More than 7000 reinforced concrete (R/C) buildings were destroyed after the Mw 7.8 Pedernales earthquake in Ecuador. In this paper, concrete materials used in such buildings are studied as a possible source of vulnerability. They have been analyzed and tested (lab and SEM). Evidence found confirmed that quality of construction materials is far from acceptable, and construction practices applied were very poor. Use of sea sand caused very high corrosion level in steel bars and mylonitization of coarse aggregates used in concrete produced very low strength materials. Those facts were the main reasons of damage and collapse of thousands of buildings.