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CIVI 321 > Durability > Flashcards

Flashcards in Durability Deck (30):
1

What is deterioration?

- Physical Manifestation of failure of a material (craking, spalling, delamination, pitting)
- Decomposition of material (disintegration, weathering)

2

What are the causes of deterioration?

- Interaction with environment (external)
- Interaction between constituents (internal)

3

What are the three types of concrete deterioration?

- Physical Det.
- Chemical Det.
- Reinforcement Corrosion

4

What causes physical deteriorations?

- Frost
- Cracking
- Fire
- Abrasion

5

What causes chemical deteriorations?

- Sulphate
- Sea Water
- Leaching
- AAR

6

What causes reinforcement corrosion?

- Carbonation
- Chlorides

7

What is corrosion?

Conversion of IRON to IRON OXIDE (rust)

8

What are the consequences of corrosion?

1. rust
2. expansion
3. stress
4. cracking
5. spalling
6. more rust
7. less steel to take the load

9

What does it mean when reinforcement are passivated?

Covered by thin oxide film

10

What are the most common causes of depassivation?

- Ingress of chloride ions
- Carbonation of concrete

11

To fight corrosion, what are they using?

- Epoxy coated bars
- Stainless steel rebar
- FRP rebar

12

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

13

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

14

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

15

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

16

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

17

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.

18

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

19

Explain Alkali-Silica Reaction (ASR)

Alkali hydroxide+Reactive silica gel = Alkali Silica Gel
Alkali silica gel filling microcracks+Moisture = expansion

20

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

21

What are the field symptoms of ASR?

- ASR induced longitudinal Cracking in soffit of bridge deck
- Longitudinal cracking in RC column

22

What are the field symptoms of AAR?

- Map cracking pattern
- Oriented cracking bridge columns

23

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)

24

What is meant by Hydrates?

Chemical reactions between cement and water
Cement+Water = Hydration products (CSH, CH)+Heat

25

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

26

What is the purpose of SCMs?

- Enhancement to mechanical properties and durability
- Refinement of porosity
- Densification of aggregate-cement paste interface

27

Explain Plastic and Drying Shrinkage.

- Very rapid loss of moisture while concrete still plastic

28

What are the factors that control Plastic and Drying shrinkage?

- Concrete and air temperature
- Relative Humidity
- Wind Velocity

29

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

30

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