these graphs it is apparent that the strength of concrete increases with the cement-content provided the voids remain constant. Usually an increase in quantity of cement changes the void-content of the mortar, or concrete; in the richer mixtures the voids are likely to be increased, but the net effect, other conditions remaining unchanged, is additional strength.

Fig. 15 shows the relation between STRENGTH AND THE VOIDS-CEMENT RATIO in which the percentage of voids in a unit volume of mortar, or concrete, divided by the absolute volume of the cement in the same volume, is plotted as abscissa and the compressive strength of the concrete as ordinate. The aggregates were artificially graded and mixed one part of cement to five parts of combined aggregate (by volume). All specimens were of normal consistency. This graph is very SIMILAR TO THAT OF THE WATER-CEMENT RATIO and for any

FIG. 15. Voids-Cement Ratio and Compressive Strength of Concrete Normal Consistency Bulletin No. 137. University of Illinois

given materials and conditions would be identical if the voids in the concrete were completely filled with water, provided that the quantity of cement was expressed in the same way. The voids in mortar, and the resulting voids in concrete are not, however, usually entirely filled with water. It appears from tests that the proportion filled averages from 75 to 90% and varies considerably for different gradings of even the same aggregate, being also somewhat affected by the richness of the mixture. Since the percentage of air-voids is decreased, as the quantity of mixing water increases beyond that required for basic watercontent, for aggregates of certain gradations an addition of 30 to 40% of water to the basic mixture may cause an increase in total voids in the mortar of only 10 to 20%. The closer correspondence between the water-cement and the voids-cement ratios, as indicative of concrete strength, occurs therefore for the wetter mixtures.

12. Measuring Ingredients. Aggregates may be measured by volume or by weight. One bag of cement, American Standard 94 lb, is considered as 1 cu ft. Volume measurement is usually more convenient and less accurate than weight measurement, which latter can be more easily made to eliminate the effect of varying degrees of moisture in the fine aggregate. When proportionment is made by volume, hoppers should be provided with vertical sides and a mechanical strike-off. Hoppers of pyramidal shape, larger at the top, are particularly undesirable, as a small error in filling produces a disproportionately large difference in volume. Whatever means are used for measurement, constant vigilance is required to secure the specified proportions in each batch. CEMENT is measured by the bag when received in jute or paper containers, and either by weight or by volume when handled in bulk. WATER is measured by the gallon and a record should be kept of the exact quantity used in each batch.

4. MIXING, CONVEYING, AND DEPOSITING CONCRETE

13. Mixing. Concrete should be mixed in a DRUM-TYPE BATCH-MIXER. Continuous mixers are not to be recommended. By HAND-MIXING, upon a watertight platform, satisfactory results can be produced, but such methods are at present archaic, and for even very small jobs a power-driven machine ist usually much more economical. If hand-mixing is the only method available, the aggregates are measured in rectangular frames designed to give the required volumes when filled and struck off level. The best practice is to mix the cement with the fine aggregate until the mixture assumes a uniform color. The coarse aggregate and water are then added, the mixing process being continued until a homogeneous mixture of the desirable consistency is obtained. Whether done by machine or by hand, the mixing should be such as to insure a uniform distribution of the materials throughout the mass, so that the mixture is uniform in color and homogeneous.

MIXERS should preferably be equipped with suitable loading hoppers, or other means of measuring the aggregate, to insure the discharge of the specified volumes or weights of material into each batch of concrete. When such an installation is impracticable, measurement of the aggregates is often accomplished by buggy, or wheel-barrow, but such means of proportionment is inaccurate at best and should be permitted only under the strictest supervision. An automatic water-control is desirable, but is practicable only when uniformly graded aggregates are available, and proper means are employed to overcome the effect of moisture in the sand. A water-measuring device, however, controlling the supply from the storage-tank, is always an essential, and should permit discharge of water into the drum of the mixer only during the period of charging. The Joint Committee, 1924, recommends that:

The mixing of each batch shall continue not less than ONE MINUTE after all the materials are in the mixer, during which time the mixer shall rotate at a peripheral speed of about 200 FT PER MINUTE. This requirement should be taken as an absolute minimum, and it is interesting to note that as the period of mixing is lengthened, the compressive strength, impermeability and abrasive

resistance are all increased as well as the workability. Where job conditions permit, and especially for floor-finishes, a mixing-period of two minutes may be a good investment.* Increasing the peripheral speed over 200 ft per minute does not appreciably benefit the concrete. All mixers should be equipped with time-locks.

Each batch of concrete should be as completely voided from the mixer as

FIG. 16. Typical Arrangements for Concrete Plants

Report of the Special Committee on Contractor's Plant for Reinforced-Concrete Construction. Proceedings of American Concrete Institute, 1921. Page 335

FIG. 17. Typical Arrangements for Concrete Plants

Report of the Special Committee on Contractor's Plant for Reinforced-Concrete Construction. Proceedings of American Concrete Institute, 1921. Page 335

possible before recharging. ENCRUSTATIONS of partially hardened concrete should be removed at frequent intervals during the hours of operation, and the mixer thoroughly cleaned at the completion of the day's run. RETEMPERING, that is, the remixing of partially hardened concrete with an additional amount * Bulletin No. 10, Structural Materials Research Laboratory, Lewis Institute.

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of water, or water and cement, should not generally be permitted, although such procedure is not necessarily as harmful as at one time believed.

Assuming that machine mixing is employed, the illustrations in Figs. 16 and 17 give the arrangement of several TYPICAL MIXING-PLANTS. Whenever practicable, it is well to locate the mixer in a central position in respect to the concrete yardage to be delivered. A location near the middle of the longer side of a building is often advantageous. For buildings of such length that distribution from a single tower is impractical or uneconomical, two towers, one adjacent to either end of the building, are used. It is ordinarily better to have several well-placed mixers of moderate size, not over 1 cu yd capacity, than to have

FIG. 18. Portable Concrete Mixter with Power-Operated Loading Skip

fewer and larger plants, as it is not desirable to convey the plastic concrete long distances before depositing. Several small mixers give also a more elastic installation. Where several mixers are used they should all be of the same type, in order to facilitate interchange of parts. They should, in all cases, be substantially mounted and in such a way as to be accessible for repairs. For comparatively small jobs of less than 4 000 cu yd it is customary to use a 4 yd mixer, a 1 yd hoist bucket, a single drum-hoist, power equivalent to a 50-h.p. electric motor, and presuming that the concrete is to be distributed in buggies, a WOODEN TOWER only a little more in height than the top floor, or roof, to be constructed. For jobs containing over 4 000 cu yd a 1-yd mixer is generally

employed with at least a 1% yd hoist-bucket, a single drum-hoist, power equivalent to a 75 h.p. electric motor, and a tower of either wood or steel. STEEL TOWERS are usually used for gravity distribution; either steel or wood, but usually the latter, for wheel distribution. The receiving hopper, into which the hoist-bucket discharges, should have from one and one-half to twice the capacity of the bucket.

For small jobs, and especially where the units of work are scattered, a PORTABLE MIXER may be the best choice. These are usually furnished with

FIG. 19. Portable Concrete Mixer Equipped with Loading Platform

either a loader operated by power, see Fig. 18, or with a loading platform, as illustrated in Fig. 19. A bag-batch machine is a popular size for jobs requiring up to 50 cu yd a day. In fact it may often be economical to employ two or more machines of this size on some types of work rather than a larger single plant. A POWER LOADING MACHINE can be operated more rapidly than one of the loading platform type and has the advantage of being charged at a lower level, but this type is more expensive and requires comparatively more fuel for operation.

In congested districts, where truck-load deliveries can be obtained to advantage, it is usually desirable to dig a hole for the mixer, feeding the aggregates by