  For dust aggregates in protoplanetary discs a transition between sticking and
bouncing in individual collisions at mm to cm size has been observed in the
past. This lead to the notion of a bouncing barrier for which growth gets
stalled. Here, we present long term laboratory experiments on the outcome of
repeated aggregate collisions at the bouncing barrier. About 100 SiO$_2$ dust
aggregates of 1 mm in size were observed interacting with each other.
Collisions occured within a velocity range from below mm/s up to cm/s.
Aggregates continuously interacted with each other over a period of 900 s.
During this time more than $10^5$ collisions occured. Nearly 2000 collisions
were analyzed in detail. No temporal stable net growth of larger aggregates was
observed even though sticking collision occur. Larger ensembles of aggregates
sticking together are formed but were disassembled again during the further
collisional evolution. The concept of a bouncing barrier supports the formation
of planetesimals by seeded collisional growth as well as by gravitational
instability favouring a significant total mass being limited to certain size
ranges. Within our parameter set the experiments confirm that bouncing barriers
are one possible and likely evolutionary limit of a self consistent particle
growth.
