Durability Flashcards
What is deterioration?
- Physical Manifestation of failure of a material (craking, spalling, delamination, pitting)
- Decomposition of material (disintegration, weathering)
What are the causes of deterioration?
- Interaction with environment (external)
- Interaction between constituents (internal)
What are the three types of concrete deterioration?
- Physical Det.
- Chemical Det.
- Reinforcement Corrosion
What causes physical deteriorations?
- Frost
- Cracking
- Fire
- Abrasion
What causes chemical deteriorations?
- Sulphate
- Sea Water
- Leaching
- AAR
What causes reinforcement corrosion?
- Carbonation
- Chlorides
What is corrosion?
Conversion of IRON to IRON OXIDE (rust)
What are the consequences of corrosion?
- rust
- expansion
- stress
- cracking
- spalling
- more rust
- less steel to take the load
What does it mean when reinforcement are passivated?
Covered by thin oxide film
What are the most common causes of depassivation?
- Ingress of chloride ions
- Carbonation of concrete
To fight corrosion, what are they using?
- Epoxy coated bars
- Stainless steel rebar
- FRP rebar
What are the steps to repair concrete deterioration?
- Concept
- Surface preparation
- Cleaning Reinforcement
- Corrosion Inhibitor
- Bond Coat
- Polymer Mortar (coarse)
- Polymer Mortar (fine)
- Anti-Carbonation Coat
What are the most common signs of freeze/thaw deterioration?
- Development of cracks sub-parallel to the surface
- Cracks throughout the concrete (delamination)
- Gaps around aggregates in the surface regions of concrete
What are the signs of Freeze/Thaw Damage?
- Spalling
- Paste Failure: Fracture surface consist of broken paste and occasionally undisturbed aggregate faces.
- Aggregate failure: spalled section contains a broken aggregate at the bottom of fracture surface pit
- Paste Fractures due to aggregate expansion
- D-Cracking and Scalling: Often occurs at expansion joints
How to protect concrete?
- Keep concrete dry (not always possible)
- Reduce the amount of freezable water (by reducing capillary porosity, feasible)
- Provide a relief for pressure (air entrainment)
- Combination of the above
Explain air entrainment.
- Network of air bubbles
- Using a chemical admixture mixed with concrete
- Size usually 10um to 100um
- Dosage, type of cement, compatibility with other admixtures used can influence volume and spacing of air bubbles
What is an Alkali-Aggregate Reaction in concrete?
AAR correspond to chemical reactions between alkalis (Na2O & K2O) in the concrete pore fluid and some mineral phases in the aggregates.
Alkalis are supplied to the concrete pore fluid from sources such as?
- Cement
- Chemical Additives
- Aggregates (medium to long-term)
- Supplementary cementing materials
- Sea Water
- De-icing Salts
Explain Alkali-Silica Reaction (ASR)
Alkali hydroxide+Reactive silica gel = Alkali Silica Gel
Alkali silica gel filling microcracks+Moisture = expansion
Time for distress due to AAR
- less than 5 to more than 25 years
- Type and reactivity level of the aggregate
- Alkali content and design of the concrete mixture
- Exposure conditions
What are the field symptoms of ASR?
- ASR induced longitudinal Cracking in soffit of bridge deck
- Longitudinal cracking in RC column
What are the field symptoms of AAR?
- Map cracking pattern
- Oriented cracking bridge columns
Preventive measures against AAR?
- Use a non-reactive aggregate
- Selective quarrying & aggregate beneficiation
- Limit the alkali content of the concrete mixture: use a low-alkali cement or limit alkali content of concrete
- Use an adequate proportion of supplementary cementing material (SCMs)
What is meant by Hydrates?
Chemical reactions between cement and water
Cement+Water = Hydration products (CSH, CH)+Heat
How to reduce sulfate attack?
- Type 2: Moderate Sulfate Resistance (Lower C3A, 7.5%)
- Type 5: High Sulfate Resistance (Lower C3A, 3.5%)
- Addition of SCMs
What is the purpose of SCMs?
- Enhancement to mechanical properties and durability
- Refinement of porosity
- Densification of aggregate-cement paste interface
Explain Plastic and Drying Shrinkage.
- Very rapid loss of moisture while concrete still plastic
What are the factors that control Plastic and Drying shrinkage?
- Concrete and air temperature
- Relative Humidity
- Wind Velocity
What solution can be done to prevent plastic and drying shrinkage?
- Keep water-cement ratio low
- Prevent rapid moisture loss: wet curing, wind break and shade concrete surface
What are the thermal effects on concrete?
- Thermal exp/contraction
- Uneven thermal loads
- Restraint to volume change
- Early thermal cracking of concrete
- Thermal movement in existing cracks
