Papers by Keyword: Interfacial Transition Zone

Abstract: This study investigates the influence of curing regimes on the microstructure and mechanical properties of alkali-activated concrete (AAC) containing coarse recycled aggregates (CRA) for structural applications. Building on prior research at BITS, Pilani Hyderabad Campus, AAC specimens were prepared by replacing natural aggregates (NA) with processed and unprocessed CRAs. Class F fly ash and ground granulated blast furnace slag (GGBFS) served as precursors, activated by sodium hydroxide and sodium silicate solutions. A consistent mix design employed a 4% sodium concentration and 60:40 fly ash-to-slag ratio. The target compressive strength was 40 MPa for structural use. Curing conditions are known to affect various AAC properties, including early and long-term strength, hydration kinetics, durability, and dimensional stability. While prior research explored these aspects under different curing regimes, the influence on microstructure development in AACs with high CRA content remains under-reported, especially considering curing regime variations. This research addresses this gap by employing three distinct curing regimes: ambient temperature (30°C) for 28 days, ambient temperature with plastic wrap for 28 days, and oven curing at 75°C for 72 hours followed by 25 days at 30°C. Microstructural investigations using XRD, FESEM, and stereo microscopy were complemented by ultrasonic pulse velocity and compressive strength tests. Notably, specimens subjected to oven curing at 75°C exhibited superior performance compared to those cured at ambient temperature with or without plastic wrapping

Abstract: The micro-mechanical properties and micro-structural characteristics of the interfacial transition zones of recycled concrete with graphene oxide were studied by using nanoindentor. The results showed that the average elastic modulus and width of the interfacial transition zone of recycled concrete with graphene oxide between new paste matrix and natural aggregate were about 20GPa and 30-35μm, respectively; The mean elastic modulus and transition zone width of the interfacial transition zone between new paste matrix and old paste matrix was about 35GPa and 25-30μm, respectively. By analyzing and calculating the probability distribution of hydration products, it was found that the graphene oxide template increased the proportion of calcium silicate hydrate in the hydrate and enhanced the content of calcium hydroxide crystal group. Compared with ordinary recycled concrete, the elastic modulus distribution in the interfacial transition zone was more uniform and the microstructure was more stable. It can be concluded that graphene oxide can enhance the mechanical properties and microstructure of the interfacial transition zone of recycled concrete so as to improve the macroscopic mechanical properties of recycled concrete.

Abstract: High-performance concrete is a very specific type of concrete. Its production is sensitive to both the quality of compounds used and the order of addition of particular compounds during the homogenization process. The mechanical properties were observed for four dosing procedures of each of the three tested concrete mixtures. The four dosing procedures were identical for the three mixes. The three mixes varied only in the type of supplementary cementitious material used and in water content. The water content difference was caused by variable k-value of particular additives. The water-to-binder ratio was kept constant for all the concretes. The additives used were metakaolin, fly ash and microsilica. The comparison of particular dosing procedures was carried out on the values of basic mechanical properties of concrete. The paper compares compressive strength and depth of penetration of water under pressure. Besides the comparsion of macro-mechanical properties, the effect of microsilica and fly ash additives on micro-mechanical properties was observed with the use of scanning electron microscopy (SEM) and nanoindentation data analysis. Nanoindentation was used to determine the thickness and strength of interfacial transition zone (ITZ) for different sequence of addition of cement, additive and aggregate. The thickness obtained by nanoindentation was further investigated by SEM EDS line scanning.

Abstract: Mechanical fracture properties of interfacial transition zone (ITZ) of fine-grained composite based on cement matrix with different types – basalt, granite, marble and amphibolite – of rock inclusion were studied. Specimens with the initial stress concentrator were tested in standard three-point bending configuration. Fracture surfaces were examined with light and electron microscopes. Local ITZ response was characterized by nanoindentation in the vicinity of rock inclusions. Local elasticity, hardness and viscous properties were assessed. It has been shown that the ITZ is mechanically weaker compared to the bulk matrix in the region of ca. 0–20 μm from the inclusion for all specimen’s types. It exhibits gradual increase of elastic modulus and hardness, which can be approximately expressed by a power law. On the other hand, the creep in ITZ was found to be higher compared to the bulk matrix. The results of nanoindentation measurements are in a good agreement with overall mechanical properties, fracture response and microstructure measurements done by scanning electron microscopy.

Abstract: This paper shows a micromechanical study of interfacial transition zone (ITZ) around steel fiber in cement paste. It investigates microstructure and mechanical performance of the ITZ by a combination of nanoindentation and scanning electron microscopy (SEM). The investigated specimens were made from cement CEM I 42.5R paste with dispersed reinforcement in the form of steel fiber TriTreg 50 mm. The SEM demonstrated larger porosity and smaller portion of clinkers in the ITZ. Nanoindentation delivered values of elastic modulus, hardness and creep parameters around the fiber. An average value of elastic modulus in ITZ was at the level of 67% in comparison with cement bulk and the width of ITZ was about 40 µm. The value of hardness was found to be 60% of the average hardness of the bulk cement paste. The measured load-displacement curves were used for calculation of creep indentation parameter (CIT) and the creep compliance function. An average value of the creep compliance in the ITZ was found to be two times higher than in the cement bulk.

Abstract: Concrete is a heterogeneous material made up at mesoscale of linear elastic aggregates distributed in a mortar matrix whose behavior is time and temperature dependent. The Interfacial Transition Zone (ITZ) between aggregates and matrix also influences the overall behavior. We investigate here analytically and numerically by means of 3D simulations the creep behavior of concrete and mortar subjected to moderate temperatures at mesoscale. The numerical specimens consist in unstructured periodic meshes of polyhedral aggregates with various size and shapes randomly distributed in a box. Specific interface finite elements are introduced between aggregates and matrix to model the ITZ. Both matrix and ITZ are considered as linear thermoviscoelastic materials. Averaged stresses and strains in the matrix and aggregate phases are compared to analytical estimations obtained with classical mean-field approximation schemes applied in the Laplace-Carson space, where the ITZ are introduced via imperfect interfaces modelled with the Linear Spring Model (LSM). The effects of ITZ thickness, aggregate shape and temperature are then studied to evaluate their respective influence on mortar and concrete creep behavior.

Abstract: The Interfacial Transition Zone (ITZ) between two basic materials having differentiations in their mechanical properties has always been intriguing. The stiffness disparities between the two will result in a very distinctive area, the interface. Cement based components such as mortar and concrete consist of the cement paste and aggregates, with the ITZ at the perimeter. When compared to the cement paste, this ITZ has a higher porosity with a dissimilar crystal formation. The resulting area therefore becomes the weak link in concrete. A Finite Element Model (FEM) was developed to construct the load-displacement behavior of a single inclusion specimen and to study the crack propagation within the ITZ. The ITZ was modeled as a linkage element having a double spring, perpendicular and parallel to the ITZ surface. The individual stiffness behavior of these springs was obtained from laboratory-tested specimens. Non-linearity was generated by evaluating the principal stresses and strains at Gauss points, while the CEB-FIB 2010 code was used for the constitutive material behavior of the mortar. Iteration is conducted by the arc-length method developed by Riks-Wempners. The load-displacement curves resulting from the FEM were validated with laboratory tested specimens to compare its effectiveness and assess the correctness of the model.

Abstract: The autoclaved-curing high strength concrete was prepared with quartz powder (QP). The deterioration behavior of concrete specimens partially exposed to Na₂SO₄ solutions was analyzed by using scanning electron microscopy, energy dispersive spectrometer and X-ray diffraction as well. The results show that the interfacial transition zone cause the autoclaved-curing concrete damage. Sulfate chemical erosion is the main cause the damage of cement concrete. Hydration products of decomposition are the main cause of concrete crack mixing with QP. QP concrete is more susceptible to sulphate attack.

Abstract: The coarse aggregate volume fraction in conventional plastic concrete is controlled relatively low to ensure a required workability, but its low aggregate content play negative roll on the service properties of the concrete. In this paper, we revealed the mechanism of a coarse aggregate interlocking concrete prepared by Scattering-Filling Stone Concrete (SFSC), it can keep the fresh concrete its workability but increase its service performance by scattering coarse aggregate during the concrete casting, the experimental and engineering application of SFSC indicated that: the strength of concrete prepared with this method increases obviously whereas the shrinkage decreases significantly when 20% cement dosage is saved. The SFS process was utilized in a highway pavement, the core of the concrete is very dense and no obvious defect could be found when 8% pavement concrete was replaced by coarse aggregate. The SFSC is a type of low carbon concrete.

Abstract: The application of recycled aggregate concrete (RAC) has environmental benefits, social benefits and economic benefits, which has profound significance to the sustainable development of society. However, recycled aggregate (RA) has a large porosity, high water absorption, high crush index, etc. In addition, the interface transition zone (ITZ) is the weak link of RAC, which has limited the application of RAC. Based on the advance of RAC, by discussing the characteristic of the ITZ of RAC, this paper proposes a secondary manufacturing shell mixing approach (SMSMA), and the mechanism of this mixing approach and its effect on performance of RAC are analyzed. Compared with the test of normal mixing approach (NMA), it indicates that compressive strength and splitting tensile strength of RAC through SMSMA are generally increased by about 10% ~ 25% and 10% ~ 40% at 28 days respectively, and the chloride ion diffusion coefficient are generally declined by about 10% ~ 15% at 28 days, but the liquidity of RAC is not improved. What's more, with replacement rate of RA dropping, the improvement of the performance of RAC tends to be better.