In this study, stirring speeds during the synthesizing process of the sodium silicate/polyurethane microcapsules were optimized. The yield values of microcapsules at different stirring rates were evaluated. Based on the yield values and shape of microcapsules, optimized microcapsules were obtained. The measurements of the shell thickness of microcapsules were performed on the fractured microcapsules by using scanning electron microscopy. The optimized microcapsules were further characterized by means of optical microscopy and nanoindentation. Load vs. displacement and modulus of elasticity vs. displacement characteristics of the microcapsules were obtained. The average modulus of elasticity was found to be 633 MPa. The self-healing efficiency of the optimized microcapsules was evaluated by means of compressive strength tests. The incorporation of microcapsules reduced the compressive strength of fiber-reinforced mortar by 12–22%. However, it was observed that the microcapsules enhanced the self-healing capacity (recovery in compressive strength) of the mortar.@en

Ultra-high performance concrete (UHPC), which typically contains extremely high superplasticizer amount, tends to render a special type of dense surface layer, especially under drying conditions. Even if the inner side of fresh UHPC is still in a flowable nature, the top surface of the concrete which is exposed to ambient conditions renders a dense cover called “elephant skin” in a short time after casting. In this study, the effects of climatic conditions and ground granulated blast furnace slag (GGBS) replacement on the elephant skin formation were studied. The influence of elephant skin formation on the macropore profile was assessed by micro-computed tomography scan. Observations revealed that any drying condition that does not lead to water vapor condensation on the fresh surface can trigger the formation of elephant skin within minutes after casting. Both microscopic and micro-computed tomography investigations showed that as a result of elephant skin formation, most of the air bubbles cannot escape from the fresh matrix, and locate under the skin. These air bubbles and elephant skin itself result in a multi-layered structure in UHPC body throughout the cross-section and reduce the aesthetics of the casting surface. Increasing GGBS replacement percentage significantly reduced the superplasticizer demand which is responsible for skin formation, however, did not eliminate the problem.@en

Ultra-high performance concrete (UHPC) is an innovative cement-based composite with high mechanical performance under tensile and compressive loads, extremely low permeability, and excellent durability. Because of these features, UHPC has the potential to contribute to the development of new architectural perspectives and structural systems with prolonged service life; therefore, it is anticipated that the use of UHPC in cast-in-situ applications will increase in the near future. As a result of its high Portland cement dosage, the hydration heat of UHPC can be relatively high compared to that of conventional concrete. Thus, ground granulated blast furnace slag (GGBS) can be used in UHPC formulation for reducing Portland cement dosage thereby limiting hydration heat while also addressing ecological and engineering concerns. In the scope of this study, the effects of GGBS replacement (0%, 30%, and 60%) on the hydration heat, strength, and microstructural characteristics of UHPC were studied. Results showed that GGBS-bearing UHPCs are more sensitive to ambient temperature in respect to cumulative heat. 60% GGBS replacement reduced cumulative heat release by 36% and 28% at 20 °C and 35 °C respectively. So, the benefit of GGBS on reducing hydration heat is less pronounced in hot weather. Performance differences in strength depending on the replacement ratio were only noticeable on the first day of curing. Prolonged curing time and fiber inclusion eliminated strength differences. Microstructural investigations indicated that Ca(OH)2 can be lowered up to 0.5%, and the Ca/Si ratio of the C–S–H phase was reduced below the value of 1.0 after 90 days of curing as a result of GGBS replacement.@en

Rheology and shrinkage are the key features that should be controlled to perform a successful casting process and obtain a durable ultra-high performance concrete (UHPC). It is anticipated that cast-in-situ applications of UHPC will increase in the near future, so the studies on early-age properties of UHPC have come into prominence. However, there is still no standardized method for mix design of UHPC that can ensure the desired performance. High amount of cement and low water/binder ratio increase the shrinkage values, which also increase the crack formation risk especially at early-age. The basic variable of mix proportions that determines the rheological and shrinkage properties is aggregate volume and accordingly paste volume. In the scope of this experimental study, influence of volume fraction of quartz aggregate on rheological, shrinkage, and mechanical properties of UHPC was investigated. The results showed increasing the aggregate volume led to sharpen pseudoplastic behaviour of UHPC. To be able to produce a self-compacting UHPC, upper limit of aggregate volume was found 45%. Both autogenous and drying shrinkage can be limited by increasing the aggregate volume. Early-age autogenous and drying shrinkage are far more than the shrinkage measured after demoulding during 90 days. Necessary precautions, thus, should be taken as of casting stage.@en

To investigate the effects of viscosity-modifying admixture (VMA) on the extrudability of limestone and calcined clay-based cementitious materials, three mix designs with different dosages of VMA were proposed in this study. The ram extrusion was utilized as an extrusion model for exploring the fresh properties of printable materials. Two methods were used, based on the ram extruder setup—(a) extruding materials with the same extrusion speed at different rest times to determine how the pressure changes with time; (b) extruding materials with different extrusion speeds at the same rest time to investigate the material flow parameters using the Basterfield et al. model. The main findings of this study could be summarized as—(1) the extrusion pressure of all mix designs exhibited an increasing trend with time. At the same tested age, the extrusion pressure under 0.25 mm/s of piston speed was increased and the shape retention of the extruded filaments was enhanced by increasing the dosage of VMA; (2) the correlation between the experimental results and the Basterfield et al. model was excellent (R-squared value: 0.99). The mixture with a higher content of VMA showed an increased elongational yield stress, flow consistency, and shear yield stress.@en

High performance concrete has been often preferred in special engineering structures and in challenging composites products. Researchers have recently focused on the radiation shielding characteristics of these type concrete mixtures due to rising nuclear industry in the developing world. In the study, performance of reactive powder concrete was researched with regard to gamma-ray and neutron attenuation when its normal weight aggregate replaced with heavyweight aggregate (barite). For this purpose, reactive powder concrete mixtures were prepared 100% quartz aggregate, 100% barite aggregate and their blending 50–50%, by volume. Some physical and mechanical characteristics such as density, compressive strength, fracture energy, flexural strength and modulus of elasticity of the mixtures were determined. Gamma-ray attenuation coefficients and transmission thickness values were theoretically established for commonly known gamma energies (661.7, 1173.2 and 1332.5 keV). Optimization of the reactive powder mixtures was performed for both neutron and gamma-ray attenuation at 8 MeV. As a result, barite significantly increased the gamma-ray attenuation coefficients of reactive powder concrete. The mechanical performance of reactive powder concrete, however, was markedly reduced as a result of barite substitution. Replacement of quartz by barite aggregate has a more adverse impact on flexural strength than that of compressive strength. A mix that contains 40% barite aggregate of total aggregate volume was found as an optimum RPC mixture for simultaneously shielding neutrons and gamma rays.@en

Recently, our group attempted to develop the ternary blended (Portland cement, calcined clay and limestone) cementitious material for 3D concrete printing (3DCP). Due to the elimination of formwork during the layer-by-layer casting process, the printed material should have favorable elastic properties and green strength at the fresh state. A small amount of Hydroxypropyl methylcellulose (HPMC) based viscosity modifier agent (VMA) is used in the printable mixture to enhance the printing shape stability during the printing process. However, adding VMA may delay the hydration of cement-based materials and affect the strength development at an early age. It is necessary to determine how the VMA additions affect the early age strength development of a 3D printable cementitious material. In this paper, three mix designs with different amounts of VMA were selected to perform the uniaxial compression test at different early ages (30 min, 1, 2, 3, 4, 6h). The setting time and compressive strength tests at 1, 7 and 28 days of those mix designs were also measured. Besides, the heat flow of different mix designs was recorded by using the isothermal calorimeter. Finally, it has been found that: (1) adding VMA could contribute to increase the green strength within the first 2h after mixing water and weaken the strength development from 2h to 6h; (2) the VMA additions mainly delayed the initial set and small effects on the final set; (3) about 50% of compressive strength at 1, 7 and 28 days were reduced for the specimens with VMA. (4) the more amounts of VMA was used in the mixture, the more retarding effects on cement hydration.@en

In this study, it was aimed to produce a four-person lightweight concrete canoe to participate in the National Intercollegiate Concrete Canoe Competition organized by Istanbul Technical University. Concrete canoe competition is a prestigious activity held regularly in the USA since 1988 by ASCE, mainly aimed at sharing knowledge among civil engineering students. For this purpose, two lightweight concrete mixtures with a hardened unit weight lower than 1000 kg/m3 were designed. Despite of such a low hardened unit weight, they have 13 MPa and 18 MPa compressive strength with excellent bond properties with basalt mesh reinforcement. A nonlinear finite element model for concrete canoe was generated to determine the moment capacity in such a way that four rowers could row at the same time, taking into account the competition rules.@en

In this study, it was aimed to produce a four-person lightweight concrete canoe to participate in the National Intercollegiate Concrete Canoe Competition organized by Istanbul Technical University. Concrete canoe competition is a prestigious activity held regularly in the USA since 1988 by ASCE, mainly aimed at sharing knowledge among civil engineering students. For this purpose, two lightweight concrete mixtures with a hardened unit weight lower than 1000 kg/m3 were designed. Despite of such a low hardened unit weight, they have 13 MPa and 18 MPa compressive strength with excellent bond properties with basalt mesh reinforcement. A nonlinear finite element model for concrete canoe was generated to determine the moment capacity in such a way that four rowers could row at the same time, taking into account the competition rules.@en

In this study, it was aimed to produce a four-person lightweight concrete canoe to participate in the National Intercollegiate Concrete Canoe Competition organized by Istanbul Technical University. Concrete canoe competition is a prestigious activity held regularly in the USA since 1988 by ASCE, mainly aimed at sharing knowledge among civil engineering students. For this purpose, two lightweight concrete mixtures with a hardened unit weight lower than 1000 kg/m3 were designed. Despite of such a low hardened unit weight, they have 13 MPa and 18 MPa compressive strength with excellent bond properties with basalt mesh reinforcement. A nonlinear finite element model for concrete canoe was generated to determine the moment capacity in such a way that four rowers could row at the same time, taking into account the competition rules.@en