Concrete made with lime, however, is not suitable for damp situations, and even when used for walls above ground it is much better to use either a "Portland" or "natural" cement for the uniting material.
Concrete made with good Portland cement, in proper proportions, becomes so hard and strong that when pieces of the concrete are broken the line of fracture will often be found to pass through the particles of stone, showing that the adhesion of the cement to the stone is greater than the strength of the stone.
For the aggregates no material is better than clean, freshly broken stone, in size about as large as a hen's egg. Granite probably makes the best aggregates, but other hard stones will answer for any ordinary concrete. Soft sandstones or "freestones" are not desirable. Pieces of hard brick or dense terra cotta also make good aggregates.
Whatever material is used it is essential that it be free from dirt and that the particles be clean.
Good clean, coarse gravel is also extensively used for the mass of the concrete, and some architects and builders prefer it to broken stone, but as all gravel has more or less rounded and smooth surfaces, it would seem as though the cement must adhere more firmly to angular and broken surfaces.
A certain proportion of clean, coarse sand is also required to fill the voids between the particles of stone or gravel.
The method of making and using concrete is very simple, but owing to the fact that it is impossible to tell from an examination of the product the amount of cement that has been used, and the great temptation to the contractor to use as little cement as possible, not more than one-half or two-thirds of the amount of cement specified is generally used (unless an inspector is kept on the work), and the mixing of the materials is also often very imperfectly done.
The only proper way to make concrete is by carefully measuring the proportions of cement, sand, broken stone, etc. This may readily be done by using the common mason's wheelbarrow for a unit of measure and mixing together the specified number of barrows of each material.
For ordinary building operations, where the concrete is mixed by hand, as much concrete as may be made by two barrows of cement is all that can be worked at one time to advantage. The ordinary barrel of cement will just about fill two barrows, so that one barrel of •cement may be considered as equal to two barrows, or parts. If the proportion is specified in this way, however, the inspector should have a barrel emptied into two barrows, and then permit the barrows to be filled with the sand and gravel only to the extent that they are filled by the cement.