LIME MORTARS Brickwork endures only according to the proportional strength and quality of the bricks of which it is composed, the mortar that binds them, and the manner of the bonding. It has often beeu demonstrated that failure in brick wall construction is rarely due to the weakness of the brick, hut to that of the mortar. One of the most striking demonstrations of the relative strength of brick and mortar is afforded in a record of tests of arches made by the Austrian Association of Engineers and Architects during 1900-1905. The exceptional conditions under which the tests were made are clearly presented in the following extract from "Engineering" (London, England, 1900 : "The long arches tested were 6.65 feet wide, and 74.5 feet span, with a rise one-fifth the span. They were built in a quarry, between solid rock walls so that all deflections were due to the effect of the load on the arch. A platform supported on six sets of columns. the feet of which rested directly on the extrados of the arch, extended in each case from one abutment to the crown ; and the testing was effected by piling rails on this platform.
The first experiments were made upon an arch of cut stone, and upon one of brick. The stone used was a fairly hard limestone of excellent quality. The voussoirs of both arches were 1.97 feet thick at the crown, and 3.6 feet deep at the springings. The mortar used was mixed in the proportion of 500 pounds of Portland cement to 15 cubic feet of clean sand.
"The stone arch gave way when the load piled ou the platform reached au amount equivalent to 1.99 tons per foot run; and the brick arch, when the load reached 1.81 tons per foot run. Up to the point of rupture, the stone arch gave no signs of incipient failure; but, in the case of the brick arch, cracks declared themselves previously, which were apparently caused by the failure of the mortar, the bricks themselves being intact.
"The arches were 51 (lays old when they were broken. In failing, radial cracks appeared on the extrados of the loaded side, near the skewbaek, and on the haunch of the unloaded side.
"The conclusions were that such masonry arches will safely bear a load causing a stress of from to the ultimate crushing strength." A knowledge of the essential qualities of good lime mortars will be of great assistance to the builder of brick structures. The main function of mortar in brickwork is to unite the brick into one substantial mass; and in so doing, all the crevices between the brick will necessarily be filled up.
Secondarily, the mortar acts as a cushion to take up all the inequalities of the brick-surface. The strength of the mortar is therefore the prime consideration; the filling of the crevices to keep out air and water is a matter of good workmanship; the thiekness of the joint determines the effectiveness of the cushioning and pressure-distribution.
All the limes come, of course, from limestone; hut, according to the purity of the limestone, we obtain by calcination—common lime, hydraulic lime, and "natural" hydraulic cement. When the limestone is nearly pure carbonate of lime, common lime is the product. If the limestone contains from 10 to 20 per cent of clay, hydraulic lime results from calcining. With from 20 to 23 per cent of clay, the limestone yields a "natural" hydraulic cement.
Common lime, or "air-lime" requires air-exposure in order that it may set or harden. It will not harden under water. Hydraulic lime and cement will both harden under water.
Common lime is the result of the burning of a pure or nearly pure limestone, in which pr4acess the carbon dioxide is driven off and calcium oxide remains. When water is applied to fresh lime, it is absorbed with avidity. The absorption is attended by great heat, steaming, and the breaking-down of the lime mass. If the slaking—as the work of absorption is called—is carried on perfectly, all the lime is reduced to a fine powder, with a volume two or three Hines that of the original lime, the increase of hulk being proportional to the limestone's purity.