Main concrete durability issues in SA Chemical deterioration and - - PowerPoint PPT Presentation

Presented by:

Professor Mark Alexander

Concrete Materials and Structural Integrity Research Unit (CoMSIRU) University of Cape Town

Main concrete durability issues in SA

Chemical deterioration and attack

Deterioration of Concrete

The concrete system Intrinsic factors

• Concrete penetrability
• Binder type
• Binder content
• Water/binder ratio
• Other constituents: aggregates, admixtures, etc.
• Design mix proportioning

Aggressiveness of the exposure environment Extrinsic factors

• Production and/or construction processes, e.g.

mixing, placing, consolidation

• Curing (temperature and moisture environment)
• Early age temperature history

Deterioration by physical mechanisms Deterioration by chemical mechanisms

• Abrasion
• Erosion
• Cavitation
• Freeze-thaw
• Salt crystallisation
• Effects of cracking due to

loading or thermal/hygral effects

• Nature and concentration
• f aggressive agents
• Internal chemical instability

(incompatibility between mix constituents)

• Coupling with effects of

temperature and relative humidity

Overview of deterioration

Service life of concrete structures

Ideal vs Actual Quality (%)

Repair 1 Repair 2 Repair 3

10 20 30 40 50 60 100 80 60 40 20

Time (years)

As built quality Unacceptable level of damage Actual Performance –

Assuming adequate inspection & maintenance

Rain reducing surface salt concentration Air borne salt and occasional salt-water inundation Evaporation giving a salt concentration Wick action Diffusion of salts

from sea-water

Water table Permeation by pressure head Tidal range Capillary absorption into partially saturated concrete Splash/spray

Diffusion in response to salt concentration

Transport Processes

• Permeation – pressure gradient
• Diffusion – concentration gradient
• Sorption (capillary) and advection

(bulk flow) – moisture gradient (e.g. sorptivity)

• Evaporation – drying gradient
• Wick action – combined

evaporation and other mechanism (e.g. permeation)

• Migration – electrical potential

gradient

Overview: Chemical deterioration mechanisms

• Alkali-Silica reaction (ASR, or AAR)
• Reinforcement corrosion
• Soft water attack
• Sulphate attack (external sulphates)
• Acid attack

Alkali Silica Reaction ASR

(Alkali Aggregate Reaction AAR)

Alkali Silica Reaction ASR

Alkali Aggregate Reaction AAR

• Reactive silica (aggregate) reacts with

alkali in cement

• Formation of silica gel at aggregate

surface

• In connection with moisture: swelling
• Result: tensile stresses, cracking

ASR

▪ Typical appearance:

• Random crack pattern (“surface map cracking”)
• Leaching of reaction product
• White rim around aggregates
• Large crack widths

ASR

▪ Time

❑ Might take years to develop

▪ Structural effects

❑ Loss of strength and stiffness, cracking, deflections

▪ Typical appearance:

❑ Random crack pattern (“surface map cracking”) ❑ Leaching of reaction product ❑ White rim around aggregates ❑ Large crack widths

ASR

▪ Identification and forensic investigation

❑ Visual assessment ❑ Petrographic investigation

▪ Testing for alkali-susceptible aggregates

❑ Rapid ‘screening’ tests, e.g. AMB ❑ Performance tests, e.g. on actual

concrete mixtures

❑ Structural monitoring

Corrosion of reinforcement

▪ Chloride-induced corrosion ▪ Carbonation-induced corrosion

▪ Visual and other effects:

➢ Cracking ➢ Stains ➢ Aesthetics ➢ Spalling ➢ Delamination ➢ Loss of cross-section ➢ Reduced load capacity ➢ Structural failure

Mechanism of corrosion

▪ Reinforcement embedded in concrete is passivated by the high alkalinity of concrete (pH of 12.5 and

above)

▪ Passivation is lost when concrete pH drops (carbonation) or by the presence of salts that cause local

pitting of steel

Anode Cathode Hydroxyl flow Electron flow Corrosion site

Mechanisms of corrosion

▪ Four states of corrosion are possible for RC:

❑ Passive state (steel embedded in

uncontaminated concrete)

❑ Pitting corrosion (chloride-induced

corrosion)

❑ General corrosion (carbonation-induced

corrosion)

❑ Active, low potential corrosion (saturated

concrete)

Pourbaix Diagram

MORE ABOUT STEEL CORROSION AND CRACKING LATER

Soft Water

Soft water characterised by:

▪ The absence of dissolved salts or ions, e.g. calcium-hungry water. Soft waters

❑ e.g. mountain water may contain few or no calcium salts and thus be aggressive to concrete ❑ This is generally true of the entire seaboard around SA – coastal waters are aggressive

(underlying geology)

▪ Presence of dissolved CO2, which can be aggressive by forming carbonic acid (e.g. underground

waters that have been under high pressure, or waters in contact with vegetable matter).

▪ Difference between aggressive and non-aggressive portions of dissolved CO2 (carbonate-

bicarbonate stability).

Soft Water Attack

Soft Water Attack

Gariep Dam, Northern Cape

Soft Water Attack

Mechanism

Zone 1 is the sound zone; Zone 2 is the zone where portlandite is totally depleted; Zone 3 corresponds to the zone where portlandite is totally dissolved and C-S-H begin to be decalcified; Zone 4 represents the zone where portlandite, hydrated aluminates and sulfoaluminates phases are totally dissolved and C-S-H continue to be decalcified; finally zone 5 is the much altered zone.

Schematic model of cement-based material leaching

(Bernard et al. 2008).

▪ Leaching – dissolution of CH, destabilisation of CSH, leading to further dissolution of CH, and so on. ▪ Zonation occurs

Sulphate attack

▪ Sulphates are common in areas of mining operations, paper industries.

May also be found in soils and waters (ground water, waste water)

▪ Common sulphates found in ground water are calcium, sodium,

potassium and magnesium

▪ Sulphates (in solution with water) permeate the concrete and react

chemically with:

❑ The cement paste’s hydrated lime CH (Ca(OH) 2) ❑ Calcium aluminate hydrate C3AnH

Water-borne Sulphate

Sulphate attack

▪ E.g.: Attack of sodium sulphate Na2SO4

Ca(OH)2 + Na2SO4 → CaSO4 + 2NaOH

▪ Gypsum has a volume increase of 20% compared to Ca(OH)2

Calcium sulphate → Gypsum

▪ Ettringite formation

C3AnH + 3CaSO4 → C3A · 3CaSO4 · 32 H2O

Calcium aluminate, gypsum → Ettringite (Aft, Trisulphate)

▪ Volume increase 200 – 600%

Sulphate attack

▪ Formation of gypsum and ettringite results in expansion, stresses, cracking, scaling, loss of paste-

aggregate bond

Water-borne Sulphate Formation of Gypsum + Ettringite Disintegrating cement matrix

▪ Severity of sulphate attack is dependent on exposure conditions, concrete permeability, concrete

type, amount of water available

▪ Precautions: Type of cement - CEM I: resistance is linked to C3A content (limit to 5% or less)

Sulphate attack – influence of cement additions (extenders) & concrete quality

❑ GGBS

• Content > 70% has positive effect, due to a lower diffusion coefficient and lower

Ca(OH)2

• At lower GGBS contents, diffusion coefficient may be higher than Portland cement

concrete, hence negative effect

• Critical importance of curing

❑ Fly Ash and Silica Fume: positive effect due to:

• Decreased permeability (in particular SF)
• Pozzolanic reaction with Ca(OH)2 during hydration reduces formation of gypsum and

ettringite under sulphate attack

• Also critical importance of curing

❑ Influence of concrete quality

• Aim for low permeability
• Reduce w/c

Acid attack- organic and inorganic acids

▪ Cement

matrix components (alkaline in nature) and calcareous aggregates are soluble in acid

▪ Acid attack is the reaction between acid and (mainly) the

calcium hydroxide of the hydrated cement

❑ E.g.: Ca(OH)2 + 2HCl → CaCl2 + 2 H2O

(HCl = Hydrochloric acid)

▪ Calcium compounds of different solubility are produced, which

may be leached away

❑ Exposed aggregates, debonding of aggregates, reduced

cover

◼ Limestone or dolomite might also be dissolved

1 7 14 Velocity of dissolution pH

Acid attack

▪ Inorganic acids: chemical industry ▪ Organic acids (generally not that aggressive): e.g.

fermentation of agricultural products

▪ Sources:

❑ Contaminated water (e.g. ground water, sewage) ❑ Industry exposure

▪ What to do?

❑ Design concrete for low permeability, specify high cover (if

needed)

Septic sewage containing no dissolved oxygen H2S+2O22H++ SO4

2-

• auto-oxidation, bacterial

activity Sewage level Silt Slime layer level

H2SH++HS-2H++S2-

H2S Absorption Moist attacked surfaces Severe corrosion above sewage H2S Emission Absence of corrosion on submerged pipe surface

Sewer corrosion due to biological acid attack

Sewer corrosion due to biological acid attack

▪ Concrete sewer corrosion results from biogenic sulphuric acid

attack on pipes above the sewage level

▪ The system is a complex one consisting of anaerobic and

aerobic bacteria generating sulphides that are oxidised to sulphuric acid

▪ Cement hydration products are readily soluble in strong

sulphuric acid. This is true for both Portland and CAC-based systems

▪ Rate of attack in sewers appears to be a function of

❑ Neutralisation capacity (i.e. alkalinity) of the concrete (thus,

aggregates are important), and

❑ Ability

• f

the concrete to stifle bacterial growth – ‘bacteriostatic’ effect

Thank You