Hollow-Concrete Block Construction.

Progress, Volume II, Issue 3, 2 January 1907, Page 91

Hollow-Concrete Block Construction.

By Spencer B. Newberry.

SECOND PAPER. Block Machines. There are a multitude of machines on the market, and new advertisements appear in every issue of the cement journals. From this we may well believe that money is more easily made by the sale of machines than by their use in block manufacture. Practically all the machines offered are repetitions of the same fundamental design, and differ only in minor mechanical details. A discussion of relative merits and expression of preference would be useless Each intending purchaser should study the various machines offered and select that best adapted to his needs, giving preference to those of solid and substantial construction and taking care that he gets real value rather than " blue sky " for his good money. Block machines may be generally divided into two groups those with horizontal and with vertical cores. The former are usually considered the more convenient for making " faced " blocks though a facing can be put on with either type of machine. The use of a facing of finer, richer or coloured mixture must be called a matter of taste In the judgment of the writer it does not pay to use a facing as the additional labour and preparation of a separate mixture cost moie than to make the whole block of suitable composition to give a good surface. If the facing differs much from the body in composition, also, it is likely not to adhere perfectly, and trouble has been experienced from this cause However for water-proof anil coloured work a facing may be found an advantage, and doubtless many will give preference to a machine which allows this to be conveniently applied Machines are provided with face-plates for the production of rock-faced tool-faced, and plain blocks and various ornamental designs, for bordeib and friezes are also furnished Plates for rockfaced blocks should always be cut from a natuial stone and several different patterns of each size should be obtained, to avoid monotony in appearance of the work Man> machines are piovided with rock-face plates evidently cast fiom some moulded soft material which yields blocks of a surface more like that ol pats of butter than natural stones. Such makeshifts are abominable, and a fraud on the purchaser As to size of blocks, most machine-makers appear to have adopted the standard dimensions of 32 in in length by 9 in in height, giving two square feet of face to each block. Machines are generally made for blocks 8, 10, or 12 in. in thickness Some machines are adjustable to allow blocks of either thickness to be made. "Where the business will warrant it, however, it saves time to have a machine for each size. An 8-mch wall is abundantly strong for foundations or walls of 2-story dwellings, but in many cities the building code prohibits anything less than 12-inch walls for foundation and first story. There seems to be only small demand for the 10-inch block. Machines are geneially provided with" attachments for making corner blocks Arrangements for making 45 degree and 30 degree angle blocks, for bays and towers, are also desirable. Tamping. This is generally done by hand, by means of iron rammers Some well-equipped plants use pneumatic tampers, operated by compressed air, with considerable economy m labour and probably some improvement in density Excellent results can, however, be obtained by hand-tampmg, if the mixture is wet enough. Sticking to the plates may be largely prevented by rubbing the inside surface, when dry, with paraffine, animal or vegetable oil, or soft soap. Mineral oils, such as common lubricating oil are less effective. Handling, Hardening, and Storage. For good results, blocks must be kept under roof, protected from sun and wind, and frequently sprinkled, for at least five days, and preferably foi seven days They may then be piled up outside, and in dry weather should be kept moist by occasional spi ink-

ling with hose for at least three weeks more.Blocks should never be built into a wall until at least four weeks old. Freshly-made blocks will be found to shrink more than 1-16 in, in length on seasoning a month, and if put into a building when only a few days old will develop unsightly cracks m the joints or through the blocks themselves. In handling blocks from the machines the use of three-decked cars on tracks will be found a great saving in time and labour. In a factory so equipped each machine, with two men tamping and one shovelling, will easily make 130 blocks, 32 in. long and 8 in. wide, in ten hours.

Properties of Blocks. Strength Well-made Portland cement concrete, of good sand and gravel or limestone screenings, 1 to 5, will have a compression strength at 4 weeks of over 2000 lbs. per square inch, and at one year of over 3000 lbs. Tests of various mixtures, in 3-inch tubes, at the Case School of Science in Cleveland, showed the following :

It is very difficult to obtain reliable crushing tests of actual hollow blocks, owing to the difficulty of applying the load umfoimly over the whole surface Three one-quailei blocks, 8 in long, 8 in wide, and 9 in. high 10 weeks old, made of cement I], lime I

sand and gravel 6, were crushed with the following results :—: —

Two blocks, 6xBx9 in., 22 months old, showed a crushing strength of 2,530 and 2,610 lbs. per sq. in. Now a block 8 in. wide and 32 in. long weighs 120 lbs., and has a surface of 174 sq. in. Assuming a crushing strength of 2000 lbs. per sq. in. we find by calculation that 2900 blocks could be piled one upon another before the lowest would give away, making a total height of 2175 ft. Allowing a factor of safety of 5, a height of 435 ft. would be admissable This shows how greatly the strength of concrete blocks exceeds any demands which could possibly be made upon it. As stated above, if it were not for the question of water absorption, much poorer mixtures than are generally used would give abundant strength.[ Where water-proof qualities

are no object, and good gravel, containing plenty of coarse pebbles, is to be had, mixtures as poor as 1 to 8 or 1 to 10 will often be found to answer.

Water Absorption. One of the chief faults often observed m work done with concrete blocks is their tendency to absorb water to such an extent as to cause dampness to appear on the interior surface. This is especially noticeable in blocks poor in cement and made too dry. The fault may be overcome by use of suitable materials and sufficient care in manufacture. (To be continued).

Italian railroad authorities are experimenting with a device for use at railway stations, by means of which all tickets are printed and stamped with their price in the presence of the passenger, a record of each sale being, at the same time, made on a roll of paper inside the machine. Each machine is equipped for printing tickets to more than 400 stations

Destiny has turned many a man down while he was, waiting foi something to turn up.

Net area. 49.7 sq. in. 49.7 sq. m. 49.7 sq. in. Max. load. 99,700 lbs. 89,800 lbs. 76 200 lbs. Stress per sq. in, 2,000 lbs. 1,805 lbs. 1,530 lbs.

U 5 """ .Si? ' t/3 (5 h -4 H nJ >h u & tn (D Cement 1| ) Sand and gravel 6 j 3200 I 2 17 4 10 Cement 1-H Hydrate lame . . \ > Sand and gravel 6 ) 3880 2 18 4 10 Cement \\\ Limestone I screenings . 6 >■ (Poured m porous mould ) / 2000 2 05 5 04

Compression Strength ai Six Weeks