424 publications from this institution
In this study, micronized sand (MS) is selected as replacement of Portland cement to reduce the amount of cement used in concrete. This filler is considered as chemecally inert to water and hydration products. However, this material can influence the microstructure, affect the transport property and eventually change the service life of concrete structures significantly. In this paper, rapid chloride migration (RCM) tests are carried out. Chloride migration coefficients of concrete samples with and without MS are measured. The results, obtained from a wide range of finenesses (between 2400 and 13000 cm2/g) and replacement levels (10% to 30%), show that migration coefficients of concrete samples with and without MS are different. The DuraCrete model is applied to predict the service life of structures made with this blended material. Migration coefficient of samples with 30% replacement increased and the required cover thickness of the structure to be increased in order to have 100 years’ service life.
In this paper, ternary and binary blended cement pastes as well as pure Portland cement paste were prepared by Portland cement, ground granulated blast-furnace slag (GGBFS), limestone powder and water, which were then hydrated from 1 to 91 days at 20 °C in a sealed environment. At each curing age, the hydration kinetics of cement and slag was determined by XRD/Rietveld and selective dissolution method (EDTA), respectively. The content of CaCO3 (limestone) was quantified by Thermogravimetric analysis (TGA) technique. The degree of hydration of cement clinker was distinctly accelerated by the single addition of slag or limestone within 91 days of hydration. The coexistence of slag and limestone in ternary blended cement accelerated the hydration of cement clinker within the first 14 days of hydration, but lowered the degree of hydration of cement clinker after 91 days of hydration compared with other pastes. The degree of reaction of slag in blended cement pastes was about 8% and 35% after 1 and 91 days of hydration, respectively, which was almost not influenced by the addition of limestone powder. A small amount of limestone, i.e. around 2% of the total solid raw materials, was reacted in pastes, and mainly occurred at the early age. Based on the experimental investigation, the results show that the hydration of calcium silicate phases of cement in pastes was enhanced by the presence of limestone, but hampered by slag. The hydration of calcium alumina phases of cement was greatly accelerated by the addition of slag, and also enhanced by the presence of limestone powder in binary blended limestone cement paste at early age. However, the coexistence of limestone with slag in ternary blended cement paste restrained the hydration of calcium alumina phases of cement.
In this paper, 871 data were collected from literature and trained by the 4 representative machine learning methods, in order to build a robust compressive strength predictive model for slag and fly ash based alkali activated concretes. The optimum models of each machine learning method were verified by 4 validation metrics and further compared with an empirical formula and experimental results. Besides, a literature study was carried out to investigate the connection between compressive strength and other mechanical characteristics. As a result, the gradient boosting regression trees model and several predictive formulas were eventually proposed for the prediction of the mechanical behavior including compressive strength, elastic modulus, splitting tensile strength, flexural strength, and Poisson’s ratio of BFS/FA-AACs. The importance index of each parameter on the strength of BFS/FA-AACs was elaborated as well.
Hot-pressed geopolymers have been introduced as ultra-high early strength ceramic-like materials with an almost pore-less structure. This study investigates the combined effects of pressure and heat on geopolymerization kinetics, microstructure, physical properties, and mechanical performance of geopolymers. The results demonstrate that about 65% of the trapped air is easily removed from the fresh geopolymer matrix using an initial impact pressure. When the pre-compacted matrix is subjected to the hot-pressing, it is further densified in the range of 1–10% as a result of continuous free-water evaporation and compaction. This process not only reduces the size and volume of porosity but also changes the continuous pore network to a closed one. Besides, the increase in hot-pressing promotes higher geopolymer gel formation and accelerates the polycondensation which possesses relatively high mechanical strength up to 160 MPa immediately after the processing. Collectively, these results demonstrate that the hot-pressing technique is a promising processing route for designing geopolymer with a diverse range of mechanical properties in a short and fast manner.
Cracks in reinforced concrete provide preferential access for aggressive substances into the concrete. Therefore, the corrosion of reinforcement bars is accelerated. Besides, carbonation, sulfate attack, and alkali–silicate reaction take place deep inside the concrete. Fortunately, from previous experiments, it was found that cracks in concrete can be healed with water and Ca is a main chemical element of the reaction products of self-healing. However, the ion concentrations in water can be various depending on the sources of water. There is still a lack of information on the effect of ion concentrations on self-healing. In this article, the effect of Ca 2+ ions on self-healing was investigated experimentally. Ca(OH) 2 was added into water as a healing agent. Self-healing behavior of cracks with saturated Ca(OH) 2 solution was explored and compared with that with distilled water. In order to gain deeper insight into the mechanism, the reaction products of self-healing were characterized by energy-dispersive spectroscopy, Fourier transform infrared spectroscopy, and thermogravimetric analysis. In addition, the filling fraction of cracks as a function of time was determined by means of backscattered electron image analysis. The efficiency of self-healing induced by saturated Ca(OH) 2 solution was evaluated and compared with that with distilled water.
Corrosion of steel reinforcement is the main cause of deterioration in reinforced concrete structures. It can result in cracking and spalling of the concrete cover. After the damaged cover is repaired, reinforcement corrosion might continue and even accelerate. While the development of the corrosion cell is difficult to control, the damage can be possibly delayed and controlled by use of a suitable repair material. The lattice fracture model is used in this paper to investigate the performance of strain hardening cementitious composite (SHCC) in concrete repair systems exposed to ongoing corrosion. Numerical results were verified by experimental tests when SHCC, nonreinforced material (repair mortar), and commercial repair mortar are used as repair materials. In experiments, reinforcement bars (surrounded by a repair material) were exposed to accelerated corrosion tests. The influence of the substrate surface preparation, the type of repair material, the interface, and the substrate strength on the resulting damage and failure mode of repair systems are discussed. In general, SHCC repair enables distributed cracking with small crack widths, up to several times smaller compared to repair mortar. Furthermore, more warning signs prior to the final failure are present in the SHCC repair system.
In this paper, different mechanisms of self-healing, i.e. self-healing based on adhesive agents, self-healing based on bacteria, self-healing based on autogenous self-healing were described. Their required conditions were summarized. The previous investigations showed that all mechanisms of self-healing are effective to some extend under particular conditions. In this paper, concrete structures were categorized according to serving conditions. Potential self-healing mechanisms are pointed out according to the required conditions of each self-healing mechanism.
In order to assess the durability of concrete structures, it is essential to investigate the ionic diffusivity under unsaturated conditions. Simulation of the ioinc diffusivity in unsaturated cement-based materials is big challenge due to the complicated microstructures and the difficulties in modelling of multi-phase fluid flow in cement-based materials. In this study, the 3D microstructure of cement paste is obtained from X-ray μCT with a high resolution. In the simulation, the Shan-Chen multi-phase lattice Boltzmann model is modified to simulate the equilibrium distribution of water and gas in the obtained 3D microstructure at various degree of water saturation. A lattice Boltzmann diffusion model is applied to simulate the ionic diffusion and determine the ionic diffusivity based on Fick’s law. It was found that the ionic diffusivity shows a strong dependence on the degree of water saturation. The simulated results fit very well with the experimental data obtained from literature.
In concrete repair systems, material properties of the repair material and the interface are greatly influenced by the moisture exchange between the repair material and the substrate. If the substrate is dry, it can absorb water from the repair material and reduce its effective water-to-cement ratio (w/c). This further affects the hydration rate of cement based material. In addition to the change in hydration rate, void content at the interface between the two materials is also affected. In this research, the influence of moisture exchange on the void content in the repair system as a function of initial saturation level of the substrate is investigated. Repair systems with varying level of substrate saturation are made. Moisture exchange in these repair systems as a function of time is monitored by the X-ray absorption technique. After a specified curing age (3 d), the internal microstructure of the repair systems was captured by micro-computed X-ray tomography (CT-scanning). From reconstructed images, different phases in the repair system (repair material, substrate, voids) can be distinguished. In order to quantify the void content, voids were thresholded and their percentage was calculated. It was found that significantly more voids form when the substrate is dry prior to application of the repair material. Air, initially filling voids and pores of the dry substrate, is being released due to the moisture exchange. As a result, air voids remain entrapped in the repair material close to the interface. These voids are found to form as a continuation of pre-existing surface voids in the substrate. Knowledge about moisture exchange and its effects provides engineers with the basis for recommendations about substrate preconditioning in practice.
Autogenous shrinkage is the volume deformation of a closed, isothermal, cementitious material system not subjected to external forces.How to accurately predict the autogenous shrinkage in hardening cement-based materials is an important issue in concrete technology because autogenous shrinkage increases cracking risk and reduces the durability and service life of reinforced concrete structures.Many models simulated the autogenous shrinkage of cement-based materials based on some mechanisms such as capillary pressure and disjoining pressure.These models are normally empirical and cannot reveal the deformation behavior of cement-based materials under the internal load of the driving force of autogenous shrinkage in microscale.As a consequence, the reliability of these models are questionable.This paper proposes a numerical model to simulate the autogenous shrinkage of hardening cement paste.A cement hydration and microstructure model HYMOSTRUC3D is used to simulate the microstructure of cement paste.A pore morphology based method is applied to describe the water and empty capillary pores distribution in the microstructure.Capillary pressure in the microstructure of cement paste is calculated from relative humidity measured by experiment based on Kelvin equation.A discrete algorithm is proposed to divide the hydration time into several static times.At each static time, a lattice finite element fracture method is used to simulate the deformation of simulated microstructure of cement paste under capillary pressure imposing.The autogenous shrinkage of hardening cement paste is equal to the sum of deformation of cement paste at each static time.The autogenous shrinkage of Portland cement paste with water to cement ratio of 0.3 is predicted.The simulation results are in a good agreement with experiments.
Though discussed a lot, the interfacial transition zone (ITZ), the linking phase between aggregate and bulk paste, still undertakes ambiguity on its knowledge. In this paper, the intention is borne to investigate the influence of curing age and water binder (w/b) ratio in blended hydration systems, where due to the more complex interaction between cementitious particles, such two factors are more pronounced compared to the hydration of plain cement. In particular, Portland cement, blast furnace slag and limestone filler are blended as binders for the cementitious composites. The designed w/b ratio is 0.3, 0.4, and 0.5, respectively. Two curing ages, i.e., 28 and 56 days are discussed. Hardened microstructures are characterized with regard to ITZ and bulk paste based on the BSE-SEM image analysis. Results indicate that ITZ is alleviated as hydration proceeds. When the w/b ratio increases, more porous ITZ are found. However, the extent of ITZ seems not affected by the curing age and w/b ratio.
In this paper, an ultrasonic shear wave reflection method was adopted to mimic the microstructural evolution of hydrating Portland cement pastes with different water/cement-ratios (0.35, 0.5 and 0.6) cured under a constant temperature of 25°C. The reflection coefficient measured with this technique was correlated to microstructural parameters of the cement pastes. The appli- cation of numerical simulation has shown that the ultrasonic wave reflection technique can be used to efficiently monitor the microstructural evolution of cementitious materials.
