Materials and Structures最新文献

This study investigates the stress relaxation characteristics of bituminous mortars subjected to torsion. The bituminous mortar specimens are prepared using two distinct production processes. The first specimen is cored from a full bituminous mixture with a nominal maximum aggregate size of 13.2 mm, while the second specimen is designed to represent the mortar section of the bituminous mixture with the same nominal maximum aggregate size. Stress relaxation tests are conducted at various strain levels and temperatures. The linear viscoelastic limits of the mortar specimens are identified using scaling and superposition principles. The relaxation modulus in the linear regime is modelled using generalised Maxwell models, and the parameters are used to produce master curves. Relaxation spectra are also developed for the isothermal data. The findings reveal that specimens with smaller aggregates have a higher linearity limit and relax faster than those with larger aggregates. The generalised Maxwell model and relaxation spectra show that specimens with smaller aggregates have longer relaxation times.

Reducing the carbon footprint of cement-based materials is a crucial step for a sustainable cement industry. CO₂ emissions from the decalcification of lime during Portland cement production cannot be avoided. Thus, alternative binders such as calcium sulfoaluminate (CSA) cements are becoming more relevant, as they contain a reduced amount of cement clinker and therefore result in lower emissions than Portland cement. In the case of CSA cement, ettringite is primarily formed during hydration, which is susceptible to carbonation over its lifetime. This paper investigates for the first time the carbonation behavior of a porous insulation plaster consisting of micro hollow glass spheres embedded in a needle-like ettringite matrix with an open porosity of approximately 45%. A series of samples were produced in a field trial. On the one hand, the samples were taken from a façade that had been exposed to environmental conditions for two years. On the other hand, comparative samples were stored for two and four years under laboratory conditions. The samples were analyzed for their crystal structure and ratio, thermal conductivity and carbonation rate. Environmental conditions led to 97% carbonation of the ettringite phase after just two years, while a controlled atmosphere led to a carbonation of 76%. It was shown that effective moisture control can significantly slow down the carbonation of highly porous ettringite structures.

This study investigates the properties of concrete early strength accelerators, focusing on setting accelerators (Ca(HCOO)₂), hardening accelerators (KBr), and alkanolamines (TIPA), with applications in concrete road repairs, fast-setting concrete, and bridge connections. The investigation utilizes standard and heat curing methods to assess the early strength accelerator’s impact on slump flow, compressive strength development, water permeability, water absorption, and drying shrinkage. The results show that KBr enhance early compressive strength at both standard and 50 °C curing, while Ca(HCOO)₂ reduces setting time but does not enhance early strength under heat curing. The combination of KBr and Ca(HCOO)₂ significantly improves early strength at the early age of curing. Adding 0.05% TIPA to the 2.0% KBr and 1.0% Ca(HCOO)₂ mix can further enhances early-age compressive strength and long-term compressive strength up to 58 days of curing. Moreover, the results also show that the water permeability and water absorption remain the same when using or not using the early strength accelerator combination and heat curing method. Hence, using a combination admixture of 0.05% TIPA, 2.0% KBr, and 1.0% Ca(HCOO)₂ is suitable for applications in repairing concrete roads and connecting bridges, where high early compressive strength is required within the curing time range of 8 to 12 h. However, it is crucial to note that drying shrinkage values are higher with the use of early-strength accelerator combination and heat curing. Furthermore, comparison between samples with and without the early-strength accelerator combination shows a significant reduction in slump flow after 60 min, when the specimens are remixed for 15 s every 30 min.

To achieve efficient building construction while ensuring the structural seismic safety, this paper proposes a precast assembled concrete structure with simplified connection. The precast structure consists of uniform double-row hollow wall panels as the primary load-bearing element and cast-in-place reinforced concrete as the constraint elements. The low-cycle loading tests are conducted on comparative specimens to determine the reliability of simplified connections in the term of seismic performance and damage mechanisms of the new precast hollow shear walls. The experimental results show that the difference in ultimate load-bearing capacity between the simplified connection and localized grouting strengthening connection is only 0.78%, the simplified connection in the horizontal direction is safe and feasible. Furthermore, based on ABAQUS, the finite element model is established, combined with experiments and simulations, reveals the three-stage gradual failure process of the simplified connection specimen, i.e., the cooperative work stage, the gradual failure stage, and the weak framework stage. The calculation methods of precast shear wall’s lateral load-bearing capacity at various stages are established. The ratios of experimental and calculated values for the cooperative working and the weak framework are 1.04 and 1.10, respectively, indicating that the calculation model and calculation method are reliable. The research results provide significant references for the engineering application of connections similar to hollow wall panels.

The efficiency of shear strengthening of continuous reinforced concrete deep beams using new configurations of carbon fiber reinforced polymer (CFRP) ropes and sheets was evaluated. Test parameters included the shear span-to-depth ratio ((a/h)) of beams, maximum aggregate size ((D_{max })) of concrete and the amount of FRP reinforcement. Fifteen two-span rectangular beams of 1450 mm length with (a/h) ratios of 1 and 0.75 were cast using concrete mixtures with (D_{max }) of 19 and 10 mm. Twelve specimens were strengthened using five configurations involving near-surface mounted (NSM) continuous CFRP ropes with and without lateral CFRP dowels; externally bonded CFRP sheets applied to both side faces of the beam either directly or on grooves; and a hybrid technique comprising NSM and embedded through-section CFRP ropes. Test results revealed the superiority of using continuous triangular profiles of NSM CFRP ropes, especially for beams with higher (a/h) ratios. With the insertion of dowels, the NSM continuous ropes effectively delayed crack propagation and concrete cover separation resulting in 29, 96 and 13% increases in capacity, stiffness and toughness, respectively. Nonlinear finite element models for the test beams were developed using ABAQUS software. Predicted load-carrying capacities agreed well with the experimental values differing by merely 1–5%. In addition, analytical cracking patterns, reflected by plastic strain distribution and vertical stress trajectory, captured the experimental failure modes. This study offers new insights into the shear behavior of FRP-strengthened continuous RC deep beams, addressing a knowledge gap in how test parameters influence shear resistance and failure modes.

The two major themes in the current construction industry are digital construction and low environmental impact. As a prominent digital construction technology, concrete 3D printing has attracted increasing attention. However, the current understanding of the durability of 3D printed cement-based materials (3DPCM) remains limited, which hinders its wider application, especially as load-bearing, reinforced concrete structures. This work shares the knowledge acquired during a broad interlaboratory study regarding the durability of 3DPCM with 15 laboratories from 13 countries participating, under the framework of TC 304-ADC ‘Assessment of Additively Manufactured Concrete Materials and Structures’. Anisotropy in water absorption capacity, carbonation and chloride ingress resistance of 3DPCM were evaluated by 15 institutes with their own printable materials and printing equipment. Additionally, the impacts of cold joints on these properties were investigated and a comparison between printed and cast samples was carried out. The outcome of this study indicates that the water absorption test provides information on the bulk porosity of the samples, while the carbonation and chloride ingress tests are more effective and visually reflect the local defects, especially the layer interfaces and cold joints. The water ingress depth of cast samples prepared with printable mixtures is an order of magnitude higher compared to conventional concrete, while their carbonation and chloride ingress resistance are comparable. The sorptivity and estimated water ingress height of printed samples measured in the direction parallel to the filaments is generally higher than that measured in the perpendicular direction and in cast samples. Similarly, the carbonation and chloride ingress depth and rate of printed samples measured in the direction parallel to the filaments is generally higher than that measured in the perpendicular direction or in cast samples. The overall durability of 3DPCM is weakened by anisotropy, these effects can be addressed with targeted mixture design and processing strategies. Due to the variations in printers, printing parameters and materials, three types of cross-section geometries were observed in printed samples with cold joints. The carbonation depth that measured from the maximum carbonation ingress point near the cold joint to the sample edge effectively captures the effect of cold joints in all these three types of cross-section geometries of printed samples. Finally, the participants identified areas of improvement in the methodology and suggestions were made to refine the procedure for adoption in future research.

Fiber-reinforced composites have been increasingly used in the last decades to reinforce existing and new structures. Among them, fiber-reinforced polymer (FRP) composites have been adopted as externally applied reinforcement of existing concrete, masonry, steel, and timber structural members to increase their bending, shear, and axial capacity. FRPs have been used also as internal reinforcement of concrete structures in applications where their peculiar physical and mechanical properties make them a valid alternative to traditional steel reinforcing bars. Fiber-reinforced cementitious matrix (FRCM), also referred to as textile-reinforced mortar (TRM) and textile-reinforced concrete (TRC), composites have been proposed to overcome some of the issues related to the use of organic resins in FRPs, such as the absence of vapor permeability and poor resistance to (relatively) high temperatures. In addition, inorganic mortar plasters reinforced with FRP grids have been used in composite reinforced mortar (CRM) systems applied to masonry structures. The growing use of fiber-reinforced composites in construction has boosted research in this field, improving the state of knowledge and promoting the safe and reliable application of these innovative technologies. Nevertheless, open issues still remain, and ongoing and future research is essential to fully exploit the possibilities offered by fiber-reinforced composites in the world of construction. This paper explores the key features of fiber-reinforced composites, outlining their advantages over traditional materials and shedding light on the challenges that still need to be addressed. Emphasis is placed on the contributions provided by the author at the Politecnico di Milano over the past 15 years.

The openings in the infill wall frame and the constructional columns significantly affect their mechanical properties. Accurately assessing the seismic resistance of such systems is crucial for preventing seismic damage and improving seismic design. This study established different refined finite element models to systematically analyze the seismic performance of the infill wall frame with constructional columns and half-wall openings. The impact of constructional column types, opening sizes, and masonry strength on the failure modes, skeleton curves, displacement ductility, energy dissipation characteristics, and stiffness degradation of the structures was analyzed. The results showed that cracks were evenly distributed laterally on both sides of the wall in the specimen with the prefabricated constructional column (PMSF). The peak load-bearing capacity of the specimen with PCC was about 80% of that of the specimen with the cast-in-place column (CMSF) and 110% of that of the specimen without constructional columns (MSF). The stiffness degradation rate of the PMSF specimen was slower than that of the CMSF specimen but faster than that of the MSF specimen. The equivalent viscous damping ratio of the PMSF specimen was about 75% of that of the CMSF specimen and similar to that of the MSF specimen. As the size of the opening increased, the peak load-bearing capacity and stiffness of each type of infill wall frames decreased. Higher masonry strength increased the strength and stiffness of the structure, but made it more prone to brittle failure. Finally, a simplified mechanical model of the infill wall frame structure considering the impact of constructional columns and half-wall openings was proposed. The research provides theoretical support for the engineering application of prefabricated constructional columns and has significant engineering value in improving the seismic design level of masonry infill walls and assessing the safety of existing buildings.

Corrosion of reinforcement bars compromises the durability of reinforced concrete structures by deteriorating their mechanical properties. This study examines the effects of uniform corrosion on TMT bars (grades Fe 500D, Fe 550D, and Fe 550SD) with diameters ranging from 10 to 25 mm using an accelerated corrosion method. Corrosion-induced mass loss, changes in cross-sectional area, and surface morphology are evaluated using gravimetric measurements and 3D scanning technique. Further, the depth-wise influence of microstructural layers on mechanical properties of rebar is analyzed. Tensile tests show that yield and ultimate strength are strongly dependent on mass loss, while strain capacity is governed by cross-sectional heterogeneity. Notably, 25 mm bars exhibit significantly higher heterogeneity compared to smaller diameters. Degradation equations are developed to relate the yield strength, ultimate strength, and ultimate strain of corroded bars to mass loss and cross-sectional area reduction. Yield and ultimate strength exhibit a strong linear correlation with both parameters, while ultimate strain decreases exponentially, showing a stronger correlation with critical cross-sectional area loss. The developed degradation models based on mass loss and critical cross-sectional area loss, address limitations of previous studies and provide improved general and diameter specific reduction factors.

Understanding the degradation of cement-based tile adhesive mortars under weather conditions is essential for improving facade tiling systems, especially in the context of increasing climate variability. However, owing to the inherently complex microstructure and properties of these cement adhesives, capturing their degradation mechanisms within tiled systems has proven challenging. Here, we employed confocal fluorescence microscopy in conjunction with mechanical testing and analytical characterization techniques (SEM/EDS, TGA, FTIR) to obtain a microscale-level understanding of the weathering-induced deterioration process of cement adhesives in tiling systems. Under prolonged exposure to an accelerated weathering regime, we identified a three-stage degradation pathway: (1) initial wetting and a rapid decline in adhesion performance; (2) onset of micro-cracking at the adhesive–tile interface, facilitated by voids entrained beneath the tile surface; and (3) progressive widening of interfacial cracks, accompanied by the formation of disjointing cracks, leading to near-complete adhesion loss. Key factors contributing to these degradation stages were identified, including phase transformations, the distribution of micro-pores and voids, and loss of polymeric phase at the tile interface. Based on these findings, we proposed a microstructure–property model that elucidates the weathering behaviour of tiling systems and offers a foundation for developing strategies to enhance their long-term durability—towards safer and more resilient building façades.