The Irish Builder from Dublin, Dublin, Ireland • Page 9

244 THE IRISH BUILDER. Sept. 1, 1875. tensively nscd of all concrete aggregates; although, for reasons stated, I hold them to be inferior to some other aggregates, they all make exceedingly good concretes. The flat and angular land gravels, from not being so mnch rounded and polUhed by attrition, make the soundest concretes, pro.

vided they are free from loam or clay. The addition of a sufficient quantity only of clean sharp sand, to fill the spaces between the larger rounded pieces, makes the best gravel concrete; but care must be taken that too much sand is not nsed, otherwise the distribution of cement is so much greater that a larger proportion is required. Care must also be taken that very fine sand be not used at all, for this is simply an adulteration of the cement that will cause very poor concrete. Where sand cannot be easily obtained, good and strong concrete is made with round gravel only. The cement, uniting the particles of gravel only, is not diluted or over distributed by sand, but uniting the particles of gravel at their points of contact only.

This kind of concrete has been proved by testing to be capable of resisting much greater crushing force than ordinary bricks. But surface cementing is necessary to prevent water from entering and running through between the particles of this kind of concrete. Respecting every kind of aggregate, it is of the greatest importance that it should be free from dirt, dust, or earthy matter of any kind, and any defects of this sort must be removed by washing. Respecting the correct proportions for mixing, it will be seen that a considerable variation is possible, from the varying nature of the aggregates, and I have used from one-third to one-twentieth of cement, but the propoition which has been found sufficient to give a coating of cement to each particle of ordinary gravel concrete is one-eighth, and this is the proportion that is almost invariably nsed. For roofs, floors, stairs, and other Bpecial work, one-fifth or one-sixtb cement is used, in order to induce a more, rapid setting and strengthening, to ensure a thorough coating to every particle, and that there may be no unfilled spaces one-eighth of cement is sufficient for wall building.

A larger proportion may be considered waste. A few months ago a piece of concrete was shown to me as a sample of good rich concrete. The joint of cement between the pieces of gravel was from 1 in. to 1 in. in thickness.

I could only remark that if the best concrete is a solid mass of neat cement, then the adulteration by means of gravel not been carried to a great extent in this sample. The water nsed for mixing concrete should be clean. It is possible to destroy much of the strength of cement by using dirty or polluted water. Sea-water has not been found injurious, but is supposed to delay the setting. Respecting the quantity of water to be used, there is more danger to be apprehended from using less than from nsing more than the right qnautity.

The correct quantify of water is of course sufficient to convert the cement into a thin paste that shall completely coat and cause to adhere all the particles of the aggregate. With absorbent aggregates extra water must be used, or the absorbent materials should be watered before -nixing with cement begins. Allowance must be made for evaporation in hot weather. I have seen concrete that had apparently snhicient water when mixed, utterly spoiled by rapid evaporation under a hot sun. Unless so much water were used as to wash away the cement, or make it so thin as to cause it to run through the aggregate (which is almost too absurd to consider possible with some workmen), the only danger from too much water is to delay the setting, because tho cement must first exude the excess of water not required for hydration.

I prefer concreta mixed by hand, as it is important to have the materials all mixed dry before mixing with water. Crystallisation or setting begins at once with the addition of the water, and as little time as possible spent in thoroughly mixing and placing into the mould or apparatus after water his been added. There ara shorter time than a mixtare of cement and and, or gravel. Simultaneous tests, with varying proportions, show in favour of the largest proportion of cement, but the concrete mixtures gradually gain upon the neat cement, and ultimately reach and even surpass it in strength. The aggregates useful for making concrete are very various, and easily obtainable in almost every locality.

The economy of concrete is chiefly due to the facility it gives for utilising almost whatever material can be most easily obtained, and to the fact that some of the best concrete aggregates are mere waste, useless for any other purpose, or een troublesome and expensive to dispose of. Pit and river gravels, with their coarse sands, beach shingle and sand, burnt clay ballast, broken stone, quarry waste, flints or pebbles, broken bricks, tiles, and waste pottery ware, broken slag, clinkers, and mill cinders, all of these have been largely used, severely tested, and fully proved to be fit materials for making good substantial walls, floors, roofs, and stairs. The best aggregates are those which can be most readily made into a concrete resembling in texture and structure the natural conglomerate stones, that is, it should be of all sizes and forms, angular and coarse fractured, so as to fit well together, and having ail spaces between large pieces filled with smaller particles, and leaving no interstices. The materials most readily fulfilling these conditions are old bricks, tiles, and pottery, rock and quarry refuse, slag and clinkers, all broken by Drake's patent stone-breaker, which, having been designed expressly for this purpose, produces fragments of various sizes from about 11 inch cubes down to particles the size of coarse sand. With this material only a minimum of cement is required, sufficient to form a thin coating around every particle, and give continuous adhesion, thus making the strongest possible concrete without waute of cement, and leaving no interstices for percolation of water through the concrete.

Old pottery and tiles and vitrified firebricks, old building ar il stones that break with rough fractures, make the strongest concrete, as broken, proved by Mr. Grant's, Mr. Kircaldy's, and numerous other tests. Flints and other stones breaking with smooth fractures, and also slag, make concrete a little strong, but still stronger than rounded gravel concrete. A fragmentary slag is produced at the Middlesbrough iron work, by running the molten slag as it leaves the furnaces ou to revolving discs, upon which jets of water play, thus causing the Blag to break up into small pieces about the size of almonds and Spanish nuts.

With this fragmentary slag, we have constructed some of the most satisfactory concrete buildings yet done. New master's houses at Marlborough College (Mr. Street, architect) were built with broken flint, concrete, and lump chalk packing. Many stones too soft and liable to decay, if used as ordinary building stones, make excellent concrete, the cement protecting them from the influence of weather and many of them containing metallic oxides and large proportions of soluble silica appear to nnite chemically with the cement and form almost indestructible hydrosilicates. Next in order of merit, after broken bricks, I place burnt clay ballast.

It has the same advantages of tough surfaces, absorbent iiature, and variety in sizes but before using this material it is necessary to prove that it is thoroughly well burnt. Stiff clays may be burnt with about five peckB of coal to the yard cube of ballast but light, loose, and shaley clays require more coal; some can hardly be burnt with double the quantity of coal. That most excelleut concrete can be made with burnt clay, you con see by examining tome pieces before you, which have been cut from the concrete of my house during alterations. The cost of making clay ballast is from la. to 2s.

6d. per cubio yard. Land, water, or lea gravels are the most ex OX BUILDING IX CONCRETE Concrete is composed of two distinct kinds of iratcrial. 1. The aggregate to be concreted or held together.

2. The matrix, or cementing material, mixed with the aggregate to hold the whole together in the homogeneous state called concrete. The second is evidently the moot essential and important, and there is no known material so valuable for this purpose as Portland cement. Besides Portland cement, there are other valuable cementing materials. Lias and other hydraulic limes have been much used for fonndation concrete, and even for building walls, but their use for this latter purpose has always been limited, and in modern times has almost ceased, because of the length of time required for setting and gaining strength.

But the recent large increase in the cost of Portland cement led to experiments, and to the use of the best limes for wall-building concretes, with greater success than hitherto. Selenitio cement prepared from lias and other hydraulic limes used together with a small proportion of Portland cement, has been found to make good wall-building concrete at a cheaper rate, and almost as quick setting, strong, and good in other respects as entire Portland cement concrete. In districts where the best lias is found, it has been used without the selenitic process but, together with Portland cement, producing very stron excellent, and cheap walls. ut the nse of these limes in combination with Portland cement should only be attempted by those who thoroughly understand and know chemically and practically the exact conditions, proportions, and application necessary for successful use. Portland cement, however, is now so well and honestly made, by so many good manufacturing firms, is so easily tested, and us correct treatment and use are so well and widely known, and there is, besides, such a wide margin of strength in its favour, as commonly applied in concrete building, that only ordinary care and intelligence are required to ensure success in its use for all purposes of concrete building.

The various roll-known mechanical and chemical tests are very valuable when it is required to ascertain the exact strength and qualities of a sample of cement for a certain special purpose, or to prove a suspected cement. The best and most easily applied working test is to make np into concrete, in the proportions that are to be used in the actual work, some of the cement and aggregate that are on the site for use. A few hours are enough to enable an experienced workman to detect if anything is wiong in the mixture, and if any defect is suspected the cement should at once be subjected to the usual mechanical and chemical tests. But at least a welt is required for these rough concrete test blocks to prove to engineer, architect, or employer that the concrete is quite good and dependable, and in the meantime tL 3 blocks should not be handled or disturbed in any way. Much harm is done, time wasted, and annoyance caused by the habit of inexperienced persons in picking and poking at cement samples before they have had time to get strength.

To such persons quick-setting, ght-weight cements would appear to promise good results but it is now well known by everyone practically acquainted with the use of cement, that the well-burned, heavy, finely-ground, slow-setting cements are the best for concrete purposes. Respecting tho mechanical tests of nrat cement, it has been found that some of these showing the highest results when tested for resistance to tensile and transverse breakage strains, have shown very inferior results when niado into concrete, and tested for the same strains in comparison with cements that cave inferior results when tested neat, but which proved very superior in concrete. Noat cement reaches its full strength in a much Br Mr. Cherle rmike. Reed meeting of Ciril u4 Mechanics) Society..