Answer :
Final answer:
The diameter of the tamping rod for ASTM C138 is not provided in the given context, but the full ASTM C138 standard should be consulted for the accurate specification. The use of a tamping device is critical for achieving uniform compaction.
Explanation:
The diameter of the tamping rod specified by ASTM C138, which deals with the test method for determining the unit weight of concrete, is not directly stated in the excerpts provided. However, a tamping device is generally used to compact layers of soil or concrete to help achieve uniformity and conform to the required specifications. It's important to measure the diameter and length of the specimen being tested to ensure accuracy and compliance with standards. According to an older "rule of thumb" mentioned, the size of devices such as rods used in soil testing may be related to the dimensions of the area they are used on, such as 8 diameters side-to-side and 15 diameters deep for compaction purposes.
To answer the question regarding the specifics of the tamping rod diameter for ASTM C138, it would be best to consult the full text of the standard. Practitioners must adhere to the guidelines provided within the actual ASTM C138 document to ensure the correct equipment is used during testing.