Assessing the ITZ microcracking via scanning electron microscope and its effect on the failure behavior of concrete Tu ¨lin Akc ßaog ˘lu a, * , Mustafa Tokyay b , Tahir C ßelik a a Civil Engineering Department, Eastern Mediterranean University, Mag ˘usa, Mersin 10, Turkey b Civil Engineering Department, Middle East Technical Eastern Mediterranean University, Ankara 06531, Turkey Received 23 May 2002; accepted 28 May 2004 Abstract The influence of aggregate size and water-to-cement (w/c) ratio of the matrix on the structure of interfacial transition zone (ITZ) and the interaction between the ITZ and the matrix on the failure process of concrete under uniaxial compression were studied. The ITZ microcracking and the failure process of concrete were investigated experimentally by means of compressive and indirect tensile testing, stress – volumetric strain measurements and microscopic analyses on the model concrete containing single spherical steel aggregate with three different w/c ratios. At low w/c ratios, the rigid and smooth surface texture aggregates made by the ITZ have a significant structural difference compared to the mortar. This was more pronounced for larger aggregates. Higher structural differences between the mortar matrix and ITZ in low w/c ratio composites resulted in accelerated ITZ microcracking at high stress level. The effect of condensed microcracking in a narrower ITZ was reflected in the lower critical stress levels for the low w/c ratio composites with larger aggregates. D 2004 Elsevier Ltd. All rights reserved. Keywords: Aggregate; Interfacial transition zone; Tensile properties; Microcracking 1. Introduction Concrete is a highly complex and heterogeneous com- posite material. The properties of the concrete depend on the properties of its component phases [aggregate, matrix and interfacial transition zone (ITZ) between the aggregates and the matrix] and the interactions between them. ITZ, which is structurally and mechanically different than the matrix, plays a critical role in determining the mechanical properties and failure behavior of concrete composites. The properties of the aggregates (type, shape, surface conditions, etc.), cement and admixtures and particularly the water-to-cement (w/c) ratio of the mixture are the main factors that form the structure of ITZ and thus its properties [1–8]. The bond strength was found closely related with the type [1,2], shape and surface texture of the aggregate used [3,4,8]. The effect of w/c ratio on the microstructure, thickness and mechanical properties of ITZ was investigated by many researchers [1,6,7,9,10]. It was concluded that by lowering the w/c ratio, the microstructure of the interface could be significantly improved through reduction in poros- ity. There is also a general agreement that the lower the w/c ratio, the thinner the ITZ [1,6,10] and the greater can be the influence of the type of aggregate [6] on the overall proper- ties of ITZ. Furthermore, for larger aggregates, thickness of the ITZ becomes larger [11]. In researches [1,5,8,12 – 14] carried out to determine the influence of aggregate, matrix and ITZ properties on the tensile and compressive strengths of concrete, it was found that the interfacial bond was the deciding factor for the tensile strength and played little role on the compressive strength. The influence of the type and the surface texture of the aggregate, on the compressive strength of concrete, however, become paramount as the quality of the mortar is improved. In connection with the ultimate stress, the significance of investigating the damage process has increased. The fracture of concrete depends on the properties of the particular phases and their interactions. The ITZ properties, has a particular importance on the cracking of concrete. As the w/c ratio increased, the porosity in ITZ is also increased, resulting in the initiation and development of cracks in this zone [9]. The relatively smooth and rounded aggregate provides a weak obstacle to crack initiation and 0008-8846/$ – see front matter D 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.cemconres.2004.05.042 * Corresponding author. Tel.: +90-542-8807302; fax: +90-392-3651574. E-mail address: tulin.akcaoglu@emu.edu.tr (T. Akc ßaog ˘lu). Cement and Concrete Research 35 (2005) 358 – 363