2. Concrete Properties presentations.ppt

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Unformatted text preview: Properties of Concrete A. Constituents of Concrete 1. Portland Cement 2. Aggregates 3. 4. 5. Fine Coarse Water Air Admixtures Properties of Hardened Concrete A. Compressive Strength B. Stress Strain Relationship Stress - Strain fcr, Compressive Strength P Stress, A X 75 – 80% of ultimate X 30 – 40% of ultimate l Strain, l Failure I. Factors Affecting Concrete Strength 1. Water-Cement Ratio high w/c, low strength low w/c, high strength 2. Type of Cement high early, type III low heat, type IV Factors – Concrete Strength (con't) 3. Aggregates - Material Strong – felsite, traprock, quartzite Medium – limestone, granite Soft – sandstone, marble - Shape Strong – angular Weak - rounded Factors – Concrete Strength (con't) 4. Curing Conditions moisture temperature 5. Age – typically gains strength w/ age, rate of level off after some time 6. Rate of Loading very slow rates – reduces strength very, very fast (e.g. earthquake loading) - increases strength gain will almost ACI 301-16 (Specifications for Structural 30 or more tests Concrete) to establish fcr and s where, s = standard deviation 1/2 2 x i x s n1 Statistical Variations Mean = fcr (Required average compressive strength) 30 Number of Tests 25 20 Nominal (Specified) f’c Normal Distribution 15 10 5 Concrete Compressive Strength Calculating fcr (in psi) Larger of: 1) fcr = f’c + 1.34s Or 1) fcr = f’c + 2.33s – 500 if fcr = 0.9f’c + 2.33s if f c 5000 f c 5000 Probabilities Eqn 1) provides a probability of 1 in 100 that the average of 3 consecutive tests will be below the specified strength (f’c). Eqn 2) provides a probability of 1 in 100 that an individual test will be more than 500 psi (or 10%) below the specified strength (f’c).. Calculating f’cr Less than 30 tests to establish f’cr and ss (ACI 301-16) Use same table used for 30 or more, but apply modification factors for the sample standard deviation No. of tests Modification factor (MF) Less than 15 Use table 15 1.16 20 1.08 25 1.03 30 or more 1.00 Calculating f’cr Less than 15 tests to establish f’cr and ss (ACI 301-16) Specified compressive strength, psi Required average compressive strength, psi f’c < 3000 f’cr = f’c + 1000 3000 ≤ f’c ≤ 5000 f’cr = f’c + 1200 f’c > 5000 f’cr = 1.10f’c + 700 Quality Control Standard deviation is a function of quality control (QC) Coefficient of Variation (COV) ss COV 100% x COV < 10% - Excellent QC COV > 20% - Poor QC Example An engineer needs a concrete compressive strength of 6000 psi for the columns used in the design of a 10 story office building. A trial mix is batched and there are 22 test specimens. The average compressive strength of the specimens is 6400 psi. If the standard deviation of the compressive strength of the samples is 460 psi, what is the required average compressive strength and is the tested average strength sufficient. Example f’c = 6,000 psi, samples = 22, ss = 460, MF = 1.06 f = f’ + 1.34s f = 0.9 f’ + 2.33s cr c s cr c s fcr = 6,000 + 1.34(1.06)(460) fcr = 6653 psi fcr = 0.9(6,000) + 2.33(1.06)(460) fcr = 6536 psi f’c = 6653 psi (larger of the two) Is this Concrete okay? II. Tensile Strength of Concrete 1. Concrete – strong in compression weak in tension 2. Tensile Strength = 8-15% of Compressive Strength 3. Standard Tests - Beam - Split Cylinder A. Beam Tensile Test Side View P 8" End View P 8" 8" Plain Concrete Beam 30" Loaded until fails due to cracking on tension side 6" 6" Beam Tensile Test – con't. P 8" P 8" Stress Distribution on Cross-Section 8" C N/A y T Beam Tensile Test – con't. P 8" P 8" Stress Distribution on Cross-Section 8" C y T N/A Flexural Tensile Strength . . . or Modulus of Rupture: Range = 8 to 12 f c fr M 6 M 6 8P 2 0.222 P 66 2 S bh Split Cylinder Tensile Test Standard 6" x 12" compression test cylinder is placed on its side and loaded in compression along the diameter 2P Splitting Tensile Strength:f ct ld Range = 6 to 8 f c Relationship between Compressive & Tensile Strengths of Concrete ACI says . . . (1) For calculating deflections: use modulus of rupture: f r 7.5 f c (2) For calculating strength – use lower value: f r 6 f c III. Time Dependent Properties (Shrinkage & Creep) 1. Shrinkage – - Drying Shrinkage Due to loss of adsorped water layer from surface of particles which forms around cement particles - Carbonation Shrinkage Occurs in carbon-dioxide rich environments (such as parking garages) Factors Affecting Shrinkage 1) Water Content~ higher water content, more shrinkage 2) ~ higher cement content, more shrink Cement Content 3) Cement Fineness~ finer – more surface area – more shrinkage ~ aggregates restrain shrinkage Factors Affecting Shrinkage (con't.) 4) Member Shape~ large volume w/ small surface area – less shrinkage 5) ~ largest for RH less than 40% Relative Humidity partially recoverable upon rewetting the concrete Shrinkage vs. Time Rate decreases w/ time Shrinkage Strain Time III. Time Dependent Properties (Shrinkage & Creep, con't.) 2. Creep~ increase in strain under constant load with time Load Removed Elastic Recovery Strain Creep Recovery Creep Strain Initial Elastic Strain Load Applied Time Residual Strain (permanent deformation) More on Creep Reinforcing Steel - doesn't creep - restrains concrete creep IV. Properties of Reinforcing Steel Steel ~ strong in tension Reinforcing bars are usually round, with deformations on surface Why Deformed ??!! BOND !! Design Assumption is that concrete & steel bond together perfectly they deform together (if properly developed). Smooth Bar Deformed Bar Chemical bond Mechanical bond Produced according to ASTM standards (size, chemical & mechanical properties) ASTM A 615 –most common ASTM A 706 – special applications (weldability, bendability, ductility) ASTM A 996 – rail & axle steel (rare) More on Reinforcing Steel Available in 4 grades (grade = yield strength in ksi) - Gr. 40 ~ most ductile, old fashioned, small - Gr. 50 - Gr. 60 ~ most common for buildings & bridges - Gr. 75 ~ used in large columns And more on Reinforcing Steel . . . Commonly called "rebar" Sizes – nominal diameter in 1/8ths of an inch For example: #4 bar has a diameter of 4/8ths inch (or ½") True up to bar size #9. From #10 & up, diameters are slightly larger Size & grade marks are rolled into bars Also available in metric sizes Stress – Strain Behavior of Steel Wire Fabric Stress, ksi Strain Hardening Yield plateau Es = 29,000 ksi .01 .02 Strain, in/in .02 .04 Another topic down! ...
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  • Spring '18
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