Concrete Design Conceptual Questions

Concrete Design Conceptual Questions Name several of the admixtures use in concrete and the purpose ️Air-Entraining: Helps with freeze/thaw cycles; Accelerating Admix: Reduces curing time; Retarding Admix: Slows curing time; Superplasticizers: Maintains workability with reduced water/cement ratio What factors influence creep of concrete? ️High water to cement ratio; High cement content; Low aggregate content; Long term sustained loading; High air-entrainment What can be done to reduce creep? ️Increasing the thickness of a member because members with smaller thicknesses experience large amounts of creep Why do surfaces of reinforcing bars have rolled-on deformations? ️To provide a space for concrete to adhere with the bars and make a strong bond with the bar What are 3 factors that influence the magnitude of the earthquake load on a structure? ️Mass of the structure, Stiffness of the member, Natural frequency Why are epoxy-coated bars sometimes used in the construction of reinforced concrete? ️They provide corrosion resistance. What are the advantages of the strength design method as compared to the allowable-stress or alternate design method? ️A more realistic factor of safety is used in strength design; The strength method permits more flexible designs; What is the purpose of strength reduction factors? Why are they smaller for columns than beams? ️To overcome analysis approximation, To allow variations in the design equations of understrength methods; Columns primarily take compressive loads and beams incur bending, which means they experience tension, compression, and shear. What are the basic assumptions of the strength design theory? ️The plane section before bending remains plain after bending, All tensile stress are taken by steel only, tensile strength of concrete is ignored, The stress-strain relationship underworking load is a straight line, The value of Young's Modulus for concrete is constant Why does the ACI Code specifiy that a certain minimum percentage of reinforcing be used in beams? ️By providing a minimum reinforcement it will give sufficient warning before failure due to cracking and deflection and sudden failure won't occur Distinguish between tension-controlled and compression-controlled beams. ️Tension: Steel fails before concrete reaches its max strength; Failure is because of yielding of steel in which tensile strain is equal to or greater than 0.005; Deflections are larger and will give warning of failure; Members are ductile Compression: Concrete fails before steel because of over reinforcement; Strain in concrete fibers reaches yield strain, which equals 0.003; Deflections will not be noticeable and failure will occur without warning; Members are brittle Explain the purpose of the minimum cover requirements for reinforcing specified by the ACI Code. ️In order to protect the reinforcement from water/salt infusion and from corrosion, a minimum distance has to be maintained between the edge of the member and the reinforcement. This distance is called cover distance.