Civil Engineering Reference
In-Depth Information
Figure 5-34
Water
solubility of carbon
dioxide [117].
An increased pH value due to mixing with water, which has been in contact with alkaline
construction materials, also leads to lime precipitation. Possible sources of hydroxide ions
in water are concrete in the form of shotcrete or in-situ concrete and accelerators with
alkaline contents.
The calcium hydrogen carbonate reacts with the calcium hydroxide to form calcium car-
bonate and water.
Ca(HCO
3
)
2
+ Ca(OH)
2
→ 2CaCO
3
+ 2H
2
O
Accelerator admixtures containing alkali are normally based on easily soluble potassium
and sodium aluminates. Sodium hydroxide, for example, reacts with calcium hydrogen
carbonate to form calcium carbonate, sodium hydrogen carbonate and water.
Ca(HCO
3
)
2
+ Na(OH) → CaCO
3
+ NaHCO
3
+ H
2
O
Not one but many of these processes take place in a typical drainage system. In different
locations in the drain run, the processes can combine and result in heavier precipitation
and finally to sintering. The form of the sinter can range from milky (easily transported by
the water flow) through sugary (with a fragile structure) to the most commonly encoun-
tered hard sintering.
5.4.2 Design of tunnel drainage for low sintering
Tests in the design phase.
In order that the risk to a tunnel drainage system can be esti-
mated in advance, it is necessary to make use of geological and hydrogeological investiga-
tions to estimate and interpret sintering problems.
According to DIN 4020 [54a], the structure and properties of soil and rock in the ground
or in quarries for construction materials and also the ground water conditions should be
adequately understood. Samples of soil and rock are evaluated as spot checks and permit
probable conclusions about the areas between them..
In order to be able to estimate the danger of sintering, the geological investigation should
include the approximate determination of the lime content with dilute hydrochloric acid
according to DIN 4022 [55].
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