Class 25.1 CIVE 2110 Concrete Material_definitions f'c

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Nov 25, 2013 (3 years and 6 months ago)

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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.