This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
6. Of the above four qualities, only two - durability and strength - are susceptible of laboratory testing, and even for these qualities the best known laboratory tests are not conclusive. The deterioration and partial failure of the masonry in some of the best known cathedrals of Europe, which commanded the best available talent in their construction, are startling illustrations of the impracticability of determining from laboratory tests the effect on stone of long-continued stress, combined perhaps with other destructive influences. Although the best technical advice was obtained in selecting the stone for the Parliament House in London, and the stone selected was undoubtedly subjected to the best known tests, it was apparently impossible to foresee the effect of the London atmosphere, which is now so seriously affecting the stone. Several of the tests to be described below should be considered as being negative tests. If the stones fail under these tests, they are probably inferior; if they do not fail, they are perhaps safe, but there is no certainty. A long experience, based on a knowledge of the characteristics of stones which have proven successful, is of far greater value than a dependence on the results of laboratory tests. The tests attempt to simulate the actual destructive agencies as far as possible, but since a great deal of stonework which was apparently satisfactory when constructed and for a few years after, has failed for a variety of reasons, attempts are made to use accelerated tests, which are supposed by their concentration to affect the stone in a few minutes or hours as much as the milder causes acting through a long period of years.
It is generally said that stones having the least absorption are the best. The absorptive power is measured by first drying the stone for many hours in an oven, weighing it, then soaking it for, say, 24 hours, and again weighing it. The increase in the weight of the soaked stone (due to the weight of water absorbed), divided by the weight of the dry stone, equals the ratio of absorption. The granites will absorb as an average value a weight of water equal to about 1/750 of the weight of the stone. For sandstone the ratio is about 1/24.
The test for absorption has but little value except to indicate a closeness of grain (or the lack of it), which probably indicates something about the strength of the stone, as well as its liability to some kinds of disintegration.
The only real test is to wash, dry, and weigh test specimens, very carefully; then soak them in water, and expose them to intensely cold and intensely warm temperatures alternately. Finally wash, dry, and weigh them. If the freezing has resulted in breaking off small pieces, or possibly in fracturing the stone, the loss in weight or the breakage will give a measure of the effect of cold winters. However, as such low temperatures cannot be produced artificially except at considerable expense, and as a sufficient degree of cold is ordinarily unobtainable when desired, such a test is usually impracticable.
An attempt to simulate such an effect by boiling the specimen in a concentrated solution of sulphate of soda and observing the subsequent disintegration of the stone, if any, is known as Brard's test. Although this method is much used for lack of a better, its value is doubtful and perhaps deceptive, since the effect is largely chemical rather than mechanical. The destructive effect on the stone is usually greater than that of freezing, and might result in condemning a really good stone.
The most difficult and uncertain matter to determine is the probable effect of the acids in the atmosphere.
These acids, dissolved in rain water, soak into the stone and combine with any earthy matter in the stone, which then leaches out, leaving small cavities. This not only results in a partial disintegration of the stone, but also facilitates destruction by freezing. If the stone specimen, after being carefully washed, is soaked for several days in a one per cent solution of sulphuric and hydrochloric acid, the liquid being frequently shaken, the water will become somewhat muddy if there is an appreciable amount of earthy matter in the stone. Such an effect is supposed to indicate the probable action of a vitiated atmosphere. Of course it should be remembered that such a consideration is important only for a structure in a crowded city where the atmosphere is vitiated by poisonous gases discharged from factories and from all chimneys.
A test made by crushing a block of stone in a testing machine is apparently a very simple and conclusive test, but in reality the results are apt to be inconclusive and even deceptive. This is due to the following reasons, among others:
(a) The crushing strength of a cube per square inch is far less than that of a slab having considerably greater length and width than height.
(b) The result of a test depends very largely on the preparation of the specimen. If sawed, the strength will be greater than if cut by chipping. If the upper and lower faces are not truly parallel, so that there is a concentration of pressure on one corner, the apparent result will be less.
(c) The result depends on the imbedment. Specimens which are rubbed and ground with machines that will insure truly parallel and plane surfaces, will give higher results than when wood, lead, leather, or plaster-of-paris cushions are employed.
(d) The strength of masonry depends largely on the crushing strength of the mortar used and the thickness of the joints. Other things being equal, an increase in the crushing strength of the stone (or brick) which is used does not add proportionately to the strength of the masonry as a whole; and if the mortar joints are very thick, it adds little or nothing. Since the strength of the masonry is the only real criterion, the strength of a cube of the stone is of comparatively little importance.
In short, tests of two-inch cubes (the size usually employed) are valuable chiefly in comparing the strength of two or more different kinds of stones, all of which are tested under precisely similar conditions. A comparison of such figures with the figures obtained by others will have but little value unless the precise conditions of the other tests are accurately known. Under any conditions, the results of the tests will bear but little relation to the actual strength of the masonry to be built.
These are generally the surest tests, and should never be neglected if the choice of stone is a matter of great importance. Field stone and outcropping rock which have withstood the weather for indefinite periods of years, can usually be relied on as being durable against all deterioration except that due to acids in the atmosphere, to which they probably have not been subjected in the country as they might be in a city. On the other hand, however, large blocks of stone can seldom be obtained from field stones. If a quarry has been opened for several years, a comparison of the other surfaces with those just exposed may indicate the possible disintegrating or discoloring effects of the atmosphere. A stone which is dense and of uniform structure, and which will not disintegrate, may be relied on to withstand any physical stress to which masonry should be subjected.