  The High Luminosity LHC (HL-LHC) phase is designed to increase by an order of
magnitude the amount of data to be collected by the LHC experiments. The
foreseen gradual increase of the instantaneous luminosity of up to more than
twice its nominal value of $10\times10^{34}\ {\rm cm}^{-1}{\rm s}^{-2}$ during
Phase I and Phase II of the LHC running, presents special challenges for the
experiments. The region with high pseudo rapidity ($\eta$) region of the
forward muon spectrometer ($2.4 > |\eta| > 1.9$) is not equipped with RPC
stations. The increase of the expected particles rate up to 2 kHz cm$^{-1}$ (
including a safety factor 3 ) motivates the installation of RPC chambers to
guarantee redundancy with the CSC chambers already present. The current CMS RPC
technology cannot sustain the expected background level. A new generation of
Glass-RPC (GRPC) using low-resistivity glass was proposed to equip the two most
far away of the four high $\eta$ muon stations of CMS. In their single-gap
version they can stand rates of few kHz cm$^{-1}$. Their time precision of
about 1 ns can allow to reduce the noise contribution leading to an improvement
of the trigger rate. The proposed design for large size chambers is examined
and some preliminary results obtained during beam tests at Gamma Irradiation
Facility (GIF++) and Super Proton Synchrotron (SPS) at CERN are shown. They
were performed to validate the capability of such detectors to support high
irradiation environment with limited consequence on their efficiency.
