T2-1: Cover Flashcards
What is steel in concrete normally?
It is normally passive
High pH (12.15-13.5) changes normal electrochemical ‘rusting’ reaction: stable Fe3O4 layer prevents further corrosion
When can corrosion occur
When passivity is disrupted:
- ingress of chlorides
- reduced pH, reaction with CO2: carbonation
Water and oxygen are available at the steel
Describe the chemistry of RC corrosion
- CL- (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
How is RC corrosion prevented, at the design stage? (4 things)
- High cement content:
- low w/c, CRM/SCM
- protect against carbonation, lower permeability
- low w/c reduces inherent porosity of the concrete (permeability) - Minimise cracking
- Low CL- agg., cement
- Adequate cover
How is RC corrosion prevented, at the operation stage? (2 things)
- Good casting quality control
- supervise compaction
- use SCC (self-compacting concrete)
- ensure cover specs are adhered to - Manage water run-off
i.e. make good concrete and treat it well
What is Stage 1 of the design for durability in EC2?
Define exposure class and structure class:
- environmental conditions
- design life
- strength class of concrete (minimum, ‘premium’)
- geometry and quality control
What is Stage 2 of the design for durability in EC2?
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
What is Stage 3 of the design for durability in EC2?
Determine configuration of reinforcement:
- minimise crack width, for cracking control
What are the different exposure class categories?
Each class severity is associated with a minimum concrete strength class (e.g. XC3,4 = C30/37)
What is the equation for nominal cover?
What is the allowance for cover deviation normally?
10mm
This may be reduced for very accurate precasting with verifiable quality control
What is minimum cover a function of?
- Strength class
- Exposure class
Also, must add on 5/10mm for pre-stressing tendons
For this bridge design in Minnesota, what exposure class would you pick?
For this bridge design in Minnesota, what structural class would you pick?
S5 (4+2-1)
- Default = 4
- 100 years life: +2
- Strength above threhold for XD3: -1
What are special cases for determining concrete cover?
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
What does concrete durability design (EN206) ensure? (3 things)
- Specification to ensure concrete durability (as opposed to steel durability)
- Minimum cement content
- good compaction
- chemically ‘buffer’ carbonation - Maximum w/c ratio and/or minimum strength class
- limits permeability
- ensures general quality
What are three other/special requirements for cover design?
What things interacted to create punching shear failure in this case?
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
What is the easiest measurement of porosity?
Strength
