1.2 Structure of the document
2.1.2 Moisture storage in the concrete microstructure
2.1.2.1 Mechanisms regulating moisture ingress
The pore structure of the hardened cement paste is exposed to the moisture of the sur- rounding environment. The interaction between moisture of the surrounding air and hardened cement paste is regulated by two mechanisms.
• At low relative humidities prevails the adsorption of water vapour molecules on the pore walls. The adsorption describes a process in which the accumulation of substances on the surface of a solid takes place. The most common relation used to describe adsorption was proposed by Brunauer et al. [34] as an extension of the Langmuir’s theory of monomolecular adsorption [92]. Based on multimolecu- lar adsorption, Brunauer et al. derived an expresion to calculated the volume of adsorbed gas in a porous solid known as the BET equation (see Eq. 2.1). This equation is used in DIN ISO 9277 [N12] as standard procedure for determining the specific surface area of porous and non-porous solids.
p/psat
v·(1−p/psat)= 1
vm·C+C−1
vm·C·p/psat (2.1) In Eq. 2.1 pandpsat are the partial pressure and saturation pressure of the gas,v is the volume of gas adsorbed,vm is the volume of gas adsorbed when the entire adsorbent surface is covered with a complete unimolecular layer andCis a constant dependent on the heat of adsorption and the temperature.
• At high relative humidities(p/psat>0.5)and depending on the size of the pores, the volume of gas adsorbed by the solid does not coincide with the values given by the BET equation. The actual volume is higher because of capillary condensa- tion. Capillary condensation is the process that follows the multilayer adsorption
2.1 Role of water in the concrete microstructure
after the point at which pore spaces become filled with condensed liquid from the gas. Due to surface tension of the liquid, a meniscus forms at the liquid- gas interface. Over a curved liquid-vapour interface the vapour can condense by partial pressures below the saturated vapour pressure, as denoted by the law of Kelvin-Thomson [62, 74]. This law describes the relationship of the ratio of va- pour pressure between a curved and a flat liquid surface according to the radius of curvature. Eq. 2.2 constitutes the most common mathematical description of the law of Kelvin-Thomson. This equation is based on a geometric simplification of the pore space. The pores are depicted as capillary tubes in which spherical menisci are formed [82].
p psat
=exp
− 2·σ·cosθ rc·ρw·RD·(T+273)
(2.2) In Eq. 2.2 p and psat are the partial pressure and saturation pressure of the gas, σ is the surface tension of water,θ is the contact angle,rcis the capillary radius, ρwis the density of water,RD is the gas constant for water vapour, andT is the temperature of the system.
2.1.2.2 Sorption isotherms
The amount of water stored in the concrete microstructure is characterized by the sorp- tion isotherm of the material. The sorption isotherm indicates the relationship between the relative humidity of the environment and the equilibrium moisture content of the material at a given temperature. For hardened cement paste the sorption isotherms are typically s-shaped corresponding to the type II according to the IUPAC classification of adsorption isotherms [17]. The characteristic properties of a sorption isotherm for hy- groscopic porous materials are schematically summarized in Fig. 2.5. In the top diagram of Fig. 2.5 the sorption isotherm is divided in two regions. A first region can be iden- tified in a range between dry state and approximately 95 % RH, in which the water is bounded by adsorption. In this region, also known as sorption moisture or hygroscopic region [85], the lower range up to about 15 % RH is determined by a monomolecular adsorption of water on the solid surfaces followed by a linear increase of the volume of adsorbed water through multimolecular adsorption, which ends at about 50 % RH. The following progressively increasing range from approximately 50 % to 95 % RH is attrib- uted to the capillary condensation. In the second region, for relative humidities above around 95 %, the sorption isotherm rises very steeply suggesting that in the macropore range, where no capillary condensation occurs, the pores are filled with unbounded wa- ter [82]. According to Künzel [85] in this region two subregions can be identified, a super-hygroscopic region corresponding to the filling of water up to free water satura- tion and a supersaturated region in which the filling continues until all pores are full with water reaching the maximum saturation.
There is no sharp transition between free and bound water. Usually free water is con- sidered to be the part of moisture exhibiting physical properties similar to those of pure water. On the other hand bound water denotes that part of moisture that is bonded by sorption forces in the material, whose physical properties are significantly different from those of pure water.
Monomolecular
adsorption Multimolecular adsorption
Capillary condensation
T= const.
Capillary radius [m]
3 5 10-9 2 5 10-810-7
Relative humidity, RH [%]
0 25 50 75 100
T> T0
T= T0 T< T0 Desorption
Adsorption
Moisturecontent[%]
Free water
Water bound through adsorption
Figure 2.5: Schematic representation of a sorption isotherm for hygroscopic porous materials [82].
Top: Ranges of moisture adsorption indicating the approximated pore radius according to a modell of capillary tubes [84]. Bottom: Influence of temperature and hysteresis effect between adsorption and desorption
The bottom diagram of Fig. 2.5 depicts two important characteristics of the sorption isotherm: the influence of temperature and the hysteresis effect between adsorption and desorption. These characteristics need to be considered more precisely.
• Due to the fact that adsorption is an exothermic process [33], according to the principle of Le Chatelier5 and the van’t Hoff equation [4], it can be stated that the adsorption is hindered when temperature is increased, or rather, a temperature
5 A system at equilibrium, when subject to a disturbance, responds in a way that tends to minimize the effect of the disturbance.
2.1 Role of water in the concrete microstructure
increase promotes desorption. This implies that at higher temperatures concrete reduces its capacity of storing water, the water molecules cannot be held by the internal surfaces of the concrete in the same amount when additional energy (tem- perature) is added to them. Therefore, for the same concrete the "warm" sorption isotherms lie under the "cold" ones. Few articles have been published regarding the effect of temperature on water sorption isotherms of cementitious materials.
Nevertheless, the work from Hundt and Kantelberg [77] and more recent works from Ishida et al. [78] and Poyet [121], evidenced experimentally the impact of temperature on the sorption isotherms of hardened cement paste, mortar, and con- crete. Furthermore, with the recent advent of molecular dynamic simulations, the effect of temperature on the water content within and between CSH grains can be approached numerically (see Bonnaud et al. [25]).
Another aspect that can be identified from the effect of temperature on the sorption isotherms is the fact that by constant moisture content, an increase in the tempera- ture of the concrete will lead to an increase of the relative humidity in the pore cavities of the concrete. This can be easily visualized by drawing a horizontal line to connect two sorption isotherms at different temperatures in the bottom part of Fig. 2.5. Bažant [6, 10, 11] introduced the hygrothermic coefficient of concrete K, which describes the variation of the relative humidity dhdue to a change of temperaturedT at a given water contentu. A simplified expression to calculateK for concrete was proposed by Bažant and Najjar [10] and is given in Eq. 2.3.
K= dh
dT
u=const
=0.0135·h· 1−h
1.25−h (2.3)
• The hysteresis effect in sorption isotherms between adsorption and desorption has been observed at both high and low vapour pressures. Classical experiments on this issue were conducted by Powers and Brownyard [120] and Feldman and Sereda [53] but also more recent investigations from Espinosa [49] (see also Espinosa and Franke [50]), Adolphs et al. [2] and Baroghel [16] have brought more experimental evidence on this effect. Recently, an analytical approach to explain the hysteresis effect at low humidity ranges was proposed by Bažant and Bazant in a series of two papers (see Bažant and Bazant [12] and Bazant and Bažant [18]).
Based on two mechanisms, they explain the hysteresis at low vapour pressure without assuming any pore collapse nor partial damage to the cement paste mi- crostructure. Sorption and desorption in calcium silicate hydrates have also been studied using numerical molecular dynamics simulations. Works on this subject have been recently published by Bonnaud et al. [23, 24] and Brochard et al. [27].
Based on the results from an extensive experimental program, Espinosa [49] developed a method to predict the hygroscopic water content in hardened Portland cement paste and
mortars at changing climatic conditions. For the development of the method which have been called the inkbottle pore method (IBP-method), a new model for cement paste based on the investigations of the structure of CSH carried out by Stark et al. [146, 147] and her own conducted experiments on sorption isotherms was deduced. The IBP- Method corresponds to the mathematical implementation of this cement paste model and takes into account the hysteresis effect of the sorption isotherms.