**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 5.3.2.2 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
- Cousins
- Compressive strength, Fcr