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ACD > T4: Common Causes of Cracking > Flashcards

T4: Common Causes of Cracking Flashcards

(60 cards)

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1
Q

What are the four main categories of cracking causes?

A
  1. Casting
  2. Direct
  3. Indirect
  4. Attack
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2
Q

What is needed for cracking to occur in concrete?

A

Concrete subjected to tensile stress that exceeds the tensile strength of the concrete
- or the tensile strain exceeds the tensile strain capacity

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3
Q

What are three types of failure modes for unreinforced concrete subject to an axial tensile stress?

A
  • Bond failure
  • Aggregate failure
  • Cement paste failure

NB. if possible, use clean (dust and silt free) crushed rock as aggregates

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4
Q

Define concrete creep

What effect can creep have in early age/green concrete?

What name is given?

A

Long-term deformation under sustained loading

Creep effects in early age/green concrete can reduce tensile strains; reduce likelihood of cracking

‘Creep relief’

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5
Q

What is autogenous healing

A

Self-healing:
- some early-age cracks can be sealed by autogenous healing
- water leaks through cracks, dissolves calcium hydroxide, which combines with CO2 to form calcium carbonate

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6
Q

Describe how poor detailing/ workmanship can cause cracking (x2)

What is caused?

A
  • Steel may not be adequately supported
  • Reduction in effective depth

Causes reduction in shear and BM capacity

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7
Q

What type of failure is this?

A

Hogging flexural failure:
- cracking of concrete in tension and crushing of concrete in compression
- localised buckling failure of longitudinal reinforcement

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8
Q

What type of failure is this?

A

Sagging flexural failure:
- cracking of concrete in tension and crushing of concrete in compression

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9
Q

What type of cracking is this?

A

Shear cracking

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10
Q

What is this (unusual) type of structural cracking?

A

Longitudinal splitting of a RC column:
- cover to main rebar must be at least equal to bar diameter
- otherwise load cannot transfer from steel into concrete

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11
Q

What type of cracking is this?

A

Plastic settlement

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12
Q

What causes plastic settlement cracking to occur?

A

When concrete exhibits bleeding

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13
Q

What is concrete bleeding?

When does it occur?

A

The process where free water in freshly poured concrete rises to the surface due to the settlement of heavier solid particles like cement and aggregate

Occurs after the concrete has been placed and finished

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14
Q

For plastic settlement cracks:

a) how early on does it occur
b) where does it commonly occur

A

a) 20 mins to 2/3 hours

b) near external restraints (e.g. wall)

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15
Q

What are two common examples of where plastic settlement cracking occurs?

A

Flat slabs near to columns
- drop-head column

Waffle slabs

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16
Q

What type of cracking is this?

A

Plastic shrinkage

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17
Q

How does plastic shrinkage cracking occur?

What is left?

A

Occurs when there is bleeding of the concrete:
- bleed water evaporates from the surface of a slab (in hot, dry conditions)
- leaves a high cement content with little coarse aggregates
- shrinkage

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18
Q

What type of cracking is this?

A

Plastic settlement (and corrosion)

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19
Q

For plastic shrinkage cracks:

a) how early on does it occur
b) where does it commonly occur

A

a) 30 mins to 12 hours

b) On exposed surfaces

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20
Q

Describe the difference between the causes of plastic settlement and plastic shrinkage cracking

A
  • Plastic settlement cracking occurs due to the volume reduction of the concrete as water moves upwards and solids settle (bleeding)
  • Plastic shrinkage cracking is caused by rapid surface drying and subsequent shrinkage (bleed water evaporates)
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21
Q

What is a type of indirect cracking?

A

Cracking due to restrained deformations

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22
Q

Describe cracking due to restrained deformations

What is most concrete restrained by (x2)?

A
  • Concrete contracts (when cooling) during first few days of hydration
  • If concrete is restrained (ie. not free to shrink) this creates tensile strains
  • Leads to cracking

NB. most concrete is restrained by the previous pour, or the ground on which it is cast

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23
Q

[NAQ] Diagram of cracking due to restrained deformations, on the stem of an earth retaining wall

A
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24
Q

What is an extremely common combined cause of cracking

A

Combined effects of:
- thermal contraction
- hydration (drying) shrinkage

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25
What are the structural implications here?
No structural problem, but **durability issues**
26
What is commonly included in the design of retaining walls, to prevent cracking?
**Movement joints** NB. this one hasn't worked
27
What types of cracking could this be?
28
What could be the cause of cracking on this bridge, and why?
Due to **restrained deformations** - bridge has **no intermediate joints** and **insufficient longitudinal reinforcement** - can't accommodate the tensile stresses created by the restrained movement
29
What type of cracks could this be?
Localised shrinkage cracks
30
When do localised shrinkage cracks occur (x3)? - with respect to cement/aggregate content, curing When else can they occur?
- Where this is a **high cement content** + **low coarse aggregate content** + **poor curing** - Can also occur where aggregates with a **high fines content** are used - This produces a **low tensile strength**; little cracking resistance
31
What is the cause of this?
**Acidic groundwater** - **dissolves the cement** that binds the sand particles together - stone returns to its **original form** (sand)
32
What does acid attack do? What does its rate depend on?
**Dissolves** the **cement paste** Rate depends on **concentration of acidity**
33
Name a source of acid attack
Industrial process plants and chemical plants
34
What is happening here?
**Sulfate attack** - formation of **expansive ettringite**
35
What does this image show?
TSA (**Thaumasite**) form of **sulfate attack** - results from formation of **expansive thaumasite crystals**
36
What does this image show?
**Alkali-silica reaction** - the 'main one'
37
Describe alkali-silica reactions What is formed?
An **expansive reaction between cements** containing high quantities of: a) **sodium** and **potassium** **oxides** b) reactive **silica aggregates** An **expansive gel** is formed, leading to cracking
38
Name two types of reactive silica aggregates
Flint Chert
39
What does ASR form
A random array of **surface cracks** - such cracking may be due to other effects (e.g. shrinkage)
40
What technique can be used to identify ASR?
**Petrographic analysis** techniques
41
What does ASR cause?
A reduction in **compressive strength** and reduced **bond**
42
What does this image show?
Frost attack
43
What causes frost attacks? What is the result (x2)?
Repeated **freeze-thaw cycles** cause: - a **fatigue-type failure** of the **cement paste** - breakdown of the **aggregate/cement** bond .
44
How would you prevent frost attack (x2)?
- **Seal the cracks** - Have **drainage** in place
45
Describe the process of reinforcement corrosion (x5)?
- **Impurities** in steel result in areas of bar with higher **electrode potential** than others - **Potential difference** between these sites (= voltage) - Many **anode** and **cathode sites** form along the bar in the presence of an **electrolyte** ... - Weak acid **reduces the alkalinity** of the concrete cover - Steel becomes **de-passivated** and corrodes
46
Describe the anode and cathode sites, and what occurs at each
Anode = **high electrode potential** site - **ionisation** occurs Cathode = **lower electrode potential** site - electrons flow through electrolyte **to cathode** (combine with water and oxygen)
47
What is used to find whether concrete is carbonated? What is the pH of carbonated/ not carbonated concrete?
**Phenolphthalein** indicator Carbonated pH < **9** Not carbonated pH ~ **13**
48
Which area of this concrete is carbonated
Colourless = Carbonated
49
What are the four main stages of rebar corrosion?
ie. corrosion → cracking → spalling
50
What does this image show?
Rust staining
51
What is concrete curing? What influence does it have on carbonation depth?
The process of **maintaining a moist and cool environment** for freshly poured concrete to ensure it **develops its strength and durability** **Longer** curing time = **lower** carbonation depth
52
What influence does the water:cement ratio have on carbonation depth?
**Higher** W/C ratio = **higher** carbonation depth
53
How would you really simply describe the carbonation process?
pH is reduced and corrosion starts
54
What does this image show?
Chloride attack
55
What causes chloride attack?
A **marine environment** - **water and oxygen** - combined with **abrasive effect** of wind and water
56
What does chloride-induced corrosion lead to (x4)?
- **Expansive rust products** - Cracking - Spalling of concrete - Severe reduction in structural performance
57
What is causing the deterioration here? What is 'the problem'
- **Leakage** of **expansion joints** - Sodium chloride **de-icing salts** 'are the problem'
58
What type of deterioration is this (x2)?
Cracking Rust staining
59
What is causing deterioration here?
Combined **carbonation** and **chloride** exposure
60
Name types of cracking that are: a) generally controlled on site rather than by the designer (x2) b) occur in the first few days, and up to two weeks, after the concrete is poured
a) plastic shrinkage, plastic settlement b) early thermal contraction and shrinkage

ACD (5 decks)

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