CONCRETE MASONRY. With special reference to the use of natural bank gravel. GEO. S. PIERSON, C. E., A. M., KALAMAZOO, MICH. The following table illustrates the growing increase in the production of Portland cement in the United States: The total value of this product last year, at the points where it was used, was probably not less than $20,000,000. No doubt much of it was not used to the best advantage nor with proper regard to the service to which it was put. Cement concrete is being put to more varied uses each year, and the present facilities for manufacturing cement and the probable price at which it will be offered will, no doubt, extend its use to purposes not thought of to-day. The practice of re-enforcing concrete with metal will largely extend its use, for this adapts it for use as beams, floors, roofs, panels, tanks, and for many other uses to which it could not otherwise be safely or, economically put. No doubt it will be used in many structures to supplant wood, as lumber grows poorer in quality and higher in price, and the demand for more permanent structures increases. The varied uses to which concrete is put, demand different mixtures. In some cases, a concrete with but a small proportion of cement, and which requires no surface finish, will answer the purpose; in other cases, strength is the important consideration; in others, a smooth surface finish; and in others, the power to resist abrasion; but whatever the use to which con crete is to be put, the desired results should be economically secured. One of the principal items of cost in concrete masonry is the aggregate. Michigan has many natural deposits of gravel and sand from which excellent concrete may be made. Often such deposits are found near by the work. These deposits often occur containing gravel and sand in proper proportions for use in concrete. Sometimes the natural mixture is in every way better and more economical for use in concrete than one required by the usual specifications of one, three, and five, because in the process of deposition the material was well washed, and accumulated in a way that tended to make it compact, and to contain a small proportion of voids. Often such deposits may be improved as aggregates by adding a small proportion of sand if they are too coarse, or of coarse gravel if they are too fine, and considerable cement can thus be saved with no detriment to the quality of masonry. The writer was engaged in the design and construction of a concrete waste weir last season, and as the price of cement was high at the time and the material for the aggregate must be brought from a distance, it seemed worth while to make a more than usually careful examination of material which could be secured for an aggregate. Samples were taken from several deposits and analyzed, with the result shown below: It is difficult to secure a fair sample from the face of a pit where the gravel is sloughing and running down the face, as the coarser particles are separated and carried farther down the slope. The material also varies in composition as it lies in most pits, and must vary somewhat as it is delivered for use on the work. This objection is not peculiar to natural bank gravel, however. The same objection may be urged against the crushed stone often used as an ingredient of the aggregate, and against the sand used with it. Although it is improbable that the analyses represent precisely the conditions of the aggregate as it is received on the work, a comparison of the analyses with each other and with the samples to which they refer, appears to be of considerable assistance in deciding the relative value of material from different deposits, and in detecting variations in the quality of material as it is delivered. Sample No. 1 contains excellent material, but the sample indicates that the addition of quite a proportion of coarse sand would result in a stronger and more impervious concrete, a smoother finish and saving in cement. Sample No. 2 is poorest of all. It contains too small a proportion of large particles, too great a proportion of fine sand, and some clay. The voids in it are greater than in any of the others, and the weight per cubic foot, and also the specific gravity of the particles, less than in any of the other samples. Sample No. 3 contains too small a proportion of coarse particles for economy. Sample No. 4 contains a larger proportion of particles from one-eighth inch to one-half inch than any of the other samples. Probably a larger proportion of sizes three-quarter inch to one-inch would be desirable in massive work, and require less cement, though probably not so smooth a finish could be obtained. The voids in this sample were less than in any of the others examined, the weight per cubic foot greatest, and the specific gravity of the particles highest. This material was fairly accessible, and was used on the work shown in the photo graph reproduced on the opposite page. Tests of the material were made frequently during construction, and it was found that the addition of a small proportion of a rather fine, gray sand, which was found in the excavation, slightly reduced the voids and made a smoother finish next the forms. The voids in all the samples were measured with the material dry and loose. As placed in the work the voids would, of course, be much less. A convenient rule for determining approximately the voids in gravels, one which can be conveniently applied in the field, is the following: I have found that the natural gravels in this vicinity have an average specific gravity of about 2.65. This corresponds to a weight per cubic foot, if there were no voids, of 166 pounds. If a known measure of gravel be weighed, and its actual weight per cubic foot ascertained, then the voids in the condition in which it was weighed will be Per cent of Voids: 166-weight per cubic foot Gravels differ considerably in the character of the particles. Some are made up of well-rounded and smooth particles, and in some the particles are more angular and the surface rougher, which is advantageous. Under a compound microscope with one-inch objective, the sand and fine gravel in sample No. 4 seems sharper than in any other of the samples, with No. 1, No. 3, and No. 2 following in the order named. The gravel in sample No. 3 seems roughened by fine sand particles which adhere to the surface, and this would probably give a better adhesion to the matrix. Probably natural bank gravels as they are taken from the pit or mixed with small proportions of fine or coarse material as a corrective can be used in concrete to better advantage than they usually are. This material can be selected or mixed so as to adapt the concrete to a wide range of uses, from rough and massive work to work which is light and finished in molds which require sharp and perfect details. The labor which can be judiciously spent in selecting or balancing the aggregate will be determined by the current price of cement and the use to which the concrete is put. |