Materials and Structures最新文献

An experimental investigation was conducted at Swinburne University to study the effects of fiber hybridization on the direct shear behavior and mechanical properties of ultra-high performance fiber reinforced concrete (UHP-FRC) members exposed to elevated temperatures. The effect of two key variables on the direct shear performance and mechanical properties of UHP-FRC were investigated. The key variables include: the type of fiber hybridization (hybrid of either hooked end steel fiber (HESF) and straight steel fiber (SSF) only or with jute fiber (JF) as well), and temperature applied on the specimens (room temperature, 200 °C, 400 °C and 600 °C). The direct shear stress—slip performance, ductility and toughness were determined. Furthermore, the thermal–mechanical properties of UHP-FRC such as residual direct tensile strength, residual compressive strength, spalling resistance and mass loss were investigated. Scanning electron microscope micrographs were used to evaluate the microstructure of UHP-FRC matrix reinforced with different types of fiber and the morphological alteration in the surface of JF under high temperatures. The tests results indicate that the residual direct shear stress decreased significantly after exposure to high temperatures. Moreover, thermal spalling occurred for specimens subjected to 600 °C and reinforced with hybrid of HESF and SSF only. While, using hybrid of HESF, SSF and JF was an effective method to prevent the thermal spalling under 600 °C and enhance the residual direct shear stress of UHP-FRC. Preliminary empirical expressions have been developed in this study to predict the shear transfer capacity of UHP-FRC as a function of compressive strength, steel fiber content, natural fiber content and temperature.

Carbonation-induced reinforced steel concrete corrosion is a prominent concern related to engineering design and maintenance. The Durability Index (DI) approach was developed in South Africa to address this concern and enhance the durability performance of reinforced concrete structures. This approach relies on durability index tests, which are associated with transport mechanisms linked to specific deterioration processes. The carbonation of concrete is primarily influenced by the microstructure and transport characteristics of the concrete. Environmental exposure conditions also influence the rate of carbonation. The focus of the research reported here was to develop a carbonation model that could predict the rate of carbonation of concrete exposed to, or sheltered from, rain, with the permeability coefficient (k) from the Oxygen Permeability Index (OPI) test (DI test) as the key unifying variable. The model development was based on natural carbonation data and the drying profiles (experimentally measured) of 48 different concretes. Concrete microstructure was varied by varying the water-to-cement ratio, curing conditions, and by using SCMs. The resulting carbonation model was able to predict the rate of carbonation of concrete, allowing for different exposure conditions. A unique feature of this model is its use of a single material property, the 'k' value, to effectively address both CO₂ diffusion and the drying process within concrete. The model displayed sensitivity towards the influence of variation in CO₂ concentration, concrete microstructure, and the environmental exposure conditions, making this a simplified, effective and practical concrete carbonation prediction model.

This review by Working Group 1 of the RILEM TC 309-MCP discusses recent advances in the beneficial carbonation treatment of recycled concrete aggregates (RCA). The impact of carbonation on RCA properties as well as the microstructure and performance of concrete and other construction materials made thereof is critically reviewed. The increasing focus on environmentally friendly building practices has led to a greater interest in the CO₂ uptake associated with carbonation processing. Furthermore, emphasis is placed on the importance of adopting tailored strategies to optimise the carbonation process based on the quality and type of RCA. Evidence in the literature highlights the beneficiation potential of carbonation processing in improving RCA properties and performance, which translates in variable degrees of enhancement of the performance of concrete or otheinitial; position: relative; float: left; top: 0px; left: 0px; z-index: 1 !important; pointer-events: none;"/>er applications made thereof. The review concludes that, to date, significant techno-economic challenges remain to be addressed to improve the competitiveness of the technology, notably in terms of upscaling and refining life cycle assessment data.

This study systematically investigated the effects of different dosages of an air-entraining admixture (616 AEA) on the fresh and hardened properties of shield synchronous grouting materials. An appropriate 0.075% dosage significantly reduced apparent density by 7.4% and increased air content by 613.3%, while maintaining good flowability and anti-segregation. XRD and thermal analysis revealed the AEA did not alter hydration product composition/formation, indicating its mechanism was introducing air bubbles to modify the pore structure. At 0.075%, an ideal multi-scale pore structure formed, with 28-day of 7.7 MPa and water-to-land strength ratio of 89.6%, meeting engineering requirements. Advanced techniques like low-field NMR, mercury intrusion porosimetry, and ultra-depth imaging comprehensively characterized pore structure evolution, consistently demonstrating increased porosity and larger pores with higher AEA dosages. The findings elucidate the underlying mechanism by which the AEA optimizes the pore structure, providing guidance for mix design optimization and enhancing comprehensive performance for sustainable underground construction applications.

In this work, a novel electromagnetic wave absorption geopolymer was created with efficient utilization of two solid wastes iron tailings and blast furnace slag, meanwhile the mechanism and impacts of iron tailings content, water–solid ratio and specimen thickness on their electromagnetic wave absorption and mechanical property were systematically investigated. It was found the pores, micro-cracks and unreacted particles in the specimens are benefitial for electromagnetic wave absorption but against to compressive strength, even though the compressive strength reaches 69.7 MPa with the iron tailings content 50% and water–solid ratio 0.4. The addition of iron tailings significantly enhances the electromagnetic wave absorption properties of the geopolymers, and increasing iron tailings content improves the number of pores, micro-cracks and the permeability of geopolymers. The electromagnetic wave absorption properties of the geopolymer initially increase and then decrease with the increase of specimen thickness and water–solid ratio. With an iron tailings content 70%, water–solid ratio 0.4 and thickness 30 mm, the effective absorption bandwidth (< -5 dB) was optimized to 10.44 GHz with a minimum reflection loss of −13.34 dB. A new mechanism for electromagnetic absorption in iron tailings has been proposed, in which the electromagnetic wave absorption of geopolymers is mainly dominated by magnetic loss and spatial propagation loss. This study provides higher competitiveness and comprehensive utilisation of iron tailings in the field of electromagnetic wave absorbing building materials, and has great potential for applications in military and other fields affected by high electromagnetic wave frequencies.

It is important to fully restore the performance of aged styrene–butadiene–styrene (SBS) modified bitumen (SMB) and reduce its construction temperatures in high-value recycling the waste SMB mixtures. This study aims to assess the performances regeneration and warm-mixing effects on aged SMB by using a compound rejuvenator, i.e. reactive warm-mix rejuvenator (RWR), which consisted of rubber oil, epoxy-terminated polybutadiene ether, cardanol (CA), modified polyethylene (PE) wax and antiaging agent. Two commercial rejuvenators were employed to compare with the RWR. The rejuvenating effects were evaluated through frequency sweep test, multiple stress creep test, cracking temperature test, linear amplitude sweep test, and chemical and morphological structure analyses. The warm-mixing effect was explored by the rotational plate viscosity test. Results indicate that RWR can react with oxygen-containing functional groups on broken molecular chains of SBS polymer with the catalysis of triethanolamine, which enables fractured crosslinking network structure to be repaired successfully. Meanwhile, light components supplied by RWR is able to restore the bitumen matrix of aged SMB to a similar level of original SMB. The RWR is able to effectively restore the viscoelasticity and plateau region of phase angle of aged SMB to the level that is mostly close to that of original SMB, while those two commercial rejuvenators are barely satisfactory. When the RWR content is 12%, the rejuvenated SMB exhibits the satisfactory high- and low-temperature performances, and the better fatigue resistance ability by comparing with original SMB. The modified PE wax in RWR has a lubrication effect on the interaction between macromolecular polymer chains, which gives rise to significant reduction in construction temperatures of rejuvenated SMB incorporating the RWR.

The chemical basics of the ASR are largely revealed and widely accepted, but the nature of the expansion mechanism is still not yet sufficiently well understood. Recent observations showed that ASR products could be considered as colloidal systems. In order to clarify if and to what extent this is the case and whether it could help to better understand the nature of the ASR products and the mechanism of ASR expansion in concrete, 10 ASR products of different composition, water content and synthesised at two temperatures (40 and 60 °C) were investigated over a period of 1.5 years. The ASR products were studied by means of NTA, SEM, ²⁹Si NMR, XRD and an osmotic cell test. The results show that ASR products contain particles of colloidal size, mainly between 50–600 nm and of different shape. The particles are unable to pass pores with a size smaller than themselves what represents a mechanism of semi-permeability in all concrete constituents with respective pore sizes, resulting in the Donnan effect and osmosis. The particles are irreversibly linked by the addition of Ca, which leads to a decrease in the particle concentration, the formation of crystalline phases and thus to a decrease in the osmotic potential of the ASR products. Based on the colloidal nature of the ASR products, expansion caused by ASR in concrete can be explained osmotically.

Concrete carbonation has been proven to be a potential path for reducing the carbon footprint of cement industry. However, since carbonation reaction significantly alters the chemical composition and microstructure of cement-based materials, it is necessary to carefully assess its effects on the transport properties and durability of concrete materials. The goal of this work is to clarify the effects of accelerated carbonation on both the pore structure and long-term water absorption behavior of cement-based materials using CEM II/B-M (T-LL) as the binder. Experimental results show that exposure to (text {CO}_{2}) at a concentration of over 65% for 90 days leads to substantial carbonation of (text {Ca(OH)}_{2}) and other calcium-bearing phases including C–S–H gels. Accelerated carbonation results in a refined pore structure of cement paste, marked by decreased porosity but increased specific surface area accessible to both (text {N}_{2}) and (text {H}_{2}text {O}). The long-term capillary absorption of non-carbonated mortar observes the square root of time law in the initial stage and then markedly deviates down, which can be well captured by the modified Richards equation accounting for water sensitivity. In contrast, the long-term absorption into carbonated mortar consistently follows the square root of time law, which could be quantified using the conventional Richards equation. This suggests that after accelerated carbonation, the pore structure of cement mortar is less sensitive to water regain, potentially attributed to the changes in the nanostructure of C–S–H gels caused by carbonation. Additionally, carbonated mortar exhibits lower sorptivity and inherent permeability than non-carbonated mortar, indicating that accelerated carbonation decelerates the water transport in cement-based materials.

This study investigates the influence of Plain and 0.1% NanoFibrillated Cellulose (NFC) modified repair mortar mixtures on the bond strength and flexural performance of composite and monolithic beam specimens. First, the effect of the NFC on the slant shear and flexural bond strength of repair mortar overlays was assessed. Thereafter, repair mortar thicknesses ranging from 25 to 50 mm were overlaid on concrete substrates, and flexural strength and toughness of specimens were evaluated. Furthermore, the effect of hybrid combination of the NFC and steel macro fiber on the flexural toughness and strain evolution of monolithic Fiber Reinforced Concrete (FRC and FRC + 0.1% NFC) beams were also evaluated. Test results showed that the NFC enhanced the slant shear and flexural bond strengths of repair mortar by about 35% and 43%, respectively. Flexural strength capacity of composite beams generally increased as the repair mortar thickness was raised from 25 to 50 mm. Relative to the single-layer FRC beam, concrete substrate overlaid with 50 mm thick Plain + 0.1% NFC mortar showed about 56% increase in flexural strength. However, a post-crack toughness superior to that of the single-layer FRC beam was only achieved using 25–35 mm thick Plain + 0.1% NFC repair mortar as overlays. Furthermore, with the combination of NFC and steel fiber as discrete reinforcements in FRC, the bending strength, static modulus and toughness of beams were also enhanced.

In response to the ever-evolving demands of end-users within the construction sector, also due to the heightened global awareness regarding the pivotal role of the construction industry in sustainability ramifications, it has become imperative to wield strategic tools to steer the market toward farsighted choices. A notable example is represented by innovative cementitious materials, which are progressively captivating market interest due to their potential for enhanced overall sustainability performance. Henceforth, a crucial role is played not only by sustainability evaluation tools like Life Cycle Assessment (LCA) and Life Cycle Costing (LCC) analyses but also by the integration of the latter into a more comprehensive approach able to promptly gauge the ecological and economic performance of the intended structural application. Some investigations have started exploring this opportunity, positing novel approaches that proffer immediate evaluations. These methods center around a range of indices that pivot upon ecological implications, along with performance indicators such as compressive strength. In light of this, the current study introduces a pair of novel indices with a more inclusive purview, encompassing not only environmental considerations but also costs and durability performance. One index, aimed at evaluating the feasibility of utilizing advanced construction materials as an alternative to traditional and consolidated options includes the aforementioned parameters on a cubic meter scale. In pursuit of this objective, part of the investigation is focused on the comparison between the mix designs of Ordinary Portland Cement Concrete (OPCC) and Ultra High Performance Concrete (UHPC), with CEM I or CEM III alternatively. The outcome revealed the limits of this first approach as it does not include some essential parameters, and OPCC performed better than UHPC in general. On the other hand, a complementary index has been proposed, seeking to optimize the mix design to be used to build structural elements or components and scale up to the level of the structural application. Thus, to check the consistency of the latter, UHPC roof panels, constructed by employing CEM I or CEM III alternatively, are then compared to panels made with ordinary reinforced concrete. The option containing CEM III registered better results in terms of holistic sustainability. The overall scope of this study is to encourage a more comprehensive, immediate, and all-encompassing evaluative approach, favouring the spread of advanced construction materials within the entire supply chain of the construction industry.