Abstract
The Lucas-Washburn equation is still being applied by a significant number of researchers for the modelling of water absorption in cementitious materials. A modern approach considers the extended Darcy’s law leading to the Richards equation instead. Three main assumptions are implied by the application of the Lucas-Washburn equation: the flow occurs in one direction only, the material is separated into one fully wet and one fully dry region, and pores are modelled as an assembly of parallel tubes of a particular radius. Its application to analyse experimental results allows defining these three assumptions as mere simplifications. Therefore, all the parameters comprised in the Lucas-Washburn model are apparent. Consequently, a very limited description of the transport properties of the material can be achieved. For many engineering purposes this would not be an issue, but for an intrinsic description of the material a more realistic model is required. This paper discusses the limitations of the Lucas-Washburn equation, and the advantages of the Richards equation regarding the modelling of water absorption in cementitious materials. The comparative analysis reveals the versatility of the Richards equation, with an approach that considers the material as a continuum and describes it through measurable parameters.
Highlights
Transport properties of concrete are defined by its pore size distribution and connectivity
The sorptivity is a qualitative descriptor of transport properties of concrete, with a low rate of water absorption indicating satisfactory properties to ensure an acceptable lifespan of a concrete structure
The variation of the hydraulic diffusivity can be obtained independently. This is the approach followed in [9], where an improved model based on RE is proposed for describing water absorption in cementitious materials, with water uptake increasing with t0.25
Summary
Transport properties of concrete are defined by its pore size distribution and connectivity. The WSC of concrete can be applied as an index for a performancebased approach for durability of reinforced concrete structures It is still very limited as a performance-based design tool for durability as it is unable to provide reliable estimations of values for other transport properties of concrete. The WSC is generally computed as the slope of the fitting line to the amount of water uptake per unit area as a function of the square root of the time along which concrete was in contact with water. This approach is applied to analyse capillary absorption in most of porous materials and in cementitious materials. The RE is based on a balance of mass in the porous material
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