You can set the tensile strength to 0 (although we normally recommend a small positive value, e.g., 1/4 or 1/10 of concrete tensile strength), but zero cohesion is very unusual. Also, please set the friction coeff. to a realistic value (typically, 0.3-0.4). Please understand it is very important to set the stiffness values according to the recommendations in the manuals. You can find more information about using contact elements in paragraph "I have problems when using the Interface (GAP) material on contacts" on our ATENA Known Issues page at http://www.cervenka.cz/products/atena/known-problems .