Constitutive laws addressing concrete durability and phenomena such as shrinkage, creep, AGR and so on very often consider water influence through the introduction of a parameter related with water content (quantity of water embedded within the concrete porous network) or with relative humidity RH). Both can be linked through a sorption or capillary curve, which is rather long to provide, because of long drying time for concrete. On site, direct measurement of these kinds of value are not easy to perform. Some people use TDR probes, others will use "pressure pulse decay" techniques, embedded RH probes or other systems.
I have an open question: from a theoretical point of view, what is the more relevant parameter between water content (related to the total amount of water) and RH (can be linked with capillary pressures) to simulate the role of water in concrete ageing phenomena? My opinion is that it is not obvious to define a internal RH in very small cement paste pores, I would prefer to handle with water content, but it can be discussed. Thanks for your opinion.
I would say the more relevant parameter is the water content (volumetric ratio of water in pore) since this value has direct impact on the diffusivity and permeability of cement-based porous material. However, RH is easy to obtain as environmental factor and it can be used as one indicator of pore saturation for concretes. The relation between RH ans pore saturation is from the water vapor sorption isotherm curves (WVSI) of the material. From this curve one can read the pore saturation corresponding to the environmental RH.
Here comes also the geometrical problem. From practical point of view, if the concrete is very permeable (low grade in strength), the pore saturation can be assumed to be in eqilibrium with the external RH. Thus using RH to characterize the water saturation condition (through WVSI) is equivalent to using pore saturation. If the concrete is very impermeable or the concrete element in question is rather thick, one can not simply use the environmental RH to stand for the pore saturation, even through WVSI, since water content gradient exists in the concrete.
Thank you Kefei. That is nice to hear from you (I was involved in the joined R&D program between EDF and Navier Institute a long time ago...)
For shrinkage (and probably also creep), RH is the most relevant quantity. Drying (or autogenous) shrinkage is directly proportional to the capillary pressure that can be calculated from the RH, while the degree of saturation S changes comparatively little in the interval of RH interesting for drying.
Consider also that calculating S from RH might not be obvious because of hysteresis in the sorption isotherms. You would need to know whether you are on the adsorption or desorption curve or a scanning curve, for which you need to know the whole history of the concrete and it is also influenced by the temperature.
The defintion of internal RH, even in small pores, is not ambiguous. Consider that the cement paste will be locally in RH equilibrium in a given moment of time and the sensor will come in equilibrium with the cement paste. You are not probing the RH within a small pore, there is no ambiguity.
On the other hand, RH measurements are difficult to perform, especially at high RH and for large temperature changes (risk of condensation on the sensor). It is also difficult to obtain embedded sensors that are both accurate and stable for long term measurements, the sensors we are using in the lab need frequent calibration.
Thank you for your contribution, Pietro. I will think about your argument related to equilibrium..
I want to know more about effects of temperature and humidity on strength of concrete (Especially high performance concrete.). Where can I get this information?
Hello Jyoti. Your question is interesting but addresses a large scope of current R&D on HPC. I am not an expert in HPC, but you will find some open resources on the web related to temperature, such as https://www4.uwm.edu/cbu/Papers/2002%20CBU%20Reports/CBU%202002-07.pdf or http://www.sciencedirect.com/science/article/pii/S0379711205001402. There are a lot of references that can provide relevant information. The role of moisture is often address for fire concerns too.
Good luck in your research work!
You need to consider BOTH! For a number of processes involved in concrete aging the state of water is decisive. Then you need to consider RH! For other processes the water content, or the degree of saturation, may be the relevant representation of water effects.
You can of course use the sorption isotherm to translate between the two. That is not always easy. Hysterisis effects are already mentioned. The sorption isotherm is also temperature dependent and the alkalis make the relationship somewhat complicated. We know a lot about all this, however!
Yes, it is not easy to measure RH but it is fully possible to do it with a very low uncertainty. We have some systematic errors when measuring on low-w/c-concretes but a total uncertainty of a few % RH is fully possible. Compare that to measuring moisture content in a heterogenous material!
RILEM TC-248 MMB is writing a state-of-the-art-report on moisture measurements. Follow our work!
Thank you Lars-Olof. I will follow carefully the progress of your work with RILEM TC. Is there any public paper available so far, just to look at?
Best regards
The measure (RH of Degree of Saturation) to be considered for a particular case would depend on the nature of exposure being considered. For instance, for concrete exposed to rain, the choice of degree of saturation becomes obvious, if we are trying to study the concomitant moisture ingress. Even during the subsequent drying phase under the influence of ambient temperature and humidity conditions, concrete would retain a substantial amount of pore water.
On the other hand, if our intention is to study the effect of temperature and humidity cycles on the state of moisture distribution in concrete, a more appropriate choice would be to model the pore humidity which can then be related to moisture content through sorption/desorpion characteristics of the material.
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