Proceedings of the International rilem conference Materials, Systems and Structures in Civil Engineering 2016

393

International RILEM Conference on Materials, Systems and Structures in Civil Engineering 

Conference segment on Service Life of Cement-Based Materials and Structures

22-24 August 2016, Technical University of Denmark, Lyngby, Denmark 

 

except for G4, the sample size and shape effects on the initial mass loss can be accurately 

taken into consideration by multiplying the mass loss by the corresponding drying radius. 

However, this is only true if similar concrete and surface conditions are applied to all 

geometries. 

 

4.2. Stage 2: main drying initiation 

The second stage of drying consists in a linear increase of the mass loss as a function of the 

square root of time. The slope of this increase is related both to the concrete properties 

(porosity, permeability, diffusivity, …) and to the humidity gradient between the concrete and 

the ambient air. As for the first stage, the effect of the sample size and shape can be accurately 

taken into account by the drying radius. A higher drying radius induces a lower slope. By 

multiplying the drying radius by the slope of the mass loss curve during this second stage, all 

geometries present the same behaviour, as shown in Figure 4c and 4d. The end of this stage is 

not clearly defined, and probably occurs at a point where the effective permeability is 

significantly decreased in comparison with its initial value. This occurs when the overall 

humidity has significantly decreased in the whole sample, which could occur close to a 

specific mass loss. Further results should provide additional answers regarding this 

observation.  

 

4.3. Stage 3: advanced drying and permeability decrease 

As stated previously, the third stage consists in a significant decrease of the mass loss, 

resulting in a progressive decrease of the slope of the mass loss versus square root of time 

curve. At a given time, this phenomenon is more clearly observed in Figure 6a for geometries 

with lower drying radius. Indeed, G4 and G3 have already lost a significant amount of water, 

in contrast with G1 or G2.  

 

Figure 6: Whole mass loss curves (dots = experiments, dashed lines = simulations) 

 

As for the first and second stage, the simulation captures this behaviour. However, as the 

mass loss increases, an overestimation of the simulation can be observed, especially when it 

overpasses 1%. This is due to the parameters expressed in equations 4, 5 and 8. These 

equations relate the relationship between saturation degree, relative humidity, and effective 

permeability and diffusivity. Indeed, all parameters from these equations were extracted from 

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394

International RILEM Conference on Materials, Systems and Structures in Civil Engineering 

Conference segment on Service Life of Cement-Based Materials and Structures

22-24 August 2016, Technical University of Denmark, Lyngby, Denmark 

 

another study [1]. Therefore, even if the same concrete was used, these equations should be 

adapted to this study. In particular, the decrease of effective permeability when decreasing the 

saturation degree should be more important. This point will be discussed further when longer 

term experiments have been performed on all sample sizes and shapes. 

 

 

5.  Conclusion and perspectives 

 

Mass loss experiments on various specimen size and shape are performed. The model 

developed and used in [1] is improved by taking into account the boundary layer of air at the 

sample-air interface during drying. The curves of mass loss as a function the square root of 

time can be divided in three stages identified in this study: a non-linear acceleration initial 

stage, a linear drying initiation, and an advanced drying consisting in a decreased slope of the 

curve. These three stages can be accurately reproduced by the model, independently on the 

sample size and shape. 

Further works are required in order to confirm the relevance of this model for longer term 

experiments, and for identifying mechanisms at the origin of the switch between the second 

stage and the thirst stage. Additional results regarding the relative humidity gradient inside the 

specimens will allow a mode complete description of these phenomena, as well as an 

improved strategy for identifying the parameters of the model. Finally, this validity of this 

model should be challenged with drying/wetting cycles experiments. 

 

 

References 

[1] Hilaire A., Etude des déformations différées des bétons en compression et en traction, du 

jeune au long terme : application aux enceintes de confinement, PhD Thesis, ENS Cachan, 

2014 

[2] Huang, Q.; Jiang, Z.; Gu, X.; Zhang, W. & Guo, B., Numerical simulation of moisture 

transport in concrete based on a pore size distribution model, Cem Conc Res , 2015, 67, 31-43 

[3] Briffaut, M., Etude de la fissuration au jeune âge des structures massives en béton : 

influence de la vitesse de refroidissement, des reprises de bétonnage et des armatures, PhD 

Thesis, ENS Cachan, 2010 

[4] Mualem, Y. (1976). A new model for predicting the hydraulic conductivity of unsaturated 

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[5] van Genuchten, M. A Closed-form Equation for Predicting the Hydraulic Conductivity of 

Unsaturated Soils. Soil Science Society of America Journal, 44(5):892–898, 1980 

[6] Millington, R., Quirk, J. (1961). Permeability of porous solids. Transactions of the 

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[7] Oxfall, M.; Johansson, P. & Hassanzadeh, M. Long-term hygrothermal performance of 

nuclear reactor concrete containments – Laboratory evaluations of measurement setup, in situ 

sampling, and moisture flux calculations, Cem and Conc Composites , 2016, 65, 128 – 138 

[8] Zhang, J.; Wang, J. & Han, Y. Simulation of moisture field of concrete with pre-soaked 

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, F. & Balabani , G. Modeling influence of hysteretic moisture 

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