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Mechanical and durability properties of concrete depend on paste composition, paste volume, the physical characteristics of aggregate, and the nature of the interfacial transition zone (ITZ). The ITZ is usually regarded as the weakest region in concrete, influencing both mechanical properties and durability, and is the reason why the stress deformation behavior of concrete differs from that of its individual components, i.e., hydrated cement paste and aggregate. The processes responsible for the formation of the ITZ are not well understood. Indirect evidence of the bonding mechanisms due to mechanical interlocking aided by the aggregate surface texture has been established by comparing the bond strength of fractured rock surfaces with that of polished rock surfaces. Due to the importance of interfacial transition zone on the mechanical properties of concrete and the lack of a simple and accurate method of measurement, it is necessary to develop a test for measurement of aggregate-cement paste interface. For this purpose, an apparatus was designed, based on the direct shear test and aggregate-cement paste interface strength of different concrete aggregates was measured. For this purpose, test specimens were preparing in based of direct shear test. Due to the difficulties involved in studying ITZ in normal concrete, laboratory specimens were prepared by casting cement paste on aggregate specimen. The aggregates were sawed into cylinder with height and diameter of 60 mm. A thin layer of cement with the same diameter was applied over aggregate in cylinder mould, and then layer of concrete apply over it. Therefore upper aggregate part was sawed aggregate and the other part consists of concrete and cement paste interlayer. The later procedure was used to prepare all specimens. Based on above discussions, 90 samples were made by ten aggregate types for three curing time. To measure the interfacial force, the shear box was positioned inside the servo-controlled direct shear machine and the specimens after 7, 14 and 28 days of curing in surface saturated-dry condition. The test was carried out using shear box with 30×30 cm2 dimensions. After carrying out test, The test results of ITZ strength showed values between 1 and 18 MPa at 7, 14 and 28 days. The highest ITZ strength of 15.64 Mpa at 28 curing days was measured in the concrete specimens prepared with the dolomite aggregates while the lowest ITZ strength was in diorite and lumashele aggregates. The result show that, ITZ strength of concrete is strongly influenced by the aggregate properties such as mineralogical source, compressive strength, porosity, water adsorption, shape, and surface texture. Mineralogical source of aggregate caused high degree of hydration in concrete at all ages, but is more appreciable in old age. Shape and surface texture porosity and water adsorption controls ITZ strength of concrete in all ages. Therefore surface texture porosity of aggregate has important in physical interlocking of aggregate with cement paste. Also, mechanical properties of aggregate are important in ITZ strength in all ages, this effect is visible in late age. In order to prepare high-strength concrete, the aggregate with high ITZ strength could be used. Therefore, in high-strength concrete, the aggregate-cement paste interface plays an important role in the strength.

Keywords: concrete, Interfacial Transition Zone (ITZ) strength, Aggregate, Cement paste

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