concrete. In other cases the construction was: 4-in. base of 1-3-7 stone concrete; 1-in. middle layer of 1-2-4 gravel concrete, and -in. top layer of 1-2 sand mortar. All these constructions have been satisfactory; the pavement is not slippery. The cost to the city by contract for the three-layer construction has in two cases been as follows:
Church St., 8,000 sq. yds.: Per sq. yd. 4-in. base 1-3-7 concrete $0.57 1-in. 1-2-4 and -in 1-2 mixture 0.32 Excavation 0.10 ——- Total $0.99
Albert and Wyandotte Sts., 400 sq. yds.: Per sq. yd.
4-in. base 1-3-7 concrete $0.66 1-in. 1-2-4 and -in. 1-2 mixture 0.39 Excavation 0.10 ——- Total $1.15
The cost of materials and rates of wages were about as follows:
Portland cement f. o. b. cars Windsor, per bbl. $2.05 River sand, per cu. yd. 1.15 River gravel, screened, per cu. yd. 1.25 Crushed limestone, to 3 ins., per ton 1.15 Labor, per day 1.75 to 2.00
At these prevailing prices the contractor got a fair profit at the contract price of $1.15; at 99 cts., any profit is questionable, according to City Engineer George S. Hanes, who gives us the above records. Expansion joints are located from 20 to 80 ft. apart and are filled with tar.
Richmond, Ind.—The first concrete pavement was built in 1896 and since then it has been used extensively, especially for wide alleys and narrow streets where traffic is heavy and concentrated in small space. The method of construction has varied from time to time but the construction shown by Fig. 120 is fairly representative. Usually a 1-3-5 concrete is used for the base, 5 ins. thick, and a 1-2 mortar for the top coat, 1 ins. thick. In 1904 this pavement cost the city by contract 16 cts. per sq. ft. or $1.54 per sq. yd, with wages and prices as follows: Stone on the work, $1.25 per cu. yd.; gravel and sand, $0.75 per cu. yd.; cement, $2.25 per barrel; common laborers, 16 cts. per hour, and cement finishers, 40 cts. per hour.
CONCRETE CURB AND GUTTER.
Current practice varies materially in constructing concrete curb and gutter. The more common practice is to lay the curb and water table in one piece, or as a monolith, but this is by no means universal practice. In much work the curb wall and the water table slab are constructed separately, the construction joint being sometimes horizontal where the curb wall sits on the slab and sometimes vertical where the water table butts against the wall. Again it is the common practice to construct curb and gutter in sections, laid either alternately or in succession, separated by sand joints to provide for expansion and contraction, but this is not universal practice, much of such work being constructed as a continuous wall with no provision for temperature movements except the natural breaks at driveways. All of these types of construction appear to have given reasonable satisfaction, but exact data for a final comparison are not available, so that we are forced to reason on general principles. Such a course of reasoning indicates that the best results should be expected where the curb and water table are built in one piece and in sections of reasonable length separated by expansion joints.
FORM CONSTRUCTION.—The form construction for curb and gutter work is determined by the general plan of construction followed,—whether monolithic or two-piece construction. In monolithic construction two types of forms are employed, sectional or box forms and continuous forms. A good example of box form is shown by Fig. 121. This form was designed for a curb 14 ins. high at the back, 6 ins. high in front and 24 ins. from face of curb to outer edge of gutter, constructed in sections 7 ft. long. The form, it will be observed, is a complete box, in which alternate sections of curb are molded and after having set are filled between using the same form but dispensing with the end boards which are replaced by the completed sections of curb. A fairly representative example of continuous form is shown by Fig. 122; in this construction a continuous line of plank is set to form the back of the curb and another line to form the face of the gutter slab, both lines being held in place by stakes. When the gutter slab concrete has been placed and surfaced the form for the front of the curb is set as shown and the upper portion of the curb wall concreted behind it. The method in detail of constructing curb and gutter, with this type of form, at Ottawa, Ont., is described in a succeeding section. Here the joints were formed by inserting a partition of 3/8-in. boiler plate every 12 ft., which was withdrawn just previous to finishing up the surface; the sections between partitions were concreted continuously. Another method is to make the partitions of plank, concrete every other section, then remove the partition plank and concrete the remaining spaces against the previously finished work. A different method of supporting the plank forming the face of the curb wall, is to clamp it to the back form (Fig. 123), spacers being inserted to keep the two their proper distances apart. The forms shown by Figs. 121 to 123 are for monolithic curb and gutter. In two-piece construction where the curb wall is constructed on the finished gutter slab practically the same method of construction is employed as is illustrated by Fig. 122 except that no attempt is made to concrete the curb wall before the slab concrete has begun to set. The more common and the preferable method of two-piece construction is illustrated by Fig. 124; the curb proper is built first using the simple box form shown at the right hand, then the water table is built using the completed curb as the form for the back and a board held by stakes as a form for the front. This board is set with its top edge exactly to the grade of the finished water table so as to serve as a guide for one end of the template, the other end of which rides on the top of the finished curb wall. Forms for curves at street intersections are best constructed by driving stakes to the exact arc of the curve and bending a 3/8-in. steel plate around them or bending and nailing 7/81-in. strips. Soaking the wood strips thoroughly will make them bend easily. The cost of form work in constructing curb and gutter is chiefly labor cost in erecting and taking down the forms.