1
Class #25.1
Civil Engineering Materials
–
CIVE 2110
Definitions
Material Properties
Concrete Compressive Strength, f’
c
Fall 2010
Dr. Gupta
Dr. Pickett
2
Advantage
Disadvantage
Advantage
Disadvantage
Shapes
Any shape
Must make forms
Manufactured
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
3
What is Reinforced Concrete?
Definition
:
A construction material composed of:
Course Aggregate
–
particles > 0.25“ diameter, retained on #4 sieve.
Fine Aggregate
–
Sand particles < 0.25” diameter, pass #4 sieve.
Water
Cement powder
cement paste,
Forms a gluing paste, when mixed with proper amount of water
Reinforcement bars
–
steel (if no reinforcement, use ACI 318, Ch.22)
Two Methods of Reinforced Concrete Construction:
Cast

in

Place
:
members are constructed at their final location;
A form (wood) or mold (steel) is built in the shape of the member,
Reinforcement bars are placed inside form (mold);
Concrete is poured into form (mold).
Pre

Cast
:
members are constructed off

site;
Members are transported to their final location,
Members are erected and joined to form a structure.
4
Cast

In

Place Concrete
I

75, Suder Ave. ramp
McCormac, 8
th
ed., p.73
5
Pre

Cast Concrete
Veterans Glass City Skyway
bridge
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
One

way slab
Two

way slab
One

way slab
MacGregor, 5
th
ed., Fig. 4

1
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
2
/L
1
> 2

two

way slab; L
2
/L
1
< 2
L
2
L
1
One

way slab
MacGregor, 5
th
ed., Fig. 4

34
MacGregor, 5
th
ed., Fig. 4

36
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. 13

2
MacGregor, 5
th
ed., Fig. 5

22
9
The design Engineer must:
specify the
exterior dimensions
of members so that
the members have;
Adequate
strength
to resist
loads
,
ACI 318, Ch. 9

21.
Adequate
stiffness
to prevent excessive
deflections
,
ACI 318, Sect. 9.5.
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
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. 7.5.
Concrete cover;
In half

inch increments,
Rebar diameter = 9/8”
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. 7.5.2.1;
0.375 inch
, effective depth, d
㠠8湣n
,
0.5 inch
, effective depth, d
> 8 inch
,
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
13
Material Properties
In a concrete beam:

Cracks
occur in areas of
Tension
,

Beam will have sudden
Brittle
failure
unless
Steel
reinforcement is
present to take
Tension.
MacGregor, 5
th
ed., Fig. 1

4
14
Material Properties
Concrete is:
Strong
in
Compression,
Weak
in
Tension
,
Cracks occur in Concrete when:
Tensile Stress can be due to:
Loads
Restrained shrinkage during curing
Temperature changes
Concrete
of
Strength
Tensile
Concrete
in
Stress
Tensile
'
10
.
0
05
.
0
c
t
f
f
15
Material Properties
= Specified Compressive Strength of Concrete
Nominal
strength (
n
) is based upon
D
esign Strength
≥ Required Strength
Reduced Nominal Strength ≥ Factored Up Load
n
≥ U
ACI 318, Sect. 5.3;
In order to validate a specified
, concrete plant must have;
Strength test records
12 months old,
A sample standard deviation,
established from 30 consecutive compressive strength tests
2 cylinders tested per test
'
c
f
'
c
f
'
c
f
s
s
16
Material Properties
ACI 318, Sect. 5.3;
In order to validate a specified
;
A Required Average Compressive Strength, , must be obtained;
For
Use the
larger value
computed from Eq. (5

1) and Eq. (5

2);
Eq. (5

1)
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. (5.2) is based on a probability of 1

in

100 that an individual test
may be more than 500 psi below specified.
s
c
cr
s
f
f
34
.
1
'
'
'
c
f
'
cr
f
psi
f
c
5000
'
500
33
.
2
'
'
s
c
cr
s
f
f
'
c
f
'
c
f
17
Material Properties
ACI 318, Sect. 5.3;
In order to validate a specified
;
A Required Average Compressive Strength, , must be obtained;
For
Use the
larger value
computed from Eq. (5

1) and Eq. (5

3);
Eq. (5

1)
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. (5.3) is based on a probability of 1

in

100 that an individual test
may be < specified.
s
c
cr
s
f
f
34
.
1
'
'
'
c
f
'
cr
f
psi
f
c
5000
'
s
c
cr
s
f
f
33
.
2
90
.
0
'
'
'
c
f
'
90
.
0
c
f
18
Material Properties
ACI 318, Compressive Strength Test;
Standard Cylinders;
Concrete samples taken per ASTM C172,
Concrete samples molded, cured per ASTM C31,
Concrete strength tested per ASTM C39;
6”x12” cylinders,
Fill cylinder with concrete,
Allow concrete to harden in cylinder,
24 hours, 60
˚
80
˚F, no moisture loss,
Strip the cylinder mold,
Place cylinder in a curing room (100% humidity)
or water tank at
72
˚F,
After 28 days,
Load 2 cylinders in compression at rate of 35 psi/sec.
Record failure load, calculate failure stress.
Compr
ApC
max
Compr
ApC
max
6”
12”
19
Cracking & Failure Mechanisms
Concrete (and all
Brittle
materials)
fail
on the plane of
Max Normal Tension
Stress
Will have Tension cracks
parallel to applied load,
on plane of
ApC
ApC
P
P
Apply
a Normal Stress in
Compression
–
concrete
Compression Cylinder Test
:
ApC
Tension
max
Compr
ApC
max
Compr
ApC
max
Tension
max
T
max
Plane of max Tension
20
Mohr’s Circle Method
–
Failure Modes
Apply
a Normal Stress in
Compression
–
Split Cylinder Test
:
max
ApC
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
˚
ApC
min
2
max
ApC
2x90
˚
Steel
Ductile
Concrete
Brittle
tension
90
˚
Plane of
max
Tension
Tension
max
ApC
Tension
max
Compression
Tension
ApC
21
Mohr’s Circle Method
–
Failure Modes
Apply
a Normal Stress in
Tension
:
ApT
max
ApT
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
˚
45
˚
45
˚
45
˚
90
˚
0
min
2
max
ApT
2x45
˚
Steel
Ductile
Cast Iron
Plexiglass
Brittle
Plane of
max
Tension
max
Tension
max
Tension
max
Tension
max
max
max
Tension
Compression
22
Mohr’s Circle Method
–
Failure Modes
ion
slightTens
max
Brittle concrete fails on plane of max normal (tension) Stress.
Failure stress located at: 2x90˚=180˚on Mohr Circle
ApC
min
2
max
ApC
2x45
˚
2x90
˚
tension
Shear Stress
Normal Stress
Principal
Stress
Neutral Axis
90
˚
tension
Plane of
max
Tension
Concrete
Brittle
23
Cracking & Failure Mechanisms
Concrete cracking process;

4 stages
:
(MacGregor, 5
th
ed., pp. 41

43)
(0) Overall Cracking Process
;

individually, cement paste & aggregate
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.
ApC
ApC
But, in the concrete mixture,
the cement paste & aggregate together
produce a non

linear stress

strain curve,
that appears slightly
ductile,
due to the gradual micro

cracking
within the mixture and
redistribution of stress throughout
the concrete mixture.
(MacGregor, 5
th
ed., Fig. 3.13)
24
Cracking & Failure Mechanisms
Concrete cracking process;

4 stages
:
(MacGregor, 5
th
ed., pp. 41

43)
(1) No

Load Bond Cracking
during curing;

cement paste shrinks,

shrinkage restrained by non

shrinking aggregate,

shrinkage causes tension in the concrete,

No

Load Bond Cracks
occur along interface
between cement paste and aggregate,

cracks have little effect on concrete at low loads,

stress

strain curve remains nearly linear up to
'
3
.
0
0
c
ApC
f
'
3
.
0
c
ApC
f
ApC
25
Cracking & Failure Mechanisms
(MacGregor, 5
th
ed., Fig. 3.1)
'
3
.
0
c
f
'
5
.
0
c
f
'
75
.
0
c
f
'
c
f
26
Cracking & Failure Mechanisms
Concrete cracking process;

4 stages
:
(MacGregor, 5
th
ed., pp. 41

43)
(2) Stable Crack Initiation
;

Bond Cracks
occur from one aggregate to
another piece of aggregate,

cracks are stable,

cracks will propagate only if load is increased,

additional load is redistributed to un

cracked portions,

causes gradual curving of stress

strain curve.
ApC
'
5
.
0
3
.
0
c
ApC
f
27
Cracking & Failure Mechanisms
Concrete cracking process;

4 stages
:
(MacGregor, 5
th
ed., pp. 41

43)
(3) Stable Crack Propagation
;

Mortar Cracks
occur between Bond Cracks,

cracks develop parallel to the compressive load,
due to
local stress reaching (Mohr Circle),

crack do not grow during constant load,

cracks propagate only with increasing load,

stress

strain curve continues to curve.

the onset of this stage is called the
Discontinuity Limit
.
ApC
'
75
.
0
5
.
0
c
ApC
f
Tension
max
28
Cracking & Failure Mechanisms
Concrete cracking process;

4 stages
:
(MacGregor, 5
th
ed., pp. 41

43)
(4) Un

Stable Crack Propagation
;

Mortar Cracks
lengthen with constant load,

additional cracks form,

few undamaged portions remain
to carry additional load,

cracks propagate without increasing load,

this is an
unstable
condition,

stress

strain curve becomes very non

linear,

eventually, stress

strain curve begins to flatten,

failure will occur.

The onset of this stage is called
Critical Stress
at
ApC
'
75
.
0
c
ApC
f
'
75
.
0
c
ApC
f
29
Cracking & Failure Mechanisms
Concrete cracking process;

4 stages
:
(MacGregor, 5
th
ed., pp. 41

43))
(4) Un

Stable Crack Propagation
;

Critical Stress;

significant
lateral strains caused by
large amount of micro cracks,

volumetric strain increases, significantly,

causes outward force on lateral confining reinforcement,

spirals,

lateral ties,

confining reinforcement becomes in Tension,

confining Steel restrains concrete expansion and disintegration,

puts column in a state of Triaxial Compressive Stress.
ApC
'
75
.
0
c
ApC
f
'
75
.
0
c
ApC
f
30
Uni

Axial vs. Bi

Axial Loadings
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
MaxTension
31
Uni

Axial vs. Bi

Axial Loadings
(MacGregor, 5
th
ed., Fig. 3.12)
Bi

Axial Compression; from points A

C

A’

Delays the formation of

Bond Cracks

Mortar Cracks

Stable crack propagation

longer time

higher load
Due to Bi

Axial Compression;
failure at point C
≈
'
07
.
1
c
f
32
Tri

Axial Loadings
(MacGregor, 5
th
ed., Fig. 3.15)
(MacGregor, 5
th
ed., Fig. 3.16)
3
3
.
4
Failure
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,

spirals

ties

confining Steel restrains concrete
expansion and disintegration,

reinforcement becomes in Tension,
as it restrains concrete expansion

puts column into
Triaxial Compression
33
Cracking & Failure Mechanisms
Confining reinforcement ;

saved Olive View Hospital
from complete collapse;

saved building in Philippines
from complete collapse;
34
Cracking & Failure Mechanisms
Confining
reinforcement ;

double spiral
reinforcement
used in
bridge piers
by CALTRANS,

puts column
into a state of
Triaxial
Compressive
Stress.
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