Brohcure_Concretal_01_2016_engl

carbonation and concrete quality

15

12

C 12/15

C 20/25

9

C-max

C 30/37

6

3

C-max

C 35/45

0 0 5 10 15 20 25 carbonation time in years

ases strength but reduces the pH value to below 9.5. This is of no significance in high-quality concretes with correctly positioned reinforcement as the depth of penetration is only slight and the rein- forcement layer is not reached. It has been scientifically demonstrated that, in good concrete, the carbonation pro- cess comes to a complete standstill after a certain amount of time (Cmax). In the case of poor quality concretes and inadequate depth of cover, carbonati- on may reach the reinforcement. Once the pH value drops below 9.5, the passivation present on the surface of the steel is eliminated. If sufficient water is present, rusting may occur on the steel. Since rust has a larger volume than steel, the overlying layer of concrete suffers spalling. Steel corrosion in concrete is an electrolytic process which only takes place if sufficient water is present in liquid form. Rust damage does not occur on concrete surfaces in dry areas, as the water necessary for electrolysis is not present.

Decay Potential causes of concrete decay may be introduced during manufacture. For example, incorrect water/cement ratios, poor compaction, leaky shuttering and insufficient curing cause increased concrete porosity and thus greater permeability to water, oxygen and carbon dioxide. In addition, the steel reinforcement is often not carefully set in place. Absent spacers, reinforcing baskets displaced in the shuttering, warped mats etc. result in inadequate depth of cover and thus rust damage.

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