Introduction. Buildings and structures are exposed to groundwater to varying degrees. The design of the expansion joint in the deepened part of the building must reliably protect it from the penetration of water and aggressive environments.

Waterproofing keys are among the primary measures for protecting concrete and reinforced concrete structures, but methods for assessing the effectiveness of this kind of materials have not yet been proposed. Also in the scientific literature there is no data on the water resistance of keys depending on its cross-section and the number of anchors.

Aim. To propose a method for assessing the waterproofness of waterproofing keys and to determine the dependence of the waterproofness of a key in a concrete sample on its width and the number of sealing anchors.

Materials and methods. Two keys of different widths and with different numbers of anchors were chosen as the test object: 240 mm wide key (number of anchors – 4 pcs); 320 mm wide key (number of anchors – 6 pcs).

For testing, the keys were welded in the form of a rectangular parallelepiped and concreted on the bottom and top sides, thus forming a closed loop for water inside. Water was supplied using a pump with a frequency pressure converter. The pressure was raised in steps of 1 water column meter for an hour until water leaks.

Results. In the case of testing of key No. 1, leakage occurred at a pressure of 0.07 MPa. Key No. 2 withstood greater pressure and leaked at 0.09 MPa. In both cases, leaks occurred at the junction of the key and the concrete.

Conclusions. Thus, the number of sealing anchors directly affects the water pressure that the key can withstand in real-life conditions. The standard method for determining water resistance according to State Standard 12730.5 for the construction of an expansion joint turned out to be ineffective due to the high initial test pressure of 0.2 MPa.