In this study, concrete of many reinforced concrete buildings damaged during the 17th August 1999 Izmit Earthquake and 27th June 1998 Adana-Ceyhan earthquake has been investigated. Low concrete quality has been a major cause in the total collapse of minor or heavy damage in reinforced concrete buildings in the area affected by the earthquakes. Core specimens, which were extracted from the buildings which collapsed or were damaged during Adana-Ceyhan earthquake were tested for the compressive strength of concrete. In addition, further experiments were carried out on the sea sand, which had been left unused on the construction site of the collapsed buildings in Avcilar, Istanbul, for determining its effect on the compressive strength of concrete. In those buildings, in which sea sand was used, aside from low concrete strength, significant loss of cross-sectional areas of steel reinforcement bars caused by the corrosion due to the presence of excessive chlorine ions was observed. It is noticed that in addition to its yielding low compressive strength concrete, sea sand is more dangerous from the standpoint of reinforcement corrosion, which may result in failure of the structure in a period of 10 or 20 years even under static loads. (C) 2004 Elsevier Ltd. All rights reserved.