T2-2: Cracking

Question 1

What are three categories of cracking, and what are they caused by?

Answer 1

Structural cracking - caused by applied loads/overloads

Intrinsic cracking - caused by intrinsic internal mechanisms

Progressive cracking - caused by progressive deterioration mechanisms

NB. content focuses on structural/intrinsic cracking

Question 2

What are four types of cracking preconditions (movement and restraint)?

Answer 2

1. Plastic: bleeding
2. Thermal: temperature gradients
3. Drying shrinkage: water movement CSH
4. Autogenous cracking: hydration

Question 3

Identify different types of cracks

Answer 3

Question 4

What causes cracking?

Answer 4

An induction of tensile stress

Question 5

For the preconditions of cracking, what two agents are needed?

Answer 5

1. Agent for dimensional change
2. Agent for restraint

Question 6

What two things are considered for the cracking agents for dimensional change?

Answer 6

Water movement:
- bleeding, hydration, curing evaporation, drying

The thermal coefficient of expansion

Question 7

What is considered for the cracking agents for restraint (external and internal)?

Answer 7

External restraints:
- ground, previous casting, adjacent bays

Internal restraints:
- differential dimensional change in large sections, rebars, change of section shape

Question 8

[NAQ] restraint factors, R

Answer 8

These different casting sequences can cause restraint

Question 9

Does cracking increase or decrease permeability?

Answer 9

Increase - water retention

Question 10

How does cracking reduce concrete durability (x2)?

What are the risks (following cracking) in:
- soffits/walls
- floors

Answer 10

• Cracks are pathways for water, air, chlorides, carbonation
• This reduces the time to initiation of corrosion of steel
• In soffits/walls, there is a risk of surface spalling
• In floors, there is decreased wear resistance

Question 11

What are three types of intrinsic cracking?

Answer 11

1. Plastic
2. Early-age thermal shrinkage cracks (1 day - 3 weeks)
3. Drying (and/or autogenous) shrinkage cracks (weeks or months)

Question 12

What are two types of plastic cracks (intrinsic)?

How long do they take?

Answer 12

1. Plastic settlement cracks (10 min - 3 hr)
2. Plastic shrinkage cracks (30 min - 6hr)

Question 13

In early-age thermal cracking, how is heat generated?

What is the possible temperature rise?

Answer 13

Heat is generated as concrete cures

50C or more is possible

Question 14

What is a feasible temperature difference, to cause thermal shrinkage cracks?

Answer 14

40C

Question 15

What is the restraint in early-age thermal cracking?

What is restraining what?

Answer 15

Temperature gradients:
- the surface cools quicker than internal: internal restrains the surface
- or external restraint

Question 16

Explain external restraints

Where are stresses from (x2)?

Answer 16

The top is free to contract on cooling: base restrained

Stresses:
- Substrate (compressive)
- Casting (tensile)

Question 17

Where might internal restraints be significant?

Answer 17

For large pours, such as in mass concrete dams etc

Question 18

What is the equation for restrained strain (thermal)?

Answer 18

Question 19

[NAQ] restraint factors

What restraint factor will full restraint have?

Answer 19

1.0

Question 20

[NAQ] example use of tensile strain capacity equation, to find max. temp before cracking appears

Answer 20

Question 21

What are three ‘design mitigations’ for early-age thermal cracking?

Answer 21

1. Avoid external restraint; correct casting sequence to avoid trapped bays
2. Use a low heat binder (LHPC, GGBS/PFA)
3. Use a low cement content
   - reduces heat of hydration and thus stresses

Question 22

What are two ‘management mitigations’ for early-age thermal cracking?

Answer 22

1. Large pours
   - insulate to avoid uneven heat loss/T-gradients
2. Smaller pours
   - cool quickly to dissipate heat
   - active cooling: ice

Question 23

Which of these cracks are early-age thermal cracks?

Answer 23

G and H

Question 24

[NAQ] limits on ΔT for varying restraint factors (BS8110)

Answer 24

Range 7 to 56C

Question 25

Describe drying shrinkage cracking - at what RH is there permanent shrinkage (and why) What things (x2) lead to more shrinkage

Answer 25

- If the **removal of water** leads to **<30% RH**, there are **new bonds in CSH**; permanent shrinkage - **High w/c** & **small sections** leads to more shrinkage (high w/c leads to more porous structure) - See S&D §20.1.2.4, 20.1.3

Question 26

How are the restraint considerations in drying shrinkage strain similar to in thermal?

Answer 26

Similar equation

Question 27

Which of these cracks are drying shrinkage cracks?

Answer 27

I

Question 28

Describe autogenous shrinkage strain What leads to shrinkage?

Answer 28

- Cracking that occurs due to the **volume shrinkage of the cement paste** during the **early stages of hydration**, even before significant drying shrinkage takes place - Related to drying shrinkage - The **loss of free water** consumed by the hydration reaction, leads to shrinkage - If <0.01% <

Question 29

When is autogenous shrinkage strain more significant? (2 things) Give an example of a type of concrete that exhibits significant autogenous shrinkage strain

Answer 29

If there is a **high binder content** and **low W/C ratio**: - like in **HPC** (high-performance concrete), high f_ck

Question 30

What is the equation for total shrinkage strain?

Answer 30

NB. see slides 71-75 for process of using this equation, plus graphs

Question 31

For cracking*, what is needed?

Answer 31

An agent for dimensional change + an agent for restraint

Question 32

Describe the difference in how early-age thermal shrinkage cracks and drying shrinkage cracks form

Answer 32

In **early-age thermal shrinkage** cracks, the **heat generated in concrete cures** start to dissipate In **drying shrinkage cracks**, over weeks/months the **hydration reactions slow down** and **water exits** the structure

Question 33

What type of cracking is the most common (e.g. in a building)

Answer 33

**Drying shrinkage** NB. it is highly unlikely drying shrinkage cracks go too deep into the concrete; **not usually a structural problem** .

Question 34

What happens in higher-strength concrete (e.g. f_ck = 90 on graph)

Answer 34

Generally, **less shrinkage** occurs (improved material properties and **reduced porosity**) Now split between drying shrinkage and autogenous shrinkage

Question 35

What does a hight w/c ratio do with respect to porosity and evaporation of water

Answer 35

High w/c = higher capillary porosity = larger pores = greater evaporation of water