Concrete durability

Question 1

Reinforced concrete structures have a limited life time. describe the stages of degradation.

Answer 1

• Stage 1: Macroscopic cracking (5-10 yrs)
• Stage 2: Visual rust strain (10-15 yrs)
• Stage 3: Surface delamination & spalling (15-20 yrs)
• Stage 4: Severe spalling and steel exposed (15-25 yrs)

Corrosion of reinforcement steel must be avoid, since it can lead to the collapse of the structure

Question 2

Describe Carbonation of concrete and how to calculate the rate of carbonation

Answer 2

• Carbonation: Concrete is a porous material, and the CO2 in the air may therefore penetrate via the pores to the interior of the concrete. There, a chemical reaction will take place with the calcium hydroxide.
• Rate of carbonation: Increase of carbonation depth with time

Question 3

Describe the Corrosion of steel in concrete (simplified model)

Answer 3

Due to carbonation of the concrete or by the action of chloride ions, the passive film may be destroyed locally or over greater surface areas. A third mechanism is a reduction of alkalinity due to the leaching out of alkalis by streaming water.

Question 4

Describe the Penetration of chlorides into concrete

Answer 4

• Besides CO2, chloride ions (originating from sea water or de-icing salt) may penetrate through the pores to the interior of the concrete.
• Chloride intrusion is due to either diffusion taking place in totally or partially water filled pores or capillary suction of chloride-containing water.

Question 5

describe the Influence of cracks

Answer 5

• In the region of cracks, carbonation and chlorides tend to penetrate faster towards the reinforcement than in uncracked concrete.
• In the case of normal crack widths at the concrete surface of up to 0.4 mm, self healing as a result of calcium, dirt and rust deposits within the cracks can frequently be observed. In this case, any on-going corrosion at the reinforcement is likely to come to a halt.
• The thickness of the concrete cover is of major importance with regard to the influence of cracks. The crack widths (if they are less than 0.4 mm) are less important

Question 6

Influencing parameters of the corrosion of reinforcement.

Answer 6

All the processes influencing corrosion of reinforcement are more or less controlled by transport processes:

a) carbonation: diffusion of CO2 in air-filled pores
b) penetration of chlorides: diffusion of chlorides in water-filled pores and capillary suction of chloride-containing water into air-filled pores
c) corrosion of reinforcement: diffusion of oxygen in air-filled pores