A straight Yes/No (which a Check Box is) is either -1 (Yes) or 0 (No).
Even if you assign a "Default Value" to a check box, once it has been checked, or unchecked, the value reverts to -1 or 0.
If you use a hidden control with an "immediate if", you can effectively accomplish the task and it will look right to the user.
The hidden control determines the value when the check box is checked.
For example, if Check7 is the check box I want to equal 7, then I'll have a hidden text box with the following statement as the control source:
When the check box is checked, the text box has the value of 7. When it's not checked, the text box is empty.
As for concatenating the values, your best bet is to use a function in code
If you use Text1, Text2, Text3, etc. for your text box names, you can use a loop to rifle through all the text boxes in only a few lines of (in straight Visual Basic, it's even easier, because VB allows you to index your controls).