  Let $\Gamma$ be a finitely generated group and $G$ be a noncompact semisimple
connected real Lie group with finite center. We consider the space $\mathcal X$
of conjugacy classes of reductive representations of $\Gamma$ into $G$. We
define the {\it translation vector} of an element $g$ in $G$, with values in a
Weyl chamber, as a refinement of the translation length in the associated
symmetric space. We construct a compactification of $\mathcal X$, induced by
the marked translation vector spectrum, generalizing Thurston's
compactification of the Teichm\"uller space. We show that the boundary points
are projectivized marked translation vector spectra of actions of $\Gamma$ on
affine buildings with no global fixed point. An analoguous result holds for any
reductive group $G$ over a local field.
