The bricks were quite strong, averaging from 13,000 to 15,000 lb. per sq. in. tested flatwise between steel. The piers were built by a common mason, with only ordinary care; and they were from a year and a half to two years old when tested. Their strength varied with their height; and in a general way the experiments show that the strength of a prism 10 ft. high, laid in either lime or cement mortar, is about two thirds that of a 1-foot cube. A deduction de rived from so few experiments (22 in all) is not, however, conclusive. The different lengths of the piers tested occurred in about equal numbers. The piers began to show cracks at one half to two thirds of their ultimate strength. The mortar was tested when fourteen months old.
Unfortunately the mortar was very poor, being but a little stronger in compression than such mortar should have been in tension. "The cement was purchased in the open market, and was not tested." On account of the poor mortar the results are less than those given on subsequent pages for younger masonry; but the results in Table 53 are of interest as showing the strength that may be obtained in actual practice unless the utmost care is taken.
In interpreting tests of the strength of brick masonry, it is con venient to remember that the best brick-work weighs about 144 lb.'per cu. ft., and that therefore each foot in height of a brick wall gives a pressure at its base of one pound per square inch. On this basis the masonry in the first line of Table 53 failed under a com pression equivalent to that of a prismatic wall 1,551 feet high—more than a quarter of a mile.
Later tests with the Watertown machine gave results as in Table 54. The tests of the bricks used in these experiments are reported in Table 8, page 42.
Notice in Table 54 that in several cases the strength at one month is greater than that at six months. The only explanation is that the anomaly is due to undetected variations in making and testing the piers. A considerable variation in the results is one of the characteristics of tests on brick and brick masonry.
The highest strength of brick masonry tested at Watertown preceding June 30, 1907, was 5,608 lb. per sq. in., for a pier 12 inches square consisting of hard-burned common brick laid in neat portland mortar, tested when seven days old. A number of piers have stood more than 4,000 lb. per sq. in.
Cornell Tests. Six experiments made at Cornell University t on piers 13 inches square (one and a half brick), laid in 1 : 2 portland cement mortar, gave an average crushing strength of 821 lb. per sq. in. for brick having a crushing strength of 3,270 lb. per sq. in. when tested flatwise with plastered surfaces, the average strength of the masonry being 0.25 of that of the brick. The same series of experiments gave 894 lb. per sq. in. for brick having a crushing strength of 3,750 lb. per sq. in., the average strength of masonry being 0.23 of the strength of the brick. The piers varied from eleven to thirty-four courses high, but the difference in height'seemed to make no appreciable difference in the strength. The thickness of the mortar joints ranged from 0.22 to 0.28 inch.
Toronto Tests. Nine tests made in Toronto, Canada,* on piers composed of common brick having a crushing strength flatwise from 1,222 to 5,372 lb. per sq. in., laid in lime mortar, gave an average crushing strength of 339 lb. per sq. in. when 2f months old. The average strength of the masonry was 0.16 of the strength of the brick.
University of Illinois Tests. Table 55 shows the results of the tests of fourteen brick piers made at the University of Illinois: The piers were 12i inches square, and practically 10 feet high. With the vitrified shale brick the thickness of the joints varied from 0.30 to 0.40 inch, and with the under-burned surface-clay brick from 0.44 to 0.46 inch. All of the piers were forty-three courses high, except two which were forty and three which were forty-two. The age when tested varied from 62 to 69 days.