Is it legal for my interviewer to ask for my previous construction salary?

One of the primary reasons people switch jobs is to earn more money, so there's nothing wrong with trying to advance your earnings and your construction career.  What you ought to do is put a sentence in your cover letter like, "My current construction salary is $X per year, which is a large part of the reason for my interest in securing another position.  My anticipated construction salary is negotiable within the $X+10%K to $X+15%K range." then see what happens.