Prediction of the hydraulic diffusivity from pore size distribution of concrete Bulu Pradhan a , M. Nagesh b , B. Bhattacharjee c, * a Department of Civil Engineering, Indian Institute of Technology, Delhi, Newdelhi-110 016, India b School of Building Science and Technology, CEPT, K. L. Campus, Navarangpura, Ahmedabad-380 009, India c Department of Civil Engineering, Indian Institute of Technology, Delhi, Hauz Kauz, New Delhi-110 016, India Received 18 March 2003; accepted 20 October 2004 Abstract A model is presented in this work through which variation of hydraulic diffusivity of concrete with relative water content can be obtained from pore size distribution as an input. The specific water capacity and hydraulic conductivity of concrete are expressed in terms of pore size characteristics, considering laminar flow due to capillary suction through tortuous elliptic tubes, oriented equally in three orthogonal directions. Hydraulic diffusivity being the ratio of hydraulic conductivity and specific water capacity is thus expressed in terms of pore size characteristics. The input pore size distributions have been determined experimentally for normal strength concrete mixes through mercury intrusion porosimetry. Using the model the variation of hydraulic diffusivity with relative water content is determined for three cases viz. 1) ideal continuous wetting, 2) ideal continuous drying and 3) random access of pores by water. These results are then compared with an experimentally obtained variation. D 2005 Elsevier Ltd. All rights reserved. Keywords: Diffusivity; Pore size distribution; Porosity; Model; Hydraulic conductivity 1. Introduction Deterioration of concrete is largely mitigated by ingress of water through it. Most of the reactions which result in deterioration of concrete take place in the presence of water. Thus the ingress of water is a necessary precondition for the deterioration and the degradation of concrete structures in service. This fact has been recognized for a long time and hence permeability and absorption tests are adopted for durability performance appraisal of concrete structures [1,2]. Essentially, these tests provide only qualitative information regarding durability performance. However, for service life prediction, often it is necessary to estimate the moisture content profile within the concrete structure at different ages during its service life. Moisture profile may include both vapor phase and liquid phase. In case of rain penetration and partially submerged structures, the ingress of water in liquid phase plays a dominant role [3]. The ingress of water into concrete is governed by the theory of unsaturated flow through porous media in general and the generalized Darcy’s law for unsaturated flow in particular [4]. The material coefficient that is encountered in such modeling is the hydraulic diffusivity D(h ) and plays an important role in the context of water ingress in concrete. The ingress of water and the migration of fluid into concrete in general, take place through an interconnected pore system. Thus porosity and pore size distribution of concrete governs its permeation properties including hydraulic diffusivity [5]. In this work a model for hydraulic diffusivity is presented, in which the input data is the pore sizes and their corresponding cumulative pore volumes. The pore sizes and their corresponding cumulative pore volumes, which represent the pore size distribution, were experimen- tally determined through mercury intrusion porosimetry for 0008-8846/$ - see front matter D 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.cemconres.2004.10.043 * Corresponding author. Tel.: +91 11 2659 1193; fax: +91 11 2658 1117. E-mail addresses: bishwa@civil.iitd.ernet.in, bishwa _ b@hotmail.com (B. Bhattacharjee). Cement and Concrete Research 35 (2005) 1724 – 1733