We address a problem of network design with minimum cost, and uniform all-to-all demands between the vertices. We deal with the case of concave increasing link cost function f depending of the capacity over directed arcs. We obtain lower bounds for this problem. In the generic case $f:x\mapsto x^\alpha$, where $\alpha\in[0;1]$, we exhibit some families that constitute an 1.12 asymptotical approximation of the optimal network.