~FORMS AND CENTERS.~--Forms and centers for conduit work have to meet several requirements. They have to be rigid enough not only to withstand the actual loads coming on them, but to keep from being warped by the alternate wetting and drying to which they are subjected. They have also to be constructed to give a smooth surface to the conduit. To be economical, they have to be capable of being taken down, moved ahead and re-erected quickly and easily. The carpenter costs run high in constructing conduit forms, so that each form has to be made the most of by repeated use.
Three different constructions of traveling forms are described in the succeeding sections. For small work, such forms appear to offer certain advantages, but for conduits of considerable size their convenience and economy are uncertain. The experience with the large traveling form employed on the Salt River irrigation works in Arizona was, when all is said, rather discouraging. The authors believe that for work of any size where the concrete must be supported for 24 hours or more, forms of sectional construction will prove cheaper and more expeditious than any traveling form so far devised.
No cla.s.s of concrete work, perhaps, offer so good an opportunity for the use of metal forms as does conduit work. The smooth surface left by metal forms is particularly advantageous, and there is a material reduction in weight and a large increase in durability due, both to the lack of wear and to freedom from warping. Steel forms of the Blaw type shown by Fig. 247, have been used for conduits up to 25 ft. in diameter.
The form ill.u.s.trated, Fig. 247, was for a 12-ft. 3-in. sewer; in this case a roof form alone was used, but full circular and egg-shape forms are made. The Blaw collapsible Steel Centering Co., of Pittsburg, Pa., make and lease steel forms of this type.
[Ill.u.s.tration: Fig. 247.--Blaw Collapsible Steel Centering for Conduit Construction.]
Sectional wooden forms for conduits of large diameters are shown by the drawings in several of the succeeding sections. Figures 248 and 249 show such forms for small diameters. The form shown by Fig. 248 is novel in the respect that after being a.s.sembled a square timber was pa.s.sed through it lengthwise, occupying the holes B and having its ends projecting and rounded to form gudgeons. The form was mounted with these gudgeons resting on horses, so that it could be rotated and thus wound with a narrow strip of thin steel plate. Thus sheathed, the form was lowered into the trench and the concrete was placed around it. When the arch had been turned, the wedges A were driven in until the ribs C dropped into the slots a and clear of the steel sh.e.l.l; the arch form was then pulled out and finally the invert form, leaving the steel sh.e.l.l in place to hold the concrete until hard. The strip of steel was then removed by pulling on one end until it unwound like cord from the inside of a ball of twine. Steel strips 6 ins. wide and 1/24 in. thick were used successfully in constructing a 5-ft. egg-shaped sewer in Washington, D. C. The forms were made in sections 16 ft. long, and were taken out as soon as the concrete had been placed.
[Ill.u.s.tration: Fig. 248.--Sectional Steel Wrapped Wooden Form for Conduit Construction.]
[Ill.u.s.tration: Fig. 249.--Invert Form for Conduit Construction.]
The form shown by Fig. 249, is an invert form, used in constructing the sewer shown by Fig. 249, built at Medford, Ma.s.s., in 1902, by day labor.
The concrete was 1-3-6 gravel. The forms for the invert were made collapsible and in 10-ft. lengths. The two halves were held together by iron clamps and hook rods. The morning following the placing of the concrete the hook rods were removed and turnbuckle hooks were put in their places, so that by tightening the turnbuckle the forms were carefully separated from the concrete. The concrete was then allowed to stand 24 hours, when the arch centers were set in place. These centers were made of 7/81-in. lagging on 2-in. plank ribs 2 ft. apart, and stringers on each side. Wooden wedges on the forward end of each section supported the rear end of the adjoining section. The forward end of each section was supported by a screw jack placed under a rib 2 ft. from the front end. To remove the centers, the rear end of a small truck was pushed under the section about 18 ins.; an adjustable roller was fastened by a thumb screw to the forward rib of the center; the screw jack was lowered allowing the roller to drop on a run board on top of the truck; the truck was then pulled back by a tail rope until the adjustable roller ran off the end of the truck; whereupon the truck was pulled forward drawing the center off the supporting wedges of the rear section. Each lineal foot of sewer required 1 cu. yds. of excavation which cost 74.2 cts. per foot, and 1 cu. ft. of brick arch which cost $12.07 per cu. yd., or 44.2 cts. per lineal foot of sewer. The invert required 4 cu. ft. of concrete per foot, which cost as follows:
Item. Per cu. yd.
Portland cement at $2.15 per bbl. $2.292 Labor mixing and placing 3.017 Cost of forms 0.187 Labor screening gravel 0.471 Carting 0.592 Miscellaneous 0.146 ------ Total $6.705
The cost of the invert was thus $1.002 per lin. ft. of sewer.
Collapsible metal forms for manholes and catch basins are made by several firms which make block and pipe molds. A cylindrical wooden form construction is shown by Fig. 250. The outside form consists of three segments of a cylinder made of 2-in. lagging bolted to hoops. Bent lugs on the ends of the hoops, were provided with open top slots and were bolted together through 13/8-in. bars which extended the full length of the form between lugs. The a.s.sembled form was collapsed by pulling up on the bars, thus lifting the bolts out of the slots. The inner mold is also made in three sections with strap hinges at two of the joints and at the third joint a wedge-shaped stave. The other details are shown by the drawing. To mold the top of the basin two cone-shaped forms are used, an outer form made in one piece and an inner form made in sections. Some 26 catch basins were built in Keney Park, Hartford, Conn., by Mr. H. G. Clark, at a cost of $7 apiece for concrete in place, and there was closely 1 cu. yd. of concrete in each.
[Ill.u.s.tration: Fig. 250.--Form for Circular Catch Basin or Manhole.]
~CONCRETING.~--Except for pipes of small diameter, the concreting is done in sections, each section being a day"s work. Continuity of construction has not proved successful, except for pipes of moderate size, in the few cases where it has been tried. Examples of continuous construction methods are given in succeeding sections. Methods of molding and laying cast concrete pipe are also best shown by the specific examples given further on. In concreting large diameters, the work may be done by molding successive full barrel sections, or by molding first the invert and then the roof arch, each in sections. The engineer"s specifications generally stipulate which plan is to be followed. Construction joints between sections are molded by bulkhead forms framed to produce the type of joint designed by the engineer; the most common type is the tongue and groove joint.
[Ill.u.s.tration: Fig. 251.--Cross-Section of Pinto Creek Irrigation Conduit.]
For small diameters built with traveling forms, a comparatively dry concrete is essential, but when the centers are left in place until the concrete has set, a wet mixture is preferable, as it is more easily placed and worked around the reinforcement in the thin sh.e.l.ls. Mixers are commonly specified even for small work, because of their generally more uniform and h.o.m.ogeneous product. Portable mixers hauled along the bank and discharging into the forms through chutes, furnish a cheap and rapid arrangement where the section being built has a considerable yardage. The examples given in succeeding sections present various methods of mixing and placing concrete in conduit work.
[Ill.u.s.tration: Fig. 252.--Traveling Form for Pinto Creek Conduit.]
~REINFORCED CONDUIT, SALT RIVER IRRIGATION WORKS, ARIZONA.~--The pipe had the cross-section shown by Fig. 251, and formed a syphon carrying water under the bed of a creek. The concrete was a 1-2-4 fine gravel mixture, mixed by hand on boards 150 ft. apart along the line. The sh.e.l.l was reinforced as shown.
The forms consisted of an outside form constructed as shown by Fig. 251, by inserting 2-in.5 ft. lagging strips in the metal ribs. The inside form was designed to permit continuous work by moving the form ahead as the concreting progressed. It consisted as shown by Fig. 252, of an invert form on which an arch form was carried on rollers. The invert form was pulled along by cable from a horsepower whim set ahead, being steered, aligned and kept to grade by being slid on a light wooden track. It had the form of a long half cylinder, with its forward end beveled off to form a scoop-like snout. The arch center consisted of semi-circular rings 2 ft. long, set one at a time as the work required.
Each ring, when set, was f.l.a.n.g.e-bolted to the one behind, and each was hinged at three points on the circ.u.mference to make it collapsible. In operation, the invert form was intended to be pulled ahead and the arch rings to be placed one after another in practically a continuous process. So that the arch rings might continue supported after the invert form was drawn out from under them, invert plates similar to the arch plates were inserted one after another in place of the sh.e.l.l of the invert form. The plan provided very nicely for continuous work, but continuous work was found impracticable for all but about 2,500 ft. of the 6,000 ft. of conduit built. The reason for this seems to have been at least in a great measure, the slow setting cement made at the cement works established by the Government, at Roosevelt. In building the first 300 ft. of conduit, a commercial cement was used and a progress of 120 lin. ft. of pipe per 24 hours was easily made. This work was done in June. Later, but still in warm weather, using the Government cement and 70 ft. of arch plates, not more than 70 ft. of pipe could be completed in 24 hours; if the plates were taken down sooner, patches of concrete fell out or peeled off with them. As the weather grew colder, this difficulty increased, until finally, the idea of continuous work was abandoned and for some 3,500 ft. of conduit only one 8-hour shift per day was worked. In December and January the plates had to remain in place three days, so that the progress was only 24 ft. per day; in warm weather this rate was increased to 40 ft. per day.
Costs were kept on two sections of one of the lines and the figures shown in the accompanying table were obtained.
A gang consisted of a foreman at $175 per month, a sub-foreman at $3.50 per day, and the following laborers at $2.50 per day: one bending the reinforcement rings; two placing the reinforcement; four taking down, moving and erecting the stationary plates; four placing the concrete and outside lagging; two wheeling concrete; six mixing concrete; one wheeling sand and gravel; one watering the finished pipe; four laying track for the steering apparatus, moving the superstructure and hangers, mixing boards, runways, etc.; one pointing and finishing inside the pipe; and one on the whim and doing miscellaneous work. The labor was princ.i.p.ally Mexican, and only fairly efficient.
It is important to note that the costs given in the table are labor costs only of mixing and placing concrete and moving forms; they do not include engineering, first cost of forms, concrete materials, reinforcement or grading.
May, "06. July, "06.
Wages 714 1,009 Cost Per Per Lin. Ft. Lin. Ft. Per Cu.
Day. Cost. Cost. Lin. Ft. Yd.
{ Laying track for { steering alligator $ 5.00 $ 71.48 $ 43.98 $0.0670 $0.16 4 men { Moving and erecting { superstructure 5.00 299.94 358.44 0.3821 0.93 4 men Moving plates 10.00 202.50 253.44 0.2646 0.65 Repairs to alligator 58.50 2.50 0.0354 0.08 1 man Bending rings 2.50 32.87 59.87 0.0538 0.13 2 men Placing reinforcement 5.00 126.94 138.13 0.1538 0.38 12 men Mixing and placing concrete 30.00 709.68 949.74 0.9631 2.34 1 man Watering finished pipe. 2.50 45.00 78.27 0.0716 0.17 1 man Painting and brush-coating inside 2.50 96.50 117.37 0.1241 0.31 Blacksmith"s work 30.00 25.00 0.0319 0.08 1 man Whim 2.50 23.87 28.75 0.0306 0.07 1 man Screening and hauling sand and gravel 2.50 183.13 300.00 0.2804 0.68 --------- --------- ------- ----- Total $1,880.41 $2,335.49 $2.4584 $5.98
~CONDUITS, TORRESDALE FILTERS, PHILADELPHIA, PA.~--At the Torresdale plant of Philadelphia filtration system the clear water conduits are reinforced concrete. The following description is composed from information furnished the authors in 1904 by the Bureau of Filtration, Mr. John W. Hill, then chief engineer. The lengths of the several conduits are as follows: 576 ft. of 7-ft., 782 ft. of 8-ft., 1,050 ft.
of 9-ft., and 1,430 ft. of 10-ft. horseshoe conduit. All sizes of conduit have the same cross-sectional form--the cross-section of the 9-ft. conduit is shown by Fig. 253, and all are reinforced by expanded metal arranged as indicated. The concrete is a 1-3-5, -in. stone mixture. The conduits were first designed with circular sections, but before construction had been begun on these plans, experience had been obtained in building a circular sewer that made a change to the horseshoe section appear desirable. In the circular sewer work, great difficulty had been found in properly placing and ramming the concrete in the lower quarters of the circular section.
[Ill.u.s.tration: Fig. 253.--Section of 9-ft. Conduit, Philadelphia Filter Plant.]
_Forms._--The forms used for the several sizes of conduit were all of the same general type, but improvements in detail were made as successive sizes were built. The last form to be designed was that for the 9-ft. section and this was the best one; it is shown by Fig. 254.
The forms were built in sections from 12 ft. to 13 ft. long. They were covered with No. 27 galvanized sheet iron, and this covering was found of advantage both in giving a smooth finish and in prolonging the life of the centers. The important feature is the construction in sections which could be set up and broken down by simply inserting and removing the connecting bolts. Three sets of forms were made for each size of conduit.
[Ill.u.s.tration: Fig. 254.--Form for 9-ft. Conduit Philadelphia Filter Plant.]
_Procedure of Work._--The first operation in building a section of conduit was to set to exact line and grade and the length of the form in advance of the finished work the bulkhead shown by Fig. 255. In this s.p.a.ce the invert concrete was deposited and formed to a plane 1 in.
below the finished invert bottom. The two bottom sections of the form were then a.s.sembled and located by bolting one end to the last preceding form and inserting the other end into the bulkhead. About two tons of pig iron were then placed on the invert form to keep it from floating while the liquid granolithic mixture was being poured into the 1-in.
s.p.a.ce between the form and the invert concrete. In building up the sides a facing form was used for placing the granolithic finish. This consisted of "boards" of sheet steel ribbed transversely on one side with -in. pipe and on the other side with 1-in. pipe. Two boards were used on each haunch, slightly lapping in the center, as follows: The board was placed with the small ribs against the form and the larger ribs kept the expanded metal just 3 ins. from the face of the form. A 6-in. depth of concrete was placed between the metal board and the outside form or planks, then 6 ins. of granolithic was poured into the 1-in. s.p.a.ce between the center and the board and finally the board was raised 6 ins. and the concrete and granolithic mixture tamped together.
With the board in its new position, another layer of concrete and granolithic was placed. Toward the crown the granolithic mixture was made stiff and simply plastered onto the mold. The expanded metal was cut into sheets corresponding to the length of the sides of the form and lapped 6 ins. in all directions; the bulkhead having a slot as shown to permit the metal to project 6 ins. from the face of the concrete in order to tie two sections together and also having a rib which formed a mortise in the face of the sh.e.l.l of concrete to key it to the succeeding section.
[Ill.u.s.tration: Fig. 255.--Bulkhead Form for Conduits, Philadelphia Filter Plant.]
All the conduits were built in sections from 12 ft. to 13 ft. long, and there was very little, if any, difference in the labor required to build a section, in from eight to ten hours, of any of the three sizes. One foreman and 18 men on the top of the trench mixed and handled the concrete and granolithic mortar while one foreman, one carpenter and seven men in the trench set the forms and placed and rammed the concrete for one section in generally eight hours. About one-third of the concrete for the whole work was mixed in a portable cubical mixer of cu. yd. capacity, and the remainder was mixed by hand. Owing to the relatively small amount of concrete used per day, about 20 cu. yds., it was found that there was practically no difference in the cost of machine mixing and of hand mixing. The 9-ft. conduit as an average of the three sizes, contained 20 cu. yds. of concrete, 1,200 sq. ft. of expanded and required 125 bags of cement for a section 13 ft. long. The cost of the work excluding excavation and profit, but including forms, metal, concrete materials and labor, was about $10.50 per cu. yd.
~CONDUIT, JERSEY CITY WATER SUPPLY.~--In constructing the 8-ft.
reinforced concrete conduit for the Jersey City water supply, use was made of forms without bottoms. Each form was made of segmental sections 12 ft. long of wood covered with sheet steel. They were set end to end in the trench, resting on 6-in. concrete cubes which were finally permanently embedded in the invert concrete. In each form there was a scuttle about 2 ft. square at the crown, and the bottom was open between the curves of the invert haunches. The form being set and greased and the reinforcement placed, the concrete was deposited on the outside and forced by means of tamping bars down the curve of the invert haunches until it filled the whole s.p.a.ce between the form and the earth and appeared at the edges of the bottom opening in the form. Concrete was then thrown through the scuttle and the invert screeded into shape. The concreting of the sides and crown of the arch was then completed, using outside forms except for about 5 ft. of the crown, the scuttle, of course, being closed by a fitted cover. The centers were left in place about 48 hours. The concrete was a 1 cement 7 sand and run of the crusher 2-in. broken stone mixture, and was made so wet that it would flow down an incline of 1 on 8. The mixing was done in portable Ransome mixers, set on the trench bank alongside the work and discharging by chute into dished shoveling boxes provided with legs to set on the erected forms. Coal scoops were used in shoveling from the box into the forms and were found superior to shovels in keeping the relative proportions of water and solids constant.
~TWIN TUBE WATER CONDUIT AT NEWARK, N. J.~--In constructing the Cedar Grove Reservoir, at Newark, N. J., two conduits side by side were built across the bottom from gate house to tunnel outlet. A section of one of the conduits showing the form construction and the arrangement of the reinforcement is given by Fig. 256. The concrete was a 1-2-5 1-in.
stone mixture and the reinforcement was No. 10 3-in. mesh expanded metal. The method and cost of construction are given as follows, by Mr.
G. C. Woollard, the engineer for the contractors.
[Ill.u.s.tration: Fig. 256.--Conduit for Cedar Grove Reservoir, Newark. N.
J.]
"The particular thing that was insisted upon by both Mr. M. R. Sherrerd, the chief engineer of the Newark Water Department and Mr. Carlton E.
Davis, the resident engineer at Cedar Grove Reservoir, in connection with these conduits, was that they be built without sections in their circ.u.mference, that the whole of the circ.u.mference of any one section of the length should be constructed at one time. They were perfectly willing to allow us to build the conduit in any length section we desired, so long as we left an expansion joint occasionally which did not leak.
"The good construction of these conduits was demonstrated later, when the section stood 40 lbs. pressure to the square inch, and, in addition, I may say that these conduits have not leaked at all since their construction. This shows the wisdom of building the conduit all round in one piece, that is, in placing the concrete over the centers all at one time, instead of building a portion of it, and then completing that portion later, after the lower portion had had an opportunity to set.
"The centers which I designed on this work were very simple and inexpensive, as will be gathered from the cost of the work, when I state that this conduit, which measured only 0.8 cu. yd. of concrete to the lineal foot of single conduit, cost only $6.14 per cu. yd., built with Atlas cement, including all labor and forms and material, and expanded metal. The forms were built in 16 ft. lengths, each 16 ft. length having five of the segmental ribbed centers such as are shown in Fig. 256, viz., one center at each end and three intermediate centers in the length of 16 ft. These segments were made by a mill in Newark and cost 90 cts. apiece, not including the bolts. We placed the lagging on these forms at the reservoir, and it was made of ordinary 24 material, surfaced on both sides, with the edges beveled to the radius of the circle. These pieces of 24 were nailed with two 10d. nails to each segment. The segments were held together by four -in. bolts, which pa.s.sed through the center, and 1-in. wooden tie block. There was no bottom segment to the circle. This was left open, and the whole form held apart by a piece, B, of 32 spruce, with a bolt at each end bolted to the lower segment on each side.
"The outside forms consisted of four steel angles to each 16 ft. of the conduit, one on each end, and two, back to back, in the middle of each 16 ft. length. These angles were 23, with the 2-in. side on the conduit, and the 3-in. side of the angle had small lugs bolted on it at intervals, to receive the 212 plank, which was slipped down on the outside of the conduit, as it was raised in height. The angles were held from kicking out at the bottom by stakes driven into the ground, and held together at the top by a 2-in. tie-rod.
"The conduit was 8 ins. thick, save at the bottom, where it was 12 ins.
The reason for the 12 ins. at the bottom was that the forms had to have a firm foundation to rest on, in order to put all the weight required by the conduit on them in one day or at one time, without settling. We therefore excavated the conduit to grade the entire length, and deposited a 4-in. layer of concrete to level and grade over the entire length of the conduit line. This gave us a good, firm foundation, true and accurate to work from, and this is the secret of the good work which was done on these conduits. If you examine them, you will say that they are one of the neatest jobs of concrete in this line that has been built, especially with regard to the inside, which is true, level and absolutely smooth. [The authors can confirm this statement.] When the conduit is filled with water, it falls off with absolutely no point where water stands in the conduit, owing to its being out or the proper amount of concrete not being deposited.
"The centers were placed in their entirety on a new length of conduit to be built, resting upon four piles of brick, two at each end as shown.
The first concrete was placed in the forms at the point marked X and the next concrete was dropped in through a trap door cut in the roof of the conduit form at the point marked Y. This material was dropped in to form the invert, and this portion was shaped by hand with trowels and screeded to the exact radius of the conduit. The concrete was then placed continuously up the sides, and boards were dropped in the angles which I have mentioned, and which served as outside form holders till the limit was reached at the top, where it was impossible to get the concrete in under the planking and thoroughly tamped. At this point the top was formed by hand and with screeds.
"Each 16-ft. length of this conduit was made with opposite ends male and female respectively, that is, we had a small form which allowed the concrete to step down at one end to 3 ins. in thickness for 8 ins. back from the end of the section, and on the other end of the section it allowed it to step down to 3 ins. in thickness in exactly the opposite way, making a scarf joint. This was not done at every 16 ft. length, unless only 16 ft. were placed in one day. We usually placed 48 ft. a day at one end of the conduit with one gang of men. This was allowed to set 24 hours, and, whatever length of conduit was undertaken in a day, was absolutely completed, rain or shine, and the gang next day resumed operations at the other end of the conduit on another 48 ft. length.
This was completed, no matter what the weather conditions were, and, towards the close of this day the forms placed on the preceding day were being drawn and moved ahead.
"The method used in moving these forms ahead for another day"s work is probably one of the secrets of the low cost of this work, and it is one which we have never seen employed before. The bolt at A, Fig. 256, was taken out, and the tie brace B thrown up. We had hooks at the points C. A turnbuckle was thrown in, catching these hooks, and given several sharp turns, causing the entire form to spring downward and inwards, which gave it just enough clearance to be carried forward, without doing any more striking of forms than pulling the bolt at A. This method of pulling the forms worked absolutely satisfactorily, and never gave any trouble, and we were able to move the forms very late in the day and get them all set for next day"s work, giving all the concrete practically 24 hours" set, as we always started concreting in the morning at the furthest end of the form set up and at the greatest distance from the old concrete possible in the 48 ft. length, as the furthest form had, of course, to be moved first, it being impossible to pa.s.s one form through the other.