Flashcards in Durability Deck (30):
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?
What causes chemical deteriorations?
- Sea Water
What causes reinforcement corrosion?
What is corrosion?
Conversion of IRON to IRON OXIDE (rust)
What are the consequences of corrosion?
6. more rust
7. 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?
- 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?
- 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?
- 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