Development of a Precision Statement for ASTM A1061

Abstract

An interlaboratory study involving 19 laboratories was conducted to quantify the precision of ASTM A1061-16, Standard Test Methods for Testing Multi-wire Steel Prestressing Strand. This standard includes methods for measuring strand yield strength, elastic modulus, elongation, and breaking strength. Strand specimens were 0.375, 0.500, and 0.600-in. [9.5, 12.7, and 15.2 mm] diameter Grade 270 [1860] low-relaxation seven-wire steel prestressing strand compliant with ASTM A416. The reported results were used to examine how the methods are implemented in practice and how precise the results are when the methods are implemented correctly, resulting in the development of a precision statement proposed for adoption into the standard. Precision statistics were calculated for yield strength, elastic modulus, elongation, and breaking strength. Methods for obtaining the yield strength, elastic modulus, and breaking strength were found to be acceptably precise, with reproducibility limits less than 4, 10, and 3 %, respectively, of the mean reported values. Methods for obtaining elongation were highly imprecise, resulting in a reproducibility limit near 50% of the mean reported value. Compliance with requirements of ASTM A1061 was also an issue. At leas tone result was classified as valid from 74, 82, 32, and 100 % of laboratories that submitted results for yield strength, elastic modulus, elongation, and breaking strength, respectively. It was found that the frequency with which strands fracture within a distance of 0.25 in. [6 mm] of the grips is very dependent on the type of grips used, with V-grips without cushioning material resulting in strand fracture near grips in 78 % of tests. Other methods of gripping strand resulted in no more than 35 % of specimens fracturing within a distance of 0.25 in. [6 mm] of the grips. This may be cause to disallow use of serrated V-grips without cushioning material, as fracture near grips was shown to correlate with a statistically significant reduction in breaking strength and elongation for some strand diameters. Finally, use of the 0.2 % offset method to determine yield strength, currently not an accepted method, resulted in added variability and small (1 to 5%) but consistent increases in yield strength compared to other methods that were statistically significant. Several changes to ASTM A1061 are proposed aimed at improving the clarity of the standard.