Yet another objection is made to the method of testing hitherto and still adopted by most engineers.

It is pointed out that, as the principal function of cement is to produce adherence between portions of the materials used, its capacity to do this should be ascertained, that the strength of its adherence to the materials should be tested; i.e. its adhesive strength, not merely its cohesive strength, that is, the strength with which its own particles cohere.

Mr. Mann has made a great many valuable experiments on the cohesive strength of cement forming it as a joint between two pieces of limestone and finding what weight was necessary to tear them apart.

The following table gives the results of some of his experiments, and he points out that it tends to show that under the ordinary cohesive tensile test a really good cement may be rejected. For example, in experiment No. 9 the cement had a very low cohesive strength after twenty-eight days - only 309 lbs. per square inch - which might have caused its rejection, whereas its adhesive strength - 110 lbs. per square inch - was greater than any of those tried with it.

Comparison Of Adhesive And Cohesive Strength

He found also that "with one or two exceptions, the quick-setting cements manifested a greater development of adhesive strength than the slow, while, in the case of cohesive strength, quick-setting seems generally to produce an opposite effect." 2

In the experiments tabulated below, Mr. Mann shows very clearly that the adhesive strength of a very finely ground cement is much greater than that of one coarsely ground.3

Comparative Cementitious Strengths Of Sifted And Unsifted Cements

In the next Table Mr. Mann shows the strength of adhesion of Portland cement to various materials.

1 M.P.I.C.K, vol. lxxi. p. 262. 2 M.P.I.C.E., vol. lxxi. p. 262.

3 M.P.I.C.K, vol. lxxi. p. 260.

Strength of Adhesion of Portland Cement to Various Materials.1

In lbs. per square inch.

N.B. - No. 176 sieve was made of silk and had 30,976 meshes per square inch.