Concrete sampling - How many molds or cylinder required for concrete sampling? according to ACI 318

Samples of concrete must have taken for each class of concrete at least once a day (minimum requirement). 

sample could have taken base on 2 conditions below

• By volume - one sample for each 150 yd³ (115m³)
• By square - one sample for each 5000 ft² (465m²) surface area for slab and wall

When the total quantity of given class in concrete less than 50 yd³ (38m³) sample is not necessary to be taken as long as satisfactory strength test result submitted and approval by client (building official)

for example, today in your project you cast blinding concrete use class A concrete 20 yd³ than cast pedestal use class B 20 yd³ so  the total quantity 40 yd³ less than 50 yd³ so you may not have taken a sample as long as compressive strength test record from previous structure submitted and approved by client (building official). Remember don't be confidence to do it before get written approval!

Specified compressive strength of concrete determine by average of 2 cylinders from same sample at 28 days age

Remember 

• Sample to be taken in strictly random to ensure validation and acceptance of concrete
• You may have taken more than 1 sample. Standard above is minimum requirement
• 1 sample consist of 3 cylinders (1 for prediction in 7 days or 14 days and 2 for final 28 days)

Note:

• 1 cubic yard = 0,76 meter cubic
• 1 square feet = 0.093 meters square
• fc' = Specified compressive strength of concrete 

Reference: ACI 318 Building code requirements for structural concrete and commentary