Post on 14-Apr-2018
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Portland CementConcrete (PCC)
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Topics to be Covered
PCC Topics Covered
Basic Principles of Conventional PCC
Introduction to Alternatives
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Introduction
Quality of Concrete:
Chemical Composition of PC Hydration and
Development of Microstructure,
Admixtures, and AggregateCharacteristics
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Introduction
Quality Strongly Affected by: Placement
Consolidation
CuringPerformance of PCC (or Durability)
Depends on:
Mixing Method
Transportation
Placement
Curing in Field
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Proportioning of PCC M ixes
Designers Specify PCC Strength or Modulus of
Elasticity
Materials Engineer Designs Mix (Proportioning,
Mixing, Placement and Curing)
Proportioning Affects Plastic as well as Hardened
PCC Performance
Unless Specified, Strength is
Avg. Strength of Three Tests
Specimen Size is 6 by 12 in.
Compressive Strength after 28 days of Curing
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Proportioning of PCC M ixes
PCA Specifies Three Qualities Acceptable Workability of Freshly
Mixed PCC (Plastic PCC)
Durability, Strength, and Uniform
Appearance of Hardened Concrete
Economy
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Proportioning of PCC M ixesHow to Determine Proportions of Cement,
Water, Fine and Coarse Aggregates, and Use ofAdmixtures
Several Mix Design Methods
From: Arbitrary Volume Method (1:2:3Cement:Sand:Coarse Aggregate)
To: Weight and Absolute-Volume ACI Methods
Weight Method is Simple and Based on Unit Wt. of
PCC Absolute-Volume Uses Sp. Gr. Of Each Ingredient
Absolute-Volume Method is More Accurate
Main Difference Between Two Methods Amount ofFine Aggregates
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Basic Step of Wt. and Vol. Methods
1. Evaluate Strength Requirements
2. Determine Water-Cement Ratio
3. Evaluate Coarse Aggregate Requirements
a) Maximum Aggregate Size
b) Quantity of the Coarse Aggregate
4. Determine Air Entrainment Requirements
5. Evaluate Workability Requirements of thePlastic Concrete
6. Estimate the Water Content
Requirements
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Basic Step of Wt. and Vol. Methods
(cont.)
7. Determine Cement Content and Type
8. Evaluate the Need and Application Rate of
Admixtures
9. Evaluate Fine Aggregate Requirements
10. Determine Moisture Corrections11. Make and Test Trial Mixes
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Aggregates Gravels, crushed rock, and sands, etc
May occupy 75% of normal mixes
Will influence all aspects of the concrete
Durability
Structural performance
Cost
Two main categoriesFine < 5mm
Coarse > 5mm
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Aggregate Quali ty
Aggregate should not contain materials
which are likely to
Decompose/change in volume (e.g. coal, clay)
React with cement paste (e.g. certain siliceous
compounds (ASR))
Affect appearance of concrete (e.g. salt,
pyrites)
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Aggregate Cleanliness
Should be free from dust, clay, etc
Sea dredged aggregate may be contaminated
with chlorides
Excessive washing is costly and may wash away
fines
Shape will affect workability and durability
Gradation (well-graded, gap-graded etc)
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Types of Aggregate
naturally occurring or industrial products
1. Normal density aggregates (most widely used)
2. Lightweight aggregates pumice,
expanded clayLeca,
PFA - Lytag,
Expanded Slag - Pellite
3. High density aggregate (e.g., lead)
4. Fibres (e.g. asbestos, wood, steel, glass, polymers)
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Water
If you can drink it it is OK!
Sea water can sometimes be
used for mass concrete, but notreinforced concrete
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Admixtures Added to concrete during mixing to modify
particular properties of concrete
Accelerators - (CaCl) NaCl, formate triethenolamine
Retarders - Gypsum, sugars, lignosulphates
Air Entrainers - Wood resins/soaps, fats and oils
Water reducers (plasticizers)Others - Corrosion Inhibiting Admixtures
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Strength Requirements
Variations in material, batching and
mixing of PCC results in strength
deviations Structural designer does not consider
variability
If material is provided with an avg.
strength, half of placed material will
be weaker than desired
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Strength Requirements
Three Quantities Needed Specified Compressive
Strength Variability or Standard
Deviation of Plant
Allowable Risk : ACI SuggestsA Risk of 10%
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Strength Requirements
90% of area under the curve has to be to the right of
specified compressive strength fcr= fc +1.34s
fcr : Required Avg. Compressive Strength
fc: Specified Compressive Strength
s: Standard Deviation
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Strength Requirements
For Mixes with Large s in Strength
fcr= fc +2.33s3.45 (MPa) or 500 (psi)
fcr : Required Avg. Compressive Strength
fc: Specified Compressive Strength
s: Standard Deviation
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Strength Requirements
Standard deviation at least from 30 strength
tests
If not available use modification factors and use
linear interpolation for intermediate No. of tests
Multiply modification factor with s
Number of Tests Modification Factor, k
15 1.1620 1.08
25 1.03
30 or More 1.00
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Strength Requirements
For Fewer Than 15 Tests
Specified fc MPa (psi) fcr MPa (psi)
< 20.7 (
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Water-Cement Ratio Requirements
Use Historical Data
Non-Air Entrained
Air Entrained
Water-Cement Ratio
Compressiv
eStrength
If Pozzolan is Used: Water-Cement plus Pozzolan Ratio
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Water-Cement Ratio Requirements
Prepare Three Trial Batches to Develop
Relationship Similar to Previous Figure
Use Table For Estimating Water-Cement Ratios
for Trial Mixes
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Water-Cement Ratio Requirements
For Small Projects Use Table in Lieu of
Trial Mixes (Conservative Table)
Not For Trial Batches
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Water-Cement Ratio Requirements
Chemical Exposure
Minimum of the Two is Selected
Type of Materials
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Coarse Aggregate Requirements
Aggregate grading has little direct effect
on strength
It does affect workability, and hence w/cratio.
Large-Dense Graded Aggregate Most
Economical Mix
Round Aggregate Require Less Water
Than Angular Aggregates
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Aggregate Grading
fundamental ideais that finer stones
fill up gapsbetween larger
stones, and
remaining space is
filled by cement
paste.
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Best Theoretical Grading
Fullers gradation
provides a dense
concrete, which is
considered harsh.
A richer mix isformed byincreasing fines.
Particle size as fraction of max
0 0.5 1.0
%passing
0
100
50
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Coarse Aggregate Requirements
Maximum allowable aggregate size depends ondimensions of structure and capabilities of
construction equipment
Situation Maximum Aggregate Size
Form Dimensions 1/5 of Min. Clear Distance
Clear Space Between
Reinforcement or PrestressingTendons
3/4 of Min. Clear Space
Clear Space Between
Reinforcement and form
3/4 of Min. Clear Space
Unreinforced Slab 1/3 Thickness
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Coarse Aggregate Requirements
Gradation of fine aggregate defined by
fineness modulus
Desirable
fineness
modulus
dependson coarse
aggregate
size
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Air Entrainment Requirements
PCC Exposed to Freeze-Thaw Condition and De-icing
Salts
In Some Cases to Increase Workability
Level of Entrainment Depends on Level of Exposure
Mild
Moderate
Severe
W k bilit
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Workability
The ease with which a concrete mix canbe handled from mixer to its finally
compacted shape
Consistency - fluidity
Mobility - ease of flow
Compactability - ease of compaction
Internal work required to produce full
compaction.
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Water Content Requirements
For Given Slump Depends on Maximum Size
and Shape of Aggregates and Air Entrainer
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Water Content Requirements
Water Requirements are for Angular Aggregates
Reduced Water for Other Shapes
Take into Account Free Moisture and Absorption
Aggregate Shape Reduction in Water Content
Kg/m3 (lb/yd3)
Sub-angular 12(20)
Gravel with Crushed Particles 21(35)
Round Gravel 27(45)
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Cement Content Requirements
334 Kg/m3 (564 lb/yd3) Min. for Severe Freeze-Thaw
385 Kg/m3
(650 lb/yd3
) Min. for PCC Under Water
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F ine Aggregate Requirements
Weight Design Mix Method Uses Table Weight of Fine Aggregate is Determined by Subtracting
from Total Weight of Other Ingredients
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Overview
1. Properties of Fresh Concrete Workability,
Segregation,
Bleeding, and
Heat of Hydration
2. Properties of Hardened Concrete
Strength
Deformation Creep
Shrinkage
W k bil i t R i t
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Workabil i ty Requirements
Generally implies the ease with which aconcrete mix can be handled from mixer to
its finally compacted shape
Consistency - fluidity Mobility - ease of flow
Compactability - ease of compaction
Internal work required to produce fullcompaction.
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Tests to Measure Workabil i ty
Four widely used tests
Slump Test (US) Compacting factor test
Vebe time test
Flow test
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Slump Test
Developed in 1913 in US,
by Chapman
Required
Slump cone
Tamping Rod
Ruler
Suitable for normal mixesof medium to high
workability
100
200
300
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Slump Test (cont)Method
Concrete put in conein 3 layers, each layertamped 25 times
Top struck off
Cone carefully liftedoff
Slump measuredNot suitable for dry
mixes
True slump
Slump (mm)
Shear slump Collapse slump
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Compacting Factor Test
Thought to be more sensitivethan the slump test
Suitable for all mixes
Method
mixed concrete put in top hopper
allowed to fall into 2nd hopper
then cylinder
cylinder stuck off, concreteweighed and compared with
weight of fully compacted cylinder
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Other tests
Vebe test - time for standard cone to be
compacted flat by glass plate on vibrating
table for workable concrete the Vebe time = approx 3s
Flow test - the measured spread in mm of a
standard cone on a dropping table (40mm, 15times)
Neither of these popular on site
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W k bil i t R i t
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Workabil i ty Requirements
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Segregation
The tendency forsand-cement mortar to separate from coarse aggregate
cement mortar to separate from fine aggregate Caused by
Excessive vibration
Dropping fresh concrete from a heightPoor grading
High workability
Mixes with no air entrainment
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Bleeding Tendency for water to rise to the surface This will cause weakness or dustiness of the
surface of the finished concrete, or a line of
weakness between pours Bleeding affected largely by the properties of the
cement.
Avoided by
a finer cementhigh C3A content
richer mix
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Heat of Hydration
Exothermic reaction during setting can
cause a significant temperature rise in
large concrete pours. This causes expansion, then setting, then
contraction.
If the pour is restrained, or has atemperature differential, cracking may
occur
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Laboratory Testing of
Concrete
Measuring Air Content in Fresh
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Measuring Air Content in Fresh
Concrete
Mixing and Handling Can SignificantlyAlter Air Content of Fresh Concrete
Field Tests are Performed
Various Test Methods:
Pressure Method (ASTM C231)
Volumetric Method (ASTM C173)Gravimetric Method (ASTM C138)
Chace Air Indicator (AASHTO T 199)
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Pressure Method (ASTM C231)
Based on Boyles Law
Measures Reduction in
Volume due to Applied
Pressure
Change in VolumeTranslates to Air
Content
Not Suitable forLightweight Aggregates
St St i R l ti
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Stress Strain Relation
Ec= 4,731 (fc)0.5 Poissons Ratio =0.15 to 0.20
Ec= 57,000 (fc)0.5
C i St th T t
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Compressive Strength Test
ASTM C39
Specimen Size 6 by 12 or 4 by 8 in.
Rate of Loading 20 to 50 psi/s Increasing Specimen Size Reduces
Strength
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Compressive Strength of PCC
Predominantly affected by the amount
of pores in the hardened concrete.
Water-cement ratio is the main
determinant of strength.
Split Tensile Test
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Split Tensile Test
ASTM C 496
Compressive Load Applied Along VerticalDiameter Until Failure
Failure Occurs Along Vertical Diameter inTension
Typical Indirect Tensile Strength, 2.5 to 3.1Mpa (360 to 450 psi)
T = 2P/( Ld)T= Tensile Strength, Mpa (psi)
P= Load at Failure, N (psi)
L = Length of Specimen, mm (in.)
D = diameter of Specimen, mm (in.)
Flexural Strength Test
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Flexural Strength Test
ASTM C 78
Typical Specimen Size 6 by 6 by 18 Load applied at a Rate of 125 and 175 psi/min
R = PL/(bd3)
R= Flexural Strength, MPa (psi)
P= Load at Failure, N (psi)
L = Span Length, mm (in.)d = Avg. Width of Spec., mm (in.)
b = Avg. Depth of Spec., mm (in.)
R= 0.62 to 0.83 (fc)0.5
R= 7.5 to 10 (fc)0.5
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Creep
Time
Creep
Elastic deformation
on loading
Immediate
elastic
recoveryCreep
recovery
Permanent
deformation
Load sustained Load removed
C
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Creep Magnitude of creep is affected by
More cement in mix - more creep
Higher w/c ratio - more creep
Higher relative humidity - lower creep
Greater age - lower creep
Rapid Hardening - lower creep
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Permeability & Porosity
both related to pore spaces in the concrete
Both cement paste and aggregate contain pores, and
in addition there may be voids due to incompletecompaction
Cement paste is made up of gel & cement particles.
Gel ~ 28% pores with a permeability of ~ 7 x 10-16 m/s.
Cement paste has 0 to 40% interconnected capillary
pores, with a permeability 20 to 200 times higher than
gel.
Absorption, Permeability &
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Absorption, Permeability &Diffusion
All these three factors related to ease with
which a fluid will pass through cement
paste along capillary pores.
Absorption is process by which concrete takes in a
liquid by capillary attraction
Permeability quantitatively characterises ease by
which a fluid passes through it
Diffusion is where a vapour gas or ion can pass
through concrete under the action of a concentration
gradient
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Permeability of Cement paste
Age Coeff of perm.
days (m/s)
fresh 2 x 10-6
5 4 x 10-10
6 1 x 10-10
8 4 x 10-11
13 5 x 10-12
24 1 x 10-12
ult. 6 x 10-13
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Effect of Water-Cement Ratio
W/c ratio
0.2 0.4 0.6 0.8
Perm
(10-14m/s)
0
50
100
150
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Mixing
Drum mixer - common on site
Pan Mixer - larger sizes, industry, labs etc
By hand - to be avoided where possible
Ready mix - most often used for sites
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Placing
By skip, wheelbarrow, shute, shovel orconcrete pump
Place at final position - do not vibrate intoposition
Vibrate using poker - approx 10 seconds at0.5 m intervals
Level with wooden float, leave for a while,then finish with steel float
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Testing Hardened
Concrete in-situ
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Visual Inspection+
Probably the most important assessment
Equipment
Notebook, camera, binoculars, ladder To observe
Cracks, spalling, honeycombing
Rust stains, flaking paint, efflorescenceDelamination, a planar crack at rebar depth
tap with a hammer and listen for a dull sound
or use infra-red thermography or radar
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Assessing reinforcement
corrosion Half Cell Potential Measurements
A half-cell is a device for assessing
reinforcement corrosion in concreteSimply a piece of metal in its own solution
Cu in CuSO4 solution
allows electrical potentials to be assesseddoes not work for carbonated concrete!
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Carbonation
Equipment
Phenolphthalein Solution
Spray solution on freshly exposed concretewill turn pink where alkali is present
Limitations
Phenolphthalein turns pink at pH 9, but de-passivation can take place at pH 11
Surface must be freshly exposed - destructive
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Chlorides
Usually involves taking powdered samplesand measuring
total (acid soluble) chlorideswater soluble
But chlorides can be squeezed out ofconcrete
free
ion concentration
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Resistivity
As corrosion is electro-chemical, the
resistance of the concrete will have a
bearing on the corrosion rate A four probe resistivity meter can be used
two outer probes pass a current
inner probes measure voltage difference
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Curing
If left in contact with water, concrete will
continue to gain strength for many months
Otherwise all free water evaporates or isused up in the hydration process, and no
further hydration can continue
Curing ensures that water for hydration isavailable as long as possible
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Curing
Age (months)0 6 12
100
50
0
Air-cured, saturated at test
Air-cured, dry at test
Moist cured, moist at test
Moist-cured, dry at test
Water curing after 9 monthsAir-cured after 1 month,
dry at test
Air-cured after 1 and 3 months,dry at test
Shrinkage
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g
3 principal types of shrinkage/expansion:
Plastic Shrinkage - caused by settlement of solids
and loss of free water from plastic concrete.
Autogenous Shrinkage - Cement gel has a lower
volume than the water and cement that makes it. So
at a constant water content shrinkage takes place.
Drying Shrinkage - Loss of water from cement gel,
after loss of water from pores and capillaries.
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Drying Shrinkage
Expansion in water
Shrinkage in air
Shrinkage on dryingAlternate wetting and drying
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Avoidance of Cracked Concrete
If concrete is restrained, movement joints
or anti-crack reinforcement must be used.
Heat of hydration, and drying shrinkagemust be minimised.
If concrete is not restrained, differential heat
of hydration and drying shrinkage should beminimised.
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Concrete Durability
Definition
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Definition
Resistance to physical and chemicaldeterioration of concrete resulting from
Interaction with environment - external
Interaction between constituents - internal
Protection of embedded steel from
corrosion processes
Durability
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Durability
Concrete Deterioration
Physical
Deterioration
Chemical
Deterioration
Reinforcement
Corrosion
Reinforcement Corrosion
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Significant corrosion of steel will take place
only as a result of electro-chemical or galvanic
action.
In the absence of dissimilar metals, corrosion is
initiated by local imperfections in the metal
(e.g. different steel crystalline structures) or
local differences in the concentration ofelectrolyte.
Reinforcement Corrosion
R i f
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Reinforcement
Corrosion (Initiation)
H2O dropletair
Steel rebar AnodeCathode Cathodeelectrons electrons
Fe2+ Fe2+
Rust (Fe(OH)3)2H2O+O2+4e
- = 4(OH)-
Fe2+ + 2(OH)- = Fe(OH)2
then Fe(OH)3
R i f
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Once rust has been
formed, the steel
surface beneath it
becomes deficient in
oxygen and becomes
the anode.
Corrosion thencontinues under the
rust covering.
Reinforcement
Corrosion (Continuation)
Fe2+
New rust
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Durability
Concrete Deterioration
Physical
Deterioration
Chemical
Deterioration
Reinforcement
Corrosion
Carbonation Chlorides
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Carbonation
Step 1 H2O+CO2 = HCO3- + H+
HCO3- = CO3
2- + H+
Step 2 Ca(OH)2 + 2H+ + CO32-
= CaCO3 +2H2O
This neutralisation reaction penetrates
gradually from the concrete surface.
Penetration = k x time1/2
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Factors affecting carbonation
Humidity - ideally 50-70%
lower, not enough water
higher water inhibits CO2 diffusion Temperature - worse in hot environments
Concentration of CO2 gas in atmosphere
Normally 0.03% but increasing annually
Higher in cities, due to motor vehicles andfossil fuel burning
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Carbon Dioxide
The most important
greenhouse gas
Global concentration
has increased from 270
to 350 ppm since 1700
Expected 500 ppm by
2050
C b ti i d d t l
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Carbonation-induced steel
corrosion Occurs due to the breakdown of alkaline
conditions
Requires over 75% relative humidity But significant carbonation occurs at lower
humidity's than this
So corrosion will only be significant ifalternate wetting and drying is present
T i l t f t ti f
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Typical rate of penetration of
carbonationAge (years)Depth ofCarbonation
(mm)20Mpa
Concrete
40 Mpa
Concrete
5
10
15
20
0.5
2
4
7
4
16
36
64
So cover is vitally important
F t ff ti b ti
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Factors affecting carbonation
(cont) There seems to be some dispute about how
important the concrete mix is, but
A high w/c ratio will lead to a greater depth ofpenetration
A sulphate-resisting cement may lead to 50%
greater penetration
A PBFC may lead to 200% greater penetration
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Other effects of carbonation
Increase in strength, as new free water may
assist continued hydration of cement
Carbonation shrinkage Associated small weight gain
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Durability
Concrete Deterioration
Physical
Deterioration
Chemical
Deterioration
Reinforcement
Corrosion
Carbonation Chlorides
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Chlorides
Very high concentrations can lead todeterioration of concrete,
Ca(OH)2
is leached from the cement paste
increasing porosity and decreasing strength
In sufficient concentrations Cl- ions canbreak down the passive oxide film on the
rebar, and allow the corrosion process tostart
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Sources of Chlorides
Contact with sea water
From de-icing salts
From beach or sea dredged aggregates
From accelerators (chloride-based
admixtures now prohibited, however)
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Factors affecting chloride attack
Concentration of chlorides - corrosion will notoccur below a threshold level (somewhere
between 0.1 and 0.4%)
Humidity, alternate wetting and drying
Temperature - worse in hot climates
Concrete permeability and chloride binding
capacity, cement content and typePFA and GGBS will help resist chloride ingress
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Type of Cement
0
30
60
30 50 70
Strength (Mpa)
Coeffofchloridediffusion(cm2s-1x
10-9)
OPC
PFA 30%
GGBS 45%
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Durability
Concrete Deterioration
Physical
Deterioration
Chemical
Deterioration
Reinforcement
Corrosion
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Durability
Concrete Deterioration
Physical
Deterioration
Chemical
Deterioration
Reinforcement
Corrosion
Sulphate Acid Sea water Alkali-
aggregate
reaction
Leaching
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Sulphate attack
Sources - Ground water, Industrial fill, Lake
and sea water
Reactions Sulphates + Calcium Hydroxide = Calcium Sulphate
(gypsum)
Sulphates + Calcium Aluminate = Ettringite
Strength loss and expansive degradation
result
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Prevention of sulphate attack
Use PFA or GGBS
Use low heat or sulphate-resisting cement
Produce a good quality concrete
Use a physical barrier
wrapping or bituminous or other coatings
Note - salt weathering on sabkha
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Durability
Concrete Deterioration
Physical
Deterioration
Chemical
Deterioration
Reinforcement
Corrosion
Sulphate Acid Sea water Alkali-
aggregate
reaction
Leaching
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Durability
Concrete Deterioration
Physical
Deterioration
Chemical
Deterioration
Reinforcement
Corrosion
Sulphate Acid Sea water Alkali-
aggregate
reaction
Leaching
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Durability
Concrete Deterioration
Physical
Deterioration
Chemical
Deterioration
Reinforcement
Corrosion
Sulphate Acid Sea water Alkali-
aggregate
reaction
Leaching
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Durability
Concrete Deterioration
Physical
Deterioration
Chemical
Deterioration
Reinforcement
Corrosion
Sulphate Acid Sea water Alkali-
aggregate
reaction
Leaching
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Durability
Concrete Deterioration
Physical
Deterioration
Chemical
Deterioration
Reinforcement
Corrosion
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Durability
Concrete Deterioration
Physical
Deterioration
Chemical
Deterioration
Reinforcement
Corrosion
Cracking Frost Attrition Fire
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Durability
Concrete Deterioration
Physical
Deterioration
Chemical
Deterioration
Reinforcement
Corrosion
Cracking Frost Attrition Fire
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Durability
Concrete Deterioration
Physical
Deterioration
Chemical
Deterioration
Reinforcement
Corrosion
Cracking Frost Attrition Fire
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Durability
Concrete Deterioration
Physical
Deterioration
Chemical
Deterioration
Reinforcement
Corrosion
Cracking Frost Attrition Fire
Good durability by design
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Good durability by design
Adequate falls and drainage of slabs
reduces time of contact with water etc
Adequate cover for exposure conditions
protects against carbonation and chlorides Well designed mix with sufficient cement
reduces permeability and increases alkalinity
Properly designed dense mix
prevents segregation, and plastic shrinkage, and reducespermeability
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Good durability in construction
THE FOUR Cs
Ensure design Cover is maintained
Ensure sufficient Cement and proper w/c
ratio
Ensure adequate Compaction so there is nohoneycombing
Ensure good Curing so that design strengthis attained (esp. At surface)