Main concrete durability issues in SA Chemical deterioration and attack Presented by: Professor Mark Alexander Concrete Materials and Structural Integrity Research Unit (CoMSIRU) University of Cape Town
Overview of deterioration Deterioration of Concrete The concrete system Aggressiveness of the exposure environment Intrinsic factors • Concrete penetrability Deterioration by Deterioration by • Binder type chemical mechanisms physical mechanisms • Binder content • Nature and concentration • Abrasion • Water/binder ratio of aggressive agents • Erosion • Other constituents: aggregates, admixtures, etc. • Internal chemical instability • Cavitation • Design mix proportioning • Freeze-thaw (incompatibility between • Salt crystallisation mix constituents) Extrinsic factors • Effects of cracking due to • Coupling with effects of • Production and/or construction processes, e.g. loading or thermal/hygral effects temperature and relative mixing, placing, consolidation humidity • Curing (temperature and moisture environment) • Early age temperature history
Service life of concrete structures Ideal vs Actual As built quality 100 Actual Quality (%) 80 Performance – Assuming adequate 60 inspection & maintenance Unacceptable 40 level of damage Repair 1 Repair Repair 3 2 20 Time (years) 0 10 20 30 40 50 60
Transport Processes Air borne salt and occasional Rain reducing surface salt-water inundation salt concentration Evaporation giving a salt Diffusion in concentration response to salt concentration • Permeation – pressure gradient • Diffusion – concentration gradient Capillary absorption into Water table • Sorption (capillary) and advection partially saturated concrete (bulk flow) – moisture gradient Permeation by Splash/spray (e.g. sorptivity) Wick action pressure head • Evaporation – drying gradient Tidal range • Wick action – combined evaporation and other mechanism (e.g. permeation) • Migration – electrical potential gradient Diffusion of salts from sea-water
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 ▪ Typical appearance: ❑ Random crack pattern (“surface map cracking” ) ❑ Leaching of reaction product ❑ White rim around aggregates ❑ Large crack widths ▪ Time ❑ Might take years to develop ▪ Structural effects ❑ Loss of strength and stiffness, cracking, deflections
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 Hydroxyl flow Anode Cathode 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) MORE ABOUT STEEL CORROSION AND CRACKING LATER Pourbaix Diagram
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 CO 2 , 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 CO 2 (carbonate- bicarbonate stability).
Soft Water Attack
Soft Water Attack Gariep Dam, Northern Cape
Soft Water Attack Mechanism ▪ Leaching – dissolution of CH, destabilisation of CSH, leading to further dissolution of CH, and so on. ▪ Zonation occurs 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 Schematic model of cement-based material leaching decalcified; finally zone 5 is the much altered (Bernard et al. 2008). zone.
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 Water-borne Sulphate ▪ 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 C 3 A n H
Sulphate attack ▪ E.g.: Attack of sodium sulphate Na 2 SO 4 Ca(OH) 2 + Na 2 SO 4 → CaSO 4 + 2NaOH Calcium sulphate → Gypsum ▪ Gypsum has a volume increase of 20% compared to Ca(OH) 2 ▪ Ettringite formation C 3 A n H + 3CaSO 4 → C 3 A · 3CaSO 4 · 32 H 2 O 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 Disintegrating cement matrix Formation of Gypsum + Ettringite ▪ 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 C 3 A 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 Velocity of dissolution calcium hydroxide of the hydrated cement ❑ E.g.: Ca(OH) 2 + 2HCl → CaCl 2 + 2 H 2 O (HCl = Hydrochloric acid) ▪ Calcium compounds of different solubility are produced, which may be leached away ❑ Exposed aggregates, debonding of aggregates, reduced cover pH 1 7 14 ◼ Limestone or dolomite might also be dissolved
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 )
Sewer corrosion due to biological acid attack H 2 S+2O 2 2H + + SO 4 2- Moist attacked - auto-oxidation, bacterial surfaces activity H 2 S Absorption Severe corrosion Sewage level H 2 S Emission above sewage H 2 S H + +HS - 2H + +S 2- Septic sewage containing Slime layer level no dissolved oxygen Absence of corrosion on Silt submerged pipe surface
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 of the concrete to stifle bacterial growth – ‘ bacteriostatic ’ effect
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