CONCRETE, the name given to a composition, variously made, but in general use among architects as an artificial foundation fin- buildings.
'Flue convenience of obtaining a firm and solid bottom by the formation of it compact mass,. concrete ; and the faeility with which this composition is made and used, have led to its alnnist. universal adoption in all situations where the requisite materials can be procured. The proportions, and the species of material vary, of course, in ditferent localities, and in the practice of different architects, but the principal ingredients, good lime, elean sharp river-sand, and pebbles well mixed, will not NI to make a good concrete.
Semple recommends to take 80 parts of pebbles—each about 7 or S ounces in weight-40 parts of sharp river-sand, and 10 of good lime; the last to he mixed w ith water to a thinnish consistenee, and grouted in. The concrete used by builders in the neighbourhood of London, is made of Thames ballast, as taken front the bed of the river; this is found to consist nearly of 2 parts of pebbles to 1 of sand, and from one-seventh to one-eighth part of lime. Mr. Godwin says the best method of making concrete is to mix the lime, pre viously ground, with the ballast in a dry state ; sufficient water being thrown over it to effect a perfect mixture ; it should then he turned over two or three times with shovels, put into barrows, and wheeled away for instant use. It is advisable to employ two sets of men to peril n-In this operation, with three men in each set, one man fetching the water, &c. while the other two turn over the mixture to the second set, and they, repeating the process, turn over the concrete to the barrow-men. After being put into the ha rn1WS, it should be wheeled rap planks, so raised as to give it a fitll of solute yards, and thrown into the foundation, by which means the particles are driven closer together, and greater sididity is given to the whole mass. Soon after being thrown in. the mixture is observed usually to be in commotion, and much heat is evul with a copious emission of vapour.
The conerete should be thrown on in layers, the first being allowed to set, before a second is thrown down. A barrow load spreading over the its fall, will firm generally a stratum) of from 7 to t) inches thick. and a cubical yard of concrete will tnke about :10 leet eube ballast, and feet Of the latter no more should be used than is absolutely necessary to effect a perfect mixture of the ingredients. 111It water accelerates the induration.
The expediency of using concrete as a substitute for stone, brick, and other materials for building, or const t•uctions above ground, has been much discussed, and a great variety of opinion has prevailed on the subject. In the " Prize Essay upon the Xature and Properties of Concrete and its 'Indica tioa to Construction," Mr. Godwin has given much valuable information, but we t hint; the opinions he has there ventured as to the use of concrete fin. walls, &c., will hardly be adopted by architects generally. "A. prudent man." says Mr. Bar tholomew, "will not heap up walls a second time, altogether of concrete, lie will not exchange masonry of good strong mortar, and good strong stone or brick, for a heap entirely of mortar, and that '• tees maigre." A careful examination
will discover that in every instance in which concrete walls have been used, more or less of instant ruin has occurred, the lintels over the apertures of the first story giving way before even those of the second story have been laid ; and when those breaches have been repaired, they have re appeared ; and even through the solid walls, rents have instantly occurred : experience proves that gravel lying in a bed, and there growing. as it were, without the means of flow or escape, is sufficient to support the most enormous weight of fabric; but the same gravel detached, cannot be piled up, so as to form either solid upright walls, or horizontal beams." Concrete has been also used both as " rough concrete," and in blocks, in extensive works, as river-walls, breakwaters, &c.., and has been recommended for such purposes by engineers of eminence. In the " Professional Papers of the Corps of Royal Engineers,'' Captain Denison describes some works of thk kind, and. in the ex perin tents he had the opportunity of witnessing, some very results are obtained as to the practical application of concrete to the construction of river •alls at Woolwielt and Chatham. In one instance at W4001 wich, it has been applied in mass, the wall having been con structed in the same manner as the Brighton sea-wall ; in both the other instances at Woolwich and Chatham, the con crete was formed into blocks, which were allowed ample time to set and harden betbre they were built into the fitce of the wall. At Woolwich, the river-wall is for the most part founded upon piles ; its height above the piles is about 24 feet ; the thickness at bottom it feet. at top 5 feet, with a slope or batter in front of 3 feet in 22. The face of the wall is composed of the blocks laid in cement. in courses 18 inches in height ; the headers and stretchers in the course being each 2 feet 6 inches long : the former having a bed of 2 feet. while the latter have only 1 foot ; behind the facing, the rough concrete was in to complete the thickness of the wall and I it er-fi qts. Both the blocks and the rough concrete were composed of iime and gravel, in the proportion of 1 to 7 and brought to the proper consistence with boiling water ; but the blocks were, or ought to have been, made with Abertbaw lime, Dorking lime being used for the rest of the work. The blocks were cast in moulds, and were submitted to pressure while setting. ; a coating of fitter stuff being given to the face for the sake of appearance. The whole of the wall was built by tide-work, and in the lower part therefore the backing of rough concrete had hardly time to set lteibre it was covered with the tide ; the water, however. in this instance, appeared to all'ect th•t surfi/ce of the mass only, the interior at the depth of a few inches appearing dry, and of a moderate degree of hardness, when examined after the retiring of the tide.