DURABILITY Flashcards
There are three basic destructive attacks on concrete. Name these!
- Frost attack
- Corrosion on reinforcement
- Chenical attack on concrete
There is one property of concrete which is decisive for the resistance against all types of attacks on concrete. Which?
The tightness of the concrete
Name five important preconditions for a high tightness of concrete!
- A low W/C
- Aggregates with suitable grading are used
- The concrete must be sufficiently compacted
- The moist-curing must be executed carefully and for a sufficiently long period
- Cracking on the concrete is prevented
How does vct affect concrete tightness?
A low W/C makes the concrete tighter
Describe how and why all steps in the “concrete process” are important for the durability
Design stage: It’s important to choice a low W/C, a high strength class and to have a concrete cover
Manufacture: Choice of suitable grading for the aggregate
Execution stage: Careful reinforcement work, compasction and moist-curing
Describe the mechanism in connection with frost attack on concrete
It is the expansion of water at freezing
What is the spacingfactor
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Present a diagram showing the relationship between the spacing factor and the degree of damage for frost attack in pure water and frost attack with water with salt
pg.
The air-content is of very great importance for the frost-resistance of concrete. There are however some parameters that can affect the needed air-content level and thus have an impact of the frost resistance. Describe the influence on the needed air-content level for the following parameters:
- Environment- presence of salt
- Consistency
- Transport-compaction
Environment: Avoid silica-fume and flyash in the cement. Vct should be less than 0,45 if its a salty environment
Consistency: Stiff-> small pores and less air-loss during transportation and compaction-> higher frost resistance
loose-> Big pores and bigger air-loss during transportation and compaction-> lower frost resistance
Transport-compaction: It will give air-loss do not over-vibrate
How does the vct affect the frost-resistance
W/C should be kept low below 0,55
How does the curing affect the frost-resistance
Keep the concrete wet for at least 48 hours or until the concrete has obtained 55 % of its specified cub-strength. The moisture-curing is vary important for the quality of the sufrace and thus for the frost-resistance
What is carbonation of concrete
Carbonates are formed in the concrete gradually from the surface inwards, due ti the penetration of CO2 for the air. It reacts with the calciumrich compounds in the cement. The reaction causes a the PH of porewater to fall to a level that no longer protect the reinforcement
A well embedded reinforcement bar is from the beginning protected from corrosion. Why
Because of the high PH in concrete
Name two factors that strongly affect the carbonation rate
The higher W/C and the drier the concrete the higher rate
To ensure a long service life it is extremly important to have a low W/C why?
A high vct will affect the rate of carbonation and the rate of chloride-penetration
Describe the corrosion model which includes the “initation stage” and the “propagation stage”
pg 7
Two factors that strongly affect the rate of carbonation in concrete are the W/C and the moisture conctent. Describe the influence of these factors!
A high W/C gives a higher rate of carbonation. Moisture concten should be less than 75-80% for no corrosion. RH between 90-95% give high risk of corrosion.
The rate of penetration of chlorides into concrete depends mainly on one factor, which?
The permability of the concrete
Cracks extending to the reinforcement should be avioded as far as possible. In which case is this expecially important? Which type of cracks is especially harmful?
It is especially important when chloride is present. Cracks that are parallel to the reinforcement bar are harmful.
When the propagation stage has been reached, the moisture content of concrete is decisive for the corrosion process. Describe how the moisture content will affect the rate ( or risk) of corrosion.
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Which is the best way to protect concrete from reinforcement corrosion? How is it done?
- .Appropriate concrete-cover and W/C
- Never use use water containing chlorides
- Never use admixtures containing chlorides
- The concrete must be compacted carefully and throughly
- Moist-cure the concrete carefully
- The concrete must be entirely free from cracks. If cracks occur they must be sealed in durable way
Chemical attack on concrete is divided into two types. Name these
- Attasck by substances which dissolve the cement-based materials
- Attack by substances which react with different substances in the concrete and cause a volume increase
Two factors are decisive for the ability of concrete to resist aggressive substances. Name these
- the permability of concrete. This is decisive for the rate of development of damages.
- The chemical composition of the concrete. -> The composition of cement and aggregate
Which is the attack mechanism behind leaching of lime?
softwater
Name two inorganic acids that will dissolve the constitutents of the cement matrix.
hydrochloric acid (HCI) and Sulphuric (H2SO4)
How can carbonated water affect concrete?
it dissolves calcium hydroxide (CaOH)
Describe the attack mechanism in connection with sulphate attack
Sulphate ions attack lime and aluminates in the cement matrix. They cause internal cracking
How can sulphate attack on concrete pe prevented?
Use sulphate resistant cement. Tightness is also important.
Describe the attack mechanism in connection with aggregate
some type of aggregate may react with the cement matrix under unfavourable conditions to form reaction products that swell
How can aggregate reactions on concrete be avoided
Avoid reactive aggregates
Use sement with low alkalicontent
