Which specific separation method is recommended for determining the grading of materials in C136?

The recommended method for determining the grading of materials as specified in ASTM C136 is sieving. This process involves passing the sample through a series of sieves with different mesh sizes. By using this method, you can effectively separate the material based on particle size, allowing for a detailed analysis of the distribution of different sizes within the sample.

Sieving provides a straightforward and reliable way to classify aggregates according to their particle size and to determine the gradation. The results obtained from sieving are essential for ensuring that the materials meet the specifications set out in construction standards, which can affect the performance and durability of the concrete or asphalt in which they are used.

In contrast, centrifugation is primarily used for separating components in a fluid based on density. Settling pertains to the settling of particles in a liquid due to gravity, which is not as precise for grading as sieving. Decantation is used to separate liquids from solids or liquids from liquids based on density differences, but it does not provide the particle size information necessary for grading. Therefore, sieving is the most appropriate method for this purpose.