Masonry Magazine January 1966 Page. 45
At 70°F the use of calcium chloride more than doubles the one day strength of concrete and produces subsequent gains of approximately 50 percent at three days and 30 percent at seven days. This means faster production of concrete units, earlier form removal in the case of concrete construction, and reduced time of curing for any concrete or mortar.
In addition to the greatly increased early strengths of concrete resulting from use of calcium chloride there are definite increases in ultimate strength. Tests by the Portland Cement Association showed an increase of 9 percent after three years, the California Department of Highways showed an increase of 10 percent after five years and the National Bureau of Standards indicated an increase of 8 percent in one year.
Rate of strength development at temperatures below 70°F is particularly important. You will note that I stress below 70°. Many cement users visualize snow and ice when they think of cold weather concreting. Actually any time the temperature dips below 70°F in the 24 hours after placing concrete, a lag in strength gain takes place. In many section of the country temperatures well below 70°F are experienced nine months of the year.
As previously mentioned, the use of calcium chloride results in savings of one-half the time required for concrete to gain a given strength at 70°F. This comparison is much greater at lower temperatures. At 40°F calcium chloride admixed concrete has the strength in one day equal to plain concrete after five or six days.
We would like to point out here that calcium chloride should not be considered as an antifreeze, for in the small quantities recommended it has little effect in lowering the freezing point. When concrete or mortar is being placed at freezing temperatures, provision should be made for normal protective measures, such as covers and heating. The acceleration of set caused by calcium chloride will of course substantially reduce the time costly protection is required.
I would like to point out briefly two other applications of calcium chloride where the properties described above are valuable. First, is in the use of calcium chloride with air-entrained concrete. As we will note later, air-entraining has been generally accepted and is well established as a means of increasing durability. However, entrained air does result in a decrease in strength, in richer mixes, and particularly at early ages. The use of calcium chloride in connection with air-entrainment increases the strength of the resulting concrete above that of plain concrete thus more than making up for strength loss.
The other point we would like to mention is that calcium chloride may also be used quite effectively with high early strength portland cements. Such use is recommended where exceptionally high early strengths are desired. An example of this is, using the one day strength of a standard mix as 100 percent the use of either 2 percent calcium chloride with standard cement or the use of high early strength cement results in strengths of approximately 240 percent. Using a combination of high early strength cement and 2 percent calcium chloride strengths of nearly 600 percent can be obtained in one day.
Calcium chloride is supplied in 25, 80 and 100 pound moisture-proof bags. In portland cement mixes it may be used in either dry or solution form. The quantities recommended are from one to two percent of cement according to prevailing temperature. For masonry cement these quantities should be cut in half. At temperatures above (continued on page 48)
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ASONRY January, 1966 47