T2-2: Cracking Flashcards
What are three categories of cracking, and what are they caused by?
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
What are four types of cracking preconditions (movement and restraint)?
- Plastic: bleeding
- Thermal: temperature gradients
- Drying shrinkage: water movement CSH
- Autogenous cracking: hydration
Identify different types of cracks
What causes cracking?
An induction of tensile stress
For the preconditions of cracking, what two agents are needed?
- Agent for dimensional change
- Agent for restraint
What two things are considered for the cracking agents for dimensional change?
Water movement:
- bleeding, hydration, curing evaporation, drying
The thermal coefficient of expansion
What two things are considered for the cracking agents for restraint?
External restraints:
- ground, previous casting, adjacent bays
Internal restraints:
- differential dimensional change in large sections, rebars, change of section shape
[NAQ] restraint factors, R
These different casting sequences can cause restraint
Does cracking increase or decrease permeability?
Increase - water retention
How does cracking reduce concrete durability?
- 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
What are three types of intrinsic cracking?
- Plastic
- Early-age thermal shrinkage cracks (1 day - 3 weeks)
- Drying (and/or autogenous) shrinkage cracks (weeks or months)
What are two types of plastic cracks (intrinsic)?
- Plastic settlement cracks (10 min - 3 hr)
- Plastic shrinkage cracks (30 min - 6hr)
In early-age thermal cracking, how is heat generated?
What is the possible temperature rise?
Heat is generated as concrete cures
50C or more is possible
What is a feasible temperature difference, to cause thermal shrinkage cracks?
40C
What is the restraint in early-age thermal cracking?
Temperature gradients:
- the surface cools quicker than internal: internal restrains the surface
- or external restraint
Explain external restraints
Where are stresses from?
The top is free to contract on cooling: base restrained
Stresses:
- Substrate (compressive)
- Casting (tensile)
Explain internal restraints
For large pours, such as in mass concrete dams etc
What is the equation for restrained strain (thermal)?
[NAQ] restraint factors
[NAQ] example use of tensile strain capacity equation, to find max. temp before cracking appears
What are three ‘design’ mitigations for early-age thermal cracking?
- Avoid external restraint; correct casting sequence to avoid trapped bays
- Use a low heat binder (LHPC, GGBS/PFA)
- Use a low cement content
What are two ‘management’ mitigations for early-age thermal cracking?
- Large pours
- insulate to avoid uneven heat loss/T-gradients - Smaller pours
- cool quickly to dissipate heat
- active cooling: ice
Which of these cracks are early-age thermal cracks?
G and H
[NAQ] limits on ΔT for varying restraint factors (BS8110)
Describe drying shrinkage cracking
- If the removal of water leads to <30% RH (relative humidity), there are new bonds in CSH; permanent shrinkage
- High w/c, small sections leads to more shrinkage
- See S&D §20.1.2.4, 20.1.3
How are the restraint considerations in drying shrinkage strain similar to in thermal?
Similar equation
Which of these cracks are drying shrinkage cracks?
I
Describe autogenous shrinkage strain
- Symbol in image
- It is related to drying shrinkage
- The loss of free water consumed by the hydration reaction, leads to shrinkage
- If <0.01% «_space;drying shrinkage in normal strength concrete*
When is autogenous shrinkage strain more significant? (2 things)
If there is a high binder content and low water-cement ratio:
- HPC (high-performance concrete), high f_ck
What is the equation for total shrinkage strain?
NB. see slides 71-75 for process of using this equation, plus graphs
For cracking*, what is needed?
An agent for dimensional change + an agent for restraint
Describe the difference in how early-age thermal shrinkage cracks and drying shrinkage cracks form
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
What type of cracking is the most common (e.g. in a building)
Drying shrinkage
NB. it is highly unlikely drying shrinkage cracks go too deep into the concrete; not usually a structural problem
What happens in higher-strength concrete (e.g. f_ck = 90 on graph)
Generally, less shrinkage occurs (improved material properties and reduced porosity)
Now split between drying shrinkage and autogenous shrinkage
