Hardened Concrete Properties u Strength u compressive strength psi u tensile strength psi u flexural strength u compression >> tension since concrete is notch sensitivite

Factors Affecting Strength u Curing conditions, humidity u temperature u w/c, (inversely related) Abram’s law u air content, (inversely related), short and long term u aggregate characteristics, roughness,grading, minerological. u cement type, composition, fineness, type I vs. type III u cement content (directly related) u Strength porosity relationship u mixing water

Strength and Curing time in air entire time moist cured entire time in air after 3 days in air after 7 days Strength %

Factors Affecting Concrete Strength

Durability u Resistance to freezing and thawing u Cracking u Internal Problems u Rebar Corrosion

Resistance to freezing and thawing-Major factors u Air Entrainment u w/c, low water -cement ratio/ water content u volume stability, stiff aggregates with low coefficient of thermal expansion

Air Entrainment u Water gains 9% in volume upon freezing u nighttime freezing followed by daytime thawing, u approximately 40 cycles per year, average. max of 200 cycles per year. u fatigue loading of ice formation within pores u Air Entraining Admixture (AEA) Must provide: u Pore size u Pore spacing u Pore specific surface area

Internal voids

Durability-Cracking u path for harmful material to get into concrete u sulfates- soils u cause severe expansion, and deterioration u chlorides -deicing salts u initiate corrosion u Excessive shrinkage

Durability-internal problems u Alkali-silica reaction u excessive sulfates

Rebar corrosion NaCl + -

Mechanism of corrosion

Corrosion Protection u Proper cover of at least 2” u lower w/c u denser concrete u avoid using chlorides

Review u Strength u Durability u Water Tightness u water cement ratio u Permeability u Volume Stability u shrinkage deformation with no load applied u creep deformation under sustained loading

Load Induced Volume Changes u Instantaneous, 1D Secant modulus Tangent modulus

Load Induced Volume Changes u Time dependant Creep deformation Deformation Time

Creep in Concrete

water Creep

Consequences of creep u Loss in pre-stress u possibility of excessive deflection u stressing of non load bearing members

Economy u Cement Content u 50-60$/ton u Aggregates u 5-6 $/ton u minimum cement required at the minimum water cement ratio, with the maximum strength and durability