Keithj
Member
- Messages
- 817
- Location
- Witnesham, Suffolk
I was surprised to be told on another thread that I'd got it wrong, and that concrete is "permeable". I'd spent an awful lot of money having this house repaired as a result of copious amounts of concrete being applied to cover some areas of rot - and the resulting almost complete rotting away of the sole plates and some other timbers. At that time, the surveyor, the conservation officer, and the technical expert who supervised the repairs all explained that concrete is "impermeable". I accepted their advice. Three years later, the repairs have reduced the resistive damp readings on the interior timbers from "very high" (8 - 10 on a scale from 0 to 10) to "very low" (0 or 1).
As part of my objective of continuous education, I therefore went off to learn more about concrete permeability. I found scientific research which showed that concrete IS permeable, and measured it. There is a formula that defines the permeability: Dq/dt = K (Δ H A)/(Lν)
To go with this, there was a table of tests on a common mix of concrete. The table also showed the range as the concrete sets:
Age - Permeability (cm/s 10^-11 )
Fresh 20,000,000
5 days - 4,000
6 days - 1,000
8 days - 400
13 days - 50
24 days - 10
Ultimate - 6
The article explained that different mixes behave differently, but all are in the scale around ten to the minus 11 centimetres per second.
A different research project suggested that steel reinforced concrete is more permeable than non-reinforced.
The ultimate value for concrete above is less permeable than some familiar stone materials:
marble 2.39 x 10^-10
granite 5.35 x 10^-9
sandstone 1.23 x 10^-8
The researchers didn't give a value for "lime render", but since it is nearest in structure to sandstone in their list, that will probably serve as a broad indication. The difference in permeability between concrete and lime is thus, very approximately, (1.23 x 10^-8)/(6 x 10^-11) or a factor of about 5,000. That clarifies it for me. I'll stick with 5,000 unless I find some research on lime compared with concrete.
It also explains other aspects of a concrete floor.
I'll go and lie down now.
As part of my objective of continuous education, I therefore went off to learn more about concrete permeability. I found scientific research which showed that concrete IS permeable, and measured it. There is a formula that defines the permeability: Dq/dt = K (Δ H A)/(Lν)
To go with this, there was a table of tests on a common mix of concrete. The table also showed the range as the concrete sets:
Age - Permeability (cm/s 10^-11 )
Fresh 20,000,000
5 days - 4,000
6 days - 1,000
8 days - 400
13 days - 50
24 days - 10
Ultimate - 6
The article explained that different mixes behave differently, but all are in the scale around ten to the minus 11 centimetres per second.
A different research project suggested that steel reinforced concrete is more permeable than non-reinforced.
The ultimate value for concrete above is less permeable than some familiar stone materials:
marble 2.39 x 10^-10
granite 5.35 x 10^-9
sandstone 1.23 x 10^-8
The researchers didn't give a value for "lime render", but since it is nearest in structure to sandstone in their list, that will probably serve as a broad indication. The difference in permeability between concrete and lime is thus, very approximately, (1.23 x 10^-8)/(6 x 10^-11) or a factor of about 5,000. That clarifies it for me. I'll stick with 5,000 unless I find some research on lime compared with concrete.
It also explains other aspects of a concrete floor.
I'll go and lie down now.
