Experimental evaluation of concrete strength in existing buildings

An extensive field and laboratory study is carried out to evaluate the concrete strength of existing building stock in Turkey. The concrete strength of each building was determined by core sampling and laboratory testing. Randomly selected components for core sampling were identified to meet with minimum requirements of standards or pre-standards for number of tests. A total of 1679 randomly selected core samples were extracted from 167 public buildings for laboratory testing. The average and coefficient of variation values of each building were obtained. The upper-bound value of the expected concrete strength is calculated as the mean less one standard deviation. It is observed that the expected concrete strength values are drastically lower than the specified ones. Also, the variability in concrete strength values is considerably high compared to the specified strength of buildings. Almost one-half of the investigated buildings have concrete strength values as low as that of the collapsed buildings during past earthquakes in Turkey. Considering that the current study validates low concrete strength in a significant number of existing buildings as observed in different buildings from various regions of Turkey during the earthquakes at Duzce and Kocaeli in 1999 and at Bingol in 2003, a considerable portion of existing building stock is at serious risk in the case of a seismic event.