43. In the tests of such structural materials as wood and steel it will not usually be difficult to determine the suitability of the material for the intended purpose, provided the test pieces truthfully represent the members to be used. It is known that so long as these members are protected from oxidation and over-loading they will retain their qualities, and there is always a reasonably clear understanding of what these qualities should be. On the other hand, in the testing of cement, one may be perfectly sure that from the moment the cement is manufactured until long after it has been in service in the structure its properties will be ever changing; and, further, the qualities which it is desirable the cement should possess are not always clearly in mind.
The desirable elements in a cement may be stated as follows: 1st, That when treated in the proposed manner it shall develop a certain strength at the end of a given period. 2d, That it shall contain no compounds within itself which may, at any future time, cause it to change its form or volume, or lose any of its previously acquired strength. 3d, That it shall be able to withstand the action of any exterior agency to which it may be subjected that would tend to decrease its strength or change its form or volume. When it is determined that a cement has these three qualities, it is certain that it is safe to use it, but it is further desirable to know that the cement in question will accomplish the given object as cheaply as any other cement.
The cohesive and adhesive strengths of cement are not usually considered in the design of the structure into which cement enters. The design of a masonry arch does not comprehend any adhesive strength in the cement, except as it may be recognized as an additional factor of safety, and a masonry dam is so designed that there shall be no tension at the heel. These facts are due in a large measure to the very imperfect knowledge we have of the behavior of cements in various contingencies. With the increasing use of concrete, as in arches, locks, floors, roofs, etc., the tensile and transverse strengths of cement are coming to be relied on to a certain extent; and as its properties become better known, and as means of recognizing these properties become more certain and widespread in their application, cement will be more extensively employed in a scientific and economical manner.
Cement may be compared in one sense to timber and cast iron. A large factor of safety is employed when dealing with these materials because of hidden defects that may exist. The defects which lie hidden in cement may be even greater than these in proportion to its possible strength, and defects in cement are often more treacherous because their development may be deferred for some time. The importance of knowing whether the cement fulfills the second and third requirements noted above is therefore evident.
45. Having considered the qualities a cement should have, we may proceed to the detailed consideration of the various tests employed to disclose the presence or absence of these qualities. The strength a given cement will develop is investigated by chemical analysis, by obtaining the specific gravity and fineness, and by actual rupture tests, whether they be tensile, compressive, transverse, or shearing. By tests for change of volume and by chemical analysis, it is sought to determine whether a cement has within itself elements of destruction. For the power to withstand external agencies there are no adequate tests, though chemical analysis is considered an aid. The methods of use, the proportions of the materials, their incorporation and deposition are of great importance in insuring against external causes of injury.
In order that uniformity should prevail in the methods employed in testing cements, various societies have discussed the subject in detail, usually through committees, and much valuable work has been done along this line. The engineers of public works in many European countries have adopted specifications and laid down more or less detailed rules for testing. The Corps of Engineers, U. S. A., has recently adopted a similar code of rules.
The International Society for Testing Materials, with which the American Society for Testing Materials is affiliated, has considered the subject and still has committees at work upon it. The New York section of the Society of Chemical Industry has recently formulated a method for analysis of materials for the Portland cement industry. The American Society of Civil Engineers received a report in 1885 from a committee appointed to consider methods of cement testing, and in order to keep the subject abreast of the latest developments in the manufacture and use of cement, a second committee was appointed several years ago, which has been making a thorough discussion of the subject, and has submitted a preliminary or progress report.
47. Notwithstanding that so much has been done toward unification of methods, it may never be possible to determine accurately the value of one cement as compared with another tested in a different laboratory; though in tests of iron and steel no such difficulty is experienced. Certainly, as at present carried out, strength tests of cement are purely relative tests and do not show the absolute strength which may be developed in the structures; nor can the results be compared with the results obtained in other laboratories and any fine distinctions of quality drawn. To attempt to carry out acceptance tests in such a way as to show directly the strength which will be developed in actual construction, is only to introduce causes of irregularity in the tests.