Concrete of many reinforced-concrete buildings damaged during the 17th August Marmara Earthquake has been investigated. In most of these buildings, 1) concrete was produced on site by volumetric methods based on judgment of workers, 2) only one natural group of aggregate was used without paying attention to its granulometry, 3) vibrators of any kind were not used during placement of fresh concrete, 4) sufficient cure measures were not taken after placement, and 5) in many buildings in close proximity to sea shores, sand quarried directly from a nearby seaside only was used. As a result of all these malpractices, the concrete quality of the collapsed and damaged but still standing buildings was observed to be far below the C20 class requested in the specification for structures in earthquake areas. In those buildings where sea sand was used, aside from low concrete strength, significant loss of cross-sectional areas of steel reinforcement bars caused by corrosion due to excessive chlorine ion presence was observed. It is noticed that next 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.