Concrete Construction Part 5

TABLE XII.--INGREDIENTS IN 1 CUBIC YARD OF CONCRETE.

(Sand voids, 40%; stone voids, 45%; Portland cement barrel yielding 3.65 cu. ft. paste. Barrel specified to be 3.8 cu. ft.)

--------------------------------+-------+-------+-------+-------+-------+------ | 1:2:4 | 1:2:5 | 1:2:6 | 1:2:5| 1:2:6| 1:3:4 --------------------------------+-------+-------+-------+-------+-------+------ Bbls. cement per cu. yd. concr't| 1.46 | 1.30 | 1.18 | 1.13 | 1.00 | 1.25 Cu. yds. sand " " | 0.41 | 0.36 | 0.33 | 0.40 | 0.35 | 0.53 Cu. yds. stone " " | 0.82 | 0.90 | 1.00 | 0.80 | 0.84 | 0.71 --------------------------------+-------+-------+-------+-------+-------+------ Proportions by Volume. | 1:3:5 | 1:3:6 | 1:3:7 | 1:4:7 | 1:4:8 | 1:4:9 --------------------------------+-------+-------+-------+-------+-------+------ Bbls. cement per cu. yd. concr't| 1.13 | 1.05 | 0.96 | 0.82 | 0.77 | 0.73 Cu. yds. sand " " | 0.48 | 0.44 | 0.40 | 0.46 | 0.43 | 0.41 Cu. yds. stone " " | 0.80 | 0.88 | 0.93 | 0.80 | 0.86 | 0.92 --------------------------------+-------+-------+-------+-------+-------+------

NOTE.--This table is to be used where cement is measured packed in the barrel, for the ordinary barrel holds 3.8 cu. ft.

It should be evident from the foregoing discussions that no table can be made, and no rule can be formulated that will yield accurate results unless the brand of cement is tested and the percentage of voids in the sand determined. This being so the sensible plan is to use the tables merely as a rough guide, and, where the quant.i.ty of cement to be used is very large, to make a few batches of mortar using the available brands of cement and sand in the proportions specified. Ten dollars spent in this way may save a thousand, even on a comparatively small job, by showing what cement and sand to select.

It will be seen that Tables XII and XIII can be condensed into the following rule:

_Add together the number of parts and divide this sum into ten, the quotient will be approximately the number of barrels of cement per cubic yard._

TABLE XIII.--INGREDIENTS IN 1 CUBIC YARD OF CONCRETE.

(Sand voids, 40%; stone voids, 45%; Portland cement barrel yielding 3.65 cu. ft. of paste. Barrel specified to be 4.4 cu. ft.)

--------------------------------+------+------+------+------+------+----- Proportions by Volume. |1:2:4 |1:2:5 |1:2:6 |1:2:5|1:2:6|1:3:4 --------------------------------+------+------+------+------+------+----- Bbls. cement per cu. yd. concr't| 1.30 | 1.16 | 1.00 | 1.07 | 0.96 | 1.08 Cu. yds. sand " " | 0.42 | 0.38 | 0.33 | 0.44 | 0.40 | 0.53 Cu. yds. stone " " | 0.84 | 0.95 | 1.00 | 0.88 | 0.95 | 0.71 --------------------------------+------+------+------+------+------+----- Proportions by Volume. |1:3:5 |1:3:6 |1:3:7 |1:4:7 |1:4:8 |1:4:9 --------------------------------+------+------+------+------+------+----- Bbls. cement per cu. yd. concr't| 0.96 | 0.90 | 0.82 | 0.75 | 0.68 | 0.64 Cu. yds. sand " " | 0.47 | 0.44 | 0.40 | 0.49 | 0.44 | 0.42 Cu. yds. stone " " | 0.78 | 0.88 | 0.93 | 0.86 | 0.88 | 0.95 --------------------------------+------+------+------+------+------+-----

NOTE.--This table is to be used when the cement is measured loose, after dumping it into a box, for under such conditions a barrel of cement yields 4.4 cu. ft. of loose cement.

Thus for a 1:2:5 concrete, the sum of the parts is 1 + 2 + 5, which is 8; then 10 8 is 1.25 bbls., which is approximately equal to the 1.30 bbls. given in the table. Neither is this rule nor are the tables applicable if a different size of cement barrel is specified, or if the voids in the sand or stone differ materially from 40 per cent. to 45 per cent. respectively. There are such innumerable combinations of varying voids, and varying sizes of barrel, that the authors do not deem it worth while to give other tables. The following amounts of cement per cubic yard of mortar were determined by test:

----------------+------+------+------+------+------+------+------+------+------ Authority | Neat.|1 to 1|1 to 2|1 to 3|1 to 4|1 to 5|1 to 6|1 to 7|1 to 8 ----------------+-------------+------+------+------+------+------+------+------ | Bbls.| Bbls.| Bbls.| Bbls.| Bbls.| Bbls.| Bbls.| Bbls.| Bbls.

Sabin | 7.40 | 4.17 | 2.84 | 2.06 | 1.62 | 1.33 | 1.14 | .... | ....

W. B. Fuller | 8.02 | 4.58 | 3.09 | 2.30 | 1.80 | 1.48 | 1.23 | 1.11 | 1.00 H. P. Boardman. | 7.40 | 4.50 | 3.18 | 2.35 | .... | .... | .... | .... | ....

The proportions were by barrels of cement to barrels of sand, and Sabin called a 380-lb. barrel 3.65 cu. ft., whereas Fuller called a 380-lb.

barrel 3.80 cu. ft.; and Boardman called a 380-lb. barrel 3.5 cu. ft.

Sabin used a sand having 38 per cent. voids; Fuller used a sand having 45 per cent. voids; and Boardman used a sand having 38 per cent. voids.

It will be seen that the cement used by Sabin yielded 3.65 cu. ft. of cement paste per bbl. (i. e. 27 7.4), whereas the (Atlas) cement used by Fuller yielded 3.4 cu. ft. of cement paste per bbl. Sabin found that a barrel of cement measured 4.37 cu. ft. when dumped and measured loose.

Mr. Boardman states a barrel (380 lbs., net) of Lehigh Portland cement yields 3.65 cu. ft. of cement paste; and that a barrel (265 lbs., net) of Louisville natural cement yields 3.0 cu. ft. of cement paste.

Mr. J. J. R. Croes, M. Am. Soc. C. E., states that 1 bbl. of Rosendale cement and 2 bbl. of sand (8 cu. ft.) make 9.7 cu. ft. of mortar, the extreme variations from this average being 7 per cent.

Frequently concrete is made by mixing one volume of cement with a given number of volumes of pit gravel; no sand being used other than the sand that is found naturally mixed with the gravel. In such cases the cement rarely increases the bulk of the gravel, hence Table XIV will give the approximate amount of cement, a.s.suming 1 cu. yd. of gravel per cubic yard of concrete.

TABLE XIV.--SHOWING BARRELS OF CEMENT PER CUBIC YARD OF VARIOUS MIXTURES OF CEMENT AND PIT GRAVEL.

---------+------------------------------------------------------------ Spc. Vol.|Barrels of Cement per Cubic Yard of Concrete for Mixtures of of bbl. +-------+-------+-------+-------+-------+-------+------------ cu. ft. | 1-5 | 1-6 | 1-7 | 1-8 | 1-9 | 1-10 | 1-12 ---------+-------+-------+-------+-------+-------+-------+------------ 3.8 | 1.41 | 1.18 | 1.01 | 0.874 | 0.789 | 0.71 | 0.59 4.4 | 1.25 | 1.02 | 0.875 | 0.766 | 0.681 | 0.61 | 0.51 ---------+-------+-------+-------+-------+-------+-------+------------

~PERCENTAGE OF WATER IN CONCRETE.~--Tests show that dry mixtures when carefully deposited and well tamped produce the stronger concrete. This superiority of dry mixtures it must be observed presupposes careful deposition and thorough tamping, and these are tasks which are difficult to have accomplished properly in actual construction work and which, if accomplished properly, require time and labor. Wet mixtures readily flow into the corners and angles of the forms and between and around the reinforcing bars with only a small amount of puddling and slicing and are, therefore, nearly always used because of the time and labor saved in depositing and tamping. The following rule by which to determine the percentage of water by weight for any given mixture of mortar for wet concrete will be found satisfactory:

_Multiply the parts of sand by 8, add 24 to the product, and divide the total by the sum of the parts of sand and cement._

For example if the percentage of water is required for a 1-3 mortar:

(3 8) + 24 ------------ = 12.

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Hence the water should be 12 per cent. of the combined weight of cement and sand. For a 1-1 mortar the rule gives 16 per cent.; for a 1-2 mortar it gives 13 per cent., and for a 1-6 mortar it gives 10.3 per cent.

To calculate the amount of water per cubic yard of 1-3-6 concrete for example the procedure would be as follows: By the above rule a 1-3 mortar requires

(3 8) + 24 ------------ = 12 per cent. water.

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A 1-3-6 concrete, according to Table XII, contains 1.05 bbls. cement and 0.44 cu. yd. sand. Cement weighs 380 lbs. per barrel, hence 1.05 bbls.

would weigh 380 1.05 = 399 lbs. Sand weighs 2,700 lbs. per cu. yd., hence 0.44 cu. yd. of sand would weigh 2,700 0.44 = 1,188 lbs. The combined weight of the cement and sand would thus be 399 + 1,188 = 1,587 lbs. and 12 per cent. of 1.587 lbs. is 190 lbs. of water. Water weighs 8.355 lbs. per gallon, hence 190 8.355 = 23 gallons of water per cubic yard of 1-3-6 concrete.

~METHODS OF MEASURING AND WEIGHING.~--The cement, sand and aggregate for concrete mixtures are usually measured by hand, the measuring being done either in the charging buckets or in the barrows or other receptacles used to handle the material to the charging buckets. The process is simple in either case when once the units of measurement are definitely stated. This is not always the case. Some engineers require the contractor to measure the sand and stone in the same sized barrel that the cement comes in, in which case 1 part of sand or aggregate usually means 3.5 cu. ft. Other engineers permit both heads of the barrel to be knocked out for convenience in measuring the sand and stone, in which case a barrel means 3.75 cu. ft. Still other engineers permit the cement to be measured loose in a box, then a barrel usually means from 4 to 4.5 cu. ft. Cement is shipped either in barrels or in bags and the engineer should specify definitely the volume at which he will allow the original package to be counted, and also, if cement barrels are to be used in measuring the sand and stone, he should specify what a "barrel" is to be. When the concrete is to be mixed by hand the better practice is to measure the sand and stone in bottomless boxes of the general type shown by Fig. 10 and of known volume, and then specify that a bag of cement shall be called 1 cu. ft., 0.6 cu. ft., or such other fraction of a cubic foot as the engineer may choose. The contractor then has a definite basis on which to estimate the quant.i.ty of cement required for any specified mixture. The same is true if the measuring of the sand and stone be done in barrows or in the charging bucket. The volume of the bag or barrel of cement being specified the contractor has a definite and simple problem to solve in measuring his materials.

[Ill.u.s.tration: Fig. 10.--Bottomless Box for Measuring Materials in Proportioning Concrete.]

To avoid uncertainty and labor in measuring the cement, sand and stone or gravel various automatic measuring devices have been designed. A continuous mixer with automatic measuring and charging mechanism is described in Chapter XIV. Figure 11 shows the Trump automatic measuring device. It consists of a series of revolving cylinders, each opening onto a "table," which revolves with the cylinders, and of a set of fixed "knives," which, as the "tables" revolve, sc.r.a.pe off portions of the material discharged from each cylinder onto its "table." The ill.u.s.tration shows a set of two cylinders; for concrete work a third cylinder is added. The three tables are set one above the other, each with its storage cylinder, and being attached to the same spindle all revolve together. For each table there is a knife with its own adjusting mechanism. These knives may be adjusted at will to vary the percentage of material sc.r.a.ped off.

[Ill.u.s.tration: Fig. 11.--Sketch Showing Trump Automatic Measuring Device for Materials in Proportioning Concrete.]

Automatic measuring devices are most used in connection with continuous mixers, but they may be easily adapted to batch mixers if desired. One point to be observed is that all of these automatic devices measure the cement loose and this must be allowed for in proportioning the mixture.

CHAPTER III.

METHODS AND COST OF MAKING AND PLACING CONCRETE BY HAND.

The making and placing of concrete by hand is divided into the following operations: (1) Loading the barrows, buckets, carts or cars used to transport the cement, sand and stone to the mixing board; (2) Transporting and dumping the material; (3) Mixing the material by turning with shovels and hoes; (4) Loading the concrete by shovels into barrows, buckets, carts or cars; (5) Transporting the concrete to place; (6) Dumping and spreading; (7) Ramming.

~LOADING INTO STOCK PILES.~--Stock piles should always be provided unless there is some very good reason to the contrary. They prevent stoppage of work through irregularities in the delivery of the material, and they save foreman's time in watching that the material is delivered as promptly as needed for the work immediately in hand. The location of the stock piles should be as close to the work as possible without being in the way of construction; forethought both in locating the piles and in proportioning their size to the work will save the contractor money.

The stone and sand will ordinarily be delivered in wagons or cars. If delivered in cars, effort should be made to secure delivery in flat cars when the unloading is to be done by shoveling; this is more particularly necessary for the broken stone. Stone can be shoveled from hopper bottom cars only with difficulty as compared with shoveling from flat bottom cars; the ratio is about 14 cu. yds. per day per man from hopper bottom cars as compared with 20 cu. yds. per day per man from flat bottom cars.

When the cars can be unloaded through a trestle, hopper bottom cars should by all means be secured for delivering the stone. If the amount of work will justify the expense, a trestle may be built; often there is a railway embankment which can be dug away for a short distance and the track carried on stringers to make a dumping place, from which the stone can be shoveled.

Sand can be dumped directly on the ground, but broken stone unless it is very small, -in. or less, should always be dumped on a well made plank floor. A good floor is made of 2-in. plank, nailed to 46-in. mud sills, s.p.a.ced 3 ft. apart, and well bedded in the ground. Loose plank laid directly on the ground settle unevenly and thus the smooth shoveling surface which is sought is not obtained; the object of the floor is to provide an even surface, along which a square pointed shovel can be pushed; it is very difficult to force such a shovel into broken stone unless it is very fine. A spading fork is a better tool than a shovel, with which to load broken stone from piles. A man can load from 18 to 20 cu. yds. of broken stone into wheelbarrows or carts in 10 hours when shoveling from a good floor, but he can load only 12 to 14 cu. yds. per day when shoveling from a pile without such a floor. It is a common thing to see stone unloaded from cars directly onto the sloping side of a railway embankment. This makes very difficult shoveling and results in a waste of stone. Stone can usually be delivered by a steel lined chute directly to a flooring located at the foot of the embankment; coa.r.s.e broken stone if given a start when cast from a shovel will slide on an iron chute having a slope as flat as 3 or 4 to 1; sand will not slide on a slope of 1 to 1. When chuting is not practicable it will pay often to shovel the stone into buckets handled by a stiff-leg derrick rather than to unload it onto the bank. Stock piles of ample storage capacity are essential when delivery is by rail, because of the uncertainty of rail shipments. When the contractor is taking the sand and stone direct from pit and quarry by wagon it is not necessary to have large stock piles.

~LOADING FROM STOCK PILES.~--In loading sand into wheelbarrows or carts with shovels a man will load 20 cu. yds. per 10-hour day if he is energetic and is working under a good foreman. Under opposite conditions 15 cu. yds. per man per day is all that it is safe to count on. A man shoveling from a good floor will load 20 cu. yds. of stone per 10-hour day; this is reduced to 15 cu. yds. per day if the stone is shoveled off the ground and to 12 cu. yds. per day if in addition the management is poor. There are ordinarily in a cubic yard of concrete about 1 cu. yd.

of stone and 0.4 cu. yd. of sand, so that the cost of loading the materials into barrows or carts, with wages at 15 cts. per hour and a.s.suming 15 cu. yds. to be a day's work, would be:

1 cu. yd. stone loaded for 10 cts.

0.4 cu. yd. sand loaded for 4 cts.

------- Total 14 cts.

To this is to be added the cost of loading the cement. This will cost not over 2 cts. per cu. yd. of concrete; the total cost of loading concrete materials into barrows or carts, therefore, does not often exceed 16 cts. per cu. yd. of concrete.