A new method of repairing damaged structures by injecting the cracks with specially designed polymer mass (flexible two-component grout based on polyurethane resin) has been recently proposed. The technique is mainly dedicated to damaged masonries, especially historical structures where minimum intervention is permitted. The cracks are filled with the special injection, forming the flexible joints bonding the disrupted structural elements. The aim of the present paper is to show the results of the experimental study focused on properties of the polymer mass used for the injections. First, the material has been subjected to static compression tests. Then, the polymer mass has been examined dynamically under harmonic excitations with different frequencies and strain levels. The results of the study indicate that the tested polymer mass shows highly non-linear behaviour with relatively low resistance under small displacements and stiffening effect for higher strain levels. Moreover, it is substantially dependent on the strain rate having higher initial deformation modulus for higher strain rate values. Finally, the observed hysteretic behaviour of the material confirms its potential to dissipate the energy during vibrations preventing from further structural damage in the case of dynamic loading.