Theory of Concrete

The dust should be removed, since it has no strength of itself and adds greatly to the surface to be coated, and also prevents the contact of the cement and the body of the broken stone. Par ticles of the size of sand grains may be allowed to remain if not too fine nor in excess.

Portland vs. Natural Cement.

It is sometimes a question whether Portland or natural cement should be used. As a rule this question should be decided upon economical grounds, which makes it a question of relative strength and relative price. For merly prices were such that natural cement was a trifle the cheaper per unit of strength; t but at present prices Portland is usually the cheaper.

Gravel vs. Broken-Stone Concrete.

There has been much debate as to the relative merits of gravel and broken stone as the aggregate for concrete; but at the same price per unit of volume, broken stone is the better. The reasons for this are: 1. The cement adheres more closely to the rough surfaces of the angu lar fragments of broken stone than to the smooth surface of the rounded pebbles. 2. The resistance of concrete to crushing is due partly to the frictional resistance of one piece of aggregate to mov ing on another; and consequently broken stone makes a stronger concrete than gravel. Experiments show that concrete made with gravel is only 70 to 90 per cent as strong as that made of broken stone.* Wet vs. Dry Concrete. There is considerable diversity of opinion among engineers as to the amount of water to be used in making concrete. According to one extreme, the amount of water should be such that the concrete will quake when tamped; or in other words, it should have the consistency of liver or jelly. According to the other extreme, the concrete should be mixed so dry that when thoroughly tamped moisture just flushes to the surface. The advocates of a wet mixture claim that it makes the stronger and more dense concrete; while the advocates of a dry mixture claim the opposite. The difference in practice is not as great as in theory, the apparent conflict being chiefly due to differ ences in condition.

It is unquestionably true that dry mixtures of neat cement, and also of cement and sand, are stronger than wet mixtures, pro vided the amount of water is sufficient for the crystallization of the cement. It is also certainly true that in even a dry mortar or concrete, the water is considerably in excess of that necessary for the crystallization of the cement, this excess increasing with the amount of sand and aggregate. Of course an excess of water is an element of weakness. But the amount of water to be used in making concrete is usually a question of expediency and cost, and not a question of the greatest attainable strength, regardless of expense.

In using concrete for pavement foundations the following items are worthy of consideration: 1. 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 liability that wet concrete will be pronounced the more dense, even though both have the same density. 2. Wet concrete is more

easily mixed; and therefore if the concrete is mixed by hand and the supervision is insufficient or the labor is careless, or if the ma chine by which it is mixed is inefficient, wet mixtures are to be preferred. 3. 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 compacted by ramming. Dry con crete must be compacted by ramming, of it will be weak and por ous; but if the concrete is mixed wet the stones by their weight will bury themselves in the mortar, and the mortar by its fluidity will flow into the voids. 4. A rich concrete can be compacted much easier than a lean one, owing to the lubricating effect of the mortar, and hence a rich concrete can be mixed dryer than lean ones. The quaking of concrete frequently is due more to an excess of mortar than to an excess of water. 5. Lean concretes should be mixed dry, since if wet the cement will find its way to the bottom of the layer and destroy the uniformity of the mixture. 6. Machine-made concrete may be mixed dryer than hand-made, owing to the more thorough incorporation of the ingredients. 7. Gravel concrete can be more easily compacted than broken stone, and hence may be mixed dryer. 8. In mixing dry by hand there is a tendency for the cement to ball up, or to form nodules of neat cement, while in mixing wet this does not occur.

The conclusion is that sometimes wet concrete must be used regardless of any question of strength and cost; while with thor ough mixing and vigorous ramming, dry concrete is strongest but also most expensive to mix and lay.

Proportions. The usual proportions of concrete em ployed for foundations of pavements are: with natural cement, 1 part cement, 2 parts sand, and 3 to 5, usually 4, parts broken stone or gravel; and for Portland cement, 1 part cement, 2 or 3 parts sand, and 5 to 8 parts, usually 5 or 6, of broken stone.

The proportions of the concrete in a measure depend upon the thickness of the foundation. The purpose of the foundation is to distribute the concentrated pressure of the wheel over an area of the subgrade so great that the soil can support the load; and this distribution can be obtained by varying either the strength or the thickness of the concrete. Therefore the engineer may use a thin bed of rich concrete or a thick bed of lean concrete. The combina tion to be used in any particular case will depend upon the relative strength of the different grades of concrete and upon the prices of the several ingredients. In some cases it may be economical to use a thicker layer of broken stone without any cement at all (see § 562). However, if a layer of broken stone is employed for a pavement foundation, it should be compacted by rolling until the fragments do not move under the foot in walking over it.