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.

Recently, novel types of shear connectors have been investigated enthusiastically in Europe. Among the new connector geometries, clothoid-shaped shear connectors are used practically for bridge engineering. Although various schemes for evaluating mechanical performance can be used, they were established based on the pushout test database. During their time of use, the concrete slab is expected to be subjected to fully reversed cyclic stress during an earthquake. The stress history thereby differs from those of standard pushout tests, resulting in the discrepancy from the evaluation equations. To address this concern through this study, cyclic loading tests were conducted, imposing compressive and tensile stresses cyclically. A comprehensive experiment using 14 specimens with various influential parameters revealed the cyclic behavior and stress transfer mechanism. The results confirmed that the mechanical capacity of a composite dowel with a clothoid-shaped shear connector is dependent on the stress orientation. Furthermore, the applicability of earlier evaluation formulae was clarified based on established knowledge of experimentally obtained data. Because the necessity for reflecting the stress history was demonstrated, this research newly presents a formula for evaluation of the ultimate shear strength and load-displacement relation.

In seismically retrofitted reinforced concrete (RC) structures, new members are connected to existing members using interfaces with roughened surfaces and post-installed dowel bars. During an earthquake, the interfaces are subjected to cyclic shear and normal stresses. Therefore, the structural design of interfaces is essential. In a previous study, normal and shear loads were applied to interface specimens. Moreover, a shear strength estimation method was proposed; however, the cyclic behavior was not discussed. This paper presents a cyclic model of the roughened surface in retrofitted RC structures. First, an envelope model was constructed using the previous shear strength estimation method and Saenz model, a constitutive law of concrete. Subsequently, the cyclic rules were modeled. Moreover, a previous dowel model was incorporated into the proposed cyclic model to estimate the test results. Finally, the proposed model provided reasonably estimated test results; the average ratio of the test results to the model results was 1.04. In addition, as δ increased, the effect of the dowel bar intensified in terms of shear.

The Fukushima Daiichi Nuclear Power Plant lost its core cooling function due to the massive tsunami generated by the 2011 off the Pacific coast of Tohoku Earthquake, which caused core meltdown, resulting in high temperature inside the containment vessel and exposing the RPV pedestal, a reinforced concrete structure, to an unusually high temperature environment. After the earthquake, water was poured into the containment vessel to cool the molten core, and the con-crete structure was gradually cooled in the process. Since it will take at least 40 years to remove the fuel from the core, the integrity of the RPV pedestal is a major concern for the decommissioning of the Fukushima Daiichi Nuclear Power Plant. In order to assess the long-term integrity of the RPV pedestal, a horizontal loading test was conducted using a 1/6 scaled model of the RPV pedestal of Unit 1 considering the effect of the high temperature heating and subsequent wet conditions. And then, the static stress analysis of the RPV pedestal was performed considering the degradation phenomena revealed by the experiments. As a result, it was confirmed that the RPV pedestal of Unit 1 would be structurally sound for 40 years against the current design basis earthquake even if the material degradation due to severe accident and aging was considered.

Polymer modified mortar (PMM) is indispensable for repairing and reinforcing concrete structures owing to its excellent adhesion to concrete, compactness, and workability. However, PMM tends to spall when exposed to high temperatures because it contains organic polymers. In this study, a ring-restrained heating test was performed on normal cement mortar and PMM mixed with three types of polymers to investigate the factors that affect fire spalling, such as differences in fire spalling magnitude, restraint stress, and water vapor pressure. Furthermore, a tensile strain failure model based on thermal stress theory was used to evaluate temporal changes in fire spalling depth.

This study presents the effect of corrosion of prestressed concrete (PC) strands on the prestressing force and the reduction mechanism of the uniaxial force. A total of 14 strands was tested by combining a rigid frame testing machine with an electrified accelerated corrosion device. The specimens were divided into two groups having prestressing levels of 70% and 50% of tensile strength of PC strands. The expected degree of corrosion of the specimens, which was defined by the mass loss, was calculated from controlled electric current and time. A rigid frame testing machine was used to sustain the tensile force of the PC strands, and the prestressing force and deformation of the strand were continuously measured during the corrosion test. The test results indicate that the prestressing force decreases with the increase of corrosion. After the corrosion tests, tensile loading tests were carried out on the specimens that did not rupture during the corrosion test. It was found that corrosion led to the deterioration of the tensile properties of the PC strands, and the ultimate tensile capacity of the corroded PC strand was related to the fracture condition.

This paper explains the experimental research on fracture properties of concrete using TiO2 nano powder (NT) (1, 2, 3, 4%) and fly ash (FA) (10, 20, 30, 40%). Three point bend tests were conducted on 153 notched beams in a closed loop servo control machine with crack mouth opening displacement (CMOD) control and opening rate of 0.0005 mm/s. A total 17 mixes were prepared with combination NT+ FA with the above percentages. Work of fracture method (WFM) and size effect method (SEM) was used to analyze fracture parameters. The results for WFM and SEM show that fracture energy, fracture toughness and brittleness number increase with increase in NT and FA percentages, while the characteristic length decreases. In each mix varying from NT1FA10 to NT4FA40, the fracture energy increased by 20.92% and fracture toughness increased by 34.61%. The mix with 4% NT showed significant improvement of 38.46% for fracture toughness and 20.93% for fracture energy. The mix indicating brittle behavior as the characteristic length (Lch) decreases with increase in percentage of NT and FA and the length of fracture process zone (CF) decreases with increase in percentage of NT and FA which indicates the brittle behavior of material.

The infrequent setting behavior and hydration process of a Portland cement with very low alkali content were concerned. Gypsum and K2SO4 with different quantities and ratios were added into cement to adjust its sulfate and alkali content. The setting time and compressive strength of cements were tested, the hydration process and hydrates of cements were investigated by isothermal calorimetry, non-evaporable water content and XRD. The results show that gypsum addition is incapable of delaying the quick initial setting of the cement clinker. A moderate substitution of gypsum by K2SO4 in same reasonable sulfate content can promote the dissolution of gypsum, suppress the formation of h-AFm, and therefore prolong the initial setting time of the cement. Moreover, the Portland cement clinker shows a very hysteretic hydration exothermic process, accompanying with a hysteretic finial setting and a weak mechanical property in early age. An appropriate dosage of gypsum can promote the hydration of the cement and result in a reasonable final setting behavior and satisfactory strength development. The modification of gypsum on the properties of cement is influenced by its alkali content. The measures which could accelerate the sulfate-supply in cement is necessary to acquire a favorable hardening property when the cement with low alkali content is used in projects.

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.

Interactions between Cl-, SO42- and cementitious materials, reinforcement passive films influence the durability of reinforced concrete structures. Transport of three solutions (NaCl, Na2SO4, mixed) in calcium silicate hydrate (C-S-H) gel, γ-FeOOH nanopores was investigated using molecular dynamics. Solution transport in γ-FeOOH nanopores is slower than in C-S-H gel nanopores because of the lesser hydrophilicity of γ-FeOOH surface. SO42- can form ion clusters to hinder the solution transport and atomic motion, and the ion clusters appear in the solution more frequently than at the interface. Temporary adsorption of Cl-, SO42- on substrate surfaces occurs during transport because of Ca-Cl, Ca-SO4 ionic bonds on the C-S-H surface and Ho (hydroxyl hydrogen atoms) -Cl, Ho-SO4 hydrogen bonds on the γ-FeOOH surface, and these bonds are influenced by the local structure. Two substrates interact with water, Cl-, SO42- via distinct microscopic mechanisms.

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 study evaluates the impact of incorporating polypropylene fibers on the endurance of 100% ground granulated blast furnace slag (GGBFS) based alkali-activated composite (AAC) exposed to elevated temperatures. In addition, the impact of critical variants, like polypropylene fiber volume fractions, fiber length, alkaline solution monomer ratio, and curing regime, on the overall performance of the fibrous composites under high temperatures was evaluated. While the polypropylene fiber content was varied at 0, 0.15, and 0.3%, two polypropylene fiber lengths, 12 and 20 mm, were examined to assess their influence on the composite thermal stability. The alkaline activating solution monomer ratios (Na2SiO3/NaOH, denoted as SS/SH ratio) was varied at 2.5 and 3.0, and two different curing regimes were adopted; the ambient curing regime, with temperature 24±2°C and 65% RH, and the heat curing regime, with 24 h heat curing in the oven at 65°C, for all mixes. The specimens were subjected to high-temperature effects at 150, 300, 600, and 800°C, and the results of residual compressive strength, residual density, and ultrasonic pulse velocity (UPV) were examined, before and after the exposure, to assess the impact of high thermal conditions. In addition to the visual inspection, SEM analyses were carried out to evaluate the effect of different parametric variances on the microstructure properties of the fibrous AACs under high-temperature exposure. The fibrous composite variants with short (12 mm) polypropylene fibers length and most of the variants with 0.15% long (20 mm) polypropylene fibers exhibited high thermal stability and better residual compressive strength than the equivalent control non-fibrous mixes when exposed to temperatures of 150 and 300°C. Further, the increase in alkali solution monomer ratio (SS/SH) from 2.5 to 3.0 has negatively influenced the overall thermal resistance of composites across all variants.

Concrete aggregate identified as “meta-chert” was irradiated with gamma-rays and neutrons. To identify the volume expansion of the aggregate under neutron irradiation, the following analyses were performed for pristine and irradiated α-quartz and meta-chert: X-ray diffraction (XRD)/Rietveld analysis, dimension change, water pycnometry, He-pycnometry, light optical microscopy (LOM), and scanning electron microscopy (SEM). From the difference of volume expansion observed from dimension change and water / helium pycnometry, the crack opening inside the aggregate subjected to irradiation was elucidated, and this was confirmed by LOM and SEM analysis. The crack contribution to the expansion of the aggregate was significant for neutron fluence > 6.99 × 1019 n/cm2, for E ≥ 0.01 MeV. Based on the XRD analysis, changes in lattice parameters were identified and the cell volume expansion was compared with the data by helium pycnometry. Based on the density change calculation and phase calculation data, the density of X-ray amorphous phase was consistent with that of expanded crystal α-quartz.

To investigate the axial load response of reinforced concrete (RC) columns confined by carbon textile-reinforced concrete (CTRC) under chloride attack, 24 CTRC-confined square columns and 12 unconfined columns were tested under axial load, while considering the influences of dry-wet cycles, textile ratios, stirrup ratios, and section sizes. The experimental results indicate that the corrosion resistance and compression performance of RC columns under chloride ion erosion were significantly improved by CTRC. The corrosion of the RC column with CTRC confinement was remarkably reduced with a maximum of 63.9% in the chloride salt environment. The maximum increments of bearing capacity and ductility of CTRC-confined columns were 30.9% and 87.7%, respectively, compared with unconfined columns. In addition, bearing capacity, ductility, and deformation energy were also affected by stirrup ratios and section sizes. Finally, the semi-empirical and semi-theoretical analytical models of the stress-strain relationship and axial-bearing capacity were proposed based on the experimental data and theoretical analysis. The compound confinement effects of CTRC and corroded stirrups on core concrete was considered in the proposed models. The models correlated well with the experimental data.

Ground granulated blast-furnace slag (GGBS) can be used as the hydraulic phase in alkali-activated slag cement (AAS) or as partial replacement in blended cements, whose storage conditions need to be fully assessed before practical applications. This paper tries to address this problem by storing the GGBS at the controlled relative humidity (RH) using saturated solution methods at 20°C. Followed by performing the dissolution experiments in both deionized water and NaOH solution on the obtained GGBS, where both aqueous and solid phases are characterised. The experimental results indicate that the GGBS stored in a high RH will depress the dissolution of silicate networks, thus suppressing the formation of C-S-H phases, especially in the NaOH solution. The dormant period of GGBS hydrated in DI water is positively correlated to the storage RH. From the degree of hydration point of view, the RH less than 60% should be recommended for storing GGBS. However, if the faster hydration of GGBS is the purpose when it is used as an SCM or in neutral salt activated cements, an extreme low RH should be recommended. It is also found that the increased storage RH shows the potential for increasing the inner products of hydrated GGBS.

In this paper, a simplified model is proposed for the shear strength of short shear walls based on the original three-parameter kinematic theory (3PKT). The model is built on first principles – compatibility of deformations, constitutive relationships and equilibrium – and aims to combine simplicity and accuracy for structural assessment applications. The model focuses on shear failures along diagonal cracks, while other failure modes such as sliding shear, out-of-plane instability, or detailing/lap splice failures need to be evaluated separately. The simplified 3PKT is validated with 29 specimens with a wide range of properties and is compared to the ASCE (ASCE 2014) and Japanese (AIJ 2001) seismic code shear provisions. It is shown that the model captures well the effect of all key test variables, and significantly reduces the conservatism and scatter of the code strength predictions. It is also shown that the proposed approach can be particularly helpful in the assessment of structures with less-than-minimum shear reinforcement to avoid costly and disruptive strengthening interventions.

This paper aims to verify a space-averaged electric filed simulation, where the whole surfaces of reinforcing bars are blended into the 3D finite volume for identifying the electric potential to drive the macro-cell corrosion of structural concrete. Experimental verification is conducted with transparent pseudo-concrete, which has chemical pore-solutions similar to those of concrete, and with which the location of anodic poles accompanying brownish rusts and the cathodic ones surrounded by hydrogen bubbles may be visually identified. The proposed electro-chemical analysis platform to be integrated with ion transport and equilibrium is adopted for full consistency with dispersed reinforcing bars. Global macro-cell corrosions of tunnel mockup and reinforcement layers induced by electric current leakage are verified quantitatively with dispersed multi-ion concentration as well as corrosion profile.

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.

The tension stiffening behavior of reinforced concrete (RC) prisms, affected by the aggregate volume expansion induced by neutron irradiation, were numerically investigated using a rigid body spring network model. First, the model was validated by comparison with the uniaxial tension test results of wet- and dry-cured (with volume contraction of concrete) RC prisms. Subsequently, different degrees of expansion strain were applied to the aggregate elements in the RC prism model and the uniaxial tension loading was simulated again. Tension stiffening decreased under larger radiation-induced volume expansion of the aggregate owing to the corresponding decrease in the concrete tensile strength with increasing damage, this behavior changed considerably according to the restraint condition. Indeed, the Young’s modulus of the restrained concrete after aggregate expansion was larger than that of the unrestrained concrete after aggregate expansion. However, the compressive stress in the concrete after aggregate expansion was effectively transmitted to the rebar during uniaxial tension loading; this behavior indicated that RC could maintain its integrity under uniaxial tension even after 0.5% aggregate linear expansion.

Reinforced concrete (RC) hollow cylindrical members were numerically investigated to understand the impact of the radiation-induced volume expansion of aggregates on the seismic performance of the member. The rigid body spring network model was used in this analysis, and the proposed constitutive laws and used parameters were validated by comparing two horizontal loading experiments for two RC members: a reference experiment and one in which a temperature gradient developed in wall of the members. The resultant volume expansion of concrete was confirmed. After validating the methodology, different degrees of aggregate expansion strain were applied to the aggregate elements considering the reduced temperature and neutron fluence distribution inside the wall, assuming the real size of the biological shielding concrete. The RC members were then loaded horizontally. It was confirmed that the stiffness and maximum bearing capacity decreased slightly with an increase in the neutron fluence, and the deformation at the maximum bearing capacity increased slightly. Based on the rigid body spring network model calculation results, a simplified analytical model that can reproduce the shear deformation–horizontal load relationship was proposed based on the model proposed by Inada (1987).