To just load the text fonts and leave the mathematics alone, use the beton
package instead.

\documentclass{article}
\usepackage{beton}
\begin{document}
Some concrete text and normal \( x^2 - y^2 = t \) maths.
\end{document}
See the description of the concrete
package on CTAN.
To switch the sans serif font I would then suggest you load on appropriate package, e.g. helvet
:

\documentclass{article}
\usepackage{beton}
\usepackage[scaled=0.92]{helvet}
\begin{document}
Some concrete text and normal \( x^2 - y^2 = t \) maths.
\textsf{Something sans serif.}
\end{document}