Portland Cement Concrete Presentation

7/27/2019 Portland Cement Concrete Presentation

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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
http://pisces.sbu.ac.uk/BE/CECM/Civ-eng/Courses/Concrete/concmix/sld001.htm


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Three Quantities Needed Specified Compressive

Strength Variability or Standard

Deviation of Plant

Allowable Risk : ACI SuggestsA Risk of 10%


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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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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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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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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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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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For Small Projects Use Table in Lieu of

Trial Mixes (Conservative Table)

Not For Trial Batches


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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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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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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


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


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


Physical

Deterioration

Chemical

Deterioration

Reinforcement

Corrosion


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Durability


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


Physical

Deterioration

Chemical

Deterioration

Reinforcement

Corrosion


aggregate

reaction

Leaching


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Durability


Physical

Deterioration

Chemical

Deterioration

Reinforcement

Corrosion


aggregate

reaction

Leaching


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Durability


Physical

Deterioration

Chemical

Deterioration

Reinforcement

Corrosion


aggregate

reaction

Leaching


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Durability


Physical

Deterioration

Chemical

Deterioration

Reinforcement

Corrosion


aggregate

reaction

Leaching


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Durability


Physical

Deterioration

Chemical

Deterioration

Reinforcement

Corrosion


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Durability


Physical

Deterioration

Chemical

Deterioration

Reinforcement

Corrosion

Cracking Frost Attrition Fire


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Durability


Physical

Deterioration

Chemical

Deterioration

Reinforcement

Corrosion



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Durability


Physical

Deterioration

Chemical

Deterioration

Reinforcement

Corrosion



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Durability


Physical

Deterioration

Chemical

Deterioration

Reinforcement

Corrosion


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)

Portland Cement Concrete Presentation

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Transcript of Portland Cement Concrete Presentation