The theory of ionic diffusion in water-saturated concrete accompanying ion exchange with mobile or immobile adsorbed ions was constructed by using the general theory of diffusion and the condition of electrical neutrality. Analysis of the diffusion profile of chloride (Cl−) ions in concrete with the theory revealed that adsorbed Cl− ions in AFm and C-S-H are as mobile as free Cl− ions in the pore solution, so that the adsorption does not retard the ingress of Cl− ions. The existing test methods for determining the effective or apparent diffusion coefficient of Cl− ions were evaluated on the bases of the present theory and new experimental findings. It was revealed that steady-state electrochemical methods such as NTBUILD-355 and ASTM C1202 are not suitable for determining the diffusion coefficient, because the steady state methods expel preexisting diffusible ions which strongly affect the diffusion of Cl− ions. Moreover, the steady state methods overestimate the diffusion rate because the methods assume that adsorbed Cl− ions are immobile. The electrochemically-accelerated method NT BUILD 443 is also unsuitable for the diffusion test, because the acceleration is induced by the electro-osmotic flow of the external solution into concrete. The present diffusion theory necessitates the effective self-diffusion coefficient of not only Cl− ions but also all the other diffusible ions in concrete. A simple method of determining the effective self-diffusion coefficients of arbitrary ions in concrete from the diffusion profile of Cl− ions was presented.
Engineered geopolymer composites (EGC) recently emerged as a promising alternative to resolve traditional concrete’s CO2 emission and brittleness. It features a cementless-based solution and possesses pseudo-strain-hardening (PSH) behaviour with high tensile ductility. There are two main obstacles associated with the use of geopolymer: firstly, the handling of user-hostile alkaline solutions and secondly, the necessity of heat curing. This paper aims to summarize the findings on the development and characterization of an ambient cured “one-part” EGC under the influence of various parameters, namely binder proportion between fly ash (FA) and ground granulated blast furnace slag (GGBS), quartz powder/binder ratio, alkali activator/binder ratio and water/binder ratio. Fresh and mechanical properties consisting of compressive strength, uniaxial tensile performance, and microstructure analysis were conducted. The results obtained indicate that increased FA content favours the attainment of PSH behaviour of the composite due to low matrix fracture toughness and fibre-matrix interfacial bond strength. Low water content and increased alkali activator content would greatly enhance the matrix toughness and fibre-matrix interfacial bond strength. The addition of quartz powder may favour strength attainment, but excessive quantities may have unfavourable consequences on the PSH behaviour.
Electromigration of electrokinetic nanoparticles in concrete structures is an effective approach to enhancing pore structure and retarding the transportation rate of chloride ions. However, there is limited research on the impact of stable positively charged electrokinetic nanoparticles on the corrosion behavior of reinforced concrete. In this paper, the structure of reinforced concrete was optimized by controllably Al2O3@SiO2 composite electrokinetic nanoparticles driven by a direct current electric field, and the influence of synthesis parameters, such as reaction time and temperature, on the modified silica sol was discussed. After 14 days of electromigration, spherical nanoparticles were found at depths of 5 to 10 mm inside the concrete. The porosity of the concrete reduced from 26.2% to 9.9%, with the most desirable pore size decreasing from 96.3 nm to 40.3 nm. After 9 months of dry and wet cycling, the electrokinetic nanoparticle-treated samples demonstrated a higher corrosion potential and a lower corrosion current density. Electromigration of electrokinetic nanoparticles slowed the transportation rate of chloride ions and significantly reduced their content in the concrete. Additionally, the Al2O3@SiO2 layer coating on the steel further enhanced its anti-corrosion ability, effectively inhibiting the corrosion of the reinforcement in concrete.
Crack-induced damage significantly affects the chloride transport mechanism in cementitious materials. To quantitatively evaluate the crack effects, a coupled model was proposed in this paper for saturated cement paste with various uniaxial tensile damage. First, the 3D microstructure of hydrating cement paste was simulated based on a voxel-based hydration model, and its damaged spatiotemporal distribution under uniaxial tensile stress was simulated using a finite-element model. Based on the damaged cement paste, an electrical Modelling framework was presented to simulate the chloride transport. The results show that the damage spatiotemporal distribution in saturated cementitious materials with uniaxial tensile and its chloride transport evolution can be successfully modelled using the self-created model and coupled method. the damage of cementitious material is an accumulation process of cracks spread over the main crack. During the process, the crack connectivity and crack width affect chloride transport and its fracture-volume threshold value is 1.40%. Compared to studies published, the coupled model could well simulate chloride transport in saturated cementitious materials with uniaxial tensile damage.
This paper focuses on the advanced design and production of concrete energy dissipators which are used for aeration of water on spillways. Traditionally, the concrete energy dissipators assumed simple block shapes which were dictated by limited production capabilities, mainly by the planar formwork and issues related to demolding. The advent of new concrete mix design and flexible construction technologies, such as 3D printing and CNC machines, allowed to consider more optimized organic shapes for increasing the energy dissipation efficiency and reducing the volume of material in our case by 38%. Since the physical laboratory modeling is an integral part of hydraulic research, a prototyping method using small-scale models was developed for design, production, quality control and experimental assessment of concrete energy dissipators of complex shapes. This method helps to expedite the research process when newly designed or modified shapes can be readily produced and tested in the hydraulic test channel for its energy dissipation efficiency and erosion resistance. The digital production, as all geometric information is perfectly digitized, at the same time helps to define the numerical models of the spillways for advanced continuum fluid dynamics analyses.
This study aims at experimentally and analytically characterizing cracking characteristics of Engineered Geopolymer Composites (EGCs) and at identifying optimal combinations of micromechanical parameters for enhancing the composite tensile performance. Fly ash-based EGCs with different volume fractions of polyvinyl alcohol fibers are investigated. The number of cracks and residual crack widths are measured for EGC specimens uniaxially loaded to 1% and 2% tensile strain. The observed crack patterns are analyzed by a micromechanics-based model that relates matrix, fiber, and interface properties to the macroscopic composite behavior. The experimental results demonstrate the lognormal distributions of crack widths and more tightly controlled cracking compared to Engineered Cementitious Composites. The simulated fiber bridging stress-crack opening relationship (σ-δ relationship) suggests that relatively high chemical bond and low frictional bond lead to the tight crack width. The simulation results also suggest that the first-cracking strength and the subsequent micro-cracking stress during the hardening stage should be below the analytical σ-δ curve peak. Higher chemical bond is beneficial for meeting these conditions, but if it is too high, fiber rupture dominates over pull-out, which lowers the complementary energy. Lower frictional bond or slip-hardening coefficient can suppress the fiber rupture tendency.
This paper presents the measurement of the tensile strength, Young’s modulus, and linear coefficient of thermal expansion in a meta-chert. Two areas are targeted in this aggregate - a white area where quartz grains are densely compacted and a gray area where the quartz grains are coarser. The macro- and micro-scale data are compared. Young’s modulus is not significantly different between macro-scale compression and micro-scale tension loading in the white region. Macro-scale splitting and micro-scale direct tensile strengths exhibit similar bimodal behavior due to the failure mechanisms of these two distinct areas. The macro- and micro-scale coefficients of thermal expansion are similar for the gray region, reflecting the quartz nature. Conversely, the micro-scale coefficient of thermal expansion of the white area shows a different behavior.
The involvement of expansion cracks in reducing compressive properties was experimentally evaluated. Concrete specimens deteriorated by delayed ettringite formation were subjected to three loading patterns (monotonic, stepwise cyclic and sustained loadings) and digital image correlation was performed to observe the behavior of expansion cracks during compressive loading. As a result, while significantly large plastic deformation was generated in the pre-peak, the reduction in compressive properties was hardly influenced by the loading patterns. The elastic strain, obtained from the loading hysteresis, increased linearly until a maximum load was reached. Consequently, two possible stress-bearing mechanism of concrete damaged by delayed ettringite formation under compressive stress was proposed to explain the development of elastic and plastic strains and the reduction in the compressive property.
Stone powder and others waste powder causes serious harm to the environment, society, ecosystem and human well-being, applying it to concrete is a path of green and effective. In this study, a series of tests was carried out on manufactured sand concrete with different granite porphyry powder contents. The results showed that when the stone powder content was lower than 8%, the cracking resistance, compressive strength, chloride ion permeability resistance, and sulfate attack resistance of the concrete were significantly improved with an increase in the stone powder content. Although the early shrinkage rate of the concrete also increased with the increase in the stone powder content, the increase rate was slow. At the microstructure level, an appropriate amount of stone powder exhibited the ‶filling effect" and "nucleation effect," which were conducive to the improvement of the concrete mechanical properties and durability. However, with an increase in the stone powder content, the early shrinkage of the concrete increased sharply when the stone powder content exceeded 8%, and the crack resistance, compressive strength, and chloride ion permeability resistance of the concrete decreased. At this time, the stone powder had a "diversified effect" on the microstructure of the concrete, and the existence of a large amount of stone powder was not conducive to the development of certain properties of the concrete. It is feasible to prepare concrete by replacing manufactured sand with an appropriate amount of stone powder and others waste powder, which can not only improve the performance of concrete, but also save cost, energy, avoid manpower and material consumption in waste powder disposal. The research conclusion shows that the allowable value of the stone powder content can be appropriately increased according to the actual application in concrete, which is an efficient, green disposal path for stone powder and other waste powder.
Use of Engineered Cementitious Composites (ECC) has been proven to enhance structural fatigue resistance and reduce the use-phase emissions for transportation infrastructure. Carbonation curing offers an opportunity to reduce the embodied carbon of ECC via direct CO2 sequestration. In this study, the impact of carbonation curing on ECC’s fatigue resistance was examined. ECC’s CO2 uptake, static flexural behavior, flexural fatigue performance, and single fiber pull-out behavior were studied experimentally. Midspan deflection up to 3 million cycles under fatigue load, fatigue stress-life relationship, and failure mechanism for carbonation-cured and air-cured ECC were investigated. Carbonation curing was found to significantly improved the fatigue life of ECC and lowered the midspan deflection under the same stress. Further, CO2-cured ECC can achieve >20% CO2 uptake per cement mass after 24-hour carbonation curing. Carbonation curing increased ECC’s flexural strength by 32% and promoted crack width control capability, with maximum post-fatigue crack width reduced from 148 μm to 76 μm. The positive impact of carbonation curing on the fatigue behavior of ECC simultaneously lowers the embodied and operational carbon of ECC structural members subjected to fatigue loading during service.