T2-3: Control of Cracking Flashcards
What type of intrinsic cracking is especially inevitable?
Drying shrinkage
For controlling crack widths, what is the difference between having no reinforcement and having reinforcement?
No reinforcement: few, wide cracks
Reinforcement: lots of narrow cracks
Why are narrow cracks preferable?
There is less ingress of water, oxygen, chloride (CI-); less chance of corrosion
What minimum crack width
0.3mm
Describe the process for determining steel content
- Calculate the minimum steel area, using the equation
- Determine the max bar sizes using Table 7.2N (max bar diameter) and Table 7.3N (max bar spacing)
NB. for restraint cracking, must comply with Table 7.2N; for loading cracking, must comply with Table 7.3N
- “if in doubt”, use 7.2N instead of 7.3N
What bar arrangement is most effective at controlling cracking?
Small bars, closely spaced
[NAQ] check slides 85-88 for example steel design calculations
If there are large areas of unreinforced concrete in deep sections, what can be added?
What should be assumed in the steel design calculations?
Skin reinforcement
Assume k = 1, and steel stress (σ_s) = half that for main reinforcement
What else helps control cracking, and is used a lot in industrial flooring?
Fibre reinforcement
Describe fibre reinforcement
Effectively acts as a lot of closely spaced/small diameter mars distributed throughout the matrix; no skin reinforcement
What might a typical hybrid reinforcement system provide, with PP microfibres + steel macrofibres?
PP microfibres: control plastic shrinkage and add sacrificial fire spalling resistance
Steel macrofibres: provide flexural toughness and crack control
[NAQ] see end of slides for frc/frp content
