In my new house the floor has a 2x4 framed opening with pea gravel in it. After I have the piping installed what do I do with the 2x4 framed opening, Do I fill it in with concrete or leave it as is.

Are we talking about a basement shower dap out?

Yeah...

The answer will depend on whether you are installing a tub or a shower...

For a tub, most people leave the box open so you can access the piping in the future.

For a shower, you would pipe all up, fill the boxout with gravel to within an inch of the finished floor and then you would fill the space with the BEDDING SUBSTRATE you will use to set the shower base/pan in... depending on the base/pan you are using.

Let us know if a shower or tub is going in... chat more after that.

Mark

Just to help you with identifying your drains: if the dap out by the wall, it is for tub. If it is away from the wall, it is probably for shower...