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

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