T5: Flexural Cracking Flashcards
(26 cards)
Why does concrete have low resistance to cracking?
When is concrete particularly vulnerable to cracking?
It has low tensile strength
- especially during early stages of hydration
- vulnerable to cracking in first few days after pouring
What type of cracking is flexural cracking in RC members?
Structural (direct action) cracking
Summarise what causes flexural cracking
Cracking caused by excessive tensile stress created by bending (or flexural) effects, resulting from applied loads
What indicator is used to determine if the concrete structure is not performing
Surface crack widths
- Caused by bending
What type of limit state consideration is cracking?
What manner is the concrete likely to be behaving in?
Serviceability limit state consideration
- cracking may make a structure become unserviceable (it may need remedial works)
- the concrete is likely to be behaving in a linear elastic manner; there shouldn’t be any plastic deformation
What three metrics assess the significance of cracking?
- Cracks that lead to durability problems (and a consequent reduction in structural capacity)
- Cracks that lead to a loss of serviceability (or loss of function) of the structure, e.g. leakage of water
- Aesthetically unacceptable cracks
What is the limiting durability crack width in EC2?
0.3mm
NB. appearance is 0.3mm or greater
What is the limiting serviceability crack width in EC2?
0.05 to 0.2mm
[NAQ] EC2 limiting surface crack widths for different exposure classes
i.e. if in doubt, 0.3mm
For the water tightness class 1 defined in EC2, what is the amount of cracking linked to?
What does this factor mean with respect to acceptable crack width, and why?
The head of water in the structure
- greater head = smaller crack that is acceptable (due to higher chance of leakage)
What causes flexural cracking?
Where are primary cracks?
What effect can help stiffen a beam?
- When tensile strain caused by applied loads exceeds tensile strain capacity, cracks form
- Cracks tend to propagate from the tension face of the concrete, towards to the main reinforcement [primary cracks]
- Uncracked regions between primary cracks help to stiffen the beam, although secondary cracks may develop here [tension stiffening]
What three things is the crack width related to?
- The strain of the concrete
- The spacing of the cracks that have occurred
- The effect of tension stiffening
- .
What does this EC2 equation represent?
Design surface crack width (basic)
What does L and ΔL represent?
What is the strain equation, and how is it rearranged to find crack width?
L = crack spacing (=S)
ΔL = crack width (=w_k)
What does this equation represent?
The difference between mean strain in steel reinforcement (ε_sm) and the mean strain in the concrete between the cracks (ε_cm)
What does this represent?
What two values are typically used?
Load duration:
= 0.6 for short-term loading
= 0.4 for long-term
What does this represent?
Mean tensile strength of the concrete
What does this represent?
What does this represent?
What is it equal to?
What is the effective area of concrete in tension (A_c,eff)?
How is calculated?
The area of the concrete cross-section which will crack as a result of the tensile stress caused by applied loading
2.5x the “distance from the tension face of the concrete” to the “centroid of the reinforcement in tension”
What are the three equations for h_c,eff ?
Lesser of:
2.5(h-d)
(h-x)/3
h/2
What does this EC2 equation represent?
Max. crack spacing
What doe these factors represent?
What are their typical values?
k_1 is a bond factor:
= 0.8 for reinforcing bars with high concrete to steel
bond properties
= 1.6 for lower bond properties, as with plain round bars
k_2 is a factor that takes into account the nature of the distribution of the
strain in the concrete
= 0.5 for members subjected to bending (flexure)
= 1.0 for axial
What is this equation a check on?
What does EC2 recommend if the check is not passed?
Check on max crack spacing
If crack spacing (s_r,max) exceeds this value, EC2 gives the max. spacing to be 1.3(h-x)
