Concrete Construction Part 4

1.0 62.355 1,684 1,178 1,094 1,010 926 842 2.0 124.7 3,367 2,357 2,187 2,020 1,852 1,684 2.1 130.9 3,536 2,475 2,298 2,121 1,945 1,768 2.2 137.2 3,704 2,593 2,408 2,222 2,037 1,852 2.3 143.4 3,872 2,711 2,517 2,323 2,130 1,936 2.4 149.7 4,041 2,828 2,626 2,424 2,222 2,020 2.5 155.9 4,209 2,946 2,736 2,525 2,315 2,105 2.6 162.1 4,377 3,064 2,845 2,626 2,408 2,189 2.7 168.4 4,546 3,182 2,955 2,727 2,500 2,273 2.8 174.6 4,714 3,300 3,064 2,828 2,593 2,357 2.9 180.9 4,882 3,418 3,174 2,929 2,685 2,441 3.0 187.1 5,051 3,536 3,283 3,030 2,778 2,526 3.1 193.3 5,219 3,653 3,392 3,131 2,871 2,609 3.2 199.5 5,388 3,771 3,502 3,232 2,963 2,694 3.3 205.8 5,556 3,889 3,611 3,333 3,056 2,778 3.4 212.0 5,724 4,007 3,721 3,434 3,148 2,862 3.5 218.3 5,893 4,125 3,830 3,535 3,241 2,947

In buying broken stone by the cubic yard it should be remembered that hauling in a wagon compacts the stone by shaking it down and reduces the volume. Table IX shows the results of tests made by the Illinois Highway Commission to determine the settlement of crushed stone in wagon loads for different lengths of haul. The road over which the tests were made was a macadam road, not particularly smooth, but might be considered as an average road surface. The wagon used was one with a dump bottom supported by chains, which were drawn as tight as possible, so as to reduce the sag to a minimum. It will be noticed that about 50 per cent.

of the settlement occurs within the first 100 ft., and 75 per cent. of the settlement in the first 200 ft. Almost all of the settlement occurs during the first half mile, as the tests showed practically no additional settlement for distances beyond. Some of the wagons were loaded from the ground with shovels, others were loaded from bins, the stone having a 15-ft. drop, which compacted the stone a little more than where loaded with shovels, so that there was somewhat less settlement.

But at the end of a half mile the density was practically the same, whatever the method of loading. The density at the beginning and at the end of the haul can be compared by the weight of a given volume of crushed stone. For convenience, the weight of a cubic yard of the material at the beginning of the haul and at the end was computed from the known contents of a wagon.

TABLE IX.--SHOWING SETTLEMENT OF BROKEN STONE DUE TO DIFFERENT LENGTHS OF HAUL ON ORDINARILY GOOD ROAD IN WAGONS.

[Transcriber's Note: Table split]

-----------+------------+---------------------------------------------------+ Size. | Method of | | | Loading. | Per cent Settlement for Hauling. | | +---------------------------------------------------+ | |100'|200'|300'|400'|500'|600'|700'| Mile|1 Mile | -----------+------------+----+----+----+----+----+----+----+------+---------+ Screenings |15 ft. drop |....| ...|....|....|....|....|....| 11.5 | 11.5 | Screenings |15 ft. drop |....| ...|....|....|....|....|....| 12.6 | 12.6 | Screenings |15 ft. drop | 7.3| 8.3| 8.9| 9.2| 9.5|10.1|10.1| 11.2 | .... | Screenings |15 ft. drop | 5.0| 9.6|10.2|10.2|10.4|10.4|10.4| 12.4 | .... | | | | | | | | | | | | ===========+============+====+====+====+====+====+====+====+======+=========+ 1 inch |15 ft. drop | ...|....|....|....|....|....|....| 11.5 | 11.5[C] | 1 inch |15 ft. drop | 5.3| 6.2| 7.1| 7.7| 7.9| 8.0| 8.3| 9.2 | .... | 1 inch |15 ft. drop | 2.6| 3.7| 4.9| 5.3| 5.3| 5.3| 5.4| 5.4 | .... | 1 inch |Shovels | 3.5| 4.1| 4.8| 5.3| 5.3| 5.7| 6.5| 7.25| .... | 1 inch |Shovels | ...|....|....|....|....|....|....| 12.6 | 12.6 | ===========+============+====+====+====+====+====+====+====+======+=========+ 3 inch |15 ft. drop | ...|....|....|....|....|....|....| 10.1 | 10.1 | 3 inch |15 ft. drop | 3.5| 4.2| 4.5| 4.8| 5.0| 5.0| 5.0| 6.0 | .... | 3 inch |15 ft. drop | 0.5| 2.5| 2.5| 4.1| 4.3| 4.3| 4.3| 4.9 | .... | 3 inch |Shovels | ...|....|....|....|....|....|....| 12.6 | 12.6 | 3 inch |Shovels | 5.0| 5.6| 6.5| 6.5| 6.8| 6.8| 6.8| 7.1 | .... | ------------------------+----+----+----+----+----+----+----+------+---------+ [Footnote C: Same per cent of settlement for two-mile haul.]

-----------+------------+----------------- Size. | Method of | Weight per | Loading. | Cu. Yd. in Lbs.

| +----------------- | | At | At | | start.| finish.

-----------+------------+-------+------- Screenings |15 ft. drop | 2,518 | 2,840 Screenings |15 ft. drop | 2,518 | 2,886 Screenings |15 ft. drop | 2,450 | 2,770 Screenings |15 ft. drop | 2,425 | 2,780 ===========+============+=======+======== | | | 1 inch |15 ft. drop | 2,305 | 2,600 1 inch |15 ft. drop | 2,380 | 2,625 1 inch |15 ft. drop | 2,450 | 2,600 1 inch |Shovels | 2,270 | 2,445 1 inch |Shovels | 2,305 | 2,642 ===========+============+=======+======== 3 inch |15 ft. drop | 2,376 | 2,638 3 inch |15 ft. drop | 2,360 | 2,505 3 inch |15 ft. drop | 2,470 | 2,595 3 inch |Shovels | 2,270 | 2,601 3 inch |Shovels | 2,335 | 2,510 ------------------------+-------+--------

~THEORY OF THE QUANt.i.tY OF CEMENT IN MORTAR AND CONCRETE.~--All sand contains a large percentage of voids; in 1 cu. ft. of loose sand there is 0.3 to 0.5 cu. ft. of voids, that is, 30 to 50 per cent. of the sand is voids. In making mortar the cement is mixed with the sand and the flour-like particles of the cement fit in between the grains of sand occupying a part or all of the voids. The amount of cement required in a mortar will naturally depend upon the amount of voids in the particular sand with which it is mixed and since a correct estimate of the number of barrels of cement per cubic yard of mortar is very important, and since it is not always possible to make actual mixtures before bidding, rules based on various theories have been formulated for determining these quant.i.ties. In this volume the rule based on the theory outlined by one of the authors in 1901 will be followed. The following is a discussion of the authors' theory:

When loose sand is mixed with water, its volume or bulk is increased; subsequent jarring will decrease its volume, but still leave a net gain of about 10 per cent.; that is, 1 cu. ft. of dry sand becomes about 1.1 cu. ft. of damp sand. Not only does this increase in the volume of the sand occur, but, instead of increasing the voids that can be filled with cement, there is an absolute loss in the volume of available voids. This is due to the s.p.a.ce occupied by the water necessary to bring the sand to the consistency of mortar; furthermore, there is seldom a perfect mixture of the sand and cement in practice, thus reducing the available voids. It is safe to call this reduction in available voids about 10 per cent.

When loose, dry Portland cement is wetted, it shrinks about 15 per cent, in volume, behaving differently from the sand, but it never shrinks back to quite as small a volume as it occupies when packed tightly in a barrel. Since barrels of different brands vary widely in size, the careful engineer or contractor will test any brand he intends using in large quant.i.ties, in order to ascertain exactly how much cement paste can be made. He will find a range of from 3.2 cu. ft. to 3.8 cu. ft. per barrel of Portland cement. Obviously the larger barrel may be cheaper though its price is higher. Specifications often state the number of cubic feet that will be allowed per barrel in mixing the concrete ingredients, so that any rule or formula to be of practical value must contain a factor to allow for the specified size of the barrel, and another factor to allow for the actual number of cubic feet of paste that a barrel will yield--the two being usually quite different.

The deduction of a rational, practical formula for computing the quant.i.ty of cement required for a given mixture will now be given, based upon the facts above outlined.

Let p = number of cu. ft. cement paste per bbl., as determined by actual test.

n = number of cu. ft. of cement per bbl., as specified in the specifications.

s = parts of sand (by volume) to one part of cement, as specified.

g = parts of gravel or broken stone (by volume) to one part of cement, as specified.

v = percentage of voids in the dry sand, as determined by test.

V = percentage of voids in the gravel or stone, as determined by test.

Then, in a mortar of 1 part cement to s parts sand, we have:

n s = cu. ft. of dry sand to 1 bbl. of cement.

n s v = " " " voids in the dry sand.

0.9 n s v = " " " available voids in the wet sand.

1.1 n s = " " " wet sand.

p - 0.9 n s v = " " " cement paste in excess of the voids.

Therefore:

1.1 n s + (p - 0.9 n s v) = cu. ft. of mortar per bbl.

Therefore:

27 27 N = ------------------------- = -------------------- 1.1 n s + (p - 0.9 n s v) p + n s (1.1 - 0.9 v)

N being the number of barrels of cement per cu. yd. of mortar.

When the mortar is made so lean that there is not enough cement paste to fill the voids in the sand, the formula becomes:

27 N = ------- 1.1 n s

A similar line of reasoning will give us a rational formula for determining the quant.i.ty of cement in concrete; but there is one point of difference between sand and gravel (or broken stone), namely, that the gravel does not swell materially in volume when mixed with water.

However, a certain amount of water is required to wet the surface of the pebbles, and this water reduces the available voids, that is, the voids that can be filled by the mortar. With this in mind, the following deduction is clear, using the nomenclature and symbols above given:

ng = cu. ft. of dry gravel (or stone).

ng V = " " " voids in dry gravel.

0.9 ng V = " " " "available voids" in the wet gravel.

p + n s (1.1 - 0.9 v) - 0.9 ng V = excess of mortar over the available voids in the wet gravel.

ng + p + n s (1.1 - 0.9 v) - 0.9 ng V = cu. ft. of concrete from 1 bbl. cement.

27 N = ----------------------------------------------- p + n s (1.1 - 0.9v) + ng (1 - 0.9V)

N being the number of barrels of cement required to make 1 cu. yd. of concrete.

This formula is rational and perfectly general. Other experimenters may find it desirable to use constants slightly different from the 1.1 and the 0.9, for fine sands swell more than coa.r.s.e sands, and hold more water.

The reader must bear in mind that when the voids in the sand exceed the cement paste, and when the available voids in the gravel (or stone) exceed the mortar, the formula becomes:

27 N = ------ ng

These formulas give the amounts of cement in mortars and concretes compacted in place. Tables X to XIII are based upon the foregoing theory, and will be found to check satisfactorily with actual tests.

In using these tables remember that the proportion of cement to sand is by volume, and not by weight. If the specifications state that a barrel of cement shall be considered to hold 4 cu. ft., for example, and that the mortar shall be 1 part cement to 2 parts sand, then 2 barrel of cement is mixed with 8 cu. ft. of sand, regardless of what is the actual size of the barrel, and regardless of how much cement paste can be made with a barrel of cement. If the specifications fail to state what the size of a barrel will be, then the contractor is left to guess.

TABLE X.--BARRELS OF PORTLAND CEMENT PER CUBIC YARD OF MORTAR.

(Voids in sand being 35%, and 1 bbl. cement yielding 3.65 cu. ft. of cement paste.)

----------------------------------+------+-------+------+-------+------+------ Proportion of Cement to Sand |1 to 1|1 to 1|1 to 2|1 to 2|1 to 3|1 to 4 ----------------------------------+------+-------+------+-------+------+------ | Bbls.| Bbls. | Bbls.| Bbls. | Bbls.| Bbls.

Barrel specified to be 3.5 cu. ft.| 4.22 | 3.49 | 2.97 | 2.57 | 2.28 | 1.76 " " " 3.8 | 4.09 | 3.33 | 2.81 | 2.45 | 2.16 | 1.62 " " " 4.0 | 4.00 | 3.24 | 2.73 | 2.36 | 2.08 | 1.54 " " " 4.4 | 3.81 | 3.07 | 2.57 | 2.27 | 2.00 | 1.40 +------+-------+------+-------+------+------ Cu. yds. sand per cu. yd. mortar | 0.6 | 0.7 | 0.8 | 0.9 | 1.0 | 1.0 ----------------------------------+------+-------+------+-------+------+------

TABLE XI.--BARRELS OF PORTLAND CEMENT PER CUBIC YARD OF MORTAR.

(Voids in sand being 45%, and 1 bbl. cement yielding 3.4 cu. ft. of cement paste.)

----------------------------------+------+-------+------+-------+------+------ Proportion of Cement to Sand |1 to 1|1 to 1|1 to 2|1 to 2|1 to 3|1 to 4 ----------------------------------+------+-------+------+-------+------+------ | Bbls.| Bbls. | Bbls.| Bbls. | Bbls.| Bbls.

Barrel specified to be 3.5 cu. ft.| 4.62 | 3.80 | 3.25 | 2.84 | 2.35 | 1.76 " " " 3.8 " | 4.32 | 3.61 | 3.10 | 2.72 | 2.16 | 1.62 " " " 4.0 " | 4.19 | 3.46 | 3.00 | 2.64 | 2.05 | 1.54 " " " 4.4 " | 3.94 | 3.34 | 2.90 | 2.57 | 1.86 | 1.40 +------+-------+------+-------+------+------ Cu. yds. sand per cu. yd. mortar | 0.6 | 0.8 | 0.9 | 1.0 | 1.0 | 1.0 ----------------------------------+------+-------+------+-------+------+------

If the specifications call for proportions by weight, a.s.sume a Portland barrel to contain 380 lbs. of cement, and test the actual weight of a cubic foot of the sand to be used. Sand varies extremely in weight, due both to the variation in the per cent. of voids, and to the variation in the kind of minerals of which the sand is composed. A quartz sand having 35 per cent. voids weighs 107 lbs. per cu. ft.; but a quartz sand having 45 per cent. voids weighs only 91 lbs. per cu. ft. If the weight of the sand must be guessed at, a.s.sume 100 lbs. per cu. ft. If the specifications require a mixture of 1 cement to 2 of sand by weight, we will have 380 lbs. (or 1 bbl.) of cement mixed with 2 380, or 760 lbs.

of sand; and if the sand weighs 90 lbs. per cu. ft., we shall have 760 90, or 8.44 cu. ft. of sand to every barrel of cement. In order to use the tables above given, we may specify our own size of barrel; let us say 4 cu. ft.; then 8.44 4 gives 2.11 parts of sand by volume to 1 part of cement. Without material error we may call this a 1 to 2 mortar, and use the tables, remembering that our barrel is now "specified to be"

4 cu. ft. If we have a brand of cement that yields 3.4 cu. ft. of paste per bbl., and sand having 45 per cent. voids, we find that approximately 3 bbls. of cement per cu. yd. of mortar will be required.