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Class #27.1

Class #27.1. Civil Engineering Materials – CIVE 2110 Concrete Material Shrinkage Creep Thermal Properties Fall 2010 Dr. Gupta Dr. Pickett. 1. Time-Dependent Volume Changes . Concrete volume changes over time due to : (1) Shrinkage; Negative volume change due to curing, drying,

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Class #27.1

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  1. Class #27.1 Civil Engineering Materials – CIVE 2110 Concrete Material Shrinkage Creep Thermal Properties Fall 2010 Dr. Gupta Dr. Pickett 1

  2. Time-Dependent Volume Changes Concrete volume changes over time due to : (1) Shrinkage; Negative volume change due to curing, drying, - shrinkage is less in a structure than in a cylinder, - rebars restrain shrinkage, - less exposed surface area per volume, - structure is built in stages, shrinkage not simultaneous throughout structure. - in a parking garage, concrete absorbs CO2 , - shrinkage due to carbonation = shrinkage due to drying. (2) Creep; Volume change due to load over time, (3) Thermal Expansion/Contraction; Volume change due to change in temperature of mass. (MacGregor, 5th ed., pp. 70-83) 2

  3. Time-Dependent Volume Changes • (1) Shrinkage: Negative volume change, shortening, under constant temperature due to; • Drying & hardening: causes water to evaporate from cement paste, causing shrinkage to occur; - aggregate does not evaporate, does NOT shrink; Shrinkage; • Increases with decreasing humidity, • Increases with increasing cement-to-aggregate ratio, • Cement paste shrinks, aggregate does not shrink. • Increases with increasing water-to-cement ratio, • More water, less aggregate, • water evaporates, paste shrinks, aggregate does not shrink. • Increases with increasing fineness of cement, • Finer cement absorbs more water, water evaporates, more shrinkage. • Decreases with increasing member size, • More mass per exposed surface area, less shrinkage. (MacGregor, 5th ed., pp. 70-83) (Fig. 3.22a, MacGregor, 5th ed.) 3

  4. Time-Dependent Volume Changes • (1) Shrinkage: Equation for Axial Shrinkage Strain between days ts& t in plain concrete: = basic shrinkage strain, for a specific concrete & relative humidity, = coefficient, a function of time and member effective thickness, t = age of the concrete, days, t1 = 1 day ts = age of concrete at end of moist curing, days, Ac = concrete cross sectional area, in2 . u = perimeter of cross section exposed to atmosphere, in. h0 = 4 in. (MacGregor, 5th ed., pp. 70-83) (Fig. 3.24, MacGregor, 5th ed.) 4

  5. Time-Dependent Volume Changes Equation for Axial Shrinkage Strain between days ts& t in plain concrete: = basic shrinkage strain, for a specific concrete & relative humidity, = 1450 psi, = mean compressive strength, at 28 days, psi, from ACI 318, Sect. 5.3.2.1, or assuming a standard deviation of Use = smaller of or = coefficient accounting for type of cement = 50, for Type I = 80, for Type III (MacGregor, 5th ed., pp. 70-83) 5

  6. Time-Dependent Volume Changes • (1) Shrinkage: Equation for Axial Shrinkage Strain between days ts& t in plain concrete: = basic shrinkage strain, for a specific concrete & relative humidity, = coefficient accounting for relative humidity, = +0.25, for Relative Humidity ≥99% For Relative Humidity 40% < RH < 99% RH = Relative Humidity of ambient atmosphere, % RH0 = 100% (MacGregor, 5th ed., pp. 70-83) (Fig. 3.23, MacGregor, 5th ed.) 6

  7. Time-Dependent Volume Changes • (1) Shrinkage: • Example calculations: • Underground parking garage, • Floor slab, 6 in. thick, lightly reinforced, • Floor restrained on outside edge by 16 in. thick basement wall, • Walls 26 months old, moist cured, 5 days, cast against ground, • Slab 24 months old, moist cured, 5 days, not on ground, • Relative humidity, roughly constant over period, 50%, • Concrete; Type I cement, • Shrinkage for reinforced concrete ≈ 0.75 shrinkage plain concrete • Cracks developed in slab, perpendicular to wall, at roughly every 6 ft. • Assume cracks resulted from restraint by wall of slab shrinkage parallel to wall. • Calculate crack width. (MacGregor, 5th ed., pp. 74-76) 7

  8. Time-Dependent Volume Changes • (2) Elastic Strain plus Creep Strain: • Example calculations: • Concrete pedestal, plain (unreinforced), 24”x24”x10’ • Moist cured, not on ground, • Applied load 1 month after casting, • Load causes average stress = 1000 psi., • Temperature, roughly constant over period, 68˚F, • Relative humidity, roughly constant over period, 50%, • Concrete; cement content = 675 Lb/yd3, slump = 3 in. • Compute total shortening in 5 years. (MacGregor, 5th ed., pp. 79-81) 8

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