CONCRETE STANDPIPE AT ATTLEBORO
When the Attleboro standpipe was contemplated, it was absolutely necessary that some steps should be taken for better fire and domestic protection, as we had only one force-main from the pumping station to the town, a distance of nearly four miles. Under existing conditions, direct pumping was necessary at every alarm of fire. Our storage consisted of a wrought iron standpipe 30 ft. in diameter and 125 ft. high, with a total capacity of 661,000 gals.—that being about the daily consumption. The top of this standpipe is at an elevation of 247 ft., about 142 ft. above the ground at the corner of County and Park streets, that being representative of our business section of the town. The iron standpipe was erected in 1890, and although the Attleborough water is one of the best in the State for both domestic and manufacturing purposes, it contains carbon dioxide, so that it attacks unprotected wrought iron or steel, but has very little effect upon cast iron. These were the conditions of our storage system in 1904. On March 16, 1904, they were clearly presented to the commissioners when a fire occurred. The alarm was rung in at 5.40 a. m.; direct pressure was immediately put on to the main, amounting to 125 lbs. at the pumping station and 100 lbs. at the centre of the town, which was held for twenty-five minutes, when the pressure dropped to 40 lbs. Fortunately the fire was extinguished at about that time; but wc realised that there must be a serious break somewhere in the system, as the 3,000,000-gal. pump was unable to hold the standpipe pressure. We soon located the trouble, which was a split pipe in the forcemain, about one mile from the pumping station and three miles from the centre of the town. Gates were immediately closed, and our standpipe gave us a pressure of 54 lbs. It was then determined at once to make investigation as to a larger storage and an additional force-main from the pumping station to the town. The commissioners were desirous of building a reservoir of some type to hold 3,000,000 gals, of water, if possible. We employed Messrs. Snow & Barbour, civil and hydraulic engineers, of Boston, to assist us. The investigation was under the personal supervision of Mr. F. A. Barbour, and it was decided to build a reinforced concrete standpipe, because, (1) the cost would be $3,135 less—$34,000 in all; (2) because there would be no cost of maintenance; (3) because it is as nearly indestructible as a structure could he made. On September 7, 1904. the contract was awarded to the Aberthaw Construction company. Boston, Mass, under the general specifications prepared by our engineer, which incorporated those of the contractors. It was early recognised that the greatest responsibility in construction would lie in obtaining watertight walls. The concrete wall is 18 ins. thick at the bottom and 8 ins. at the top. In the construction of the standpipe the conditions were carefully studied, analyses of the sand and stone were frequently made by sifting samples through a series of sieves, and the material were proportioned in accordance with the results obtained so as to reduce the voids to a minimum. The stone was crushed on the work and the concrete mixed by a machine-mixer, and, in general, very good results were obtained both in consistency and uniformity. The foundation of the tank is concrete carried below frost line, a very good bottom of hard-pan being found at rhe depth of 7 ft. After the placing of the foundation, the erection of the steel reinforcement for the first 7 ft. of the wall was begun. This reinforcement is imbedded in the centre of the con crete wall, and is necessarily heavier at the bottom or in proportion to the possible depth of water above. The reinforcement of the walls was made several times as strong as is theoretically necessary, or, three bars lapping 18 ins., and held together by clips were used to make the circle. Near the bottom these bars were placed in two rows, spaced about four inches on centres, horizontally, the spacing increasing with the height until a point was reached where only, one row of bars became necessary. The bars were supported vertically by fifteen channel irons, with the flanges drilled at proper intervals and small rods run through these holes to carry the horizontal bars. After erecting about 7 ft. of steel reinforcement, the inside and outside forms for the concrete were placed in position. These forms were constructed of wood, 7 ft. high, and two sets were used, so that, after the concrete hardened, the lower set could be taken down and moved up above the other set. The circle of the tank was made by having twelve sections in each set, locking them together at the side by iron clamps. After getting the forms in position, the concrete was raised by a derrick to the platforms carried on the interior tower, from which it was shoveled into the space between the inner and outer forms and thoroughly rammed and spaded. After the placing of the concrete, another Section of steel was erected, the lower set of forms loosened and moved up to a new position, and the wall carried up in this fashion to its full height. As soon as the first 20 ft. was completed it was filled with water, and during the entire construction the tank was kept filled up to within about 1 ft. of the inside forms. This was done to test the tightness of the concrete, also to keep it wet to prevent cracking. There were leaks developed after we had completed 50 ft. These gradually grew less as the sediment filled the small voids. When completed, with 100 ft. head of water, there were a number of leaks; but, on the entire surface only three jets, these spurting out near the bottom, not larger than an ordinary pin. At this time there had been no attempt to put on a watertight coat of plaster on the inside; but it was just as the concrete was put in between the forms. The amount of leakage was practically nothing. The tank is covered with a Gustavino tile dome, and may be said to be a masonry structure throughout. The interior coat of plaster was applied in the spring. At no time has there been a wetting through of this standpipe over more than from one to two per cent, of the entire surface—showing that a structure of this type can be made absolutely tight. On December 27. 1905, we put the new standpipe into commission, and continued to use it until May 15. 1906. The leaks during that time were very trifling, although during extreme cold weather we noticed a scaling off on the outer surface at certain points, beginning 5 ft. from the bottom of the tank and extending to a point about 15 ft. from the bottom of the tank. This was apparently caused by pockets or cavities, that must have existed on the outside of the steel, probably caused by the slight moving of the forms when the concrete was being placed. About May 15, 1906, the Aberthaw Construction company began the plastering, on the inside of the standpipe. The first coat had 2 per cent, lime to one part cement and one part sand; the other three coats were composed of one part sand and one part cement. This was floated until a hard, dense surface was produced; then this surface scratched to receive the succeeding coat. This work was done by experts in that line. Prior to the plastering the entire inside of the standpipe was thoroughly cleaned and then picked. This was done to insure the bonding of the cement plaster to the surface. There were four coats of plaster put on, and we felt reasonably sure that it would be perfectly tight, as great care was used in applying the same. But, upon filling the standpipe, this did not give us the result we expected, as we had felt positive that we should have an absolutely watertight structure. At the time the inside work was being done the outside, where the cement had scaled off from the effects of frost, was repaired by digging roiind the outside row of steel reinforcement, putting on iron clips made of ¾-in. by ⅛-in. iron bolted through, and then cement was forced into the cavities round these clips by throwing it a distance of 4 or 5 ft. to insure the filling of the voids. This process was continued until the cement covered the entire outer surface, so that further plastering could be perfectly bonded; on this surface was placed expanded metal, forced over the clips that stood out horizontally, and then a coat of plaster was carefully troweled over the surface of this metal, and then a coat of metal placed outside of that plastering, the ends of the clips being turned at right angles to hold the same in place. After this the final outside coat was applied—thus making a very firm and compact surface equal to any part of the structure. After noting the result of the interior plastering, we were satisfied that some other method must be used to make the standpipe prefectly tight under 100 ft. head, at the same time realising that in a warmer climate we should not hesitate to accept it as it was. We decided to coat the inside with what is known as the “Sylvester process” wash, and we decided to try 35 ft. of our standpipe from the bottom up. After applying four coats of the mixture, we filled the standpipe full, and at 100-ft. head we found there were only four leaks in the 35 ft. coated. On account of this success, we decided to apply four coats more to this same surface, that making eight coats from the bottom up to 35 ft., and above that distance, four coats.