one of sienite about twenty-nine thousand pounds to the square inch.-Report of the Architect of Public Buildings, Dec. 1, 1852. 332. Rondelet, from a numerous series of experiments on the same subject, published in his work, Art de Bâtir, has arrived at like conclusions with regard to the relations between the specific gravity and strength of stones belonging to the same class. Among the results of the more recent experiments on this subject, those obtained by Mr. Hodgkinson, showing the relation between the crushing, the tensile, and the transverse strength of stone, have already been given. M. Vicat, in a memoir on the same subject, published in the Annales des Ponts et Chaussées, 1833, has arrived at an opposite conclusion from Mr. Hodgkinson, stating as the results of his experiments, that no constant relation exists between the crushing and tensile strength of stone in general, and that there is no other means of determining these two forces but by direct experiment in each case. 333. The influence of form on the strength of stone, and the circumstances attending the rupture of hard and soft stones, have been made the subject of particular experiments by Rondelet and Vicat. Their experiments agree in establishing the points that the crushing weight is in proportion to the area of the base. Vicat states, more generally, that the permanent weights borne by similar solids of stone, under like circumstances, will be as the squares of their homologous sides. These two authors agree on the point that the circular form of the base is the most favorable to strength. They differ on most other points, and particularly on the manner in which the different kinds of stone yield by rupture. 334. Practical Deductions. Were stones placed under the same circumstances in structures as in the experiments made to ascertain their strength, there would be no difficulty in. assigning the fractional part of the weight termed the working strain or working load which, in the comparatively short period usually given to an experiment, will crush them, could: be borne by them permanently with safety. But, independently of the accidental causes of destruction to which structures are exposed, imperfections in the material itself, as well as careless workmanship, from which it is often placed in the most unfavorable circumstances of resistance, require to be guarded against. M. Vicat, in the memoir before mentioned, states that a permanent strain of of the crushing force of experiment may be borne by stone without danger of impair ing its cohesive strength, provided it be placed under the most favorable circumstances of resistance. This fraction of the crushing weight of experiment is greater than ordinary circumstances would justify, and it is recommended in practice not to submit any stone to a greater permanent strain than one-tenth of the crushing weight of experiments made on small cubes measuring about two inches on an edge. Other authorities state that cut stone in cases like the voussoirs of arches and stone pillars should not be subjected to a working strain greater than th of the crushing weight of experiment. The following table shows the permanent strain, and crushing weight, for a square foot of the stones in some of the most remarkable structures in Europe. Pillars of the dome of St. Peter's (Rome).... Do. St. Paul's (London). St. Geneviève (Paris). Do. Church of Toussaint (Angers).. The stone employed in all the structures enumerated in the Table, is some variety of limestone. 335. Expansion of Stone from Increase of Temperature. Experiments have been made in this country by Prof. Bartlett, and in England by Mr. Adie, to ascertain the expansion of stone for every degree of Fahrenheit. The experiments of Prof. Bartlett give the following results: Granite expands for every degree. Sandstone. .000004825 .000005668 .000009532 Table of the Expansion of Stone, etc., from the Experiments of Alexander J. Adie, Civil Engineer, Edinburgh. 336. Strength of Mortars. A very wide range of experiments has been made, within a few years back, by engineers both at home and abroad, upon the resistance offered by mortars to a transversal strain, with a view to compare their qualities, both as regards their constituent elements and the processes followed in their manipulation. As might naturally have been anticipated, these experiments have presented very diversified, and in many instances, contradictory results. The general conclusions, however, drawn from them, have been nearly the same in the majority of cases; and they furnish the engineer with the most reliable guides in this important branch of his art. 337. The usual method of conducting these experiments has been to subject small rectangular prisms of mortas, resting on points of support at their extremities, to a transversal strain applied at the centre point between the bearings. This, perhaps, is as unexceptionable and convenient a method as can be followed for testing the comparative strength of mortars. 338. M. Vicat, in the work already cited, gives the following as the average resistances on the square inch offered by mortars to a force of traction; the deductions being drawn from experiments on the resistance to a transversal strain. Mortars of very strong hydraulic lime... .170 pounds. 40 10 These experiments were made upon prisms a year old, which had been exposed to the ordinary changes of weather. With regard to the best hydraulic mortars of the same age which had been, during that same period, either immersed in water, or buried in a damp position, M. Vicat states that their average tenacity may be estimated at 140 pounds on the square inch. 339. General Treussart, in his work on hydraulic and common mortars, has given in detail a large number of experiments on the transversal strength of artifical hydraulic mortars, made by submitting small rectangular parallelopipeds of mortar, six inches in length and two inches square, to a transversal strain applied at the centre point between the bearings, which were four inches apart. From these experiments he deduces the following practical conclusions. That when the parallelopipeds sustain a transversal strain varying between 220 and 330 pounds, the corresponding mortar will be suitable for common gross masonry; but that for important hydraulic works the parallelopipeds should sustain, before yielding, from 330 to 440 pounds. 340. The only published experiments on this subject made in this country are those of Colonel Totten, appended to his translation of General Treussart's work. The results of these experiments are of peculiar value to the American engineer, as they were made upon materials in very general use on the public works throughout the country. From these experiments Colonel Totten deduces the following general results: 1st. That mortar of hydraulic cement and sand is the stronger and harder as the quality of sand is less. 2d. That common mortar is the stronger and harder as the quantity of sand is less. 3d. That any addition of common lime to a mortar of hydraulic cement and sand weakens the mortar, but that a little lime may be added without any considerable diminution of the strength of the mortar, and with a saving of expense. 4th. The strength of common mortars is considerably improved by the addition of an artificial puzzolana, but more so by the addition of an hydraulic cement. 5th. Fine sand generally gives a stronger mortar than coarse sand. 6th. Lime slaked by sprinkling gave better results than lime slaked by drowning. A few experiments made on airslaked lime were unfavorable to that mode of slaking. 7th. Both hydraulic and common mortar yielded better results when made with a small quantity of water than when made thin. 8th. Mortar made in the mortar-mill was found to be superior to that mixed in the usual way with a hoe. 9th. Fresh water gave better results than salt water. 341. Strength and Other Properties of Portland Cement. From experiments made in England by Mr. Grant on the resistance to crushing of blocks of Portland cement, and of Portland cement mortars, the following results are deduced. 1st. The strength of the blocks in both cases increased with time. The blocks of pure cement bearing respectively nearly 4,000 lbs. on the square inch after three months; over 5,000 lbs. at six months; and nearly 6,000 lbs. at nine months. 2d. The strength of the blocks of mortar also increased with time; but decreased as the volume of sand used was increased. The blocks made with one volume of sand to one of cement bore about 2,500 lbs. on the square inch, and those made of six volumes of sand to one of cement 959 lbs. at the end of three months; whilst those made of one volume of sand to one of cement bore 4,561 lbs. on the square inch at the end of nine months, and those made of six volumes of sand to one of cement bore 1,678 lbs. on the square inch at the end of the same period. From numerous experiments made by Mr. Grant in England, on Portland cement, he draws the following conclusions: 1st. Portland cement, if it be preserved from moisture, does not, like Roman cement, lose its strength by being kept in casks or sacks, but rather improves by age. 2d. The longer it is in setting, the more its strength in creases. 3d. Very strong Portland cement is heavy, of a blue-gray color, and sets slowly. Quick setting cement has, generally, too large a portion of clay in its composition, is brownish in color, and turns out weak if not useless. 4th. The less the amount of water in working the cement up the better. |