Part 37 (1/2)
The salvage on the centers amounted to $11 per M. ft. B. M.
The spandrel arch centers were each used twice and cost per M. ft. B. M.
for
Lumber at $25 per M. ft. $12.50 Erecting at $25 per M. ft. 25.00 Moving at $5 per M. ft. 5.00 Total per M ft. 42.50
_Molding Concrete Blocks._--The bridge is trimmed throughout with molded concrete blocks, comprising belt courses, quoin stones, chain stones, ring stones, brackets and dentils. The blocks were made of a 1-2-4 concrete faced with a 1-3 mixture of Dragon Portland cement and bluestone screenings from 3/8-in. size to dust. They were cast in wooden molds with collapsible sides held together by iron rods. Each mold was provided with six bottoms so that the molded block could be left standing on the bottom to harden while the side pieces were being used for molding another block. The molding was done on a perfectly level and tight floor on mud sills, the perfect level of the molding platform having been found to be an important factor in securing a uniform casting. The blocks were molded with the princ.i.p.al showing face down and the secondary showing faces vertical. The facing mortar was placed first and then the concrete backing. Care was taken to tamp the concrete so as to force the concrete stone into but not through the facing. Mr.
Douglas remarks that the back of the block should always be at the top in molding since the laitance or slime always flushes to the surface making a weak skin which will develop hair cracks. In this work the backs of the blocks were mortised by embedding wooden cubes in the wet concrete and removing them when the concrete had set. These mortises bonded the blocks with the ma.s.s concrete backing. The blocks were left to harden for at least 30 days and preferably for 60 days and were then bush hammered on the showing faces, some of the work being done by hand and some with pneumatic tools.
Some precautions necessary in the molding and handling of large concrete blocks were discovered in this work and merit mention. In designing blocks for molding it is necessary to avoid thin f.l.a.n.g.es or the f.l.a.n.g.es will crack and break off; blocks molded with a 2 in. f.l.a.n.g.e projecting 1 ins. gave such trouble from cracking on this work that a f.l.a.n.g.e 5 ins. thick was subst.i.tuted. Provide for the method of handling the block so that dog or lewis holes will not come in the showing faces. Dog holes can be made with a pick when the concrete is three or four weeks old.
When it is not practicable to use dogs, two-pin lewises can be used. The lewis holes should be cast in the block and should be of larger size than for granite; they should not be located too near the mortar faces.
In turning blocks it is necessary to provide some sort of cus.h.i.+on for them to turn on or broken arrises will result. When the work will permit, it is desirable to round the arrises to about a 3/8-in. radius.
The following general figures of the cost of block work are available.
Foreman cutters were paid $5 per day; foreman concrete workers $3 per day; stonecutters $4 per day; concrete laborers $1.70 per day, and common laborers $1.50 to $1.70 per day. Plain and ornamental blocks cost about the same, the large size of the ornamental blocks bringing down the cost. The following is given as the average cost of block work per cubic yard:
Cement $ 1.95 Sand 0.35 Stone 1.14 Forms, lumber and making 0.80 Mixing and placing concrete 1.50 Dressing 4.73 Handling and setting 2.00 Superintendence, plant, incidentals at 25 per cent. 3.12 Condemnation at 5 per cent. 0.78 ------ Total cost blocks in place $16.37
It will be seen that the largest single item in the above summary of costs is the item of dressing. This was done, as stated above, partly by hand and partly by pneumatic tools. Hand tooling cost about twice as much as machine tooling, but its appearance was generally better. The average cost of tooling the several forms of blocks is shown by Table XIX. For 42,190 sq. ft. the average cost was 26 cts. per sq. ft. or $2.34 per sq. yd., or $4.73 per cu. yd. of block work. This tooling was done by stone cutters, and was unusually high in cost.
_Ma.s.s Concrete Work._--All parts of the bridge except the molded block trim were built of concrete deposited in place. Briefly, the molded blocks were set first and then backed up with the ma.s.s concrete deposited in forms and on centers. The only features of this work that call for particular description are those in connection with the main arch ring and the spandrel arch construction.
The main arch rings were concreted in transverse sections; Fig. 158 shows the size and order of construction of these sections. Back forms were necessary up to an angle of 45 from the spring line after which the concrete was made somewhat drier and back forms were not used. After Sections 1, 2, 3 and 4 had been concreted they were allowed to set and then the struts and back forms were taken out and the intervening sections were concreted. The large Sections 6 and 7 were concreted in five sections each, in order to permit the taking out of the timber struts supporting the sections above. The concrete in all sections was placed in horizontal layers as a rule and it is the judgment of the engineers in charge of this work that this is the preferable method.
TABLE XIX.--SHOWING COST OF TOOLING CONCRETE ORNAMENTAL BLOCKS FOR CONNECTICUT AVENUE BRIDGE.
=============================================================================== | | Per Superficial Foot of | Per Cubic Foot. | Showing Face.
+------+-----+------+-----+------+------+------+------ DESCRIPTION. | | Num-| | | | | |Number | | ber | | |Super-| |Cost |super.
1: 2: 4 Concrete Backing| Total|cubic|Total |Cost |ficial|Total |per |ft. to 1: 3 (Mortar Face). |Number|feet |cubic |per | feet |super-|super-| one |Stones|in | feet |cubic| in |ficial|ficial| cubic | Cut. |each.| cut. |foot.| each.| feet.|foot. | foot.
-------------------------+------+-----+------+-----+------+------+------+------ Brackets under Lamps and | | | | | | | | Rail Posts (Cap and Base)| 344| 16.0| 5,500|$0.27| 10.5 | 3,630|$0.41 | 0.66 Moulding under coping | 770| 5.9| 4,560| 0.30| 3.8 | 2,930| 0.47 | 0.64 Dentils between Moulding | 520| 5.5| 2,860| 0.20| 8.0 | 4,160| 0.14 | 1.45 Coping | 494| 61.2|30,220| 0.12| 35.4 |17,490| 0.21 | 0.58 Pedestal (3 courses) | 162| 27.2| 4,400| 0.15| 14.1 | 2,290| 0.29 | 0.52 Rail Posts (Top and Base)| 296| 7.1| 2,100| 0.50| 17.3 | 5,100| 0.21 | 2.43 Lamp Posts and Parapets | | | | | | | | over Piers (Top and Base)| 248| 22.9| 5,690| 0.17| 26.5 | 6,580| 0.15 | 1.16 -------------------------+------+-----+------+-----+------+------+------+------ Average of above--Totals | 2,834| 19.5|55,330|$0.17| 14.8 |43,190|$0.26 | 0.77 -------------------------+------+-----+------+-----+------+------+------+------
TABLE XX.--SHOWING COST OF Ma.s.s CONCRETE WORK PER CUBIC YARD.
[Transcriber's note: Table split]
=========================================================================== | | | Cost Delivered | | | | | on Mixer. | | Description. | | +--------+------+-------+ | | | Average | | | | | | | Yardage | | | | | | Propor-| for Days| | | | Total | | tions.| Run. | Cement.| Sand.| Stone.| Materials.| -------------------+--------+---------+--------+------+-------+-----------+ Cla.s.s A, in Piers | 1:2:4 | 150 | 1.65 | 0.39 | 1.08 | 3.12 | Cla.s.s A, in Arches | 1:2:4 | 200 | 1.65 | 0.39 | 1.08 | 3.11 | Cla.s.s B, in Piers | | | | | | | --Solid Work | 1:3:6 | 160 | 1.40 | 0.42 | 1.23 | 3.05 | Cla.s.s B, in Piers | | | | | | | --Hollow Work | 1:3:6 | 110 | 1.40 | 0.42 | 1.23 | 3.05 | Cla.s.s B, in | | | | | | | Spandrel Walls | 1:3:6 | 110 | 1.40 | 0.42 | 1.23 | 3.05 | Cla.s.s B, in | | | | | | | Spandrel Arches | 1:3:6 | 200 | 1.40 | 0.42 | 1.23 | 3.05 | Cla.s.s B, | | | | | | | in Abutments | 1:3:6 | 150 | 1.40 | 0.42 | 1.23 | 3.05 | Cla.s.s C, Filling | | | | | | | over Bridge | 1:3:10 | 145 | 0.90 | 0.31 | 1.30 | 2.51 | -------------------+--------+---------+--------+------+-------+-----------+
=============================================== | Cost of | | Mixing and Placing. | Description. +--------+--------+--------+ | | | Total | | | | Mixing | | | | and | | Mixing.| Placing| Placing| -------------------+--------+--------+--------+ Cla.s.s A, in Piers | 0.09 | 0.21 | 0.30 | | | | | Cla.s.s A, in Arches | 0.05 | 0.28 | 0.33 | Cla.s.s B, in Piers | | | | --Solid Work | 0.09 | 0.18 | 0.27 | Cla.s.s B, in Piers | | | | --Hollow Work | 0.11 | 0.36 | 0.47 | Cla.s.s B, in | | | | Spandrel Walls | 0.11 | 0.40 | 0.51 | Cla.s.s B, in | | | | Spandrel Arches | 0.07 | 0.26 | 0.33 | Cla.s.s B, | | | | in Abutments | 0.11 | 0.24 | 0.35 | Cla.s.s C, Filling | | | | over Bridge | 0.11 | 0.28 | 0.39 | -------------------+--------+--------+--------+
========================================================================= | Cost of Form Work. | | Description. +----------+-------+--------+-----------+-------------+ | | Taking| | Total | Total Cost | | Erecting.| Down | Lumber.| Form Work | per Yard.[G]| -------------------+----------+-------+--------+-----------+-------------+ Cla.s.s A, in Piers | 0.17 | 0.05 | 0.16 | 0.38 | $3.80 | | | | | | | Cla.s.s A, in Arches | 0.08 | 0.03 | 0.10 | 0.21 | 3.66 | Cla.s.s B, in Piers | | | | | | --Solid Work | 0.17 | 0.05 | 0.16 | 0.38 | 3.70 | Cla.s.s B, in Piers | | | | | | --Hollow Work | 0.77 | 0.25 | 0.64 | 1.66 | 5.18 | Cla.s.s B, in | | | | | | Spandrel Walls | 0.85 | 0.28 | 0.73 | 1.86 | 5.42 | Cla.s.s B, in | | | | | | Spandrel Arches | 0.94 | 0.30 | 0.86 | 2.10 | 5.48 | Cla.s.s B, | | | | | | in Abutments | 0.10 | 0.03 | 0.12 | 0.25 | 3.65 | Cla.s.s C, Filling | | | | | | over Bridge | 0.00 | 0.00 | 0.00 | .... | 2.90 | -------------------+----------+-------+--------+-----------+-------------+
[Footnote G: Add 25% to the cost here tabulated for superintendence, plant and incidentals.]
Considerable difficulty was experienced in building the large arches with a concrete block facing on account of the fact that the edges of the blocks are liable to chip off when any concentrated pressure is brought on them. In order to permit the ring of blocks to deform as the centering settled under its load, sheet lead was placed in the joints between blocks at the points corresponding with the construction joints between sections of the ma.s.s concrete backing. The deflection of the centers at the crown was a maximum of 3 ins. and a minimum of 2 ins.
TABLE XXI--Detail Cost of Engineering and Inspection for Different Cla.s.ses of Work.
Engineering. Inspection.
Kind of Work. Total. Unit. Total. Unit.