Preserving microstructure of concrete under load using the Wood's metal technique K.M. Nemati* Departments of Construction Management/Civil Engineering, University of Washington, Seattle, Washington, 98195-1610, USA Accepted 7 October 1999 Abstract A special experimental technique was developed which made possible the preservation of microstructure and the compressive stress-induced microcracks in concrete as they exist under applied loads. Cylindrical specimens of concrete were tested under uniaxial and triaxial compression. The resulting induced cracks were impregnated with a metal alloy, Wood's metal, that lique®es at higher temperatures (70±858C), but is solid at normal temperatures. At the stress of interest, this alloy was solidi®ed to preserve the stress-induced microcracks as they exist under load. Scanning Electron Microscopy (SEM) was employed to capture images from the cross sections of the concrete specimens. Stereology presents the geometrical statistical background for relating three-dimensional structures to their two-dimensional sections. Stereological estimates were obtained for total crack extension per unit of volume. Further, two-dimensional features of the cracks were analyzed in the section plane, such as orientation distribution, and length. 7 2000 Elsevier Science Ltd. All rights reserved. 1. Introduction Concrete is a heterogeneous, multiphase material. On a macroscopic scale it is a mixture of cement paste and ®ne and coarse aggregates, with a range of sizes and shapes. With regard to its mechanical behavior, concrete is often considered to be a three-phase com- posite structure, consisting of aggregate particles, the cement paste matrix in which they are dispersed, and the interfacial transition zones around the aggregate particles. Since the 1920s, researchers have suggested and assumed the existence of dierent kinds of defects called microcracks that would occur in concrete. How- ever, only since the early 1960s have such cracks been observed, measured, and characterized in the interior of the system [1]. In the 1970s and 1980s the develop- ment of nonlinear fracture mechanics models enabled the structure and behavior of concrete to be taken into account. In the 1980s and 1990s, further research has led to the increasingly common application of fracture mechanics in the design of beams, anchorage, and large dams. In spite of this, the theory of fracture mechanics in concrete is not yet as mature as conti- nuum theories, such as elasticity, viscoelasticity, and thermal problems. This is in part due to the limited understanding of the formation and propagation of microcracks in concrete. Early stereological approaches to analyzing damage evolution in concrete date back to the 1970s [2±4]. Such studies were able to reveal mechanisms of damage evolution in complicated load- ing cases, such as in the low-cycle fatigue domain [5]. By application of the fractal concept, recently the in- ¯uence could be assessed of the sensitivity of the quan- titative image analysis of microcracking in concrete on the spatial morphological features [6,7]. 2. Methods for studying microcracking The investigation of microcracking ranges from a macroscopic study of the behavior of cracked speci- International Journal of Rock Mechanics and Mining Sciences 37 (2000) 133±142 1365-1609/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S1365-1609(99)00099-4 www.elsevier.com/locate/ijrmms * Tel.: +1-206-685-4439; fax: +1-206-685-1976. E-mail address: nemati@u.washington.edu (K.M. Nemati).