Defect dynamics of cement mortar under repeated loading, studied by electrical resistivity measurement Jingyao Cao, D.D.L. Chung * Composite Materials Research Laboratory, University at Buffalo, Department of Mechanical and Aerospace Engineering, The State University of New York, Buffalo, NY 14260-4400, USA Received 20 September 2000; accepted 17 September 2001 Abstract Defect dynamics, as studied by electrical resistivity measurement during repeated compression of cement mortar in the elastic regime, are characterized by defect generation, defect healing and defect aggravation. Defect generation dominates in the first compressive loading and in tensile loading in any loading cycle. Defect healing dominates in all subsequent compressive loading cycles and in tensile unloading in any cycle. Defect aggravation dominates during compressive unloading in any cycle and occurs during tensile loading in any cycle. Both the interface between sand and cement and that between silica fume and cement contribute to the defect dynamics, particularly the defect healing. The defect dynamics give similar effects to the longitudinal and transverse resistivities. Upon uniaxial compression, the resistivity decreases (except for the first cycle); upon uniaxial tension, the resistivity increases. D 2002 Elsevier Science Ltd. All rights reserved. Keywords: Microcracking; Microstructure; Electrical Properties; Strain effect; Mortar 1. Introduction Defects in a solid respond to applied stresses. When the applied stress is dynamic, the response of the defects is also dynamic. The response encompasses the generation, healing and aggravation of defects [1]. Defect generation refers to the formation of defects, which usually occurs during loading. Defect healing refers to the diminution of defects. Healing can occur during compressive loading of a brittle material, such as a cement-based material. Defect aggrava- tion refers to the propagation or enlargement of defects; it can occur during removal of a compressive stress from a brittle material. Previous work on defects in cement-based materials has emphasized the effects of plastic deformation and damage. Due to the irreversible nature of these processes, the observation of their effects does not require monitoring during deformation. In contrast, this work addresses the effects of elastic deformation. Due to the reversible nature of elastic deformation, the observation of the effects of elastic deformation requires monitoring in real time during defor- mation. Moreover, the method of monitoring must be sensitive to the subtle effects of elastic deformation on the defects. For example, modulus measurement is not sensitive to the effects of elastic deformation. Because of these difficulties, there has been little prior work on the defect dynamics during elastic deformation [1]. The characterization of defect dynamics is of both scientific and technological interest, as it relates to materials science as well as material durability and reliability. In particular, it provides an understanding of the microstruc- tural effects of elastic deformation, which is commonly encountered by concrete structures during normal use. In contrast, plastic deformation and damage are not usually encountered during normal use. Hence, knowledge of the defect dynamics during repeated loading and unloading in the elastic regime is useful for the design and use of concrete structures. Previous work on defect dynamics has addressed cement paste under repeated compression in the elastic regime [1]. As indicated by electrical resistivity measurement, defect generation dominates during first loading, defect healing dominates during subsequent loading, and defect aggrava- tion dominates during subsequent unloading. This work extends the study from cement paste to mortar. Cement * Corresponding author. Tel.: +1-716-645-2593; fax: +1-716-645-3875. E-mail address: dlchung@acsu.buffalo.edu (D.D.L. Chung). Cement and Concrete Research 32 (2002) 379 – 385 0008-8846/02/$ – see front matter D 2002 Elsevier Science Ltd. All rights reserved. PII:S0008-8846(01)00686-X