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1.1 This practice presents a method for determining in-air index values (see Note 1) of the radial load and radial stiffness at low strain in each symmetric direction of a balanced geogrid (see Note 2) subjected to 360° radial strain. This determination is based on tensile test results from testing in accordance with Test Method D6637/D6637M and an analysis of the radial force balance in each symmetric direction across a circle of geogrid that is assumed to be strained uniformly in all radial directions to a low strain level.

1.2 Symmetric directions relative to the machine direction:

1.3 This practice will facilitate comparisons of the radial tensile properties of different balanced geogrids and judgment of conformity of product manufactured and acceptance of commercial shipments of geogrids by standardizing the data used and the method by which the calculations are performed.

1.4 This standard practice is restricted in application to geogrids with two sets of parallel ribs arranged at nominally 90° to each other or three sets of parallel ribs arranged at nominally 60° to each other with one set lying in the cross-machine direction.

1.5 Further, this practice is restricted to 20 % variation in the specified unit tensile properties of the different sets of ribs in a geogrid in order to enable the development of the mathematics that use these properties of each set of ribs in the geogrid. This generates index values for the unit tensile properties in the nominal directions as outlined in 1.2.1 and 1.2.2 of a geogrid that is under 360° radial strain at any designated strain level. A 2 % strain level is set as the default strain of choice as it has been found that this is the lowest strain level at which most test laboratories can achieve consistent low-strain tensile test results on geogrids.

1.6 Calculations in accordance with this practice determine the radial component of the unit tensions generated in the geogrid. Differences in the unit tensions of different rib sets will also give tangential components of force. With a difference in tension between rib sets restricted to 20 % or less, it has been found that these tangential components will be approximately 10 % or less of the values of the radial components. Therefore, the resultant of the radial and tangential component in any symmetric direction will be less than 0.5 % greater than the radial component. As this is less than the variation normally found in tensile testing of a set of rib or wide-width specimens, it does not detract from the calculated radial tensions being valid index properties of the geogrid.

1.7 This practice is not applicable to uniaxial geogrids.

1.8 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.

1.9 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.

1.10 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.

4.1 The determination of the tensile force-elongation values of geogrids provides index property values. This standard practice shall be used for the determination of radial properties, judgment of conformity of product manufactured, and acceptance of commercial shipments of geogrids.

4.2 The standard strain at which this practice is applied is 2 %, as this has been established and accepted over the years as the lowest strain at which consistent measurement of the properties of geogrids can be achieved. However, if a customer or specifier requires calculation at a lower strain, this can be done provided the higher standard deviation this can give is accepted.

4.3 In cases of dispute arising from differences in reported results when using this standard practice for acceptance testing of commercial shipments, the purchaser and supplier should conduct comparative tests and calculations to determine if there is a statistical bias between their laboratories. Competent statistical assistance is recommended for the investigation of bias. As a minimum, the two parties should take a group of test specimens which are as homogeneous as possible and which are from a lot of material of the type in question. The test specimens should then be randomly assigned in equal numbers to each laboratory for testing. The average results from the two laboratories should be compared using Student's t-test for unpaired data and an acceptable probability level chosen by the two parties before the testing began. If a bias is found, either its cause must be found and corrected or the purchaser and supplier must agree to interpret future results in light of the known bias.

4.4 This standard practice is applicable to all geogrids with two or three sets of ribs in which variation of tensile properties between different sets of parallel ribs is 20 % or less. For multiple-layered geogrids, it should be applied to each layer individually and the results for all layers summed in each radial direction.