Hollow-Concrete Block Construction.

Progress, Volume II, Issue 2, 1 December 1906, Page 47

Hollow-Concrete Block Construction.

By Spencer B. Newberrv,

FIRST PAPER. Historical. — The use of concrete for building construction dates back to the time of the Romans. Though their only cement was a mixture of volcanic scoria with slaked lime, they showed a degree of skill and boldness in the moulding of walls, arches and domes which is scarcely equalled at the present day, and many of then structures still stand as striking examples of the everlasting qualities of artificial stone. The moulding of concrete into separate blocks, to be u«ed for building in the same manner as brick or blocks of stone, appears first to have been introduced in the early part of the 19th century. Solid blocks were first made, butjj&oved heavy to handle and found but scanty use,^Hollow blocks to be used as such or filled up wkn concrete after placing in the walls, were patented by Sellars, in England, in 1875. Concrete facing slabs, with projections to secure them to the concrete filling, soon followed, and in 1878 Listh, of Newcastle, patented a very ingenious Z-shaped block. All accounts indicate that these blocks were made by pouring wet concrete, and allowed to harden many hours before removing the moulds, a somewhat costly and tedious process. The modern rapid method of moulding hollowconcrete blocks, from semi-wet mixtures of such consistency as to permit immediate removal from

the machine, is an American invention, and has been gradually developed during the past six years. By the use of this process the manufacture of blocks has been greatly simplified and cheapened. Materials. — Portland cement, owing to its uniformity, strength, and especially its promptness in hardening, is the only hydraulic material which finds any considerable use in block-making. A great point in favour of Portland cement is that it gams at least as great strength in the air as in water ; other hydraulic cements are generally unsuitable for work not kept permanently wet. At the present moderate price of Portland it is certainly cheaper in use, for a given strength, than any of its substitutes. The " aggregate " or inert coarse material used with cement.to produce concrete blocks, may be either sand and gravel or stone screenings. There is little choice between these two classes of material, if of good quality. Sand and gravel are generally cheaper, and are usually somewhat easier to perfectly mix with the cement. In the matter of strength and hardness of the resulting blocks there appears to be little or no difference. The strength of concrete depends greatly on the density of the mixture, and this is chiefly a question of voids in the aggregate used. It is well known that a mixture of cement and sand is weaker than the same mixture with the addition of coarse gravel. For example, a mixture of cement i, sand 3, will show a lower strength than cement 1, sand 3 and gravel 4, though the latter mixture contains only half as much cement as the former. This is due to the reduction of the voids in the mass by the addition of coarse gravel.

As an example of this, the writer made the following experiments on two samples of gravel .

Note the surprising increase in density on adding the coarse gravel. A concrete of cement with 6 parts of Nos. i and 2 mixed would undoubtedly have shown as great strength as 1 of cement to 2>h parts of No. 1 alone. A good indication of the value of gravel or stone screenings for concrete may be obtained by filling a cabic foot box with the material and weighing it. A solid block of one cubic foot of limestone or quartz, free from voids, would weigh 165 lbs. The amount by which a cubic foot of gravel or stone falls short of this weight represents the proportion of voids, or empty spaces contained in it. For example, if a cubic foot of gravel weighs 130 lbs., the voids would be 35-165 of the total volume, or 21.2 per cent. A few trials will show that for best results the mixture must contain a large proportion of coarse material. Very few gravels will be found that are not improved by the addition of pebbles, and the greatest strength is obtained with material that contains comparatively little sand. There is, however, another consideration which enters into the selection of materials for block concrete, and that is the appearance of the surface of

no great saving in using it. However, it forms a simple means of improving the appearance and water-resisting qualities of the work, and a few experiments will convince most block-makers that it is a valuable addition to the mixture.

Proportions. Cement. — The proportion of cement to be used will depend on the strength, appeaiance and impermeability desired. So far as strength is concerned, very poor mixtures, as one to seven or eight, may answer every requirement. Blocks made from such poor mixtures, however, absorb water like a sponge, and will not answer for the walls of dwellings, though they may perhaps be good enough for partitions, retaining walls, and buildings in which dampness is no objection. For dwellings, a poorer mixture than i to 5 is not to be recommended. Much depends, of course, on the character of the gravel or screenings employed With properly graded gravel or screenings, containing a large proportion of coarse material, a 1 to 5 mixture will be found better than 1 to 3 with cement and sand only. The writer would recommend a mixture composed as follows, by measure . Cement 1 hydrate lime \, sand and gravel 6. This is practically a 1 to 4 mixture, and will be found to possess fair water-proof qualities, sufficient at least for dwellings which are to be furred and lathed. If plastering is to be applied directly to the inside surface of the walls it is necessary that the blocks shall absorb water only very slowly,

the work. Unless a facing is used, a fair amount of fine material must be present to give the blocks an even surface. Nevertheless, it will be found that much coarser gravel or screenings may be used, with good results, than is generally supposed, and that the appearance of the work is not injured by the addition of a large proportion of coarse pebbles up to about f inch, while the strength and density are greatly improved by this addition. A one-to-five mixture of cement and gravel, containing at least 50 per cent of pebbles remaining on an screen, will be found better than a one-to-three mixture of cement and sand only. As to the character of the sand and gravel to be used, there are many tests on record which show that rounded grains give greater density and strength than sharp irregular splinters, and that a small percentage of loam or clay does no harm providing the mixing with cement is thoroughly done. • Another material which may be used with advantage in block concrete is slaked lime. In poor mixtures, as 1 to 4 and 1 to 5, the addition of lime improves the strength and lessens penetration of water ; it also makes the blocks whiter on drying. It is possible, also, to replace part of the cement used, perhaps one-third, by slaked lime, without loss of strength. The most convenient form of lime for block-makers' use is the dry-slaked or " hydrate " lime, now a common article of commerce. At present prices, hydrate lime costs almost as much as Portland cement, and theie is

so as to be only practically penetrated during a long-continued rain Such a result can be reached by giving the blocks a facing of richer material, perhaps i to 2, or by using a sufficiently rich mixture for the whole body of the blocks, such as Cement i£, hydrate lime J-, sand and gravel 5, or cement 1 , hydrate lime 1 , sand and gravel 5 . These should be effective with gravel or screenings of suitable character. The block-maker should experiment with the materials at his disposal, and thus ascertain what proportions are suitable for the special purposes he may have in view. Another method of producing water-tight blocks, which is effective with mixtures as poor as 1 to 6, is by the use of a special water-proof compound, invented by the writer. Water. — The use of a proper amount of water is essential to good work, and in this respect many block-makers are extremely careless. It is well known that a fairly wet concrete is far better than a dry one, and that too much water is better than too little. A rather dry mixture may be more convenient to use, and may enable the maker to turn out a larger number of blocks in a day, but the resulting work will always be porous, weak and crumbling. On the other hand, the extravagant claims made for the wet process, 111 which the concrete is poured into the moulds and left for many hours to harden, are, according to the writer's experience, chiefly imaginary. It is perfectly practicable to produce concrete of the highest possible quality and still to remove the blocks at

once from the moulds. The mixture must be as wet as it can be made without sticking to the plates, and without sagging or becoming distorted when taken from the machine. Blocks so made will be found at least equal m strength and hardness to any that can be made by pouring. They are also much more attractive in appearance, showing a rich, sandstone-like surface, instead of the dull, lifeless look which the wet process gives. As compared with blocks made from too dry a mixture, they are lighter in colour, denser, stronger and more impermeable. The correct percentage of water varies with the materials, but is generally from 8 to 9 per cent, of the mixture, by weight. It will be found that a mixture containing much coarse gravel or stone may be made much wetter, without sagging or sticking, than one of finer material.

Manufacture. Mixing. — The materials, carefully measured, should be thoroughly mixed dry, then the proper amount of water added and the mixing contmued. Thoroughness in this part of the operation will be found to pay for itself many times in improved work. Uneven colour or blocks results from imperfect mixing, and especially from varying the proportion of water. Use of a good concretemixer is far preferable to mixing by hand, both in saving of time and labour and in thoroughness of work done. Hand mixing by the use of a hoe must be vigorous and long-continued, or the full effect of the cement used will not be obtained.

The inferior work of many small-scale block makers is largely due to this cause. For the preparation of concrete for blocks, in which thorough mixing and use of an exact and uniform proportion of water are necessary, continuous mixing machines are unsuitable, and batch mixers, in which a measured batch of the materials is mixed the required time and then discharged, are the only type which will be found effective. The writer prefers a batch mixer of the intermittent pug-mill type, with hinged bottom discharge, as made by the Drake Concrete Machinery Co. of Chicago. If such a machine is purchased, extensible mixing arms of chilled iron, capable of being lengthened as the ends become worn, should be insisted upon. Another machine which the writer has seen in successful use is the revolving pan mixer with central discharge, made by the Elliott & Walker Co. of Wilmington, Del. (To be continued.)

wt o. I, sand and fine gravel \o. 2, coarse gravel, nut to egg size lbs. i and 2, mixed, equal weights t. per cv. ft. i 127.51D5. ioo.9lbs. i39.61b5. Pet t. voids. 22.9 38-9 15-5