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

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

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

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4 Time-Dependent Volume Changes (1) Shrinkage: Equation for Axial Shrinkage Strain between days t s & 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, t 1 = 1 day t s = age of concrete at end of moist curing, days, A c = concrete cross sectional area, in 2. u = perimeter of cross section exposed to atmosphere, in. h 0 = 4 in. (MacGregor, 5 th ed., pp ) (Fig. 3.24, MacGregor, 5 th ed.)

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5 Time-Dependent Volume Changes Equation for Axial Shrinkage Strain between days t s & 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 , 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, 5 th ed., pp )

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6 Time-Dependent Volume Changes (1) Shrinkage: Equation for Axial Shrinkage Strain between days t s & 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, % RH 0 = 100% (MacGregor, 5 th ed., pp ) (Fig. 3.23, MacGregor, 5 th ed.)

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

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

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