Properties of Hardened Concrete

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29 Νοε 2013 (πριν από 3 χρόνια και 9 μήνες)

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Dr. Kimberly Kurtis
School of Civil Engineering
Georgia Institute of Technology
Atlanta, Georgia
Properties of Hardened Concrete
Outline

Compressive strength

E

Tensile strength

Drying Shrinkage

Creep
Compression Testing

Uniaxial compressive strength of
concrete is easy to measure

It has become the standard gauge of
concrete quality (for better or worse)
Some notes about “failure”:

With most materials, failure is associated with the appearance
of cracks

Concrete intrinsically contains many cracks, which will
propagate under loading

However, cracks may or may not be visible at the surface when
concrete fails
Compression Testing

Compressive strength is determined according to
ASTM C469, where a 6x12” or 4x8” cylinder, cured for
28 days, is tested at a load rate of 20-50 psi/sec.

Can also be performed 1d, 3d, 7d, 28d, 90d.
Typical 28-day strengths are

Normal strength 3-6 ksi

High strength 6-9 ksi

Ultrahigh strength 10-18+ ksi
Factors Influencing Strength

Time

Curing conditions
Factors Influencing Strength

W/C or W/CM

Number/size voids

Cement content
Factors Influencing Strength

Cement Type (composition)

Cement fineness
Factors Influencing Strength

Use of chemical admixtures

Use of SCMs
Factors Influencing Strength

Aggregate strength

Aggregate MSA

Aggregate/paste bond
strength
Test Parameters

Specimen size

Specimen shape

Load rate
Factors Influencing Strength
Stress-Strain Behavior
Why is concrete less
brittle than the aggregate
and cement paste it is
composed of?
Stress-Strain Behavior
3500psi
4800psi
Elastic Modulus
Elastic Modulus: Estimations
Can also be estimated from compressive strength:

E
c
= 33 w
c
1.5
f
c
0.5
(ACI 318)
*
E
c
= elastic modulus of concrete,psi
W = unit weight,pcf
f
c
=28d compressive strength of standard cylinders, psi

Valid to strengths of at least 6000 psi (perhaps to as
high as 9000 psi)

The unit weight is used to account for the presence
and density of the aggregate

E
agg
is rarely known and this is a useful way to include
its effect in E
*
E
c
= 0.043
w
c
1.5
f
c
0.5
for E
c
in MPa, where w is in kg/m
3
and f
c
is in MPa
Elastic Modulus: Estimations
For normal weight concrete (145pcf),the ACI 318
equation reduces to

E
c
= 57000 f
c
0.5
for E
c
in psi

E
c
= 4.73 f
c
0.5
for E
c
in GPa where f
c
is in MPa
Typical values for E
c
are 2-6x10
6
psi for normal weight,
normal strength concrete
For lightweight concrete, there is a correction for
aggregate density

E
c
= 0.043ρ
1.5
f
c
0.5
for E
c
in GPa where f
c
is in MPa
Elastic Modulus: Models
Parallel Model
E
c
= E
p
V
p
+ E
a
V
a
Assumes ε is same in
aggregate and paste
E
c
=E concrete
E
p
=E cement paste
E
a
=E agg
Assume V
p
+V
a
=1
V
p
=vol paste
V
a
=vol agg
Series Model
1/E
c
=V
p
/E
p
+ V
a
/E
a
Assumes σ is same in
aggregate and paste
Elastic Modulus: Models
E
c

Parallel model
overestimates E
c

Series model
overestimates E
c

Combination
models (like
Hirsch or Counto,
see Ch. 9) do a
pretty good job

Deviations from
actual behavior
are believed to be
due to ITZ effects
Factors Influencing E
c

Aggregate volume

E
agg

Aggregate porosity

MSA

Aggregate shape

Aggregate surface texture

Aggregate mineralogy

Porosity of the paste

ITZ

Testing parameters (speed, moisture state)
Influence microcracking
in the ITZ
Splitting Tension

f
t
~ 8-12% of f
c

ASTM C496 or the “Brazilian
Test” is performed on 6x12”
cylinders

f
t
= 2P/πDL
Can be estimated by:
f
t
=6.7(f
c
)
0.5
for normal strength
concrete where units are psi

Splitting tension test
introduces some
compressive stress at
top and bottom of
(6x12”) cylinder

Measured strength is
10-15% higher than
nominal strength
Splitting Tension
Deformation in Concrete
EARLY AGE CONCRETE

Plastic shrinkage – shrinkage strain associated with
early moisture loss

Thermal shrinkage – shrinkage strain associated with
cooling
LATER AGE CONCRETE

Drying shrinkage -shrinkage strain associated with
moisture loss in the hardened material

Deformations occur under loading
- Elastic
- Viscoelastic (including creep)
Drying Shrinkage and Creep
Both result from movement of water in the hydrated
cement paste, which results in new bonds forming in the
C-S-H; the driving force differs.

For drying shrinkage,
environmental conditions (e.g.,
low external RH) are the
driving force

For creep, stress is the
driving force.
Drying Shrinkage

Inadequate allowance for drying shrinkage can lead to
cracking and warping or curling

Must provide adequately spaced joints in slabs and
pavements

Joints define where the crack will form, rather than allowing
for random crack formation

Can then seal joints to prevent moisture ingress
Creep
Creep can be both beneficial and problematic.

Creep of concrete in prestressed members
Prestressing steel strand embedded in concrete
P
Induced compressive stress balances
tensile stresses expected during service
Creep in concrete can
reduce the pre-stress and
possibly lead to cracking
Creep
Creep can be both beneficial and problematic.

Stress relaxation,
the complement to
creep, can reduce
stress in the
concrete at early
ages and reduce
the likelihood for
early age cracking.
Creep and Shrinkage
Drying Shrinkage
and Creep
Parameters Affecting
Drying Shrinkage and Creep
Influence of Aggregate

Aggregate volume fraction is an important parameter
ε
c
= ε
paste
(1-V
agg
)
n
where n~1.8
Influence of Aggregate

E
agg
is another important factor
Influence of Paste Properties

Prolonged hydration or hydration at elevated
temperatures increase chemical bonding, reducing
creep and shrinkage

Lower w/c concrete creep and shrink less
But, generally, these relationships are complex and
require testing to confirm anticipated behavior