134 BULLKTIX 86 



ABSORPTION TESTS 



The absorption is determined in the following manner: A piece 

 of concrete, weighing from one to two pounds, is broken out of each 

 pipe to be tested. These test pieces are dried in an oven at a tem- 

 perature of 110^ F. for at least seven days, and are then weighed. 

 They are then soaked in water for three days and weighed again. 

 The gain in weight, expressed as a percentage of the dry weight, is 

 called the absorption. 



The absorption is a measure of the watertightness of concrete. 

 The test is especially important if the pipe is to be used under con- 

 siderable water pressure or in alkali soil. With clay pipe, however, 

 the test is not a positive indication of perviousness. If the pipe is 

 thoroughly vitrified the absorption may be low and yet the pipe 

 may be pervious. 



Absorption is a measure, also, of the density, and the density of 

 concrete is an index of its strength. Given the ingredients and the 

 proportions, the strength depends directly upon how densely the 

 concrete is packed. It is possible to tamp pipe by hand so thor- 

 oughly as to give it great density ; much depends on the man who 

 does the tamping and on the rate at which the mortar is fed into the 

 molds. It is likely, even, that pipe made in the morning will be 

 denser than that made in the afternoon, and different parts of the 

 same pipe must vary in density. On the other hand, the density 

 of machine-made pipe is very uniform. 



The results of the tests for absorption are placed in Table XI 

 in order to study the effect of the porosity upon the strength. The 

 absorption for the machine-made pipe varies from 5.15 to 6 58 per- 

 cent, average 5.7 per cent. The absorption for the hand-made pipe 

 is shown to be considerably higher. 



Test pieces from pipe Nos. 4 to 7 of Table XI were boiled three 

 hours and then were left in the water for three days before weigh- 

 ing. The average absorption was 7.6 per cent. The figures are not 

 placed in the table because they are not comparable with the others. 

 The tentative specifications adopted recently by the American So- 

 ciety for Testing Materials for sewer pipe require that, after thor- 

 ough drying at a temperature not less than 110°, the specimens shall 

 be kept in boiling water five hours. Doubtless this method of mak- 

 ing the absorption test will come into general use. Boiling must 

 result in expelling all the air from the test specimens, but the 

 acceleration of the hydration of the cement is likely to make the 

 porosity appear to be greater than it is. 



