What is the correct procedure to calculate the concrete pressure of 300 x500 x 1200mm rectangular RC beam on plywood forms(i.e. on the sides of fomworks)?
Pressure is a function of temperature, pour rate, etc. ACI-318 covers formwork. The basic information that is in the ACI document is available as a free .pdf download, "Concrete Forming", from the APA Engineered Wood Association at this link (fourth publication on the list)