Tensile Strznoth

TENSILE STRZNOTH. This is the most important of the tests for cement, and in a degree it includes most of the other tests. The strength of cement mortar is usually determined by submitting a specimen having a cross section of 1 square inch to a tensile stress. The reason for adopting tensile tests instead of compressive is the greater ease of making the former and the less variation in the results. Mortar is eight to ten times as strong in compression as in tension.

The accurate determination of the tensile strength of cement is much more difficult than at first appears. Many things, apparently of minor importance, exert such a marked influence upon the results that it is only by the greatest care that trustworthy tests can be made. The variations in the results of different experienced oper ators working by the same method and upon the same material are frequently very large; and therefore careful attention should be given to the standard method of making the tests, so the results will be comparable with those obtained by others.

Neat vs. Sand Tests.

Although in practice it is the almost universal custom to mix cement with sand, tests are usually made of both neat cement and sand mixtures. There are two serious objec tions to testing cement neat. 1. Most neat portland cements decrease in tensile strength after a time. The strength of a cement is due to the aluminates of lime and the silicates of lime, the former being responsible for the setting and the early strength and the latter for the final strength. The strength due to the aluminates is not permanent, but decreases after about 28 days; while the strength due to the silicates increases slowly and does not overcome the loss due to the aluminates until about a year—see Fig. 7, page 122. This decrease is most marked with high-grade portlands which attain their strength rapidly. There is no loss of strength in natural cements, probably because the combination of the lime with the silica and alumina are different from those in portland cement. 2. A second objection to neat tests is that coarsely ground cements show greater strength than finely ground cements, although the latter mixed with the usual proportion of sand will give the greater strength.

On the other hand, more skill is required to secure uniform re sults with sand than with neat cement.

The Sand.

The quality of the sand employed in the tests is of great importance, for sands looking alike and sifted through the same sieve give results varying 30 to 40 per cent. To secure uni formity in the results, it is necessary to adopt some particular sand as a standard.

The Committee of the American Society for Testing Materials, in co-operation with similar committees from various other national engineering societies, "recommends the natural sand from Ottawa, Ill., screened to pass a sieve having 20 meshes per linear inch and retained on a sieve having 30 meshes per linear inch. The wires of the sieves are to have diameters of 0.0165 and 0.0112 inches respectively, i.e., half the width of the opening in each case. Sand having passed the No. 20 sieve shall be considered standard when not more than one per cent passes a No. 30 sieve after one minute of continuous sifting of a 500-gram sample. The Sandusky Portland Cement Co., of Sandusky, Ohio, has agreed to undertake the prepara tion of this sand, and to furnish it at a price only sufficient to cover actual cost." Formerly American engineers used crushed quartz, such as is employed in the manufacture of sand paper; but it did not prove satisfactory.

The standard sand employed in the official German tests is a natural quartz sand obtained at Freienwalde on the Oder, passing a sieve of 60 meshes per square centimeter (20 per linear inch) and caught upon a sieve of 120 meshes per square centimeter (28 per linear inch).

The sand used in ordinary building operations will usually give a greater strength than the so-called standard sand, since usually the former consists of grains having a greater variety of sizes, and consequently there are fewer voids to be filled by the cement (see Table 19, page 93).