Those cements winch contain the most lime are usually most affected by the action of sea water. Cements containing consider able alumina should not be used for work in sea water, siliceous cements, or those in which alumina is replaced by iron oxide being preferable. In France a siliceous hydraulic lime known as lime of teil is extensively used for such work.
The addition of finely ground puzzolanic materials to Portland cement has been found useful in preventing the disintegrating effects of sea water. These materials probably combine with and reduce the amounts of free lime available for combination with the sea salts. As used, they also render the mortar less permeable.
Mortars of fine sand are found to be more affected by sea water than those of coarse or graded sands.
The injurious action of sea water is dependent upon the water having access to the body of the concrete, hence it is important in such work to use concrete of maximum density, or to protect the body of the concrete by a surface of dense mortar or concrete. The Joint Committee on Concrete in its 1917 report makes the following reference to work in sea water: The data available concerning the effect of sea water on concrete or reinforced concrete are limited and inconclusive. Sea walls out of the range of frost action have been standing for many years without apparent injury. In many places serious disintegration has taken place. This has occurred chiefly between low and high tide levels and is due, evidently, in part to frost Chemical action also appears to he indicated by the softening of the mortar. To effect the best resist ance to sea water, the concrete must be proportioned, mixed and placed so as to prevent the penetration of sea water into the mass or through the joints. The aggregates should be carefully selected, graded and proportioned with the cement so as to secure the maximum possible density; the concrete should be thoroughly mixed; the joints between old and new work should be made watertight; and the concrete should be kept from exposure to sea water until it is thoroughly hard and impervious.
92. Effect of Alkalies.—In some localities in the arid regions of the Western States difficulty has been met in the use of concrete because of the disintegrating effects of alkaline waters—effects similar to those of sea water and probably due to the same causes. The most serious disintegration is found where the concrete is alter nately wet and dry, although in some cases the whole of the concrete below water has been affected.
On many irrigation projects large quantities of concrete are being used, and the problem of dealing with the alkaline salts, with which the soil is impregnated in some localities, has become a serious one. These alkaline deposits vary in character in different places, com prising salts of potassium, sodium, calcium, and magnesium. The ill effects seem to occur where sulphates are present. in considerable quantities,' which agrees with the results of studies of the action of sea water. The same precautions may be taken in selection of materials as for work in sea water, but all cements seem to be affected to some extent by contact with these salts. The use of dense con
crete, or the application of protective coatings to prevent access of the alkaline water to the interior of the mass of concrete, offers the best means of preventing disintegration.
93. Resistance to Fire.—Experience indicates that concrete, when properly used, is one of the best materials for resisting fire. The surface of concrete immediately exposed to the fire is injured and may become dehydrated, but concrete is a poor conductor of heat, and the penetration of the dehydrating effect is extremely slow-. Experiments by Professor Woolson 2 show that when a mass of concrete is subjected to high heat for several hours, the temperature 1 inch beneath the surface is several hundred degrees below that at the surface. With the temperature of 1500° F. at the surface for two hours, the temperature at 2 inches beneath the surface was from 500° to 700° F., and at 3 inches beneath the surface about 200 to 250° F.
When concrete is used as structural material, where it is liable to be subjected to serious fire risk, a layer of concrete next the exposed surface should be considered as fireproofing and not included in the section necessary for resisting stresses.
The Joint Committee in its report of 1917 discusses fireproofing as follows: Concrete, because incombustible and of a low rate of heat conductivity, is highly efficient and admirably adapted for fireproofing purposes. This has been demonstrated by experience and tests.
The dehydration of concrete probably begins at about 500° F. and is completed at about 900° F., but experience indicates that the volatilization of the water absorbs heat from the surrounding mass, which, together with the resistance of the air cells, tends to increase the heat resistance of the concrete, so that the process of dehydration is very much retarded. The concrete that is actually affected by fire and remains in position affords protection to that beneath it.
The thickness of the protective coating should be governed by the intensity and duration of a possible fire and the rate of heat conductivity of the concrete. The question of the rate of heat conductivity of concrete is one which requires further study and investigation before a definite rate for different classes of con crete can be frilly established. IIowever, for ordinary conditions it is recom mended that the metal be protected by a minimum of 2 inches of concrete on girders and columns, 11 inches on beams, and 1 inch on floor slabs.
Where fireproofing is required and not otherwise provided in monolithic concrete columns, it is recommended that the concrete to a depth of 1; inches be considered as protective covering and not included in the effective section.
The corners of columns, girders, and beams should be beveled or rounded, as a sharp corner is more seriously affected by fire than a round one; experience shows that rotmd columns are more fire resistive than square.