Large-area samples of graphene tend to be polycrystalline (PC) on some substrates. Grain boundaries with structural defects are expected to alter the structural and electronical properties of graphene. In this work, the mechanical properties of PC graphene are studied by means of density-functional theory and furthermore the electrical and thermal transport properties are addressed. To construct grain boundaries of zigzag and rotated armchair graphene sheets, molecular dynamics simulations are performed. The critical strain leading to structural failure of PC graphene nanoribbons is only half the value of pristine armchair nanoribbons. However we show that it can be significantly enhanced by the reaction of the chemically active grain boundaries with atmospheric gases. The transport properties of those systems are investigated, both parallel and perpendicular to the grain boundary, using an ab initio based atomistic model combined with Landauer transport theory and recursive Green function method. The electronic part is calculated within a tight-binding model and a force-constant approach has been applied for phonon transport.