Capillary rise is an important cause of deterioration for plaster in building. This phenomenon has been studied by the tensiometric experimental technique. Structural and hydric characteristics of plaster have been determined. Decane, a perfectly impregnate fluid, has been used to calculate the porous constant characteristic called and other structural parameters like pore radius and specific surface. Evaluation of the sensibility of plaster for water has been evaluated in a second time. The angle constant and the capillary moisture content have been determined. A comparison between results for plaster and composite plaster with fiber has been done. The addition of glass and hemp fiber does not have important effect on hydric properties (angle constant), but we have observed influence on structural charecteristics (pore radius, porosity, and degree of heterogeneity). 1. Introduction Most building materials are porous allowing damp from the ground to rise by capillary action. Brick, stone, plaster, and mortar are particularly susceptible to this form of absorption although levels of porosity vary [1]. These building materials, particularly minerals, are deficient in their crystal system that result in surface charges that attract molecules of a dipolar solvent such as water [2]. The plaster is likely to absorb moisture in significant quantity that can shorten its life but also deteriorates the durability of the buildings walls and indoor air quality. Various studies have demonstrated the influence of water (liquid and vapor phases) on the mechanical properties [3, 4] and thermal properties [5]. Washburn’s equation is a basic instrument for analysis of height penetration and mass gain of wetting liquids in time inside porous media. Experimental method based on capillary rise is also widely used for porous media characterization (i.e., pore radius and contact angle) [6]. The objective of the work is to use this experimental technique (the so-called tensiometry) to study the behavior of plaster during capillary imbibition. In the first time, structural (pore radius, porosity, and specific surface) and hydric (contact angle and moisture content) properties will be determined. Then, the influence of added glass and hemp fibers will be evaluated. 2. Theory of Capillary Imbibition The surface tension is defined to express the cohesion of a condensed material [7]. By introducing the surface tension and as forces per unit length ( ), respectively, appearing at the interface solid/vapor, solid/liquid, and liquid/vapor can be defined spread ( ) as the parameter
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