Transcript Class 25.1 CIVE 2110 Concrete Material_definitions f`c

Class #25.1
Civil Engineering Materials – CIVE 2110
Definitions
Material Properties
Concrete Compressive Strength, f’c
Fall 2010
Dr. Gupta
Dr. Pickett
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Reinforced Concrete
Structural Steel
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
Creep due to long
term load.
Shrinkage due to
curing.
Time
dependent
Strength
Weight
Stiffness
Rigid,
Less; drift, deflection,
vibrations.
More thermal
expansion and
contraction
Low tensile
strength.
Low
strength/volume
ratio.
High tensile
strength.
High
strength/volume
ratio.
Higher,
More seismic load
Lower,
Less seismic
load
Flexible ,
More; drift, deflection,
vibrations.
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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,
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 Members are erected and joined to form a structure.

Cast-In-Place Concrete
I-75, Suder Ave. ramp
McCormac, 8th ed., p.73
4
Pre-Cast Concrete
Veterans Glass City Skyway
bridge
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Reinforced Concrete Structures
One-way slab
Fig. 4-1,MacGregor,
5th edition, 2009,
Pearson/Prentice Hall
Load bearing
masonry walls.
Gravity loads
supported
Two-way slab
by columns.
One-way slab
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MacGregor, 5th ed., Fig. 4-1
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,





- two-way slab; L2/L1 < 2
L2
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;

L1
- one-way slab; L2/L1 > 2
MacGregor, 5th ed., Fig. 4-34
Piles, Spread footings.
One-way slab
MacGregor, 5th ed., Fig. 4-36
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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,
MacGregor, 5th ed., Fig. 13-2
Two-way slab
MacGregor, 5th ed., Fig. 5-22
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Dimensions and Tolerances

The design Engineer must:
 specify the exterior dimensions of members so that
the members have;
 Adequate

ACI 318, Ch. 9-21.
 Adequate

strength to resist loads,
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.
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Dimensions and Tolerances

The design Engineer should specify:
 Calculations;
3

significant digits,
Exterior dimensions of beams, columns;
 In

Slab thickness;
 In

whole inch increments,
half-inch increments,
Rebar size, length;
Rebar diameter = 9/8”
 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,
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Dimensions and Tolerances

The design Engineer should ensure
construction tolerances of:
 Exterior dimensions of beams and columns;


Slab thickness;


0.5 inch,
0.25 inch,
Concrete cover; ACI 318, Sect. 7.5.2.1;

0.375 inch, effective depth, d  8 inch,
  0.5 inch, effective depth, d > 8 inch,
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Material Properties

In any beam (concrete, steel, masonry, wood):
 Applied loads produce
Internal resisting Couple,
MacGregor, 5th ed., Fig. 1-4
 Tension
and Compression
forces form couple.

Positive bending moment,
 Axial
Compression forces
in the top regions,
 Axial Tension forces
in the bottom regions,
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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, 5th ed., Fig. 1-4
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Material Properties

Concrete is:



Strong in Compression,
Weak in Tension, f  0.05  0.10 f '
t
c
Cracks occur in Concrete when:
Tensile Stress in Concrete  Tensile Strength of

Concrete
Tensile Stress can be due to:
 Loads
 Restrained
shrinkage during curing
 Temperature changes
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Material Properties
'
f
 c = Specified Compressive Strength of Concrete

'
f
Nominal strength ( n ) is based upon c
Design Strength ≥ Required Strength
 Reduced Nominal Strength ≥ Factored Up Load

n ≥ U
ACI 318, Sect. 5.3;

In order to validate a specified


f
'
c
, concrete plant must have;
Strength test records  12 months old,
A sample standard deviation, s
s

established from 30 consecutive compressive strength tests
 2 cylinders tested per test
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Material Properties
ACI 318, Sect. 5.3;

In order to validate a specified

f c' ;
A Required Average Compressive Strength,

For

f cr' , must be obtained;
f c'  5000psi
Use the larger value computed from Eq. (5-1) and Eq. (5-2);
f  f  1.34ss
Eq. (5-1)
f cr'  f c'  2.33ss  500
Eq. (5-2)
'
cr
'
c

Eq. (5-1) is based on a probability of 1-in-100 that the average of
'
3 consecutive tests may < f c specified.

Eq. (5.2) is based on a probability of 1-in-100 that an individual test
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may be more than 500 psi below f c' specified.
Material Properties
ACI 318, Sect. 5.3;

In order to validate a specified

f c' ;
A Required Average Compressive Strength,

For

f cr' , must be obtained;
f c'  5000psi
Use the larger value computed from Eq. (5-1) and Eq. (5-3);
f cr'  f c'  1.34ss
Eq. (5-1)
f cr'  0.90 f c'  2.33ss
Eq. (5-3)

Eq. (5-1) is based on a probability of 1-in-100 that the average of
'
3 consecutive tests may < f c specified.

Eq. (5.3) is based on a probability of 1-in-100 that an individual test
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may be < 0.90 f c' specified.
Material Properties
ACI 318, Compressive Strength Test;




 ApC   maxCompr
Standard Cylinders;
Concrete samples taken per ASTM C172,
6”
Concrete samples molded, cured per ASTM C31,
Concrete strength tested per ASTM C39;
12”



6”x12” cylinders,
Fill cylinder with concrete,
Allow concrete to harden in cylinder,


Strip the cylinder mold,


24 hours, 60˚ 80˚F, no moisture loss,
 ApC   maxCompr
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.
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Cracking & Failure Mechanisms
P
Concrete (and all Brittle materials)
fail on the plane of
 ApC
Max Normal Tension Stress
Apply a Normal Stress in Compression
– concrete Compression Cylinder Test:

Will have Tension cracks
parallel to applied load,  ApC
on plane of  maxTension
 ApC
 ApC   maxCompr
 maxTension
Plane of max Tension
 maxT
 ApC   maxCompr
P
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Mohr’s Circle Method – Failure Modes
Apply a Normal Stress in Compression – Split Cylinder Test:
Ductile Material fails by Buckling.
Brittle Material fails on plane of max Steel
Ductile
NORMAL (Tension) Stress,  maxTension
Failure stress  maxTension is 2x90˚=180˚
on Mohr Circle from applied stress  ApC
 min   ApC
 max
2x90˚
Tension


Compression
 max 
 ApC
2
Concrete
Plane of
max
Tension
Brittle
90˚
 ApC
90˚
 tension
 ApC
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Mohr’s Circle Method – Failure Modes
Apply a Normal Stress in Tension:
Ductile Material fails on plane of  max
From  maxTensionto failure stress = 2x45˚=90˚
on Mohr Circle
Cast Iron
Brittle Material fails on plane of  maxTensionSteel
Ductile
Plexiglass
 maxTensionacts on plane perpendicular
to applied Tension load.
 min  0
2x45˚

 max
Tension
 max 

 ApT
2
45˚
45˚
 max   ApT
Compression
Brittle
90˚
45˚
 max
45˚
 ApT
Plane of
max21
Tension
Mohr’s Circle Method – Failure Modes
Brittle concrete fails on plane of max normal (tension) Stress.
Failure stress located at: 2x90˚=180˚on Mohr Circle
Concrete
Brittle
 tension
 tension
90˚
 min   ApC
Neutral Axis
2x90˚
 max  slightTension Plane of

2x45˚
 max 
Stress
Shear
ApC
2

Normal Stress
max
Tension
Principal
Stress
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Cracking & Failure Mechanisms
 ApC
(MacGregor, 5th
ed., Fig. 3.13)
Concrete cracking process;
- 4 stages: (MacGregor, 5 ed., pp. 41-43)
(0) Overall Cracking Process ;
th
-
-
individually, cement paste & aggregate
each have brittle, linear stress-strain curves,
during a cylinder compression test,
-
 ApC
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.
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
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the concrete mixture.
Cracking & Failure Mechanisms
Concrete cracking process;
- 4 stages: (MacGregor, 5 ed., pp. 41-43)
th
 ApC  0  0.3 f c'
(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

ApC
between cement paste and aggregate,
- cracks have little effect on concrete at low loads,
 ApC  0.3 f c'
- stress-strain curve remains nearly linear up to
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Cracking & Failure Mechanisms
f c'
0.75 f c'
0.5 f c'
0.3 f c'
(MacGregor, 5th ed., Fig. 3.1)
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Cracking & Failure Mechanisms
Concrete cracking process;
- 4 stages: (MacGregor, 5 ed., pp. 41-43)
 ApC  0.3  0.5 f c'
th
(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
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Cracking & Failure Mechanisms
Concrete cracking process;
- 4 stages: (MacGregor, 5 ed., pp. 41-43)
 ApC  0.5  0.75 fc'
th
(3) Stable Crack Propagation ;
- Mortar Cracks occur between Bond Cracks,
- cracks develop parallel to the compressive load,
 ApC
due to
local stress reaching  maxTension (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.
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Cracking & Failure Mechanisms
Concrete cracking process;
- 4 stages: (MacGregor, 5 ed., pp. 41-43)
 ApC  0.75 f c'
th
(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.
 ApC
- The onset of this stage is called Critical Stress at  ApC  0.75 f c'
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Cracking & Failure Mechanisms
Concrete cracking process;
- 4 stages: (MacGregor, 5 ed., pp. 41-43))
 ApC  0.75 f c'
th
(4) Un-Stable Crack Propagation ;
- Critical Stress;  ApC  0.75 f c'
- significant lateral strains caused by
large amount of micro cracks,
 ApC
- 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.
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Uni-Axial vs. Bi-Axial Loadings
Concrete always cracks
on plane of  MaxTension
So far, discussion has
involved
Uni-Axial loading;
Uni-Axial
compression,
points A or A’
Uni-Axial tension,
points B or B’
(MacGregor, 5th ed., Fig. 3.12)
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Uni-Axial vs. Bi-Axial Loadings
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 ≈ 1.07 f c'
(MacGregor, 5th ed., Fig. 3.12)
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Tri-Axial Loadings
Tri-axial Compression ;
- Compared to uni-axial
compression;
- higher compressive
strength,
- more ductile,
 Failure  4.3 3
(MacGregor, 5th ed., Fig. 3.15)
(MacGregor, 5th ed., Fig. 3.16)
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
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- puts column into
Triaxial Compression
Cracking & Failure Mechanisms
Confining reinforcement ;
- saved Olive View Hospital
from complete collapse;
- saved building in Philippines
from complete collapse;
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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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