Concrete Construction Part 36

Foreman $0.28 Preparing for mixing 0.08 Cleaning out forms 0.05 Handling materials 0.14 Handling and placing steel 0.08 Mixing and placing 0.87 Ramming 0.36 ----- $1.86

This gives a total cost per cubic yard for the concrete in the girders in the completed bridge as follows:

Materials $ 7.01 Erecting forms 4.73 Tearing down forms 0.61 Labor 2.57 General expense 1.60 ------ $16.52

The cost per cubic yard for the floor was:

Materials $ 6.13 Erecting forms 4.73 Tearing down forms 0.61 Labor 1.86 General expense 1.60 ------ $14.93

Included with this is an item for general expense, being expenses of the contractor in bidding on the work, car fare, and other items of expense in looking after the contract.

It will be noticed that a record is here given of three different mixtures and that the labor cost of mixing and placing increases with the richness of the mixture. This is because it takes a greater number of batches to the cubic yard. Record has also been given of cost of preparing the mixing board and other work necessary to start and clean up each day; also when stock piles could not be arranged close to the mixing board, of the cost of handling the materials. These items, it will be noticed, are large enough to be considered in estimating on new work. The cost of sweeping and cleaning out the forms has also been listed, as this work is extremely important.

The cost of the reinforcing steel is given in with the materials, but the labor of handling it and placing it in the forms is listed under labor. This naturally varies with the amount of steel needed, and with the Kahn bar it will vary from 10 cts. to 75 cts. per cubic yard, as the p.r.o.ngs of the bar must be bent into proper position and at times straightened, when bent in s.h.i.+pment. This cost seems large, but it is done with the ordinary labor, while with round rods a large amount of blacksmith work has to be done and a smith and his helper frequently must place them. The patent bars are all lettered and numbered as structural steel is, and can be placed under the direction of the foreman.

One striking lesson can be learned from the forming. It will be noticed that the cost for common labor for handling and helping to erect the forms was much larger in Example I than in Example II, although the bridge was higher in the latter instance. This was caused by the heavy timber that was used, and equaled an extra cost nearly 50 per cent. of the price of new lumber. It certainly speaks volumes against the use of unnecessarily heavy timber for concrete forms.

In bridge work the height of the floor above the stream to some extent governs the cost of the forms. This is made so by the extra lumber needed as props or falsework to support the forming, and also by the fact that men at some height above the ground do not work as quickly or as readily as they do nearer the ground. For high and long spans a derrick is sometimes needed for the work of placing the centering.

On these jobs the concrete was made so wet that with the proper tamping and cutting of the concrete in the forms the surfaces were so smooth that no plastering was needed.

~MOLDING SLABS FOR GIRDER BRIDGES.~--The bridges carry railway tracks across intersecting streets; the slabs rest on two abutments and three rows of columns so that there are two 24-ft. spans over the street roadway and one 10-ft. span over each sidewalk. The larger slabs were 24 ft. 3 ins. long, 33 ins. thick and 7 ft. wide; each contained 16 cu. yds. of concrete and weighed 36 tons. The smaller slabs were 10 ft.

9 ins. long, 17 ins. thick and 7 ft. wide; each contained 3.65 cu. yds.

of concrete and weighed 7.8 tons. The weights were found by actual weighing. They make the weight of the reinforced slab between 160 and 162 lbs. per cu. ft. The concrete was generally 1 part cement and 4 parts pit gravel. The reinforcement consisted of corrugated bars. The method of molding was as follows:

[Ill.u.s.tration: Fig. 155.--Arrangement of Tracks and Forms for Molding Slabs for Girder Bridge.]

[Ill.u.s.tration: Fig. 156.--Form for Molding Slabs for Girder Bridge.]

A cinder fill yard was leveled off and tamped, then the forms were set up on both sides of two lines of railway track arranged as shown by Fig.

155. The exact construction of the forms for one of the larger slabs is shown by Fig. 156. The side and end pieces were so arranged as to be easily taken down and erected for repeated use. About 100 floors were used and they had to be leveled up each time used as the lifting of the hardened slab disarranged them. The side and end pieces were removed in about a week or ten days, but the slabs stood on the floor 90 days, being wetted each day for two weeks after molding.

The plant for mixing and handling the concrete was mounted on cars. A flat car had a rotary drum mixer mounted on a platform at its forward end. Beneath the mixer was a hopper provided with a deflector which directed the concrete to right or left as desired. Under the hopper were the ends of two inclined chutes extending out sidewise beyond the car--one to the right and one to the left--and over the slab molds on each side. Above the mixer was another platform containing a charging hopper, and from the rear of this platform an incline ran down to the rear end of the car and then down to the track rails. A car loaded with cement and gravel in the proper proportions was hauled up the incline by cable operated by the mixer engine, until it came over the topmost hopper into which it was dumped. This hopper directed the charge into the mixer below; the mixer discharged its batch into the hopper beneath from which it flowed right or left as desired into one of the chutes and thence into the mold. The chutes reached nearly the full length of the molds and discharged as desired over the ends into the far end of the mold or through a trap over the end of the mold nearest the car.

To the rear of the mixer car came a cement car provided with a platform overhanging its forward end. Two hoppers were set in this platform each holding a charge for one batch. Coupled behind the cement cars came three or four gravel cars. These were gondola cars and plank runways were laid along their top outer edges making a continuous runway for wheelbarrows on each side from rear of train to front of cement car. The sand and gravel were wheeled to the two measuring hoppers and the cement was handed up from the car below and added, the charge was then discharged into the dump car below and the car was hauled up the incline to the mixer as already described. Two measuring hoppers were used so that one was being filled while the other was emptied, thus making the work continuous.

The molding gang consisted of 33 laborers, two foremen and one engineman. This gang averaged 7 of the large slabs per 10-hour day and at times made as many as 9 slabs. When molding small slabs an average of 12 were made per day. This record includes all delays, moving train, switching gravel cars on and off, building runways, etc. The distribution of the men was about as follows:

Handling Materials: No. Men.

Shoveling gravel into wheelbarrows 9 Wheeling gravel to measuring hoppers 9 Emptying cement into measuring hoppers 2 Handling cement to men emptying 1 In charge of loading dump car 1 On top of cement car 1 Sub-foreman in charge 1

Mixing and Placing:

Engineer 1 In charge of mixer 1 Hoeing and spreading in mold 2 Spading in mold 2 Finis.h.i.+ng sides of block 2 General laborers 3 Foreman in charge 1 -- Total men 36

This gang mixed and placed concrete for 7 blocks or 117 cu. yds. of concrete per day. a.s.suming an average wage of $2 per day the cost of labor mixing and placing was 61.4 cts. per cu. yd. or $10.28 per slab.

It is stated that the slabs cost $11.80 per cu. yd. on storage pile.

This includes labor and materials (concrete and steel); molds; loading into cars with locomotive crane, hauling cars to storage yard and unloading with crane into storage piles, and inspection, incidentals, etc. To load the slabs into cars from storage piles, transport them to the work and place them in position is stated to have cost $2 per cu.

yd. The slabs were placed by means of a locomotive crane being swung from the flat cars directly into place.

[Ill.u.s.tration: Fig. 157.--Sections Showing Construction of Connecticut Ave. Bridge.]

~METHOD AND COST OF CONSTRUCTING CONNECTICUT AVE. BRIDGE, WAs.h.i.+NGTON, D.

C.~--The Connecticut Ave. Bridge at Was.h.i.+ngton, D. C., consists of nine 150-ft. spans and two 82-ft. spans, one at each end, all full centered arches of ma.s.s concrete trimmed with tool-dressed concrete blocks.

Figure 157 is a part sectional plan and elevation of the bridge, showing both the main and spandrel arch construction. This bridge is one of the largest concrete arch bridges in the world, being 1,341 ft. long and 52 ft. wide, and containing 80,000 cu. yds. of concrete. Its total cost was $850,000 or $638.85 per lin. ft., or $10.63 per cu. yd. of masonry. It was built by contract, with Mr. W. J. Douglas as engineer in charge of construction. The account of the methods and cost of construction given here has been prepared from information obtained from Mr. Douglas and by personal visits to the work during construction.

_General Arrangement of the Plant._--The quarry from which the crushed stone for concrete was obtained was located in the side of the gorge at a point about 400 ft. from the bridge. Incidentally, it may be added, the fact that the contractor had an option on this quarry gave him an advantage of some $30,000 over the other bidders. The stone from the quarry was hoisted about 50 ft. by derricks and deposited in cars which traveled on an incline to a Gates gyratory crusher, into which they dumped automatically. The stone from the crusher dropped into a 600-cu.

yd. bin under the bottom of which was a tunnel large enough for a dump car and provided with top gates by which the stone above could be dropped into the cars. The cars were hauled by cable to the mixer storage bin and there discharged. Sand was brought in by wagons and dumped onto a platform about 50 ft. higher than the bottom of the main stone bin. A tunnel exactly similar to that under the stone bin was carried under the sand storage platform. The sand car was hauled from this tunnel by cable to the mixer storage bin using the same cable as was used for the stone cars, the cable being s.h.i.+fted by hand as was desired. Cement was delivered to the mixer platform from the crest of the bluff by means of a bag chute.

The mixer used was one of the Hains gravity type. It had four drops and was provided with four mixing hoppers at the top. The concrete was made quite wet. The proportions of sand and water were varied to suit the stone according to its wetness and the percentage of dust carried by it.

The head mixer regulated the proportions and his work was checked by the government inspector. From the bottom hopper the mixed concrete dropped into a skip mounted on a car.

[Ill.u.s.tration: Fig. 158.--Center for Connecticut Ave. Bridge (Elevation).]

To distribute the skip cars along the work a trestle was built close alongside the bridge and at about springing line level. This trestle had a down grade of about 2 per cent. from the mixer. Derricks mounted along the centering and on the block molding platform lifted the skips from the cars and deposited them where the concrete was wanted. The skip cars were large enough for three skips but only two were carried so that the derricks could save time by depositing an empty skip in the vacant s.p.a.ce and take a loaded skip away with one full swing of the boom. Altogether nine derricks were used in the bridge, four having 70-ft. booms and five having 90-ft. booms. These derricks were jacked up as the work progressed.

[Ill.u.s.tration: Fig. 158.--Center for Connecticut Ave. Bridge (Details).]

_Forms and Centers._--The forms for wall and pier work consisted of 1-in. lagging held in place by studs about 2 ft. on centers and they in turn supported by wales which were connected through the walls by bolts, the outer portions of which were removed when the forms were taken down.

The centers for the five 150-ft. arches were all erected at one time; those for the 82-ft. arches were erected separately. The seven centers required 1,500,000 ft. B. M. of lumber or 1,404 ft. B. M. per lineal foot of bridge between abutments, or 1,640 ft. B. M. per lineal foot of arch span. The centers for the main arch spans are shown in detail by Fig. 158; this drawing shows the sizes of all members and the maximum stresses to which they were subjected from the loading indicated, that is the arch ring concrete. The centers as a rule rested on pile foundations. Four piles to each post were used for the intermediate posts and two piles for the posts in the two rows next the piers.

Concrete foundations, however, were put in Rock Creek and on the line of Woodley Lane Bridge where it was impracticable to drive piles. As considerable difficulty was experienced in driving the piles, the ground consisting mostly of rotten rock, it is thought that it would have cost less if the contractor had used concrete footings throughout.

Some of the costs of form work and centering are given. The cost of lumber delivered at the bridge site was about as follows:

M. ft. B. M.

Rough Virginia pine $25 Dressed Virginia pine lagging 23 Rough Georgia, sizes up to 1212 ins. 33 Rough Georgia, sizes over 1212 ins. 35 Rough oak lumber 35

The following wages were paid: Foreman carpenter, $3.50; carpenters, $2 to $3; laborers, $1.70, with a few at $1.50. An 8-hour day was worked.

The cost, of formwork is given in summary as follows:

Lagging per M. ft. (used twice): Lumber at $23 $11.50 Erection 15.00 ------ Total cost erected $26.50

Studding and rough boards used in place of lagging per M. ft. (used twice):

Lumber at $25 $12.50 Erection 10.00 ------ Total cost erected $22.50

Wales per M. ft. (used six times):

Lumber at $36 $ 6.00 Erection 10.00 ------ Total cost erected $16.00

The total cost of the main arch span centers to the District of Columbia was $54,000 or $59 per lineal foot of arch span, or $37.33 per M. ft. B.

M. The cost of center erection and demolition was as follows:

Erection below springing line per M. ft. $15 Erection above springing line per M. ft. 25 Demolition 5