This paper discusses Core Test and Load Test as a method to evaluate Structural Strength for existing structural system. The primary objective of the two methods is to assess structural strength, safety and serviceability. Failure in beams and roof Slabs structural strength were targeted to determine the causes of the failure, and further analysis has been done to suggest the best methods to repair the defected structural system. In this paper a two-way roof slab system for two different vacant villas in Kuwait City was inspected, at different construction time intervals. As a result, weakness in concrete strength was present, and a deflection appears in roof slabs, also cracks appeared in some of slabs and beams. Therefore, decision was made to do further structural safety investigation, and study the causes of such failure. Load test and core test according to ACI-318, were carried out to measure and evaluate the strength of the existing concrete. The result indicates that the structural elements is unsafe, so a necessitate procedure should be carried-out to support these elements, like re-design, recasting, adding additional structure support. Through investigation and analysis, several reasons were found that leads for failure in concrete strength. Finally, recommendations to assess the safety of the structure design is given.