ABSORPTION TEST.
There has been a change of view in reference to the value, applicability. and purpose of the absorption test. In the early experience with brick pavement, soft and porous brick were used and the fear was expressed that the brick would crumble and disintegrate under the effect of a re peated freezing and thawing, and an absorption test with an arbitrary limit was included in the specifications. This test was used without full information of the properties of the brick and frequently without good judgment. The experience of years and tests made by repeatedly freez ing and thawing bricks have established the fact that the action of freezing and thawing is not likely to disintegrate brick of a high grade which will pass the requirements of other tests. This statement should not be interpreted to mean that the action of frost and traffic together will not cause disintegration of brick which, when dry and cold, would resist the wear of the traffic fairly well. The improper use of the ab sorption test resulted in an indiscriminate condemnation of it and also in a lack of appreciation of its value and usefulness as an auxiliary test and as a means for studying properties of the brick. The absorption test is a valuable adjunct for use in interpreting the results of the rattler and cross breaking tests and in studying the peculiarities of the particular make of brick which will be put into a pavement.
A good paving brick will absorb water quite slowly, the rate of absorption varying from hour to hour. Fig. 1 shows the rate of absorption through the period of some days, as given by Mr. F. F. Har rington. If the outside of the brick is more dense than the interior the rate of absorption is still slower. A broken brick or a rattled brick will absorb water more readily than whole brick for this reason, and such brick should be selected for the test. In some tests the brick have been partially submerged for some time to allow the escape of air. The absorption of water is more rapid in the beginning, is quite slow after 24 hours, and, still slower after 48 hours. The absolute value of the absorption power is not required, and for comparative purposes the result at the end of 24 hours, or better, at the end of 48 hours, will be sufficient. Brick which absorb but a small part of their final amount
are usually so dense that the total absorption would be very small and the variation in value for such brick will not affect comparisons. Since brick in their usual condition contain some moisture, the sample should be dried for several hours at a temperature at or above the boiling point of water. The method given below requires 48 hours, but this protracted period seems unnecessary for ordinary purposes.
The absorption test should be conducted under the following condi tions: The test will be made on five brick which have been exposed to the action of the rattler, or if these are not available, on five brick which have been broken into halves. The brick shall be dried at a temperature of 200° to 300° F. for 24 hours and then after weighing shall be immersed in water for 48 hours. Before reweighing the brick, surplus water shall be wiped from its surface. The absorption shall be expressed in per cents of the dry weight of the brick.
The idea that low absorption is a guaranty of excellency of the wear ing qualities of paving brick was held by engineers for many years. As brick are burned in the kiln the amount of their porosity becomes less and less until a point is reached when another change occurs and further burning will not decrease the porosity. The absorption test may determine or distinguish underburned brick, but overburned brick may not give a test much different from brick which have received the best degree of burning. The best limits for absorption will vary with the clay and method of manufacture and will have to be determined for every make of brick. This determination may be made by comparison with the results of other tests and by experience with the brick. In other words, no general limits can be placed for the absorption test, but special limits may be specified for particular makes of bricks used in any city. For a given brick, then, it may be said that the absorption test is able to distinguish underburned brick, and that it will be helpful in determining the length of burning permissible with a given grade and make of brick.