check your hourly rate $$$

to see if the expected overtime has been factored in it, this can only be determined by yourself.  Also, since your offer said up to 10 hours a day, the worst case is that you would work 10 hours a day, if they ask you to work 12 hours, you can tell them very clearly that your contract is to work up to 10 hours a day.  Do not expect your PM or colleagues would know your contract automatically, you have to tell them so that they do not have unrealistic expectation on you.

As for working 6X12, in my view, it is highly unlikely.  because that won't be productive, your project manager knew that, the construction may be going 24/7, it does not mean everybody on the project works 24/7.