DURABILITY

Question 1

There are three basic destructive attacks on concrete. Name these!

Answer 1

• Frost attack
• Corrosion on reinforcement
• Chenical attack on concrete

Question 2

There is one property of concrete which is decisive for the resistance against all types of attacks on concrete. Which?

Answer 2

The tightness of the concrete

Question 3

Name five important preconditions for a high tightness of concrete!

Answer 3

• 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

Question 4

How does vct affect concrete tightness?

Answer 4

A low W/C makes the concrete tighter

Question 5

Describe how and why all steps in the “concrete process” are important for the durability

Answer 5

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

Question 6

Describe the mechanism in connection with frost attack on concrete

Answer 6

It is the expansion of water at freezing

Question 7

What is the spacingfactor

Answer 7

.

Question 8

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

Answer 8

pg.

Question 9

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

Answer 9

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

Question 10

How does the vct affect the frost-resistance

Answer 10

W/C should be kept low below 0,55

Question 11

How does the curing affect the frost-resistance

Answer 11

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

Question 12

What is carbonation of concrete

Answer 12

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

Question 13

A well embedded reinforcement bar is from the beginning protected from corrosion. Why

Answer 13

Because of the high PH in concrete

Question 14

Name two factors that strongly affect the carbonation rate

Answer 14

The higher W/C and the drier the concrete the higher rate

Question 15

To ensure a long service life it is extremly important to have a low W/C why?

Answer 15

A high vct will affect the rate of carbonation and the rate of chloride-penetration

Question 16

Describe the corrosion model which includes the “initation stage” and the “propagation stage”

Answer 16

pg 7

Question 17

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!

Answer 17

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.

Question 18

The rate of penetration of chlorides into concrete depends mainly on one factor, which?

Answer 18

The permability of the concrete

Question 19

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?

Answer 19

It is especially important when chloride is present. Cracks that are parallel to the reinforcement bar are harmful.

Question 20

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.

Answer 20

.

Question 21

Which is the best way to protect concrete from reinforcement corrosion? How is it done?

Answer 21

• .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

Question 22

Chemical attack on concrete is divided into two types. Name these

Answer 22

1. Attasck by substances which dissolve the cement-based materials
2. Attack by substances which react with different substances in the concrete and cause a volume increase

Question 23

Two factors are decisive for the ability of concrete to resist aggressive substances. Name these

Answer 23

1. the permability of concrete. This is decisive for the rate of development of damages.
2. The chemical composition of the concrete. -> The composition of cement and aggregate

Question 24

Which is the attack mechanism behind leaching of lime?

Answer 24

softwater

Question 25

Name two inorganic acids that will dissolve the constitutents of the cement matrix.

Answer 25

hydrochloric acid (HCI) and Sulphuric (H2SO4)

Question 26

How can carbonated water affect concrete?

Answer 26

it dissolves calcium hydroxide (CaOH)

Question 27

Describe the attack mechanism in connection with sulphate attack

Answer 27

Sulphate ions attack lime and aluminates in the cement matrix. They cause internal cracking

Question 28

How can sulphate attack on concrete pe prevented?

Answer 28

Use sulphate resistant cement. Tightness is also important.

Question 29

Describe the attack mechanism in connection with aggregate

Answer 29

some type of aggregate may react with the cement matrix under unfavourable conditions to form reaction products that swell

Question 30

How can aggregate reactions on concrete be avoided

Answer 30

Avoid reactive aggregates

Use sement with low alkalicontent