Civil engineers apply scientific and engineering principles to improve the built environment and to provide sustainable infrastructure systems. Civil engineers analyze and design structural components of buildings and bridges; improve seismic safety; investigate soil-structure interactions; design pavements and test new asphalt and concrete mixtures; implement strategies for improving traffic flow within a transportation network; and develop treatment strategies for water, wastewater, and environmental contaminants. Civil engineers use computer design and simulation to envision new systems and to retrofit existing ones.