Coucrete Mill Construction.

Coucrete Mill Construction.

In a recent discussion of the possible dangers incident to the use of reinforced concrete in mill constri ction, Leonard C. Wason, president of the Aberthaw Construction company, of Boston, was led to speak from his experience. He at once showed that, practically, all of the possible dangers are avoidable; that they are entailed by incorrect design or poor workmanship; and that there is nothing inherently dangerous in the use of reinforced concrete. So far as strength and structural features go, it has few limitations, discrimination in use being controlcd principally by its adaptability. Recognising the common im pression that almost anybody can mix and place concrete, Mr. Wason emphasised the vital importance of employing skilled men : First—Be cause the reinforcement might otherwise be improperly placed, perhaps, too high in the beams and girders, or in the top of the floor-slabs, thereby introducing serious sources of weakness To the ignorant laborer such misplacement would mean nothing; in fact, it is not an uncommon practice to spread the concrete out over the forms and lay out the bars on top. Under such condi tions, the laborer is more apt to spread the con crete thick than thin, because, if very thin, the stone in the concrete interferes in the spreading Second—Because of the errors which may arise through the selection of the wrong size of ban for reinforcement. It is a simple matter for 2 mail to overlook the difference of an eighth of an inch in diameter, which in small bars may make a difference of 50 per cent, or more in the strength. Sometimes the wrong number of bars may he used. Third—Because an unskilled or careless foreman may read his plans erro neouslv. Sometimes, also, he may set the bars at right angles to the correct position. In columns, these bars arc usually placed near the surface to avoid flexure; but it is easier to place the concrete, if they are nearer the middle. If nearer the middle, however, the flexure is not taken care of. and the tie between the floor and column is not so rigid. Mr, Wason referred to some of the failures of reinforced concrete which have been reported, and stated that, although usually sworn to as caused by the drawing of the form-work too soon, the fact usually is that the failure has been due really to improper setting of the steel and to careless placing of the concrete. In a failure which occurred in Philadelphia about two years ago, sawdust and shavings were found in the columns, where they had fallen before the concrete was placed. The effect was to reduce the cross-sectional area of the columns by fully 50 per cent. The difference in strength between tamped and untamped concrete was shown to be 30 per cent, —due largely to the fact that, under the latter conditions, it does not flow round the reinforcement. As a consequence, voids are formed. It was also pointed out that, when the concrete is mixed dry, it does not grip the bars properly. Mr. Wason asserted that it has been proved pretty conclusively by disinterested engineers, who have carefully examined such failures, that they were due to carelessness on the part of the contractor and his workmen, and that none of them was due to incorrect principles of design. In a word, the danger lies in hidden defects, which, once covered from sight, are revealed only by disaster.

No posts to display