An analysis has been made of the factors involved in the A.S.T.M. Trapezoid Tear-Strength Test. The tear strength of a fabric measured by this test is dependent mainly on the extensibility of the threads, the breaking strength of the threads, the effective specimen length, and the number of threads per inch of fabric. An analytical interpretation of the part played by each of these factors resulted in a general equation relating tear strength to the breaking strength and extensi bility of a given fabric. The evaluation of the derived equation gave a good correlation between the calculated and measured tear-strength values of the fabrics investigated.
A spacing factor only for small voids is derived from the proportion of small voids to total voids. The formula can be used with any method of air determination in hardened concrete that includes denominating some voids as large and some as small. The formula is not dependent on the size criterion used. A graphic method of estimating the spacing factors is given: ¯L for total voids and ¯Ls for the proportion of small voids.
It has long been recognized that no aggregate is completely inert in the environment of portland cement paste. This environment is always at least as alkaline as a saturated solution of calcium hydroxide and may for certain sources of cement exceed this alkalinity [1]. We only need to consider those reactions which take place at a rate sufficient to affect the concrete during its normal life span. Carbonate aggregate reactions may be beneficial, innocuous, or deleterious.
The spacing factor for the proportional number of small air voids ¯Lps as proposed by Walker in 1980 and the air-void parameters normally obtained by ASTM Practice for Microscopical Determination of Air-Void Content and Parameters of the Air-Void System in Hardened Concrete (C 457) were each correlated with the durability factor and weight loss found by ASTM Test for Resistance of Concrete to Rapid Freezing and Thawing (C 666) Procedure A for 151 concretes. The correlation coefficients were examined, and it was found that for the durability factor the classical spacing factor from ASTM C 457 gave a slightly but not significantly better correlation than did ¯Lps. The 151 concretes were divided into three groups according to their source. In no case did the correlation obtained from ¯Lps significantly exceed the spacing factor calculated from ASTM C 457.
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