409. Tensile tests were made of briquets from mortars similar to those used in the adhesive tests and stored in damp sand, and the results are used for comparison with the adhesive tests. The cohesive strength given by the briquets is not strictly comparable with the adhesive strength shown in the tests with brick, because of the great difference in the area of the breaking sections in the two cases. It has been well established in tensile tests of cohesion that briquets of large cross-section break at a lower strength than those of small section. It is quite possible also that even with the special clip devised, cross-strains were more likely to occur in the adhesive tests than in the briquet tests. An opportunity was furnished of comparing the tensile strength of neat natural cement mortar under the two conditions, for in one case six joints "broke directly through the mortar, the adhesion being greater than the cohesion. It was found that the strength per square inch given by the briquets was at least six times that given by the large joint. This difference should be kept in mind in making comparisons in the tables between the cohesion and adhesion as given. It should also be noted that some of the highest results of adhesive strength represent in reality the strength of the brick rather than the adhesive strength of the mortar, as chips were pulled from the brick, leaving the mortar joint undisturbed. The brick were of a rather poor quality, but selected with a view to obtaining those of a uniform degree of burning.

Table 120. Adhesion Of Cement Mortar To Brick. Variations In Richness Of Mortar

Notes : — Bricks were cemented together in pairs in cruciform shape and kept in damp sand until time of test. Briquets for cohesion tests stored in same manner. Each result in cohesion, mean of five briquets. Each result in adhesion is in general mean of six results, three with common die cut brick and three with sand molded stock brick.

When adhesion exceeded 50 pounds per square inch, bricks were about as likely to break as the joint between brick and mortar.

410. Adhesion Of Neat And Sand Mortars Of Portland And Natural

Some of the results of these tests are given in Table 120. The most noteworthy point developed is that for mortars containing more than one-half part of sand to one part cement, the adhesion of the natural cement is greater than that of the Portland with the same proportion of sand, although the Portland mortar was much the stronger in cohesion. The mortars giving the highest adhesive strength are. those containing not more than one-half part sand to one part cement.

The addition of sand lowers the adhesive strength more rapidly than it does the cohesive strength. This point would be shown still more clearly if the true adhesive strength of the richest mortars was obtained, as we may be certain that the adhesion of these mortars would be shown to be considerably greater if the brick were strong enough to allow this strength to be developed. With natural cement mortars containing not more than two parts sand to one cement, the adhesion is one-sixth to one-ninth the cohesion, and with Portland mortars containing not more than one part sand, the adhesion is about one-fifteenth the cohesion. (But see § 409 in this connection).

411. Effect Of Lime Paste On Adhesive Strength Of Cement Mortars

A number of tests were made to determine the effect, on the adhesive and cohesive strength of mortars, of mixing lime paste with the cement. Tables 121 and 122 give the results of a few preliminary tests on this subject.

For the tests recorded in Table 121 the mortars were allowed to harden in dry air. From the cohesive tests it is seen that lime in form of paste to the amount of ten per cent, of the cement had little effect on one-to-four Portland mortars, but that a larger amount of lime was very deleterious for dry air exposure. The sample of natural cement used did not harden well in dry air, and the highest result is given by the lime mortar without cement. It appears that the adhesive strength of the Portland mortar was slightly increased by the addition of a small amount of lime paste, but the adhesive strength of natural was not greatly affected. The adhesive strength of the natural cement is, in general, higher than the Portland. The natural cement appeared to harden better in the joints than in the briquets, and we have, as a peculiar result, the adhesive strength exceeding the cohesion. This illustrates a statement already made, that to store briquets in dry air does not approach very nearly the ordinary conditions of use.

Table 121. Adhesion Of Cement Mortar To Brick. Effect Of Lime Paste In Mortar Hardened In Dry Air

Notes: — Brick, sand molded stock.

All briquets and brick stored in dry air.

412. In Table 122 are given a few tests of mixtures of Port-land cement and lime paste, the mortars being hardened in dry-air and in damp sand. Cohesive tests are also given of briquets hardened in damp sand, water and dry air. It appears that the addition of ten per cent, of lime in the form of paste to mortars of this sample of Portland increases the tensile strength, the effect being least when the mortars harden in dry air. The substitution of lime for one-sixth of the cement in a one-to-four mortar has little effect on the tensile strength. Larger proportions of lime result in decreased strength, and if one-half of the cement is replaced by lime, the resulting strength is only about one-half that given by the cement mortar without lime. The results of the adhesive tests show that if half of the cement in the mortar is replaced by an equal weight of lime in the form of paste, the resulting strength is increased by nearly 100 per cent., and that if smaller amounts of lime are used, the adhesive strength is increased by about 150 per cent, over that given by the cement mortar without lime.