Answer :
Final answer:
The specific number of layers for vibrating concrete according to ASTM C138 cannot be given based on the provided information, but concrete should generally be placed and vibrated in uniform horizontal layers for proper compaction without over-vibration.
Explanation:
The student's question concerning the placement of concrete layers according to ASTM C138 when using vibration does not directly correspond to the information provided, which centers on concrete flow through a hose and related calculations. Therefore, a precise answer to the specific question about the number of layers for vibrating concrete per ASTM C138 standards cannot be given based on the provided reference information.
However, it is generally recommended that concrete be placed in a series of horizontal layers of uniform thickness when vibration is employed for compaction. Each layer should be vibrated until it is thoroughly consolidated, and the surface should not move excessively, with care being taken to not over-vibrate, which can cause segregation or loss of entrained air.