Better Concrete

Progress, Volume XI, Issue 10, 1 June 1916, Page 638

Better Concrete

WHAT THE KINEMATOGRAPH SHOWS A Motion Picture Analysis of the flow on concrete from the mixers used in combining its elements shows that those elements are largely unmixed by the time they reach their destination, owing to the influence of gravity, under which the liquid part separates from the solid and the finer ingredients from the coarser. This is explained by Nathan C. Johnson, a consulting concrete-engineer of New York City, in “The Engineering iiecord.” Mr. Johnson does not say that this partial undoing of the mixer’s work is producing unsafe results, but he hints it very strongly and believes that the motionpicture analysis warns us of the necessity of improving the present methods of mixing and pouring concrete.

The pictures show that a liquid runs first into the forms, carrying dirt and scum and light particles "a poor bond," as Mr. Johnson remarks. Then "After this first rush of dirty fluid comes the gray of the finer cement and sand, in a sort of soup that constantly thickens with increasing quantities of coarser cement and sand, as the pouring goes on. Next come the heavier sand and finer sione, sometimes washed nearly clean of cement; and at the bottom of the barrow lies the heavier stone, which, refusing to flow, has to be scraped out with shovels. All this is done in a few seconds— more than a minute after the mixture was delivered by the mixer, for the cost of placing must be kept lowyet the materials have become almost completely separated. A little more time, as in a longer haul to the forms, and this separation, or 'segregation,' might be complete.

“But what of the mass in the forms? How does it lie—in layers, as it went down? What of its endurance? And what of 'its strength? Is careful design useless? Or is the concrete being used in such bulk, regardless of initial cost or actual possibilities, that nothing would matter, save almost total lack of cement? The answer is that, consciously or unconsciously, “Everybody’s Doing It,” and it .is “Getting Away With it”; and if Luck turns thumbs down, and if something happens in the far-off future, there are so many inanimate things that can be blamed when the time comes that the risk is held to be negligible and unworthy of consideration compared to present profits.

"If the practises illustrated above occurred only in isolated and infrequent instances they would have little bearing on the general quality and endurance of concretes. Unfortunately, however, they are all too prevalent; and although their impropriety is recognised when pointed out, they are yet of general daily occurrence and will continue so until more general understanding is had of their economic fallacy.

"When this understanding becomes general, such procedure in the manufacture and placing of concrete will be no more tolerated by owner, engineer, or contractor than would the distribution of fireproducing chemicals throughout the structure of an inflammable building. Under certain favourable conditions the two might prove equally potent for trouble and damage.

"The ultimate loss falls on the owner. Cement is a wonderful servant, standing much abuse. Trouble will probably be some time in coming, and

by then the contractor will be paid and away. Nor has there been any normal obtuseness on the part of the latter. He has done his work probably as well as he knew how, in full accordance with average practise, and as carefully as his bid rate-per-yard has permitted with the making of a reasonable and fair profit.

"Obviously, the remedy lies first of all in better understanding of concretes, not on the part of those whose knowledge is beyond question, but on the part of the great army of every-day workers on whom dependence must so largely rest. If improved methods of mixing can be produced which will secure better results without increased cost, the problem is much simplified, but if this cannot be accomplished, there will have to be an economic readjustment all along the line, with the securing of better-quality concrete and a proportionate recompense to the owner in the quality and endurance of the structure he has purchased."