Making and Placing Concrete

MAKING AND PLACING CONCRETE.

There is considerable diversity of opinion among engineers as to the amount of water to be used in making concrete; but in recent years there has been a decided tendency toward the use of more water than formerly. Until recently one extreme advocated the use of dry concrete, i.e., of a concrete mixed so dry that moisture would just flush to the surface under vigorous tamping; while the other extreme advocated the use of wet concrete, i.e., a concrete that would quake like liver or jelly when tamped. At present only comparatively few advocate the use of dry concrete, and most engineers prefer a plastic or quaking mixture; but a considerable number use a very wet or mushy concrete, i.e., a mixture which can not be tamped and into which a man sinks to his ankles or above in walking over it.

Dry vs. Wet Concrete.

The following is the conclusion of the most elaborate series of tests made to determine the relative strength of wet and dry concrete.* The mean crushing strength of 156 1-foot cubes of concrete made with mortar as "dry as damp earth" was 13 per cent more than the average of 148 cubes that "quaked like liver under moderate ramming," and 11 per cent more than the average of 144 cubes made with mortar of the "ordinary consistency used by the average mason." The cubes were made of five brands of portland cement and five proportions of sand varying from 1 : 1 to 1 : 5; half the cubes had a little more mortar than enough to fill the voids of the broken stone, while the other half had only about 80 per cent as much mortar as voids. One quarter of the cubes were stored in water, one quarter in a cellar, one quarter under a wet cloth, and one quarter in the open air. All were broken when approximately 2 years old. The difference in the amount of mortar made no appreciable difference in the strength.

Experiments frequently show a greater difference than the above. For example, the mean of twelve cubes of dry concrete was 51 per cent stronger than corresponding cubes of quaking concrete.t But all experiments show that the difference between the strength of wet and dry concrete is greater at earlier ages than later, and that after 3 to 6 months there is but little difference.

It is unquestionably true that with sufficient ramming, dry mix tures of neat cement, and also of cement and sand, are stronger than wet mixtures; and hence if the absolute maximum strength of con crete is desired a dry mixture should be used. But the amount of water to be used in making concrete is usually a question of ex pediency'and cost, rather than a question of the greatest attainable strength regardless of expense.

A consideration of the following principles will be useful in determining whether to use a wet or a dry concrete. 1. Dry mixtures set more quickly and gain strength more rapidly than wet ones; and therefore if quick set and early strength are desired, dry concrete should be preferred. 2. Wet concrete contains a great

number of invisible pores, while dry concrete is likely to contain a considerable number of visible voids; and for this reason there is likelihood that wet concrete will be pronounced the more dense, even though both have the same density. 3. Wet concrete is more easily mixed; and therefore if the concrete is mixed by hand and the super vision is insufficient or the labor is careless, or if the machine by which it is mixed is inefficient, wet mixtures are to be preferred. 4. Wet mixtures can be compacted into place with less effort than dry; but on the other hand the excess of water makes the mass more porous than though the concrete had been mixed dry and thoroughly com pacted by ramming. Dry concrete must be compacted by ramming, or it will be weak and porous; therefore if the concrete can not be rammed, it should be mixed wet and then the stones. by their own weight will bury themselves in the mortar, and the mortar will flow into the voids. 5. A rich concrete can be compacted much easier than a lean one, owing to the lubricating effect of the mortar; and hence rich concretes can be mixed drier than lean ones. The quaking of concrete is due more to the excess of mortar than to the excess of water. 6. Lean concretes should be mixed rather dry, since if quite wet the cement will find its way to the bottom of the layer and destroy the uniformity of the mixture. 7. Machine-made concrete may be mixed drier than hand-made, owing to the more thorough incorporation of the ingredients. 8. Gravel concrete can be more easily compacted than broken-stone, and hence may be mixed drier. 9. In mixing dry by hand there is a tendency for the cement to ball up, or form nodules of neat cement; while in mixing wet this does not occur. 10. If wet concrete is deposited in a wood form, there is liability of the water exuding between the planking and carrying away part of the cement and thus weakening the face— which should be the strongest part of the mass. 11. With reinforced concrete a wet mixture is necessary so that the mortar will certainly flow around the reinforcing steel and come into contact with its surface. 12. A great excess of water not only makes the concrete porous and therefore weak, but also tends to destroy the cement by the washing out of the finest particles. 13. If the concrete is mixed with a great excess of water, when it is deposited in place, the excess water will rise to the surface and carry with it the finest or active particles of cement, which will decrease the strength of the concrete and also deposit upon the surface of the mass a light colored powdery substance which will prevent the adherence of the next layer of concrete.