Natural Frequencies Reduction of RC Slab Subjected to Incremental Concentrated Loads

Reinforced concrete slabs are frequently used in many engineering structures such as buildings and bridges. Damages in an engineering structure including reinforced concrete slab are classified as light, moderate, and severe cracks. A special treatment is required for every typical crack-damage to strengthen its structure. The reinforced concrete slab experiencing cracks reduce stiffness as well as its strength. An experimental program was set up to establish a relation between cracks and changes of a dynamic system characteristics of the slab through natural frequencies. The aim is to perform experimental modal analysis (EMA) as a non-destructive vibration testing method for diagnosing the structural integrity of the slab. For this purpose, two RC square slabs specimens of dimensions 1200mm x 1200mm x 40mm were used.  As a first part of the project, a reinforced concrete slab was subjected to an increasing partially concentrated loads at the slab’s center to introduce cracks. After each incremental load steps (5, 10, 12kN) the slab was unloaded. Then, a free-free set-up for the slab was established by using the flexible elastic ropes (bungee cords). After the dynamic test of 5, 10 and 12kN loads, the slab was strengthened using external CFRP sheets (type TR30S). Next, experimental modal analysis was performed to establish a relation between cracks and changes in natural frequencies of the slab. The overall trend of frequency decrease after occurring damage that would be expected for all modes. However, it is not guaranteed all modes have same sensitivity to the applied load that inflicts severe cracks.