T2-1: Cover

Question 1

What is steel in concrete normally?

Answer 1

It is normally passive

High pH (12.15-13.5) changes normal electrochemical ‘rusting’ reaction: stable Fe3O4 layer prevents further corrosion

Question 2

When can corrosion occur

Answer 2

When passivity is disrupted:
- ingress of chlorides
- reduced pH, reaction with CO2: carbonation

Water and oxygen are available at the steel

Question 3

Describe the chemistry of RC corrosion (x2)

What are corrosion products, and what does it cause?

Answer 3

• CI- (de-icing, marine agg.) or carbonated layer reaches steel
• The oxide layer is disrupted and corrosion cell is initiated if water & oxygen are available
• The corrosion products are porous and expansive: spalling

Question 4

How is RC corrosion prevented, at the design stage? (4 things)

Answer 4

1. High cement content:
   - low w/c, CRM/SCM
   - protect against carbonation, lower permeability
   - low w/c reduces inherent porosity of the concrete (permeability)
2. Minimise cracking
3. Low CI- agg., cement
4. Adequate cover

Question 5

How is RC corrosion prevented, at the operation stage? (2 things)

Answer 5

1. Good casting quality control
   - supervise compaction
   - use SCC (self-compacting concrete)
   - ensure cover specs are adhered to
2. Manage water run-off

i.e. make good concrete and treat it well

Question 6

What is Stage 1 of the design for durability in EC2?

Answer 6

Define exposure class and structure class:
- environmental conditions
- design life
- strength class of concrete (minimum, ‘premium’)
- geometry and quality control

Question 7

What is Stage 2 of the design for durability in EC2?

Answer 7

Determine minimum and nominal cover as a function of exposure/structure class:
- re-bar or prestress
- allow for deviations
- concrete quality (ie. strength)
- EN206 for specialised concrete specs if freeze/thaw chemical attack expected

Question 8

What is Stage 3 of the design for durability in EC2?

What is minimised for cracking control?

Answer 8

Determine configuration of reinforcement:
- minimise crack width, for cracking control

Question 9

What are the different exposure class categories?

Answer 9

Each class severity is associated with a minimum concrete strength class (e.g. XC3,4 = C30/37)

Question 10

What is the equation for nominal cover?

Answer 10

Question 11

What is the allowance for cover deviation normally?

Answer 11

10mm

This may be reduced for very accurate precasting with verifiable quality control
.

Question 12

What is minimum cover a function of?

What changes for pre-stressing tendons?

Answer 12

1. Strength class
2. Exposure class

Also, must add on 5/10mm for pre-stressing tendons

Question 13

For this bridge design in Minnesota, what exposure class would you pick?

Answer 13

Question 14

For this bridge design in Minnesota, what structural class would you pick?

Answer 14

S5 (4+2-1)

• Default = 4
• 100 years life: +2
• Strength above threhold for XD3: -1

Question 15

What are special cases for determining concrete cover?

Answer 15

There are cover modifications/limits for casting onto uneven surfaces:
- Exposed aggregate concrete surfaces: +5mm
- Prepared ground (blinding): min 40mm
- Cast directly against soil (piles etc): min 65mm in UK

Question 16

What does concrete durability design (EN206) ensure? (2 things)

Answer 16

1. Minimum cement content
   - good compaction
   - chemically ‘buffer’ carbonation
2. Maximum w/c ratio and/or minimum strength class
   - limits permeability
   - ensures general quality

Question 17

What are three other/special requirements for cover design?

Answer 17

Question 18

What things interacted to create punching shear failure in this case?

Answer 18

Poor 1960s quality concrete, which was:
- very porous
- highly variable in quality
- wasn’t compacting properly
- variable cover of steel

• creep effect flattened ‘dome’ shape that was there to ensure water run-off; this led to lots of water presence

Question 19

What is the easiest measurement of porosity?

Answer 19

Strength