It has, however, often been suggested that, especially in the case of a heavy slow-setting cement, seven days is too short a period for its properties fully to develop, and a period of thirty days has been suggested as one which would give the cement a fairer trial.
There would be a practical disadvantage in having to keep a consignment of cement thirty days before it is accepted or rejected, otherwise the longer period might be preferable. If it were adopted, of course a far higher tensile test would be necessary.
Mr. Grant says that cement required to bear 350 lbs. per square inch after seven days should bear 450 lbs. after thirty days.
It has also been suggested that as the cement has generally to be used in the form of mortar, it should be mixed with sand before being tested. Such, however, is not the common practice in this country.
It has, however, frequently been pointed out by Mr. Grant, Mr. Colson, and others, that as Portland cement is nearly always used with a mixture of sand, it would be better to test it, as far as possible, under the condition in which it is used in practice, as it is thus that its probable behaviour when in use can best be ascertained.
The Germans have for some time made their tests on briquettes of 1 Portland cement and 3 sand after twenty-eight days' setting.
It is found that the testing of neat cement forms but little guide to their behaviour when mixed with sand, thus, "coarsely ground cement will, as a rule, give somewhat higher results when tested neat than finely ground, but when mixed with sand, say in the proportion of 1 to 3, the superiority of the finely ground cement becomes apparent."1
Sand, however, retards the hardening, and it is found that briquettes formed of 1 Portland cement to 3 sand must be left at least twenty-eight days before being tested.
It is impossible to compare tests made with mixtures of cement and sand, unless the sand is always of exactly uniform composition and quality as regards size, sharpness, and surface of grains, degree of dampness, etc., and sand so uniform in quality would be very difficult to obtain.
The practical difficulties involved in testing cement mixed with sand have prevented it from being universally adopted; there can be no doubt, however, that for large works where ample storage exists and sand of uniform quality can be ensured, more information about the future behaviour of the cement can be obtained by this test than by testing the cement neat.
Again it has been pointed out that cement when in actual use is generally subjected to compression - very rarely to tension - and that it would be more useful to test its resistance to a compression than to a tensile stress.
The apparatus for testing cement by compression is, however, cumbersome and expensive, and tests of compressive strength are never specified by engineers.
Many experiments have, nevertheless, been made on the resistance of Portland cement to compression. Mr. Grant found the measure of the "compressive strength to be about twenty times that of the tensile strength,"2 but Herr Bauschinger considers that "there is no fixed relation between the crushing and tensile strength of cement." 2
1 Grant, M.P.I.C.E. 1880, vol. Ixii. p. 104. 2 M.P.I.C.E., vol. Ixii. pp. 108, 208.