Building repair materials should comply with specific requirements: compatibility with repaired material, high adhesion and durability. Additionally, in structural repair, it should actively contribute to bearing capacity.

In concrete structures, first requirement is met by materials based on mineral binders. When choosing the material satisfying to the conditions of adhesion, one can consider expansive binders. Their increase in volume is a result of the growth of ettringite crystals. They are built also in the joint, fill asperities of ‘old’ concrete surface and penetrate open pores. It results in increase of adhesion and eliminates consequences of shrinkage. Moreover, confinement of the expansion by adjacent parts of structure causes the pressure exerted to joint, increasing the bond strength.

In order to quantitatively evaluate the adhesion of expansive materials to ‘old’ concrete, two groups of tests were carried out. First, tensile strength of cylindrical concrete specimens made of two samples connected by means of cement grout was evaluated. Cement grouts were made with different binders (Portland cement with and without resin additives and expansive cement).

Furthermore, researches of bearing capacity of joint between ordinary and expansive concrete under combined stresses (shear and tension) were carried out. Two types of samples were used: sample with slant shear interface and sample with interface shaped as the surface of a truncated cone. These samples enabled to point out the influence of the volume changes of the concrete (shrinkage and expansion).

In the paper, the results of these tests are presented. The following conclusions are drawn:

- grout based on expansive binder used as a layer between two concrete samples ensure higher adhesion level than other tested materials,

- bond strength between ordinary and expansive concretes tested in combined stress state is higher than in the case of two ordinary concretes.

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