58. Significance

Since a hard burned Portland cement will usually be heavier than a light burned one, a test of the weight per cubic foot was once thought to be of great value in judging of the degree of burning. But it has been shown repeatedly that the weight per cubic foot depends quite as much on the fineness as on the burning. It also depends on the age of the cement, and its chemical composition. As a test for quality, the determination of the apparent density has therefore been discarded. However, it is an aid in classifying a product, since Portland cements weigh from 70 to 90 pounds per cubic foot when loosely filled in a measure, while natural cements weigh from 45 to 65 pounds. A knowledge of the weight per cubic foot is also useful in reducing proportions given by weight to equivalent volumetric proportions, and vice versa.

59. Method

This test may be made with a very simple apparatus, and the results obtained, though not strictly accurate, are sufficient for all practical purposes. A metal tube, 2 feet 4 inches long, about 6 inches in diameter at the top, and 3 or 4 inches at the bottom, is supported by a frame resting on four legs. A metal cylinder, 6 inches in diameter and 6 3/32 inches deep, holding one-tenth cubic foot, is placed on the floor below the tube. A coarse sieve, through which all of the cement will pass, is placed on top of the tube and three feet above the bottom of the measure. The cement passes through the sieve, falling freely to the cylinder below, which is struck off level when full. The cement must not be heaped too much, and great care must be taken that the measure is not jarred while it is being filled or struck off. The cement is in such a light condition that a very slight jar is sufficient to cause it to settle.

The above apparatus is on the same plan as that used by Mr. E. C. Clarke on the Boston Main Drainage Works, and is described here for general use when it is desired to compare the results obtained by operators at different points. Should one wish simply to obtain a series of results on different cements which are to be compared among themselves, it is quite sufficient to sift each sample through a coarse sieve, and then with an ordinary scoop carefully fill a measure of any known capacity, without other apparatus.

Mr. Henry Faija has described an apparatus consisting of a funnel with a screw at the mouth which carries the cement horizontally to the point where it falls freely into the measure. Various other devices have been employed, but none seems to have met with universal favor.

60. To determine the relative accuracy obtainable with the simple form of apparatus first described, the author made a series of tests which may be summarized as follows: —

1st Method

Cement passed a wire mesh sieve, holes .033 inch square and fell freely two feet through a 6-inch tube into a measure holding 1/8 cu. ft. Five trials with a sample of Dycker-hoff Portland, highest weight per cubic foot, 81 lbs. 4 oz., lowest, 79 lbs. 2 oz., difference, 2 lbs. 2 oz. Three trials with Alsen's Portland, highest weight, 73 lbs., lowest, 72 lbs., difference, 1 lb.

2d Method

Measure same size filled with scoop without other apparatus, and cement not shaken or jarred in measure. Five trials with Alsen's Portland, highest result, 73 lbs. 8 oz. per cu. ft., lowest result, 72 lbs. 12 oz., difference, 12 oz. Five trials with different sample of same cement, highest, 72 lbs. 4 oz., lowest, 72 lbs., difference, 4 oz.

3d Method

Measure filled with scoop, and cement well shaken down as filling proceeded. Five trials with Alsen's Portland, highest result, 100 lbs. 8 oz., lowest, 97 lbs. 14 oz., difference, 2 lbs. 10 oz.

It appears from these tests that when the measure is filled with the scoop, the results are about as uniform as when the apparatus is used, provided the filling is always done by the same person. But the results obtained by different operators with the same sample of cement would probably vary less, one from the other, when the apparatus is employed. In other words, the personal factor is more nearly eliminated when the cement is passed through a sieve and allowed to fall freely from a given height.

61. As to the effect of age on the weight per cubic foot, it was found in one case that cement which weighed 93 1/2 pounds per cubic foot when freshly ground, weighed but 88 pounds when a few days old, and 78 and 74 pounds after six months and one year, respectively.1

Many experiments have been made to show the effect of fineness on the weight per cubic foot, but as this subject will be taken up again under "fineness," it will suffice to quote one series of tests made by Mr. E. C. Clarke,2 giving the "weight per cubic foot of the same sample of German Portland cement containing different percentages of coarse particles as determined by sifting through the No. 120 sieve".

Samples containing 0, 10, 20, 30, and 40 per cent, of coarse particles retained on No. 120 sieve gave the following weights per cubic foot: 75, 79, 82, 86 and 90 pounds, respectively.

It may be repeated that the weight per cubic foot is no longer considered an indication of quality, but should it be desired to specify a given weight, the method by which the test is to be made should also be stated.