Reinforced Concrete Design / module 14
Water-retaining and containment structures
A tank does not fail by collapsing — it fails by leaking, and the crack that leaks usually formed before it was ever filled.
What you will understand
- Explain why crack width, not strength, governs a containment structure
- Derive the critical steel ratio, and say what happens either side of it
- Calculate the restrained early-thermal strain and the crack it causes
- Distinguish a through crack from one with a compression zone
- Say what actually reduces early thermal cracking
- Explain autogenous healing and what it cannot be relied on for
Lessons
Cracking before the tank is filled
Why the critical load case happens in the first three days, and the ratio that decides whether cracking is controlled at all.
Joints, limits, and what actually works
Why the pour sequence is a structural decision, and what a crack limit is really protecting.
Module summary
- A tank fails by leaking, long before it is near its ultimate capacity
- The governing crack usually forms in the first few days, before filling
- ρcrit = fct,eff/fyk — below it, one wide crack; above it, many fine ones
- Restrained strain ε = R α T₁ K, and R is the part a designer controls
- A through crack has no compression zone to seal it, so its limit is far tighter
- Two different concrete areas appear: half the section for minimum steel, the surface zone for crack spacing
- Fine cracks heal themselves; moving cracks do not
Teaching basis and review status
These are educational models and exercises. Check their assumptions before using the results. The full technical content and every diagram have not yet been independently verified.
Concrete and steel calculations retain their stated first-generation Eurocode teaching basis. Use the standards and amendments specified for your project; do not mix generations without an appropriate design basis.
Read the lesson register and outstanding review notesCheck your understanding
Practice
Concrete with fct,eff = 2.0 N/mm² and grade 500 steel. What is the critical steel ratio, as a percentage?
Check yourself
What happens to a fully restrained member with less than the critical steel ratio?
Practice
A wall restrained at R = 0.4 with T₁ = 40°C. Taking α = 12 × 10⁻⁶ and K = 0.65, what is the restrained strain, in microstrain?
Practice
A 250 mm wall with 40 mm cover and H12 bars. What is the surface zone hc,eff, in mm?
Check yourself
Why is a higher concrete grade unhelpful in a water-retaining structure?
Practice
A 200 mm wall retains 5 m of water. What is the hydraulic gradient?