Influences of water by cement ratio on mechanical properties of mortars submitted to drying Ismail Yurtdas a , He Peng b , Nicolas Burlion b, ⁎ , Frédéric Skoczylas c a Laboratory of Mechanics, Materials and Structures, Université de Reims Champagne Ardenne, Rue des crayères, 51687 Reims Cedex b Laboratory of Mechanics of Lille, UMR CNRS 8107, Polytech'Lille — USTL, Cité Scientifique, 59650 Villeneuve d'Ascq, France c Laboratory of Mechanics of Lille, UMR CNRS 8107, Ecole Centrale de Lille, Cité Scientifique, 59650 Villeneuve d'Ascq, France Received 9 December 2005; accepted 16 December 2005 Abstract Concrete materials are submitted to drying when the relative humidity of their surrounding is decreasing. The main purpose of this study is to highlight the variation of multiaxial mechanical behaviour of mortars which depends on desiccation level and cement paste properties (quality). The behaviour under discussion includes uniaxial and triaxial strengths, elastic properties and volumetric strains due to hydrostatic loading. Multiaxial experiments, carried out on two mortars for which the only difference was the water by cement ratio (w / c = 0.5 and 0.8), show a competitive effect between the increase in material rigidity due to capillary suction and saturation gradients, and the microcracking which comes from material heterogeneity and differential shrinkages of the sample. This effect mainly depends on cement paste properties and its porosity; therefore the capillary suction effect is preponderant for a high paste quality (i.e. lower porosity) while a low paste quality would be more sensitive to microcracking. © 2006 Elsevier Ltd. All rights reserved. Keywords: Drying; Microcracking; Compressive strength; Mechanical properties; Mortars 1. Introduction The mechanical behaviour of concrete will vary as drying induces local and structural modifications. The drying process brings about desiccation shrinkage whose origins are: an increase in capillary suction [1,2]; variations in disjoining pressure [3,4]; and in surface energy [4]. These effects occur at a microscopic scale. As regards the whole structure, saturation gradients [1–8] are created due to non uniform drying. The latter derives mainly from the low permeability of concrete, and the structure geometry. On the other hand, the kinetics of drying are faster near the surface than deeper in material. This leads to non homogeneous strains that induce tensile stresses and microcracking [1–5]. Microcracking may also occur close to the interface between cement paste and aggregates which is mainly due to their differential stiffness [5,9,10]. This effect, which is weak and relatively superficial for small aggregates, leads to an important diffuse microcracking for aggregates size greater than 6 mm [10]. As a result, dried concrete can be considered as damaged prior to any mechanical loading. This damage will change the elastic properties and strength limits of the material. To avoid such a phenomenon, specific laboratory conditions must be followed: either desaturation carried out with very small increments of relative humidity or the use of samples of very low thickness [4,11,12]. As drying induced microcracking weakens the material and increases its permeability [13], it will also play an important role in the durability of concrete structures. Uniaxial compressive tests have been used to evaluate the variations due to drying [6–8,14–23], but few results obtained with conventional triaxial tests are reported on the role played by interstitial water [24]. A study of these data shows important scattering and uncertainty as regards the drying effects on strength and elastic modulus variations. Results on strength variations, which are found in the literature, are ambiguous. It is often observed that uniaxial Cement and Concrete Research 36 (2006) 1286 – 1293 ⁎ Corresponding author. E-mail address: Nicolas.Burlion@polytech-lille.fr (N. Burlion). 0008-8846/$ - see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.cemconres.2005.12.015