Journal of Advanced Concrete Technology最新文献

In recent advancements, a novel strengthening approach employing engineered cementitious composites (ECC) and fibre-reinforced polymer (FRP) materials has emerged. This method integrates ECC as the matrix, carbon fibre-reinforced polymer (CFRP) grid as the internal strengthening component, and epoxy resin for bonding the overlay to the concrete substrate. This study conducted tests on four reinforced concrete (RC) beams under a four-point load configuration. One beam served as an un-strengthened control specimen, while three were subjected to different shear strengthening methods: polymer-modified mortar (PMM), ECC, and CFRP grid-reinforced ECC matrix composites layer (FGREM). The investigation covered failure modes, load-deformation relationships, and load-strain relationships. Finite element (FE) analysis was employed to reproduce the test results. Key findings include the ability of ECC as a matrix to substantially reduce concentrated interfacial bond stresses, preventing debonding failure. The FGREM-strengthened specimen exhibited a failure mode characterized by side concrete cover separation, resulting in a notable 124% enhancement in shear resistance. The proposed FE model, incorporating interfacial behaviour, accurately simulated the performance of all specimens.

In this study, the well-established service life design code defined in fib Bulletin 34 is adapted to predict the time to initiation of reinforcement corrosion in alkali-activated concretes submerged in marine conditions. The model approach is based on the probabilistic calculation of the time needed for a critical concentration of chloride ions, migrating from the external environment towards the rebar, to accumulate at the surface of the steel reinforcement and initiate the corrosion reaction. The information required to define the parameters of the model is derived from literature data, relating the concrete mix designs with accelerated laboratory test results. The findings indicate that alkali-activated concretes with high calcium content can exhibit promising characteristics as a construction material applied for structural application in chloride-rich corrosive environments. The probabilistic approach adopted in this model provides the opportunity to assess the influence of variability in mineralogical composition and reactivity of the precursors with the alkaline activating solution, that influence the chemical evolution and microstructure of the binder matrix. The predicted service life is quite sensitive to these factors, with very high service lives predicted for some alkali-activated concretes but rather short service lives predicted for others, and this must be incorporated into any engineering assessment of future material performance.

Air voids in concrete are sometimes observed to be blocked by white precipitates. In this study, the authors conducted freeze-thaw tests on concrete mixed with lime-based expansion material and then performed microscopic observations using scanning electron microscopy, elemental analysis using energy-dispersive X-ray spectroscopy. The size distributions of air voids in the concrete during the curing period (28 days after placement) and before, during, and after the tests were also measured in order to investigate the effects of the expansion material on air-void blockage as well as the underlying processes. During the curing period, no air-void blockage was observed. However, as the concrete aged, precipitates formed on the inner surfaces of the air voids. During the freeze-thaw tests, the number of air voids with diameters of ≤0.15 mm decreased, and the air-void spacing factor increased as the relative dynamic modulus of elasticity decreased. It was found that freeze-thaw cycles caused calcium-based precipitates to form inside the air voids of concrete mixed-with lime-based expansion material, and that the air voids became blocked.

This paper discusses the hydration process of Portland cement (PC) and Calcium Sulphoaluminate cement (CSA) composite material at the early age. The setting time and mortar strength were tested, the hydration process was analyzed by isothermal calorimetry tests, and the hydration products were characterized by XRD, SEM analysis. The results show that: CSA can significantly improve the hydration rate of ultra-early age (2 h and 6 h) and improve the mechanical properties. But there are still interactions between the hydration of PC and CSA, including that the gypsum in CSA will inhibit the hydration of C₃A in PC and the CH generated by PC will promote the rehydration of AH₃. The inhibiting or promoting effect of CSA on hydration process changes with its content. The design method of PC-CSA composite material was proposed, and the reasonable mixing amount of PC or CSA was also suggested.

Due to lightweight, high-strength, and high toughness properties, Basic magnesium sulfate cement (BMSC) can be widely used in producing various structural and decorative products. Most current research only focuses on the performance of BMSC under room temperature. However, low temperature environments may have a serious impact on the performance of BMSC, such as during cold seasons or in salt lake cold regions with abundant magnesium resources, so the performance of BMSC in low temperature environment needs to be explored to expand its application. This study investigated the influence of different low temperatures (−5 to 20°C) on the compressive strength and water resistance of BMSC cement. The phase composition, microstructure and pore structure of BMSC has been analyzed using XRD, QXRD, TG/DTA, SEM, BSEM/EDS Mapping and MIP. The Results indicate that the low hydration rate, precipitation of magnesium sulfate, increase in porosity and decrease in 517 phase content are the main reasons for the low strength of BMSC under low temperature curing. Although hardened BMSCs cured at low temperatures can continue to hydrate in water to form strength phases, corrosion and cracking may occur. Replacing light burned magnesite with high active magnesium oxide can significantly improve the mechanical properties of low temperature cured BMSC. However, low temperatures reduced the crystallinity of the 517 phase, thereby increasing its water solubility and lead to poor water resistance of BMSC.

Although engineered cementitious composites (ECC) exhibit tensile strain hardening and multiple microcracking characteristics, whether the performance of ECC changes obviously under freezing-thawing conditions is important for the design and maintenance of buildings in the areas with freeze-thaw exposure. Results indicates that, as the number of freeze-thawing cycles increase, the PVA fiber/matrix interfacial bonding strength decreases, more fibers can be pulled out, thereby resulting in an increasing deformation on ECC. The results of water capillary absorption, chloride penetration and carbonation on ECC reveal that the frost damage has little effect on ECC. In addition, the steel bar-normal concrete interfacial ultimate bonding strength decreases linearly with the increase in the number of freeze-thawing cycles, the specimen splits failure. However, for the ECC, the steel bar-ECC interfacial ultimate bonding strength decreases with the increase in the number of freeze-thawing cycles, the pull-out failure occurs.

This paper investigates the mechanism of the corrosion pattern of wires on the prestressing forces of prestressed concrete (PC) strands. A rigid frame testing machine combined with an electrically accelerated corrosion device was used to test 16 steel strands. Specimens subjected to the tensile load of 50% of the tensile strength were divided into three groups according to the target corrosion level (mass loss). The rigid frame testing machine can remain the prestressing force, and the force and deformation were measured continuously during the corrosion test. The prestress was degraded in two stages; the beginning and development of the second stage being related to the maximum cross-sectional area loss and fracture type of the wires, respectively. The prestress loss of the strands was the result of the internal force balance after the deterioration of the tensile properties. It was found that the deterioration process of the ultimate tensile capacity related to the fracture type of the wire. The fracture type of PC strand was influenced by the shape and the distribution of the cross-sectional area loss of corroded wires.

Red mud (RM) is a hazardous waste generated by aluminum production. It is difficult to utilize due to its high aluminum and iron oxide contents, high alkalinity, and large specific surface. Still, extensive research is underway to explore its potential as partial replacement for cement in concrete. Due to the differences in the physical, chemical, and mineralogical characteristics of bauxites from different sources, the associated RMs are also different. Some studies have reported that unless calcined, RM produced by the Bayer process has negligible pozzolanicity. However, the appropriate calcination temperature is not unique as it will depend on the RM mineralogical composition. Here, the calcination mineralogical composition nexus and its effect on RM pozzolanicity are investigated in three types of RM produced by Bayer’s process. The RMs were calcined at 600, 800, and 1000°C for 2 hours, and were used as 15 wt.% replacement for Portland cement in mortar mixes. One of the RMs exhibited pozzolanicity without calcination while another showed increased reactivity after calcination at 800°C. The underlying mechanisms are discussed, and it is concluded that no specific calcination temperature(s) can be recommended to activate every RM. Contrary to the findings of previous studies, one of the investigated RMs, used in its virgin form at 15 wt.% replacement for cement, exhibited noticeable pozzolanic activity and achieved over 94% of the compressive strength of the control specimen at 91 days. The calcination of the same RM, irrespective of the calcination temperature, reduced its pozzolanicity.

Capturing atmospheric CO₂ into cement-based materials is a way to set off the CO₂ emissions of concrete production. This study proposes an experimental method to track the origin of cement paste that fixes CO₂ directly from the air under natural conditions. By exposing powders of well-hydrated cement paste to air, carbonated cement paste powders are obtained with different carbonation degrees. The inorganic carbon of these carbonated samples is extracted by dissolution in phosphoric acid, and the isotopic characteristics related to ¹³C and ¹⁴C are measured. The experimental results show that the ¹⁴C value of carbonated cement paste can be used as an indicator for tracing the origin of carbon.

The aging and damage of concrete buildings and structures is a problem in modern society. This is especially true for nuclear power plant buildings, which are required to have high safety standards. In this study, molecular dynamics simulations were performed to obtain mechanical properties for silicate minerals, including quartz, which is used as an aggregate in concrete. We also attempted to clarify phenomena including mechanical fracture. Mechanical properties of each mineral (Young's modulus, Poisson's ratio, and maximum stress) were obtained by performing tensile simulations on 10 silicate minerals which are α-quartz, Orthoclase, Microcline, Albite, Oligoclase, Andesine, Labradorite, Augite, Diopside and Forsterite. Minerals other than α-quartz were highly anisotropic with respect to Young's modulus. The maximum stress was highest for α-quartz, but once a fracture started, the development of large fractures progressed at once and the stress relaxed rapidly. Deformation and fracture of the mineral in response to strain were analyzed by extracting the non-affine component of the local displacement of atoms in tensile simulations. This analysis was able to explain the behavior of the stress-strain curve for each mineral. We also investigated how the composition of a mineral affects its mechanical fracture.