truth is that, while such treatment sometimes augments the strength, it often has the opposite effect. From the resume of an able paper by Prof. Duff A. Abrams entitled "Effect of Vibration, Jigging, and Pressure on Fresh Concrete" the author extracts the following:

The indications of the vibration and jigging tests should not be misinterpreted. The tests show that, after the concrete is properly placed, these methods of treatment do no good, and may be harmful, if too severe or too long continued. However, there can be no doubt of the value of such methods for getting concrete into place in intricate forms and around reinforcing bars. The tests are of value in showing that this is the only desirable function of such treatments.

The tests show that, with jigging, high strength may be secured with drier mixes than would otherwise be feasible. It is a matter of common experience that concrete of drier consistency (and consequently higher strength) can be placed by means of jigging or vibration than would be possible by the usual methods.

It is clear from these tests that if tamping, vibration, or pressure on fresh concrete is to be effective in increasing its strength three factors must be kept in mind.

(1) We must take advantage of the fact that with these methods the concrete can be placed and finished drier than with ordinary methods.

(2) Excess water which is brought to the surface must be removed.

(3) We must take advantage of the fact that aggregate of a coarser grading may be used when such methods are employed than would be practicable otherwise.

The advantages to be gained under (3) are due to the fact that, up to a certain point, a plastic mix can be secured with a smaller quantity of water, if the aggregate is as coarse as practicable. Unless these precautions are taken, tamping and vibration are of doubtful value.

Age of Cement

Prof. Abrams has lately proved that the ultimate strength of concrete in compression is a function of the age of the cement at time of mixing, the fresher it is the greater the strength. This is at variance with the idea which governed previously, viz., that cement is better for a little aging; but we are learning these days that many of our old ideas about cement and concrete were incorrect.

In Bulletin No. 6 of the Structural Materials Research Laboratory of Lewis Institute, Prof. Abrams presents a number of diagrams showing the percentages of loss of strength in compression for concrete by using cement, that had been stored from two to twenty-four months, as compared with that manufactured with perfectly fresh cement; and the amounts thereof are surprising. They average fifteen per cent for two months and fifty-five per cent for twenty-four months, with almost proportionate percentages for intermediate periods. However, the loss is not quite as bad as these figures would show, because the experiments prove that the concrete tends to recover its strength with age. For instance, when two-months-old cement is employed, the loss is nineteen per cent at one month and only eleven per cent at two years; and when two-year-old cement is used the loss is fifty-six per cent at one month and only forty-two per cent at one year.