Concrete Block Industry

Unit of Measurement.

The barrel of Port land cement shall weigh 380 pounds net, either in barrels or subdivisions thereof made up of cloth or paper bags; and a cubic foot of cement packed as received from the manufacturer shall be called 100 pounds of the equivalent of 3.8 cubic feet per barrel. Cement shall be gauged or measured either in the original package as received from the manufacturer, or may be weighed and so proportioned; but under no cir cumstances shall it be measured loose in bulk, for the reason that when so measured it increases in volume from 20 to 33 per cent, resulting in a deficiency of cement.

Proportions.

Owing to the different values of natural sand or fine crusher screenings for use in mortar mixtures, due not only to its mean effective size, but also to its physical character istics, it is difficult to do more in a general speci fication than fix the maximum proportions of good sand that may be added to cement.

Sand,

or the fine aggregate, shall be suitable silicious material passing the one-fourth-inch mesh sieve, and containing not over ten per cent of clean, unobjectionable material passing the No. 100 sieve. A marked difference will be found in the value of different sands for use in cement mortar. This is influenced by the form, size, and relative roughness of the surface of the sand grains, and the impurities, if any, contained.

Only clean, sharp, and gritty sand, graduated in size from fine to coarse and free from impuri ties, can be depended upon for the best results. Soil, earth, clay, and fine, "dead" sand are in jurious to sand, and at times extremely danger ous, particularly in dry and semi-wet mortars; and they also materially retard the hardening of the cement. An unknown or doubtful sand should be carefully tested before use, to deter mine its value as a mortar ingredient. Screen ings from crushed trap rock, granite, hard lime stone, and gravel stones are generally better than bank sand, river sand, or beach sand in Portland cement mortars (but not so when used with natural cement, unless the very fine ma terial be excluded).

So-called clean but very fine sand has caused much trouble in cement work, and should always be avoided, or, if impossible to obtain better, the proportion of cement should be increased. Stone screenings and sharp, coarse sand may be mixed with good results; and this mixture offers some advantages, particularly in making sand cement blocks.

For foundations or superstructure walls ex posed to weather, carrying not over eight tons per square foot, the maximum proportion shall not exceed four parts sand to one part cement.

This proportion, however, requires extreme care in mixing for uniform strength and will not pro duce water-tight blocks. We recommend for general work not over three parts sand, if well graded, to one part cement, and the further addi tion of from two to four parts of clean gravel stones passing the three-fourths-inch sieve and retained on a one-fourth-inch mesh sieve, or clean screened broken stone of the same sizes. These proportions, with proper materials and due care in making and curing, will produce blocks ca pable of offering a resistance to crushing of from 1,500 to 2,500 pounds per square inch at twenty eight days.

(For the best fireproof qualities limestone screenings or broken sizes should be excluded, but otherwise are all right for use.) Where greater strength is desired, particu larly at short periods, from two to six weeks, we recommend the proportions of one and one-half to three parts gravel or broken stone of sizes above given. Blocks made of cement, sand, and stone are stronger, denser, and consequently more waterproof than if made of cement and sand only, and are more economical in the quan tity of cement used.

Mixing. The importance of an intimate and thorough mix cannot be overestimated. The sand and cement should first be perfectly mixed dry and the water added carefully and slowly in proper proportions, and thoroughly worked into and throughout the resultant mortar. The moistened gravel or broken stone may then be added, either by spreading same uniformly over the mortar, or by spreading the mortar uni formly over the stones ;_and then the whole mass shall be vigorously mixed together until the coarse aggregate is thoroughly incorporated with and distributed throughout the mortar.

We recommend mechanical mixing wherever possible, but believe in the thorough mixing of cement and sand dry, before the addition of water; this insures a better distribution of the cement throughout the sand, particularly for mortar used in machine-made blocks of a semi wet consistency. For fine materials, such as used in cement blocks, it is necessary that the mechanical mixer be provided with knives, blades, or other contrivances to thoroughly break up the mass, vigorously mix the same, and pre vent balling or caking.