Masonry Magazine January 1966 Page. 46
Admixtures for All-Weather Construction
(continued from page 47)
70 degrees, one percent should be used, while at temperatures of 70° and below, two percent by weight of cement is recommended.
AIR-ENTRAINING AGENTS
Probably the most outstanding advance in concrete technology during the past twenty years has been the discovery of the benefits of air-entrainment. Experience in both laboratory and field has demonstrated the superior durability of air-entrained concrete and there should be no resitancy in using it under conditions where severe natural weathering is a problem.
Air-entrainment alters the properties of both the plastic and hardened concrete. Air-entrained concrete is considerably more plastic and workable than ordinary concrete. It can be handled and placed with less segregation and there is less tendency for bleeding and water gain. The major effect of air-entraining is due to a change in the characteristics of the concrete brought about by the presence of air bubbles in the paste.
As we mentioned earlier, air-entrainment while improving both workability and durability may have an adverse effect on strength. Within the range of air contents normally used, say from 3 to 7 percent by volume, the decrease in strength usually is about proportional to the amount of air-entrained. Where the concrete mix has been redesigned this reduction in strength will rarely exceed 15 percent in the case of compressive strength and 10 percent in the case of flexural strength.
There are two methods of achieving air-entrained concrete. First, through the use of air-entraining cement. Air-entraining cement certainly has its place in the concrete field but particularly in small concrete jobs where good engineering design and supervision is not available. However, for definite control of the quantity of air to be entrained the use of admixtures is recommended. There are now on the market a large number of commercial admixtures. Many materials including the natural wood resins fats, various sulfonated compounds, and oils, have the property of entraining air in concrete. Some of these materials, as for example the resins, are insoluble in water and must be neutralized before they can be used as admixtures. Soluble salts of the resins are now available commercially so that it is no longer necessary to neutralize them in the field. Before specifying or permitting the use of an air-entraining admixture in concrete the use should satisfy himself first that the material actually does function as an air-entraining agent, and secondly, that none of the essential properties of the concrete are seriously impaired. The ASTM has developed a specification and a method of testing air-entraining admixtures for concrete.
In closing we would like to refer to a recent article by R. E. Copeland in the September, 1965 issue of CONCRETE PRODUCTS. He discusses the requirements of mortar for basement walls and the need for high bond strength. He recommends use of Types M and S mortar made with portland cement and lime in the construction of block basement walls. For maximum tensile bond the joints should be kept moist for a couple of days or so by fogspraying the masonry two or three time a day. Rapid drying of the mortar should be prevented. Here calcium chloride offers an advantage in two ways. First, by increasing strength gain, you accomplish curing faster. Second, because of its moisture retaining properties calcium chloride reduces water loss during setting. Tests conducted by the Bureau of Public roads on mortar show that moisture loss is very rapid where calcium chloride is not used. With calcium chloride, water loss is only 32% at age 225 minutes, compared to 60% for plain mortar.
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48 MASONRY January, 198