•Leaching and efflorescence

(1)

(2)

Contents:

• Permeability & Diffusivity

• Chemical attack

• Leaching and efflorescence

• Sulfate attack

• Attack by acids and bases

• Corrosion of sewer pipes

• Alkali-aggregate reaction

• Corrosion of steel reinforcement

• Physical attack

• Freezing and thawing

Delayed Ettringite Formation (DEF) in concrete box-beam (Texas, USA)

(3)

Durability in general

• Durability depends on

• concrete quality

• service environment

• design service life

• Premature failure

• ignorance in design

• poor specification

• poor workmanship

• Quality of concrete related to

• permeability, diffusivity, absorption

• strength

• type of cementitious material: OPC, SRC, blended cements

• aggregate: reactivity

(4)

Permeability & Diffusivity

(5)

Permeability

• Play an important role in durability

• Essential for water-retaining structures and construction below grade – water tightness

• Flow of water through cement paste obeys D’Arcy’s law

= rate of flow of water

= head of water (hydraulic pressure)

= thickness of specimen

= permeability coefficient, depend on capillary porosity which is affected by w/c and degree of cement hydration

p

K h

  x



h

x

K

p

(6)

Permeability

• Depends on

• porosity

• pore size distribution – dominated by large capillary pores

• continuity of pores

• W/C (w/cm) has the most significant influence on permeability and durability

• w/c , porosity , permeability 

• Lower w/c reduces capillary porosity, reduce penetration of water and harmful substances

• Lower w/c increases strength, improve concrete resistance to cracking from internal stresses

• Permeability is also reduced by

• use of mineral admixtures

• increase in curing

(7)

Permeability

Composition of sealed & fully hydrated Portland cement paste

Volume of capillary pores  markedly for w/c > 0.42

Effect of ITZ in conc rete on permeability

(8)

Permeability

(Mindess et al 2003)

(9)

Chemical transport

• Most concrete contains capillary water

• Chemical transport will be affected by interaction between pore solution and chemicals

• Moisture content largely determine the nature and speed of penetration of chemicals into concrete

• Movement of dissolved ions with water under pressure head

• For dry or semi-dry concrete exposed to water, capillary suction pressure has the same effect on flow as a pressure head of 2.4 MPa in saturated concrete

• In near-saturated concrete, diffusion under a concentration gradient provides the principal method of transport

(10)

Diffusion

• Diffusion can be described by Fick’s second law

• C=concentration, t=time,

• K_d =diffusion coefficient, x=depth

• Penetration of Cl follows Fick’s Law very closely

• Factors affecting diffusion

• Pore structure

• Relative humidity of concrete

• Ion diffusion (chlorides, sulphates) is most effective

• when the pores in the cement paste are saturated

• Age

2 d 2

C C

t K x

 

  

(11)

Diffusion

• Tortuosity

(Provis et al.

CCR 2012)

     

2 0

2

&

D

D a

D t d r as t

d

 



     

Self-diffusivity of a random walker (free space)

D 

0

( ) D t 

 

²

r  

Self-diffusivity of a random walker (porous medium) Mean-squared displacement a.f.o. time steps

(12)

Permeability

• Water Permeability Test

• Results exhibit significant variability

• Typical tests involve movement of water through concrete specimens under high water pressures

• Permeability coefficient

• calculated according to D’Arcy’s Law

• calculated based on water penetration depth

Where K_P = coefficient of water permeability (m/s);

d = water penetration depth (m);

v = porosity of concrete [m = gain in mass (g);

A = cross-sectional area of specimen (mm²) ,  = density of water]

h = water hydraulic head (m); t = time under pressure (s).

• Difficulty

• permeability coefficient  as the pressure 

2 p

2 d v

K h t

 

 

v m

Ad 



^Specimen

Water under pressure

(13)

Permeability

• Permeability test

• Tested according to ASTM C 1585

• mass increase resulted from absorption of water as a function of time

• 3 discs ø100× 50 mm

• Sorptivity S (kg/m² h^0.5): slope of regression curve of water absorbed by a unit surface area vs SQRT of time from 1 to 24 hrs

0.0 0.1 0.2 0.3 0.4 0.5 0.6

0 1 2 3 4 5 6

Mass increase, kg/m2

√time, h^0.5

(14)

Permeability

Charge passed,

Coulombs Chloride ion penet rability

> 4,000 High

2,000-4,000 Moderate

1,000-2,000 Low

100-1,000 Very Low

< 100 Negligible

• Rapid Cl Penetrability Test (RCPT)

• According to ASTM C 1202

• 6 h charges passed (C)

• RCPT ratings

• Used as construction acceptance test

(15)

Permeability

• Rapid Migration Test

• According to NT Build 492

• 3 discs ø100×50 mm

• External potential: 30 V

• Duration: 24 hours

• Specimen split into two halves

• Open interior surface sprayed with 0.1N AgNO₃ to determine chloride penetration depth

• AgNO₃+ Cl^-  AgCl  (white) + NO₃^-

(16)

Leaching & Efflorescence

(17)

• Efflorescence

• Occur frequently on the surface of concrete when water can percolate through the concrete

• Major constituent is calcium carbonate

Ca(OH)

₂

+ CO

₂

= white crusts of CaCO

₃

on the surface

• Among hydration products, Ca(OH)₂ is most susceptible to leaching due to its

relatively high solubility (1230 mg/liter)

(18)

• Efflorescence

• An aesthetic problem in itself

• Extensive leaching of Ca(OH)

₂

exposes other cementitious constituents to chemical decomposition, results in

•  in porosity and permeability

•  strength

• Rate of leaching

• Depends on the amount of dissolved salts in percolating water

• Soft waters (rain water) – most aggressive

• Hard water (containing large amounts of Ca⁺⁺) – less dangerous

• Temperature: Ca(OH)

₂

is more soluble in cold water than in

warm water

(19)

Sulfate Attack

(20)

Sulfate Attack

• Most widespread & common form of chemical attack

• Sulfates sources

• Natural origins: ground water, sea water

• Industrial sources: mine tailings

(D. Hooton)

(21)

Sulfate Attack

• Mechanism of sulfate attack

• Dissolved sulphate penetrate into concrete

• Gypsum corrosion

• Accompanied by an expansion in volume by~120%

• Sulfoaluminate corrosion

• Accompanied by 55% increase in solid volume

• Volume expansion may be  due to water absorption when ettringite is in microcrystalline form

 

2

4 2

2 ( )

CH  SO

^

aq C SH  OH

^

aq

4 12

2

16

6 3 32

C ASH  C SH  H C AS H

(22)

Sulfate Attack

• Crystal structure of Ettringite

4 12

2

16

6 3 32

C ASH  C SH  H C AS H

(Manzano et al. J. Phy. Chem. C, 2012)

(23)

Sulfate Attack

• Consequence

• expansion

• cracking

• loss of strength due to the loss of cohesion in the cement paste and its bond with aggregate

• damage usually starts at edges and corners followed by

progressive cracking and spalling which reduce the concrete to a friable or even soft state

• Field observation shows

• Sulfate attack is not always accompanied by expansion

Underground pile subjected to sulfate attack

(Scrivener 2012)

(24)

Sulfate Attack

• Effect of different sulfates

• Type of sulfate commonly encountered: Na, K, Ca, Mg

• Mg sulfates can be more aggressive because of possible additional reactions which decompose C-S-H and calcium sulfoaluminates

• SH_x may react with MH to form crystalline magnesium silicate, no cementing property

• MH often forms in pores, reduce porosity of concrete, and hinders penetration of sulfates

• The reactions  pH of pore solution

4 12 3 ( ) 4 2 3 3

C ASH  M S aq  C SH  MH  AH

( ) 2

CH  M S aq  C SH  MH

3 2 3 3 ( ) 3 2 3 2 _x

C S H  M S aq  C SH  MH  SH

(25)

Sulfate Attack (Sea water)

• Action of sea water

MgSO₄+ CaOH  MgOH + CaSO₄

• Formation of MgOH in pores is one reason why seawater is less corrosive than might be expected

• Gypsum and ettringite are more soluble in solutions containing Cl ion; this reduces deleterious expansions

(26)

Sulfate Attack

• Control of sulfate attack

• Make dense concrete

• Reduce w/c

• Use mineral admixtures (GGBFS, Class F FA, etc) or blended

cements

• Reduce Ca(OH)₂

• Use mineral admixtures (SF, GGBFS, Class F FA) or blended cements

• Minimise C₃A content

• Use sulphate resistant cements

(Verbeck 1969)

(27)

Sulfate Attack (DEF)

• Delayed Ettringite Formation (DEF)

• Occur in concrete cured at high temperatures (>70 ^oC) in many precast plants

• A special case of sulphate attack where the source of sulphate ions happens to be internal (within concrete)

• Sulfate in cement

• Gypsum-contaminated aggregates

• At high T, redistribution of aluminate and sulphate in other phases, e.g.

C-S-H, AFm phase.

• After cooling and during service life, sulphate and aluminate are desorbed to form ettringite,  in volume

• The expansion results in formation of microcrystalline ettringite adjacent to aggregate particles

• Cracking

(28)

Sulfate Attack (DEF)

• Delayed Ettringite Formation (DEF)

Alleged DEF in concrete box-beam

(Texas, D. Hooton) 3-year old highway viaduct (Malaysia)

(Verbeck 1969)

(29)

Sulfate Attack

• Thaumasite formation from sulfate attack

• Primary risk factors

• presence of sulphate and/or sulphide

• presence of carbonate ions, from concrete aggregate, or in cement as filler, or due to carbonation of concrete

• low temperatures (generally below 15 C)

• presence of water, or very wet C-S-H + H

₂

O/CO

₃^-2

/SO

₄^-2



• Thaumasite

• soft white powder, no binding power

• Sulphate resisting cement will not be immune to this type of attack although the formation of thaumasite is decreased with decreasing C

₃

A content

• Concrete with 70-90% OPC replaced by slag performed well under conditions in which concrete with OPC alone performed poorly

3 2 15

C SC S H

(30)

Crystallization of salts

• Salts can also cause damage to concrete through the development of crystal growth pressure that arise through physical causes

• Penetration of water containing considerable quantities of dissolved salts into concrete

• Salt crystallization in pores when water evaporates

• Repeated or continued evaporation cause salts built up

• Cracking

• Where?

• Fluctuating water levels

• Concrete is in contact with ground water rich in salts

• Control

• Use low w/c, low permeability concrete

• For existing concrete, seal concrete to prevent the ingress of moisture and evaporation

(31)

• Changes in temperature and RH can cause alternate cycles of dissolution and crystallization of Na

₂

SO

₄

salts, resulting in phase changes between Na

₂

SO

₄

(thenardite) and Na

₂

SO

₄

· 10H

₂

O (mirabilite).

(R. Flatt 2002)

(D. Hooton)

(32)

- Current standards deal with evaporative

transport and sulfate salt crystallization by limiting the W/CM of concrete

- At W/CM < 0.45, the

rate of evaporative

transport rapidly

diminishes.

(33)

Acid Attack

(34)

Acid Attack

• Naturally occurring acidic ground water are not common

• Acidic waters may occur in landfilled areas or mining operations

• Highly acidic conditions may exist in industrial wastes

(35)

Acid Attack

• Concrete made with Portland cement is not resistant to attack by strong acids or compounds which may convert to acids.

• pH < 6.5, concrete can be attacked

• 4.5 < pH < 5.5, severe attack

• pH < 4.5, very severe attack

• Most vulnerable hydration products is Ca(OH)

₂

• If the acid is highly concentrated, C-S-H may also be attacked, forming silica gel

• The nature of the anion that accompanies the hydrogen ion may further aggravate the situation.

2

2 2

( ) 2 2

Ca OH  H

^

 Ca

^

 H O

 

2

2 2 2 2 2

3 CaO  2 SiO  3 H O  6 H

^

 3 Ca

^

 2 SiO  nH O  6 H O

H O2

(36)

Acid Attack

HCl(from ACI Committee 201 report)

• Examples: sulfuric acid, sulfate attack, expansion

• If the reactions of an acid and cement paste form

• Soluble products: leaching, losing binding capacity

• Expansive products: expansion and cracking

• Insoluble products: fill voids, reduce rate of deterioration

• Limestone and dolomite aggregates are also subjected to attack by acids.

(37)

Corrosion of sewer pipes

(38)

• Domestic sewage itself is usually harmless

• Problem: corrosion caused by bacteria

• Anaerobic bacteria reduces sulphur compounds to H

₂

S

• H

₂

S dissolves in water film in upper part of the pipe

• Aerobic bacteria

oxidises H

₂

S, and

produce sulphuric acid

• sulphuric acid attacks

concrete both above and at the level of

flow of sewage, but

the attack is more severe

above the flow level

(39)

• Service life of concrete with limestone aggregate > that of concrete with siliceous aggregate

• Protective treatment of surface

• Coatings

• Bitumens, resins

• Calcium aluminate cement concrete

• Epoxy mortars

• Treatment

• Sodium silicate (water glass)

• Lining of polyvinyl chloride (PVC) sheets

(ACI Committee 201 report)

(40)

Alkali Aggregate Reaction (AAR)

(41)

• Types of AAR

• alkali-silica reaction (ASR) (most common)

• alkali-carbonate reaction (ACR)

• Alkali-silica reaction

• Certain types of aggregates contain reactive silica which may react with alkalis from cement and cause damage

• symptoms

• pop-outs and map cracking with gel coming through cracks to form jelly like or hard beads on surface

• Consequences of ASR

• appearance

• serviceability

• severe cracking can facilitate

other damages

(ACI Committee 201 report)

(42)

• Mechanism of damage

(43)

• Rate of reaction depends on the size of the aggregate particles and the form of silica

• Fine particles (20 to 30 m) lead to expansion within a few months

• Large ones lead to expansion only after many years

• High temperature accelerates the reaction

• ASR takes places at high concentration of OH^- in pore water

• Cracking pattern

• irregular, spider's web

• Reaction rim formed on affected aggregate particles, destroy the bond between the aggregate and hydrated cement paste

• Diagnose

• Visual inspection

• Optical microscopy

• Scanning electron microscopy

• energy dispersive X-ray analysis

(44)

• Factors affecting expansion

• Nature of reactive silica

• Amount of reactive silica

• The pessimum % depends on

• form of reactive silica

• degree of alkalinity

• w/c

• Typical range 2-10%

• Particle size of reactive material

• Amount of available alkali

• Amount of available moisture Pessimum %

(45)

• Pessimum amount in reactive silica

• Low SiO

₂

/Na

₂

O, high pH, low solubility of CH and low Ca

²⁺

in pore solution

• form Na(K)-Si-H gel

• expansion

• Increase SiO

₂

/Na

₂

O,

•  reaction product Na(K)-Si-H

• More expansion

• High SiO

₂

/Na

₂

O, low pH, high solubility of CH and high Ca

²⁺

in pore solution

• Non-swelling Ca-Na(K)-Si-H 

• Expansion 

(46)

• Factors affecting expansion

Particle size , expansion 

Na₂O eq. <0.6% in cement, del eterious expansion usually do not occur

(47)

• Control alkali concentration (use low alkali cement)

• Control pH in pore solution

• Control the amount of reactive silica

• Avoid susceptible aggregate based on petrographic analyses and service records

• Control moisture

• reduce permeability of concrete

• reduce the water supply, reduce swelling

• Alteration of alkali-silica gel

• Use lithium & barium salts as additive (mechanism: preferential formation of non-swelling lithium & barium silicate hydrates)

• Use mineral admixtures (fly ash, slag, silica fume)

• Reduce cement content

• Reduce pH of pore solution

• Reduce permeability

• Alkali content in mineral admixtures need to be checked before being used to control ASR

(48)

Carbonation

(49)

Carbonation

• CO₂ concentrations:

• rural air 0.03%

• unventilated lab. >0.1%

• large cities 0.3% on average

Ca(OH)₂ + CO₂ → CaCO₃ +H₂O

pH >12.5 ~9

• When pH reduced to a level <~11.5, passive film is destroyed

• Depth of carbonation d = k t ^0.5

t - time (year)

k - relates to the permeability of concrete, cement type, T, RH, micro- and macro climatic conditions (frequency and duration of wetting and drying)

(50)

Carbonation

- The depth of carbonation is determined by spraying phenolphthalein solution on a freshly broken concrete surface

- Noncarbonated areas turn red or purple, carbonated areas stay colorless.

(51)

Corrosion of steel embedded concrete

(52)

• In high alkaline environments, ordinary steel products are covered by a thin iron-oxide film -FeOOH (passive film)

• impermeable

• strongly adhere to the steel surface

• make the steel passive to corrosion

• metallic iron will not corrode until the passive film is destroyed

• The passive film may be destroyed by

• Carbonation

• Chloride ions

(53)

• Sources of Cl

• sea water

• de-icing salt

• salt contaminated aggregate

• contaminated water

• admixtures

• Three forms of chlorides in concrete

• Chemically bound

• reaction of Cl with C₃A  calcium chloroaluminate

• Physically bound

• adsorbed on the surface of gel pores

• Free chlorides

• water soluble

• responsible for the initiation of steel corrosion

(54)

• Equilibrium of the three forms of Cl

• The distribution of the chlorides among the three forms is not permanent as there is an equilibrium such that some free chloride ions are always present in the pore solution.

• Passive film maybe destroyed even at pH >11.5

• Formation of soluble iron-chloride complex which results in deposition of loose porous rust

• The amount of Cl required to initiate corrosion depends on pH of pore solution

• Cl/OH  0.6, corrosion

 

Fe

2^

 Cl

^

 FeCl complex

^

 ^FeCl 

^

^ ² ^OH

^

^ ^{Fe OH}  

₂

^ ^Cl

^

(55)

• Mechanism of corrosion

• Electrochemical process

• Anode reaction (oxidation)

• Cathode reaction (reduction)

2

Fe e

^

 Fe

^

2 2

0.5 O  H O  2 e

^

2 OH

^

(56)

• Mechanism of corrosion

• At the anode (1)

(2)

• Formation of hydrated ferric oxide (rust),volume increase that cause cracking and spalling

• When O₂ is limited, reaction limited to Step (1), steel bars can be corroded without cracking the concrete cover

   

2

Fe

^

 OH

^

 Fe OH

 

2

 

3 2 3 2

2 Fe OH  2 Fe OH  Fe O  nH O

2, 2

O H O

(Broomfield, 2007)

(57)

• Development of anode & cathode areas due to different electr ochemical potentials

• Different impurity levels in steel bars

• Different residue strains

• Different concentration of oxygen or electrolyte in contact with the metal

• Rate of corrosion depends on

• Availability of O₂ and H₂O

• Resistivity of concrete

• Consequence of corrosion of embedded steel

• Stain

• Reduce cross sectional area of the steel bars, and reduce load carrying ca pacity

• Loss bond between steel and concrete

• Cracking and spalling of concrete cover

(58)

• Material selection

• Reduce the porosity of concrete

• reduce w/c

• use mineral admixtures

• Corrosion inhibitors

• Calcium nitrite: Fe²⁺ + OH^- + NO^2-  NO + -FeOOH

• -FeOOH redeposit on the surface of steel reinforcing bars to maintain the passivity of steel

• For long-term protection, high doses of calcium nitrite are needed

• Stainless steel, or other reinforcing materials

• Design: Sufficient thickness of concrete cover

• Coating and membrane

• Epoxy coating of steel reinforcing bars

• Protective layer for concrete

• Cathodic protection

(59)

Deterioration of concrete in marine environment

(60)

Freezing/Thawing (Metha and Monteiro 2006)

(61)

Freeze/Thawing

• Freezing of cement paste

• Freezing of most H₂O in saturated cement paste does not occur immediately when the concrete is cooled below 0 ^oC

• Freezing point depends on the size of pores

• Salt in pore solution also reduce freezing point

• Mechanism of Frost Attack

• Generation of hydraulic pressure

• water  ice: volume increase ~9%

• generate hydraulic pressure

• with the increase of saturation degree of concrete, the volume increase upon freezing causes damage, cracking

• cycles of freezing and thawing: cumulative effect

(62)

Freeze/Thawing

• Mechanism of Frost Attack

- Freezing damage is observed with liquids that do not expand upon freezing

- Generation of osmotic pressure

 solute concentration in pore water adjacent to freezing sites

 draw water from the more dilute pore solution in surrounding unfrozen paste

 movement of water create osmotic pressure

 cause the surrounding paste to crack.

Chemical potential of water in the form of ice < that of water in unfrozen pores, effective RH at the freezing sites is lowered, water moves towards the freezing sites  desorption of water from C-S-H

(63)

Freeze/Thawing

• Protection of frost attack

• Use Air Entrainment to protection

• Provide empty space so the excess water can move and freeze without causing damage

• Spacing factor determines the average distance water must travel to reach the free space

• Design requirements

• air content

• spacing factor (characteristic space between air bubbles) <0.2mm

(Cordon 1967)

(64)

Freeze/Thawing

• Resistance of concrete to freezing/thawing cycling depends on

• Permeability of cement paste

• Degree of saturation of the cement paste and amount of freezable water

• Below some critical value of saturation, concrete is highly resistant to frost

• Fully saturated concrete will be damaged even if it is properly air-entrained

• Rate of freezing

• Average distance from any point in the paste to a free surface

• Strength of hardened cement paste

(65)

Freeze/Thawing

• Improving concrete sustainability by designing and specifying for durability

• Making durable concrete structures has a large impact on sustainability

• the elapsed time, from construction to rehabilitation and replacement, can be extended by increase the service life.

• Improvements in concrete mixture design

• Optimize of combined aggregate gradation

• Use of WRA and superplasticizers

• Use of supplementary cementing materials

• Use of recycled aggregates