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1 Class #25.1 Civil Engineering Materials – CIVE 2110 Definitions Material Properties Concrete Compressive Strength, f’ c Fall 2010 Dr. Gupta Dr. Pickett

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2 AdvantageDisadvantageAdvantageDisadvantage ShapesAny shapeMust make formsManufactured shapes Limited shapes Fire resistance 1-3 Hr. resistance with NO coating Must add fire-proof coating Maintenance Less, No need to paint More, Must paint for corrosion resistance Time dependent Creep due to long term load. Shrinkage due to curing. More thermal expansion and contraction Strength Low tensile strength. Low strength/volume ratio. High tensile strength. High strength/volume ratio. Weight Higher, More seismic load Lower, Less seismic load Stiffness Rigid, Less; drift, deflection, vibrations. Flexible, More; drift, deflection, vibrations. Reinforced Concrete Structural Steel

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3 What is Reinforced Concrete? Definition: Definition: A construction material composed of: A construction material composed of: Course Aggregate – particles > 0.25“ diameter, retained on #4 sieve. Course Aggregate – particles > 0.25“ diameter, retained on #4 sieve. Fine Aggregate – Sand particles < 0.25” diameter, pass #4 sieve. Fine Aggregate – Sand particles < 0.25” diameter, pass #4 sieve. Water Water Cement powder cement paste, Cement powder cement paste, Forms a gluing paste, when mixed with proper amount of water Forms a gluing paste, when mixed with proper amount of water Reinforcement bars – steel (if no reinforcement, use ACI 318, Ch.22) Reinforcement bars – steel (if no reinforcement, use ACI 318, Ch.22) Two Methods of Reinforced Concrete Construction: Two Methods of Reinforced Concrete Construction: : members are constructed at their final location; Cast-in-Place: members are constructed at their final location; A form (wood) or mold (steel) is built in the shape of the member, A form (wood) or mold (steel) is built in the shape of the member, Reinforcement bars are placed inside form (mold); Reinforcement bars are placed inside form (mold); Concrete is poured into form (mold). Concrete is poured into form (mold). : members are constructed off-site; Pre-Cast: members are constructed off-site; Members are transported to their final location, Members are transported to their final location, Members are erected and joined to form a structure. Members are erected and joined to form a structure.

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4 Cast-In-Place Concrete I-75, Suder Ave. ramp McCormac, 8 th ed., p.73

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5 Pre-Cast Concrete Veterans Glass City Skyway bridge

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6 Reinforced Concrete Structures Load bearing masonry walls. Gravity loads supported by columns. Fig. 4-1,MacGregor, 5 th edition, 2009, Pearson/Prentice Hall One-way slab Two-way slab One-way slab MacGregor, 5 th ed., Fig. 4-1

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7 Reinforced Concrete Structures Floor slabs: One-way or Two-way; One-way slab: Takes load in only One direction, Slab forms top flange of T-beam joist, T-beam takes load in only One direction, Load transferred to T-beam joist, T-beam transfers load to girder, Girder transfers load to column (or wall), Column (or wall) transfers load to; Piles, Spread footings. - one-way slab; L/L 1 > 2 - two-way slab; L/L 2 - two-way slab; L 2 /L 1 < 2 L2L2 L1L1 One-way slab MacGregor, 5 th ed., Fig MacGregor, 5 th ed., Fig. 4-36

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8 Reinforced Concrete Structures Floor slabs: One-way or Two-way; Two-way slab: ACI 318, Chapter 13, Transfers load in Two directions to girder or column, Two-way slab MacGregor, 5 th ed., Fig MacGregor, 5 th ed., Fig. 5-22

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9 The design Engineer must: specify the exterior dimensions of members so that the members have; Adequate strength to resist loads, ACI 318, Ch Adequate stiffness to prevent excessive deflections, ACI 318, Sect specify the reinforcement, - size, quantity, location. ensure constructability of members; Rebars must not interfere with each other, Need space for concrete to flow around rebars, Adequate strength during – erection, curing. Dimensions and Tolerances

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10 Dimensions and Tolerances The design Engineer should specify: Calculations; 3 significant digits, Exterior dimensions of beams, columns; In whole inch increments, Slab thickness; In half-inch increments, Rebar size, length; Bar sizes are manufactured in 1/8 in. increments, Length in two-inch increments, ACI 318, Sect Concrete cover; In half-inch increments, Rebar diameter = 9/8”

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11 Dimensions and Tolerances The design Engineer should ensure construction tolerances of: Exterior dimensions of beams and columns; 0.5 inch, Slab thickness; 0.25 inch, Concrete cover; ACI 318, Sect ; inch, effective depth, d 8 inch, 0.5 inch, effective depth, d > 8 inch,

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12 Material Properties In any beam (concrete, steel, masonry, wood): Applied loads produce Internal resisting Couple, Tension and Compression forces form couple. Positive bending moment, Axial Compression forces in the top regions, Axial Tension forces in the bottom regions, MacGregor, 5 th ed., Fig. 1-4

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13 Material Properties In a concrete beam: - occur in areas of, - Cracks occur in areas of Tension, - Beam will have sudden failure reinforcement is - Beam will have sudden Brittle failure unless Steel reinforcement is present to take present to take Tension. MacGregor, 5 th ed., Fig. 1-4

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14 Material Properties Concrete is: Strong in Compression, Weak in Tension, Cracks occur in Concrete when: Cracks occur in Concrete when: Tensile Stress can be due to: Tensile Stress can be due to: Loads Loads Restrained shrinkage during curing Restrained shrinkage during curing Temperature changes Temperature changes

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15 Material Properties = Specified Compressive Strength of Concrete Nominal strength ( n ) is based upon Design Strength ≥ Required Strength Reduced Nominal Strength ≥ Factored Up Load n ≥ U ACI 318, Sect. 5.3; ACI 318, Sect. 5.3; In order to validate a specified, concrete plant must have; In order to validate a specified, concrete plant must have; Strength test records 12 months old, Strength test records 12 months old, A sample standard deviation, A sample standard deviation, established from 30 consecutive compressive strength tests established from 30 consecutive compressive strength tests 2 cylinders tested per test 2 cylinders tested per test

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16 Material Properties ACI 318, Sect. 5.3; ACI 318, Sect. 5.3; In order to validate a specified ; In order to validate a specified ; A Required Average Compressive Strength,, must be obtained; A Required Average Compressive Strength,, must be obtained; For For Use the computed from Eq. (5-1) and Eq. (5-2); Use the larger value computed from Eq. (5-1) and Eq. (5-2); Eq. (5-1) Eq. (5-1) Eq. (5-2) Eq. (5-2) Eq. (5-1) is based on a probability of 1-in-100 that the average of 3 consecutive tests may < specified. Eq. (5-1) is based on a probability of 1-in-100 that the average of 3 consecutive tests may < specified. Eq. (5.2) is based on a probability of 1-in-100 that an individual test may be more than 500 psi below specified. Eq. (5.2) is based on a probability of 1-in-100 that an individual test may be more than 500 psi below specified.

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17 Material Properties ACI 318, Sect. 5.3; ACI 318, Sect. 5.3; In order to validate a specified ; In order to validate a specified ; A Required Average Compressive Strength,, must be obtained; A Required Average Compressive Strength,, must be obtained; For For Use the computed from Eq. (5-1) and Eq. (5-3); Use the larger value computed from Eq. (5-1) and Eq. (5-3); Eq. (5-1) Eq. (5-1) Eq. (5-3) Eq. (5-3) Eq. (5-1) is based on a probability of 1-in-100 that the average of 3 consecutive tests may < specified. Eq. (5-1) is based on a probability of 1-in-100 that the average of 3 consecutive tests may < specified. Eq. (5.3) is based on a probability of 1-in-100 that an individual test may be < specified. Eq. (5.3) is based on a probability of 1-in-100 that an individual test may be < specified.

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18 Material Properties ACI 318, Compressive Strength Test; ACI 318, Compressive Strength Test; Standard Cylinders; Standard Cylinders; Concrete samples taken per ASTM C172, Concrete samples taken per ASTM C172, Concrete samples molded, cured per ASTM C31, Concrete samples molded, cured per ASTM C31, Concrete strength tested per ASTM C39; Concrete strength tested per ASTM C39; 6”x12” cylinders, 6”x12” cylinders, Fill cylinder with concrete, Fill cylinder with concrete, Allow concrete to harden in cylinder, Allow concrete to harden in cylinder, 24 hours, 60˚ 80˚F, no moisture loss, 24 hours, 60˚ 80˚F, no moisture loss, Strip the cylinder mold, Strip the cylinder mold, Place cylinder in a curing room (100% humidity) or water tank at 72˚F, Place cylinder in a curing room (100% humidity) or water tank at 72˚F, After 28 days, After 28 days, Load 2 cylinders in compression at rate of 35 psi/sec. Load 2 cylinders in compression at rate of 35 psi/sec. Record failure load, calculate failure stress. Record failure load, calculate failure stress. 6” 12”

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19 Cracking & Failure Mechanisms Concrete (and all materials) Concrete (and all Brittle materials) on the plane of fail on the plane of Stress Max Normal Tension Stress Will have Tension cracks Will have Tension cracks parallel to applied load, parallel to applied load, on plane of on plane of P P Apply a Normal Stress in Compression – concrete Compression Cylinder Test: Plane of max Tension

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20 Mohr’s Circle Method – Failure Modes Apply a Normal Stress in Compression – Split Cylinder Test: Ductile Material fails by Buckling. Brittle Material fails on plane of max NORMAL (Tension) Stress, Failure stress is 2x90˚=180˚ on Mohr Circle from applied stress 90˚ 2x90˚ Steel Ductile Concrete Brittle 90˚ Plane of max Tension Compression Tension

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21 Mohr’s Circle Method – Failure Modes Apply a Normal Stress in Tension: Ductile Material fails on plane of From to failure stress = 2x45˚=90˚ on Mohr Circle Brittle Material fails on plane of acts on plane perpendicular to applied Tension load. 45˚ 90˚ 2x45˚ Steel Ductile Cast Iron Plexiglass Brittle Plane of max Tension Tension Compression

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22 Mohr’s Circle Method – Failure Modes Brittle concrete fails on plane of max normal (tension) Stress. Failure stress located at: 2x90˚=180˚on Mohr Circle 2x45˚ 2x90˚ Shear Stress Normal Stress Principal Stress Neutral Axis 90˚ Plane of max Tension Concrete Brittle

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23 Cracking & Failure Mechanisms Concrete cracking process; - : (MacGregor, 5 th ed., pp ) - 4 stages: (MacGregor, 5 th ed., pp ) ; (0) Overall Cracking Process ; - individually, cement paste & aggregate each have brittle, linear stress-strain curves, each have brittle, linear stress-strain curves, - during a cylinder compression test, - friction between test machine head-plates and cylinder ends, - prevents lateral expansion at cylinder ends, - this restrains vertical cracking near cylinder ends, - this strengthens conical regions near cylinder ends, - vertical cracks at mid-height of cylinder do not enter conical regions. But, in the concrete mixture, the cement paste & aggregate together the cement paste & aggregate together produce a non-linear stress-strain curve, produce a non-linear stress-strain curve, that appears slightly ductile, that appears slightly ductile, due to the gradual micro-cracking within the mixture and within the mixture and redistribution of stress throughout redistribution of stress throughout the concrete mixture. the concrete mixture. (MacGregor, 5 th ed., Fig. 3.13)

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24 Cracking & Failure Mechanisms Concrete cracking process; Concrete cracking process; - : (MacGregor, 5 th ed., pp ) - 4 stages: (MacGregor, 5 th ed., pp ) during curing; (1) No-Load Bond Cracking during curing; - cement paste shrinks, - shrinkage restrained by non-shrinking aggregate, - shrinkage causes tension in the concrete, - occur along interface - No-Load Bond Cracks occur along interface between cement paste and aggregate, between cement paste and aggregate, - cracks have little effect on concrete at low loads, - stress-strain curve remains nearly linear up to - stress-strain curve remains nearly linear up to

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25 Cracking & Failure Mechanisms (MacGregor, 5 th ed., Fig. 3.1)

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26 Cracking & Failure Mechanisms Concrete cracking process; Concrete cracking process; - : (MacGregor, 5 th ed., pp ) - 4 stages: (MacGregor, 5 th ed., pp ) ; (2) Stable Crack Initiation ; - occur from one aggregate to - Bond Cracks occur from one aggregate to another piece of aggregate, another piece of aggregate, - cracks are stable, - cracks will propagate only if load is increased, - cracks will propagate only if load is increased, - additional load is redistributed to un-cracked portions, - causes gradual curving of stress-strain curve.

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27 Cracking & Failure Mechanisms Concrete cracking process; Concrete cracking process; - : (MacGregor, 5 th ed., pp ) - 4 stages: (MacGregor, 5 th ed., pp ) ; (3) Stable Crack Propagation ; - occur between Bond Cracks, - Mortar Cracks occur between Bond Cracks, - cracks develop parallel to the compressive load, due to due to local stress reaching (Mohr Circle), local stress reaching (Mohr Circle), - crack do not grow during constant load, - cracks propagate only with increasing load, - stress-strain curve continues to curve. - stress-strain curve continues to curve. - the onset of this stage is called the. - the onset of this stage is called the Discontinuity Limit.

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28 Cracking & Failure Mechanisms Concrete cracking process; Concrete cracking process; - : (MacGregor, 5 th ed., pp ) - 4 stages: (MacGregor, 5 th ed., pp ) ; (4) Un-Stable Crack Propagation ; - lengthen with constant load, - Mortar Cracks lengthen with constant load, - additional cracks form, - few undamaged portions remain to carry additional load, to carry additional load, - cracks propagate without increasing load, - this is an condition, - this is an unstable condition, - stress-strain curve becomes very non-linear, - stress-strain curve becomes very non-linear, - eventually, stress-strain curve begins to flatten, - failure will occur. - The onset of this stage is called at - The onset of this stage is called Critical Stress at

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29 Cracking & Failure Mechanisms Concrete cracking process; Concrete cracking process; - : (MacGregor, 5 th ed., pp )) - 4 stages: (MacGregor, 5 th ed., pp )) ; (4) Un-Stable Crack Propagation ; - - Critical Stress; lateral strains caused by - significant lateral strains caused by large amount of micro cracks, large amount of micro cracks, - volumetric strain increases, significantly, - volumetric strain increases, significantly, - causes outward force on lateral confining reinforcement, - causes outward force on lateral confining reinforcement, - spirals, - lateral ties, - confining reinforcement becomes in Tension, - confining reinforcement becomes in Tension, - confining Steel restrains concrete expansion and disintegration, - confining Steel restrains concrete expansion and disintegration, - puts column in a state of Triaxial Compressive Stress. - puts column in a state of Triaxial Compressive Stress.

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30 Uni-Axial vs. Bi-Axial Loadings So far, discussion has involved So far, discussion has involved Uni-Axial loading; (MacGregor, 5 th ed., Fig. 3.12) Uni-Axial tension, points B or B’ Uni-Axial compression, points A or A’ Concrete always cracks on plane of

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31 Uni-Axial vs. Bi-Axial Loadings (MacGregor, 5 th ed., Fig. 3.12) Bi-Axial Compression; from points A-C-A’ - Bond Cracks - Delays the formation of - Bond Cracks - Mortar Cracks - Mortar Cracks - Stable crack propagation - longer time - higher load - higher load Due to Bi-Axial Compression; failure at point C ≈

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32 Tri-Axial Loadings (MacGregor, 5 th ed., Fig. 3.15) (MacGregor, 5 th ed., Fig. 3.16) Tri-axial Compression ; - Compared to uni-axial compression; - higher compressive strength, - more ductile, In columns: - Uni-axial compression causes outward force on lateral confining reinforcement, lateral confining reinforcement, - spirals - spirals - ties - ties - confining Steel restrains concrete expansion and disintegration, - reinforcement becomes in Tension, as it restrains concrete expansion - puts column into Triaxial Compression

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33 Cracking & Failure Mechanisms Confining reinforcement ; - - saved Olive View Hospital from complete collapse; - - saved building in Philippines from complete collapse;

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34 Cracking & Failure Mechanisms Confining reinforcement ; - double spiral reinforcement used in bridge piers by CALTRANS, - puts column - puts column into a state of into a state of Triaxial Triaxial Compressive Compressive Stress. Stress.

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