Density of Mortar

The first law has long been understood and acted upon by all users of cement; and is useful in determining the proportion or amount of cement to be used in any particular case. The second law does not seem to be generally appreciated, although it is very useful in comparing different sands. It was discovered by Mr. Rene Feret, Chief of the Laboratory of Bridges and Roads at Boulogne sur-Mer, France.* Relation between Strength and Amount of Cement. The effect of varying the amount of cement in a unit of volume of the mortar differs according to the fineness of the cement and its capacity for taking up water, the plasticity of the mortar, and the fineness of the sand. Fig. 4, page 112, is from Mr. Feret's experi ments t and shows the strength of plastic cement mortar with various proportions of cement for three different grades of sand. The granulometric composition of the three sands is as follows: The coarse sand had 37 per cent voids, the medium 43, and the fine 44. The value for each proportion is the mean of twenty-five briquettes, broken when 5 months old.

Notice that the 1 : 0.3 fine-sand mortar is stronger than the neat cement, which is contrary to the first law mentioned in 4 236. This exception is due to the abnormally fine sand and to the very rich mortar.

Fig. 4 is useful in fixing the proportions of cement to be used in practice, since it shows the relative strength of different mixtures. The amount or proportion of cement to be used in any particular case is entirely a matter of judgment.

Relation between Strength and Density.

Having decided upon the proportion of the cement to the sand, the next step is to select the sand that will give a mortar of maximum strength and greatest density. Selecting the sand is a very important step. since sands differ greatly in their mortar-making qualities (see Table 18, page 89). The second law of § 236 affords an easy means of comparing two sands. This law is substantially only another way of stating that the sand having the smallest per cent of voids will make the best mortar.

There• are two methods of determining the relative mortar making qualities of different sands: 1. By the method of 5 201, determine the volume of mortar produced with the same weight of the different sands mixed with the same proportion of cement, and the sand giving the least volume of mortar is the best. 2. By the method of § 234, determine the density of the mortar made with each of the sands, using a constant proportion of cement, and then the sand giving the greatest density will make the strongest and cheapest mortar.

Fig. 5 shows the density of mortars made with different pro portions of three sands differing only in fineness.* Three sizes of grains were used, C, M, and F, the numerical values of which are stated in the preceding section.

The diagram is obtained by mix ing the three sizes of sand in various proportions, and using each of these mixtures in making a 1 : 3 mortar, and then determining the density of each mortar as explained in $ 234.

The density of each mortar is re corded at the point of the diagram corresponding to the granulometric composition of the sand. The pro portion of each of the three sizes in the sand is represented by the per pendicular distance from the. side opposite each vertex. For example, a point at C represents sand composed wholly of coarse grains; a point halfway between C and F represents a sand composed of equal parts of coarse and fine grains; and a point halfway from M to the base of the diagram represents a sand composed of 50 per cent of M grains, 25 per cent of C grains, and 25 per cent of F grains. The contour lines are drawn through points representing the same density.

Fig. 5 shows that the densest mor tar that can be made with these three grades of sand is composed of about 78 per cent of coarse grains, 22 per cent of fine grains, and no medium grains. The contour lines show that there are various proportions of the three sizes that give mortar of the same density.

239.

Fig. 6 shows the compressive strength of mortars made of different proportions of the three sands used in Fig. 5. A comparison of Fig. 5 and 6 shows that the lometric composition of the sand which gives maximum density also gives maximum compressive strength. The agreement is not exact, due to errors of observation; but the similarity in the general form of the contour lines in Fig. 5 and 6 indicates that the density varies as the strength.

In laboratory work the proportions of the cement and sand are uniformly determined by weighing; but there is no uniform practice of measuring the pro portions on the work. One of the three following methods is gener ally employed, viz.: (1) by weight; (2) by volumes of packed cement and loose sand; (3) by volumes of loose cement and loose sand.

1. By Weight. The most accurate, but least common, method is to weigh the ingredients for each batch. This method could be used more easily now than formerly, since at the present time cement is usually shipped in bags holding 94 pounds, while formerly it was shipped in barrels holding 380 pounds; and hence if a batch con taining one or more bags of cement is desired, it is necessary to weigh only the sand. Weighing the sand would add some complication and cost, but would give better control of the mixture. This method is said to be common in Germany, and has been used on a few jobs in this country.