478. The porosity and permeability of mortars have been thoroughly investigated by M. Paul Alexandre, who has published his results in " Recherches Experimentales Sur Les Mortiers Hydrauliques."1 The results and conclusion in the following notes on the subject are largely a resume of the systematic investigation made by M. Alexandre.

The two qualities, porosity and permeability, should not be confused, nor should it be thought that a porous mortar is always very permeable, or that a permeable mortar must of necessity be very porous. Porosity is measured by the amount of water which will be absorbed by a specimen after drying, while permeability is measured by the amount of water which will pass through a specimen in a given time under certain defined conditions of thickness, water pressure and area of face.

479. Porosity

The porosity of mortars is due to, and in fact is measured by, the volume of the voids contained. These voids may be divided into three classes, according to the causes to which they may be attributed, as follows: 1st, apparent voids, due to the mortar not being properly compacted; 2d, latent voids, due to the imprisonment of air in the mortar when made; and 3d, voids resulting from the evaporation, during hardening, of a portion of the water used in gaging.

480. Apparent voids may occur as the result of using insufficient cement to fill the voids in the sand, or, in the case of concretes, insufficient mortar to fill the voids in the aggregate. They may also be due to improper manipulation as to tamping, or improper mixing, giving an excess of matrix in one place and a deficiency in another. It was found by experiment that mortars made with coarse sand had the largest volume of apparent voids.

It has been shown elsewhere that if dry sand be moistened and agitated, the bulk of the sand is increased. This is caused partially by the imprisonment of air bubbles in the mass, and if a measure of sand so treated is filled with water, the bubbles will rise to the surface on jarring the vessel. Latent voids in mortar are due to a similar action, and hardened mortars containing such voids refuse to absorb water to replace the air bubbles, at least for a long time.

1 Extrait des Annates des Ponts et Chaussees, September, 1890.

481. A portion of the water used in mixing mortar enters into chemical combination with the cement, another portion is absorbed by the sand grains, and a third portion goes, to moisten the sand. The quantity absorbed by the grains depends upon the character of the sand, and the amount required to moisten the sand depends upon the superficial area of the grains in a given volume, being greatest for fine sands and least for coarse ones. At least one fourth of the water ordinarily used in mixing neat cement is given off later, if the hardened mortar is allowed to remain in dry air. The water required to moisten the sand, and at least a part of that absorbed by the sand grains, also dries out, leaving voids of the third class mentioned.

The apparent voids may be reduced to a very small percentage by care in the proportions and preparation of the mortar. The latent voids may amount to six or seven per cent, of the total volume. The evaporation of water may leave from six to eighteen per cent, of voids in the mass.

482. The conclusions drawn from M. Alexandre's experiments are briefly as follows: The porosity varies between wide limits according to the fineness of the sand and the richness of the mortar. It may be as low as thirteen per cent, and may exceed thirty-one per cent.

With sand of the same degree of fineness, the porosity diminishes as the proportion of cement in the mortar increases.

With the same quantity of cement per volume of sand, the porosity increases with the fineness of the sand. This is especially marked in rich mortars, where the increase in porosity may reach 50 to 100 per cent., while in lean mortars the use of a fine sand may not increase the percentage of voids more than 20 per cent.

The least porous mortars are those rich in cement and made with coarse sand. Mortars made with fine sand are relatively very porous, even when made rich with cement.

Mortars gaged dry are more porous than those of ordinary consistency, and mortars gaged wet are also likely to be more porous, unless the manipulation is such as to allow the excess water to rise to the surface of the mortar.

483. Permeability The degree of permeability of mortars is a more important property than the porosity, since not only does it affect the suitability of the mortar for certain uses, but the life of the structure may depend upon the difficulty with which water may percolate the mass.

The permeability of mortar decreases as the proportion of cement is augmented, and in the case of concretes the permeability diminishes as the percentage of mortar increases, at least to the point where the latter is in excess of the voids in the stone.

From experiments made at the Thayer School of Civil Engineering, Messrs. J. B. Mclntyre and A. L. True found that a five-inch layer of concrete containing from 30 to 45 per cent, of one-to-one Portland cement mortar, and some of the specimens containing 40 to 45 per cent, of one-to-two mortar, were impermeable with pressures of 20 to 80 pounds per square inch, maintained for two hours.

484. Mortars made with fine sand are much less permeable than those made with coarse sand. This difference is so marked that a less permeable mortar is made with one barrel of cement per cubic yard of fine sand, passing a sieve having, say, fifty meshes per inch, than with two barrels of cement per cubic yard of very coarse sand in which the grains are, say, one-tenth inch in diameter. Mortars made with sands composed of a mixture of grains of various sizes are neither very porous nor easily permeated.

Mortars mixed very dry or very wet have greater permeability than those of the ordinary consistency, and in the case of concretes, it would probably be found that a deficiency of water would result in a much more permeable mass than the use of what might be considered an excess.