Numerical Study on Chloride Ingress in Cement-Based Coating Systems and Service Life Assessment

Chloride-induced corrosion is a critical issue for RC structures. Cement-based coatings can be used to protect concrete structures with unsatisfactory quality against chloride ingress. To evaluate the effectiveness of the coatings to extend the service life of coated concrete structures, the evolution of the chloride profile in the coated concrete structures should be determined. This paper investigated the mechanism of chloride ingress into coated concrete structures (i.e., coatings made of cement paste and concrete substrate). A numerical tool is proposed for calculating the chloride profiles in the coated concrete structures. A parametric study investigated the influence of several factors on the chloride ingress: the water:cement (w:c) ratio of the coating, the thickness of the coating, and early or late application of the coating. A preliminary cost analysis of coating materials was carried out. The results showed that the effectiveness of the coatings increased with coating thickness at a drastic increase of material cost; the effectiveness of the coatings increased with the decrease of the w:c ratio at a moderate increase of material cost. In order to extend the service life of the substrate, a coating with a low w:c ratio is recommended, and the coating thickness should be designed depending on the requirements. Moreover, the exposure history of the substrate before application of the coating also has an influence on the effectiveness of the coating. To protect an existing concrete structure exposed to a chloride environment against rapid chloride ingress, it is preferable to apply a coating as early as possible, because the effectiveness of the coating is reduced by late application.Accepted Author ManuscriptMaterials and Environmen

Characterization and comparison of capillary pore structures of digital cement pastes

More and more studies are based on digital microstructures of cement pastes obtained either by numerical modelling or by experiments. A comprehensive understanding of the their pore structures, therefore, becomes significant. In this study, the pore structure of a virtual cement paste (HYMO-1d) generated by cement hydration model HYMOSTRUC 3D is characterized. The pore structure of HYMO-1d is compared to the one of CT-1d that is reconstructed by using X-ray computed tomography technique (CT scan). Both HYMO-1d and CT-1d have the same porosity. Various parameters are taken into account, viz., the specific surface area, the pore size distribution (PSD), the connectivity and the tortuosity of water-filled pores. Regarding the PSD, two concepts (i.e., the “continuous PSD” and the “PSD by MIP simulation”) are adopted. The “continuous PSD” is believed to be a “realistic” PSD; while the “PSD by MIP simulation” is affected by the “throat” and “ink bottle” pores. The results show that HYMO-1d and CT-1d exhibit a similar curve of “continuous PSD”, but distinct curves of “PSD by MIP simulation” and different specific surface areas. A lower complexity of the pore structure of HYMO-1d is indicated by a higher tortuosity of water-filled pores with reference to CT-1d. This study indicates that the comparison of pore structures between the digital microstructures should be based on multiple parameters. It also gives an insight into further studies on digital microstructures, i.e. transport properties of unsaturated materials.Else Kooi LaboratoryMaterials and Environmen

Simulation of moisture transport in hydrating cement-based overlay systems

Drying of cement-based overlay systems is a critical issue, because it causes differential shrinkage between the overlay material and the concrete substrate and may induce cracking or debonding of the overlay material. In this paper the mechanisms of moisture transport in hydrating cement-based overlay systems are studied. A model is proposed for simulating the moisture transport. A parameter study has been conducted to quantitatively investigate the influence of the thickness of the overlay material and the curing conditions on hydration of the overlay materials. The evolution of the moisture profile in the overlay system and the development of the degree of hydration (DOH) of the overlay material have been calculated. The change of water content in the overlay material is investigated, in terms of the water absorbed by the substrate, the water consumed by hydration of the overlay material and the water evaporated to the environment. The simulation results show that the water evaporation is a dominant factor that causes water loss of the overlay material, while the water absorption by the substrate plays only a minor role. Moist curing is much more effective than sealed curing (e.g. by using sealing agent) for hydration of the overlay material. The DOH of the overlay material is significantly increased with longer moist curing. Under the same curing condition (e.g. moist curing + drying), thinner overlay materials are more vulnerable to water loss and exhibit a lower DOH. It suggests that for proper hydration of cement-based overlay materials, moist curing is recommended rather than applying a sealing agent.Materials and Environmen

Inorganic powder encapsulated in brittle polymer particles for self-healing cement-based materials

Many types of healing agents have been investigated. These agents are processed in different ways, such as adhesive polymer in capsules or hollow fibre glasses, bacteria in porous aggregates and geo-materials directly incorporated in the cementbased materials. In this study, sodium silicate powder is encapsulated in polystyrene particles (polystyrene particle containing sodium silicate is defined as PS particle in short). The PS particles remain intact in the cement-based matrix before cracking. If water or moisture is available, the healing agent can be released into the crack provided that the crack passes through the PS particles. The dissolved sodium silicate reacts with calcium hydroxide in the matrix, and the healing products (C-S-H gel) can form in the crack. Furthermore, compared to the reference, for the cracked specimens with polystyrene particles, the recovery of flexural stiffness can be observed. Different sizes and mass fractions, i.e. sodium silicate / cement ratios, of PS particles used in engineered cementitious composite (ECC) mixture are studied to see their influence on mechanical properties as well as their healing efficiency. When the mass fraction of polystyrene particles is 4% of cement and the polystyrene particles have a proper slender shape, the ECC show good results in terms of flexural strength, flexural deflection capacity and recovery of mechanical properties. Therefore, encapsulation of healing agent in polystyrene could be regarded as a promising way for realising self-healing of cement-based materials.Structural EngineeringCivil Engineering and Geoscience

Drying shrinkage of alkali-activated slag concrete with natural/recycled aggregates

Each year a large amount of construction and demolition waste (CDW) is generated in the European Union. For sustainability development the CDW is recycled and re-used. To promote the use of CDW, recycled aggregates from CDW were incorporated in alkali-activated concrete (AAC), which mainly consisted of secondary materials or industrial by-products. This study investigated the influence of recycled aggregates on workability, compressive strength and drying shrinkage of slag-based AAC. Properties of conventional concrete with natural/recycled aggregates were also tested for comparison. The results showed that the pre-saturated recycled aggregates only slightly affected the workability of conventional concrete or AAC. Recycled aggregates reduced compressive strength of both conventional concrete and AAC due to extra water for pre-saturation of the recycled aggregates. The mass loss of the concrete specimens upon drying was greater for low-strength concrete than for moderate strength concrete. The recycled aggregates increased the mass loss and drying shrinkage of AAC. For conventional concrete, low-strength concrete had a higher drying shrinkage compared with moderate strength concrete. On the contrary, for AAC in this study, low-strength concrete had a lower drying shrinkage compared with moderate strength concrete.Materials and Environmen

Drying shrinkage of alkali-activated slag concrete with natural/recycled aggregates

Each year a large amount of construction and demolition waste (CDW) is generated in the European Union. For sustainability development the CDW is recycled and re-used. To promote the use of CDW, recycled aggregates from CDW were incorporated in alkali-activated concrete (AAC), which mainly consisted of secondary materials or industrial by-products. This study investigated the influence of recycled aggregates on workability, compressive strength and drying shrinkage of slag-based AAC. Properties of conventional concrete with natural/recycled aggregates were also tested for comparison. The results showed that the pre-saturated recycled aggregates only slightly affected the workability of conventional concrete or AAC. Recycled aggregates reduced compressive strength of both conventional concrete and AAC due to extra water for pre-saturation of the recycled aggregates. The mass loss of the concrete specimens upon drying was greater for low-strength concrete than for moderate strength concrete. The recycled aggregates increased the mass loss and drying shrinkage of AAC. For conventional concrete, low-strength concrete had a higher drying shrinkage compared with moderate strength concrete. On the contrary, for AAC in this study, low-strength concrete had a lower drying shrinkage compared with moderate strength concrete

Relationship between the size of the samples and the interpretation of the mercury intrusion results of an artificial sandstone

Mercury intrusion porosimetry (MIP) measurements are widely used to determine pore throat size distribution (PSD) curves of porous materials. The pore throat size of porous materials has been used to estimate their compressive strength and air permeability. However, the effect of sample size on the determined PSD curves is often overlooked. In pursuit of a better understanding of the effect of sample size on mercury intrusion into porous materials, a combined experimental and numerical approach was applied. Quartz sand and epoxy resin were mixed to form artificial sandstone. Digital microstructures of the sandstone were obtained by using X-ray computed tomography (CT scan) technique. PSD curves of the artificial sandstone with different sample sizes were determined both by MIP measurement and by simulation of mercury intrusion (i.e., MIP simulation). Percolation analysis was performed on mercury-intruded pores in the digital microstructures. The PSD curves determined both by MIP measurements and by MIP simulations show that there was a significant effect of sample size on mercury intrusion before percolation of mercury-intruded pores. The effect of sample size decreased with the increasing pressure. After the mercury-intruded pores percolated through the samples, the effect of sample size on mercury intrusion became minor. The pore throat size of the artificial sandstone was used to estimate the air permeability using the relation proposed in the literature. The calculated air permeability of the smaller sandstone sample was higher. However, in principle, the air permeability of sandstone samples should be independent of the sample size. Two main conclusions can be drawn: (1) a fixed sample size should be used in MIP measurements or MIP simulation so that the PSD curves of different samples can be properly compared, (2) sample size needs to be considered when the pore throat size determined by MIP measurement is used for estimating air permeability.Materials and Environmen

Design Considerations and Short-Circuit Characteristics of Fully Superconducting Wind Turbine Generators

Compared with partially superconducting generators, fully superconducting generators (F-SCGs) can further increase the torque density in large direct-drive wind turbine applications. Design trends of F-SCGs intend to increase the electrical loading by applying superconducting wires and boost the current density in the armature winding to meet the critical current density with a safety margin. High currents may cause a low power factor and require the power electronic converter to have a much larger capacity. In an F-SCG, furthermore, torques could be too high, and field and armature currents may exceed the critical currents during a generator short circuit. This paper studies the design of a 20 MW F-SCG with consideration of the control strategy and the power factor, and then evaluates the short circuit characteristic of the F-SCG. The results analysis shows that a capacitive load control should be adopted to avoid a significant drop in the power factor and to make full use of the current-carrying capability of superconductors. An I_{d} = 0 control can also be used with a medium current level. During the short circuit, the negative side is that the phase currents exceed the critical currents and cause quenches. The positive side is that the field currents stay below the critical currents and the torques do not exceed the mechanical limitation of three times the rated torque. Accepted Author ManuscriptTransport Engineering and Logistic

Micromechanical study of the interface properties in concrete repair systems

Structural EngineeringCivil Engineering and Geoscience

Mechanisms of autogenous shrinkage of alkali-activated slag and fly ash pastes

This study aims to provide a better understanding of the autogenous shrinkage of slag and fly ash-based alkali-activated materials (AAMs) cured at ambient temperature. The main reaction products in AAMs pastes are C-A-S-H type gel and the reaction rate decreases when slag is partially replaced by fly ash. Due to the chemical shrinkage and the fine pore structure of AAMs pastes, drastic drop of internal relative humidity is observed and large pore pressure is generated. The pore pressure induces not only elastic deformation but also a large creep of the paste. Besides the pore pressure, other driving forces, like the reduction of steric-hydration force due to the consumption of ions, also cause a certain amount of shrinkage, especially in the acceleration period. Based on the mechanisms revealed, a computational model is proposed to estimate the autogenous shrinkage of AAMs. The calculated autogenous shrinkage matches well with the measured results.Materials and Environmen