Brick No. 15.—in this brick the amount of burning seems to affect the quality very much and it is difficult to say just what range of absorption is allowable. 3%% absorption accompanies a fairly good brick, but variations on either side of this are very detrimental to the quality. The crushing and cross breaking tests for this brick are low. All of these conditions are indi cations of an undesirable brick for use as they would be delivered on the street.
Brick No. 14.—This brick has low cross breaking and high absorption. The samples tested do not indicate a first class brick.
Brick No. 16.—This brick permits a wide range of burning without much change in its quality.
The results of the absorption test show that there is generally little or no difference in the amount of water absorbed in overburned brick and well burned brick. This agrees, of course, with our knowledge of the change which takes place at vitrification. The amount of variation in absorption between brick of different degrees of harness (of the same make) which show practically the same good wearing quality by all the other tests is of interest. A favorable or wide range of absorption for the same wearing qualities must be considered advantageous to the manufacturer and also to the consumer, both by reason of the wider latitude allowed in burning and also upon the ease of inspection on the street. Other brick like No. 10 give a considerable difference in appearance with only a slight change in the qualities of the absorption test and without any marked change in the wearing qualities of the brick. The absorption test appears to be of value in studying a given make of brick or in learning of its properties and giving information bearing upon the inspection of the brick delivered on the street. For any given make of brick the specific range of absorption which will give a good article may be determined and required.
The results show that generally the rattler test made a fair determin ation of the quality of the brick, if we may judge by the appearance of the brick, the results of other tests, and the reputation of the brick. In some cases the rattler test gave a rank better than would be given by the character and appearance of the brick and by the results of the other tests. A few of the makes showed rather high rattler loss and gave
a fairly good modulus of rupture and cross-breaking strength and uniformity, and some of these brick are reported to have given excellent service under light traffic. Brick 17, 18, 19, and 20 are in this class. The range of difference between that of a single test and the mean of the duplicate rattler tests averaged from about .5 to 1% for the better grades of brick, although in one case the variation was as high as 1.8% from the mean. The variations are smaller than is usual in the rattler test, and attest the care in selecting the brick. The value of the crush ing strength was generally between three and four times the modulus of rupture. There was a fairly close agreement between these two tests. A high value in one test was accompanied by increased values in the other test. Generally speaking, it may be said that a value of 2,500 pounds per square inch for the modulus of rupture and 7,500 pounds per square inch for the crushing strength may be expected in first class paving brick. Lower values like 2,000 and 6,000 pounds respectively, are not especially objectionable. In the cross-breaking test the variation in the values for individual bricks is of interest and in some respects this variation may be considered a measure of the uniformity of the brick. As already stated, the numbers given with the cross-breaking test in Fig. 33 show the average range of variation in the modulus of rupture for individual brick from the average modulus for the given grade expressed in per cent of the mean modulus of rupture. In other words, a range of 10 per cent means that if the difference between the moclulus of rupture for each individual brick and the average modulus for that grade be expressed in per cents of this average modulus, the average of the re sults for the given grade of brick will be 10 per cent. It will be noted that for the better grades of brick this range is within 12 per cent. Attention is called to the much greater variation in bricks No. 13, 15, 16 and 17. Since uniformity of quality in a lot of brick is of considerable import ance a test of this kind may be used to rate different makes of brick on the score of uniformity.