The test for adhesion is also one of long standing, being used during that time when engineers were content with an approximate idea of what might be expected of an hydraulic product. It has been stated above that when failure occurs in a mass of masonry, it is more frequently a failure in tension than in compression; it may be added, that it is also more likely to fail in adhesion than in cohesion. Hence, an adhesive test is a very proper one to make, and will give most valuable results. In fact, it is perhaps the most rational rupture test, and were it not for the difficulties involved in its application, it would doubtless come into general use.
150. One of the greatest difficulties experienced in making adhesive tests is the preparation of the specimens of stone or other material to which the mortar is to adhere. In early experiments common brick were used, or pieces of stone were cut to the same shape as brick, and two or more pieces cemented together. In later methods the flat surfaces of two specimens are sometimes joined with their axes at right angles, thus making the cemented surface square. The upper brick being held on two supports, a load is applied to the lower brick.
151. Mr. I. J. Mann, in a paper presented to the Institution of Civil Engineers,1 described a method of testing adhesion in which are used test pieces 1 1/2 inches long by 1 inch wide by 1/4 to 3/8 inch thick. These are cemented together in a cruciform shape, and a simple spring balance machine with properly arranged levers pulls them apart. The upper block is supported at its ends and an inverted U-shaped piece bears upon the ends of the lower block. The stress is applied through a conical shaped pivot bearing on the U-shaped saddle. Mr. Mann states that test pieces may be made either of plate glass or close grained limestone, the latter being sawn into pieces of the right size.
1 Proc. Inst. C. E., Vol. lxxi, p. 251.
152. Another method is to make test pieces to fit one end of the mold used for tensile tests, and after placing the piece of stone in the mold, to fill the other end with the mortar to be tested. The objection to this method is the expense of preparing pieces of this form. It has been suggested to substitute artificial stone for the cut stone samples. Thus, suppose it is required to test the adhesion of a certain mortar to granite: mold half briquets of a mixture of ground granite with cement, and after these have well hardened, replace them in the mold and fill the other end of the mold with the mortar to be used. It is quite certain that the same result would not be obtained in this way as though the specimens were cut from a piece of solid granite.
153. One of the simplest methods of applying this test is one which the author has used for some time. The test pieces are in the form of flat plates one inch square and one-fourth inch or less in thickness. These plates being placed in the center of a briquet mold, the ends of the mold are filled with mortar. The plates may be improved by cutting shallow grooves in two opposite sides to make a more perfect fit with the sides of the mold. This may easily be done with a round file. Besides the simple form of the test pieces and consequent ease of making them, this method has the further advantage that a test may be made almost as readily and accurately as a tensile test of cohesion. Also, since the adhesive area is one square inch, the results may be compared with cohesive tests on specimens having the same area of cross-section.
154. The experiments on adhesive strength made by Mr. Mann were probably more extensive than any others published. His results are useful mainly as showing the lack of cementitious properties in the coarser grains of cement, and this point he proves very clearly by quite a large number of experiments. It was also developed that cement that had been rendered slow setting by aeration or "cooling" gave a lower adhesive strength than samples directly from the makers, which set more rapidly. But the method followed by Mr. Mann, of immersing the specimens as soon as cemented together, may have had something to do with this result; the quicker setting samples would earlier resist the injurious action which is likely to follow the immersion of such small quantities of mortar before they have set.
155. All of the things which influence the results in testing the cohesive strength must also be considered as affecting the adhesive test. The consistency of the mortar, the method of gaging, the pressure applied in cementing the specimens, and the conditions of storage until the time of breaking, will all have an influence on the result obtained. In addition to these, the character of the samples as to the kind of stone used, its structure, the physical condition of the surfaces, etc., must all be considered. It is therefore clear that many difficulties must be met before the test for adhesion can ever be included in standard tests.
156. Special tests directed toward ascertaining the comparative adhesion of cement to different varieties of stone, the effect of the various differences in manipulation, the comparative adhesion of mortars containing various proportions of sand, etc., are of undoubted value. But, before the adhesive test can be considered a normal one for cement, much of this experimental work will be required.
The results of a number of adhesive tests made under the author's direction are given in Art. 51.
Abrasion tests of cement are not at all common, and for the ordinary uses to which cement is put, its resistance to such action is of little interest except as it may imply other kinds of strength. Occasionally, however, it may be desired to have a mortar which will withstand wear, as, for instance, in making concrete walk. In such cases, tests for resistance to abrasion have some interest and value.
The test is usually made on a sample prepared as for tensile or compressive tests, by submitting it to the wearing action of an emery or grindstone, or a cast iron disc covered with sand. The number of revolutions of the stone or disc is recorded, automatically if possible, and the loss of weight is determined after a given number of revolutions.
A few tests of this kind made at St. Mary's Falls Canal are given in Art. 58.