162 Damage to concrete structures
mechanisms. It is not fully clear whether the Thiobacillus bacteria migrate
into the concrete, producing sulfuric acid much closer to the damage front,
or whether the produced sulfuric acid itself is moving inward (O’Connell
et al. 2010). Based on a comparison with pure sulfuric acid attack where
the penetration of sulfuric acid is somewhat obstructed due to the damage
process, an active contribution by the Thiobacillus bacteria seems plausible in explaining the much faster damage process in the case of biogenic
sulfuric acid attack.
It is clear that the performance of concrete with regard to biogenic sulfuric acid attack cannot be fully judged by purely chemical tests alone
because the microbial effects are not considered in this way. Tests in a real
biological environment will enable more reliable conclusions concerning
the concrete performance in sewer pipes (Monteny et al. 2000).
According to the review performed by O’Connell et al. (2010), there
seems no agreement on the parameters of significance governing the
resistance of concrete to biogenic sulfuric acid attack. A dense concrete
is typically considered to perform better because of reduced penetration
properties, although a refined pore structure could also increase capillary
suction. Debate also exists concerning cement and aggregate types and contradictory results can be found in literature. Although limestone aggregates
can be attacked by the acid, it seems that due to their buffer capacity they
can delay the concrete degradation much better than river gravel.
According to Leemann et al. (2010), who studied concrete degradation
due to the effect of bacteria oxidizing ammonium from nitrate, it is also
important to notice that calcite precipitation close to the surface, leading
to the formation of some dense layer, plays an important role in addition
to the dissolution of hydrates. They experimentally found that concrete
deterioration in the case of biogenic acid attack by the nitrifying biofilm is
correlated with the CaO content of the cement. In the case of a higher CaO
content, more calcite is formed offering a better protection against acid
attack. It is not clear to what extent this conclusion also holds for biogenic
sulfuric acid attack.
5.4 reInforceMent corrosIon
5.4.1 general
Corrosion of reinforcing steel probably is the most widely spread damage
mechanism in concrete structures. Typical features of corroding concrete
structures are the occurrence of cracks along the reinforcing steel and spalling of the concrete cover, which leave the corroding reinforcing steel visible.
Brown colouring due to outflowing corrosion products can often be noticed
as well.
mechanisms. It is not fully clear whether the Thiobacillus bacteria migrate
into the concrete, producing sulfuric acid much closer to the damage front,
or whether the produced sulfuric acid itself is moving inward (O’Connell
et al. 2010). Based on a comparison with pure sulfuric acid attack where
the penetration of sulfuric acid is somewhat obstructed due to the damage
process, an active contribution by the Thiobacillus bacteria seems plausible in explaining the much faster damage process in the case of biogenic
sulfuric acid attack.
It is clear that the performance of concrete with regard to biogenic sulfuric acid attack cannot be fully judged by purely chemical tests alone
because the microbial effects are not considered in this way. Tests in a real
biological environment will enable more reliable conclusions concerning
the concrete performance in sewer pipes (Monteny et al. 2000).
According to the review performed by O’Connell et al. (2010), there
seems no agreement on the parameters of significance governing the
resistance of concrete to biogenic sulfuric acid attack. A dense concrete
is typically considered to perform better because of reduced penetration
properties, although a refined pore structure could also increase capillary
suction. Debate also exists concerning cement and aggregate types and contradictory results can be found in literature. Although limestone aggregates
can be attacked by the acid, it seems that due to their buffer capacity they
can delay the concrete degradation much better than river gravel.
According to Leemann et al. (2010), who studied concrete degradation
due to the effect of bacteria oxidizing ammonium from nitrate, it is also
important to notice that calcite precipitation close to the surface, leading
to the formation of some dense layer, plays an important role in addition
to the dissolution of hydrates. They experimentally found that concrete
deterioration in the case of biogenic acid attack by the nitrifying biofilm is
correlated with the CaO content of the cement. In the case of a higher CaO
content, more calcite is formed offering a better protection against acid
attack. It is not clear to what extent this conclusion also holds for biogenic
sulfuric acid attack.
5.4 reInforceMent corrosIon
5.4.1 general
Corrosion of reinforcing steel probably is the most widely spread damage
mechanism in concrete structures. Typical features of corroding concrete
structures are the occurrence of cracks along the reinforcing steel and spalling of the concrete cover, which leave the corroding reinforcing steel visible.
Brown colouring due to outflowing corrosion products can often be noticed
as well.