Methods O Testing Diinability

METHODS O! TESTING DIINABILITY. It has long been recog nized that there are two ways in which a judgment can be formed of the durability of a building stone, and these may be distinguished as natural and artificial.

Natural Methods.

The natural methods must always take the precedence whenever they can be used, because they involve (1) the exact agencies concerned in the atmospheric attack upon stone, and (2) long periods of time far beyond the reach of artificial experiment.

A study of the surfaces of old buildings, bridge piers, monuments, tombstones, etc., which have been exposed to atmospheric influences for years, is one of the best sources of reliable information concerning the durability of stone. A durable stone will retain the tool marks made in working it, and preserve its edges and corners sharp and true.

Another method is to visit the quarry and observe whether the ledges that have been exposed to the weather are deeply corroded, or whether these old surfaces are still fresh. , In applying this test, consideration must be given to the modifying effect of geological phenomena. It has been pointed out that "the length of time the ledges have been exposed, and the changes of actions to which they may have been subjected during long geological periods, are unknown; and since different quarries may not have been exposed to the same action, they do not always afford definite data for reliable compara tive estimates of durability, except where different specimens occur in the same quarry." North of the glacial limit, all the products of decomposition have been planed away and deposited as drift formation over the length and breadth of the land. The rocks are therefore, in general, quite fresh in appearance, and possess only a slight depth of cap or worthless rock. The same classes of rock, however, in the South are covered with rotten products from long ages of atmospheric action.

Artificial Methods of Testing Durability.

The older artificial methods of determining durability were based upon the assumption that the relative durability of stones is proportional to their crush ing strength, their absorptive power, their resistance to freezing, and their solubility in acids; but it is now known that stones differ widely in durability which do not differ much if any in one or more of the above respects, and consequently the determination of the above elements for several building stones is only an approximate method of determining their durability. In making the experiments

each element acts by itself, while in the structure the stone is exposed to the combined action of all the methods of attack; and their action may be very different separately than simultaneously.

In recent years it has been claimed that the best artificial method of determining the probable durability of a stone was to study its surface or a thin, transparent slice under the microscope. Undoubt edly this method gives valuable information, but it still remains true that in the present state of our knowledge the methods of deter mining the durability of building stones by laboratory tests are unsatisfactory and the results are unreliable. The object sought is important, and therefore the difficulties should stimulate greater care in making the experiments and in interpreting the results.

In a general way the weight and crushing strength throw some light upon the durability of a stone.

The heavier a stone the more dense it is, and, other things being the same, the more durable it is; but to this there are some exceptions. The more dense it is, the less water it will absorb, and hence it is less likely to be affected by frost and the acids of the atmosphere. The 'weights of the several classes of stones are given in Table 1, page 6.

As a rule the stronger stones are the more durable, but there are numerous and sometimes marked exceptions to this rule. The crushing strength of building stones is given in Table 2, page 11.