sewers, unimportant work in damp or wet situations, or in heavy work in which the working loads will not be applied until long after completion, natural cement may be employed to advantage. Slag cement is best adapted to heavy foundation work that is immersed in water or at least continually REQUIREMENTS FOR HIGH-GRADE CEMENTS

damp. This kind of cement should never be exposed directly to dry air, nor should it be subjected either to attrition or impact.

The preceding tables give the average weights of hydraulic cements and the various requirements for high-grade cements.

Sand.-Dry sand weighs from 80 to 115 lb. per cu. ft.

Moist

sand occupies more space and weighs less per cubic foot than dry sand.

The voids of ordinary sand range from one-fourth to onehalf of the volume. The more uneven the grains in size, the

smaller the percentage of voids.

The fineness of sand is measured by determining the percentage passing through five sieves, the first having 400 meshes, the second 900, the third 2,500, the fourth 6,400, and the fifth 28,900 per sq. in. When the grains range from to in., the sand is called coarse; when from in., fine; and when fromtoin., very fine. When it is composed of sizes varying within these limits it is termed mixed sand.

MORTARS

to

Lime mortar is ordinarily composed of 1 part of slaked lime to 4 parts of sand. This kind of mortar should not be used in foundation work below the water-line, or in continually damp situations; neither should it be used in freezing weather. MATERIALS REQUIRED PER CUBIC YARD OF MORTAR

Portland-cement mortar is composed of Portland cement and sand in proportions that vary from 1 part of cement and 1 part of sand to 1 part of cement and 6 parts of sand, this variation being according to the strength of the mortar desired. The common proportion for ordinary masonry is 1 part of cement to 3 parts of sand. For pointing face joints, 1 part of cement to either 1 or 2 parts of sand is used.

Natural-cement mortar is usually composed of 1 part of cement and 2 parts of sand. This proportion is found to possess sufficient adhesion and resistance to crushing for ordinary masonry

above ground.

In the preceding table are given the quantities of materials required to produce 1 cu. yd. of compacted mortar. The proportions are by volume, a cement barrel being assumed to contain 3.6 cu. ft.

Mortar Impervious to Water.-Both lime and cement mortar absorb water; consequently, they disintegrate under the action of frost. Impermeability of the mortar may be increased by carefully grading the sand and increasing the amount of cement. The addition of a small amount of lime tends to reduce the volume and number of the voids and thus aids in reducing the permeability. Practically impermeable mortar may be made by adding to the mortar a mixture of alum and soap. The proportions usually employed are lb. of pulverized alum to each cubic foot of sand, and lb. of potash soap to each gallon of water. The alum and soap combine and form compounds of alumina and fatty acids that are insoluble in water. The strength of the mixture is but little inferior to the strength of the mortar of the same proportions.

Strength of Mortar.-The strength that mortar should possess is of three kinds; namely, compressive, cohesive, and adhesive. The degree to which it should possess any one of these depends on the position in which it is employed. In ashlar masonry, resistance to compression is all that is required; in uncoursed rubble masonry and in brick masonry, it must possess adhesiveness, or the capacity of adhering to the surface of the stones or brick in order to prevent their displacement. In masonry of all classes that may have to develop transverse stresses, it must possess cohesiveness or tensile strength.

in.

The tensile and the compressive strength of a given mortar depend on the adhesive strength of the cementing medium and on the character of the aggregate. Coarse and fine sand in the proportion of about 4 parts of coarse grains (to in diameter) and 1 part of very fine grains (less than in. in diameter) usually produce the strongest mortar. Screenings from broken stone usually produce stronger mortars than sand,

because of their greater density. Mixtures of sand and screenings often produce stronger mortar than either material alone. With the same aggregate, the strongest and most impermeable mortar is that containing the largest percentage of cement in a given volume of the mortar. With the same percentage of cement in a given volume of mortar, the strongest, and usually the most impermeable, mortar is that which has the greatest density, that is, which in a unit volume has the largest percentage of solid materials.

In the accompanying table is given a fair average of the tensile strength that may be expected from mortars of Portland and natural cements that are made in the field and with a sand of fair quality but not especially prepared.

The strength of Portland-cement mortar increases up to about 3 mo.; after that period, it remains practically constant for an indefinite time. Natural-cement mortar, on the

TENSILE STRENGTH OF CEMENT MORTARS

Proportions

Tensile Strength, in Pounds
per Square Inch

Portland Cement Natural Cement

Cement
Parts

Parts

Sand 7 da. 28 da. 3 mo. 7 da. 28 da. 3 mo.

other hand, continues to

increase in strength for 2 or 3 yr., its

greatest strength being about 25% in excess of that attained in 3 mo. The strength of slag-cement mortar averages about three-quarters of that of Portland-cement mortar.

The compressive strength of cement mortar is about eight times its tensile strength, and the strength of mortar in crossbreaking and shear may be taken at about one and one-half to two times the tensile strength.

The adhesion of 1-2 Portland-cement mortar, 28 da. old to sandstone averages about 100 lb. per sq. in.; to limestone, 75 lb.; to brick, 60 lb.; to glass, 50 lb.; and to iron or steel, 75 to 125 lb. Natural-cement mortars have nearly the same adhesive strength as those made with Portland cement.

CONCRETE

Concrete consists of cement, water, sand, and large or small fragments of broken stone, gravel, or cinder. The plastic cement, either by itself or with the sand, is called the matrix and the hard material the aggregate.

Cement for Concrete.-The cement used for concrete work is almost exclusively hydraulic cement, generally Portland cement. Natural cement is not so strong and reliable as Portland. It sets more quickly, but takes longer to obtain its ultimate strength. It is used where economy demands it, but should never be placed under water. In civil-engineering work it is seldom employed, except in the form of mortar. very good substitute for Portland cement in concrete for use under water is pozzuolana cement. This cement never gets very hard, but it withstands the action of sea-water even better than Portland cement. It will, however, soon fail if subjected to much attrition and wear.

A

Water for Concrete.-The wetter the concrete is the easier it will be put in place, but mixtures that are too wet are not so strong as medium mixtures. The quantity of water that will make the best mixture is such that after the concrete has been put in place and rammed, it will quake like jelly when struck with a spade, and water will come to the surface. If the concrete is wetter than this, the water will have a slight chemical effect on the cement, and, moreover, the sand and cement will tend to separate from the broken stone.

In cinder concrete, owing to the porosity of the cinders, it is necessary to use a little more water, so that the cement