Due to growing shortages of fly ash (FA) and sustainability concerns, this study investigates the use of sewage sludge ash (SSA) as a high-volume supplementary cementitious material (SCM) in concrete. SSA is a by-product of sewage sludge incineration and contains reactive oxides (e.g., CaO, SiO2, Al2O3) that can participate in cement hydration, making it a promising alternative to FA. The research involved a comparative analysis of concrete mixtures incorporating either SSA or FA at high replacement levels. Concrete mixtures with 20%, 40%, and 60% replacement of cement by SSA or FA were tested for compressive strength, hydration behavior, pore structure, and micro-mechanical properties. Mechanical performance was evaluated through compressive strength tests, hydration behavior was monitored (including heat of hydration and thermogravimetric analysis), and microstructural characteristics were examined (pore size distribution and nanoindentation). Results showed that SSA accelerates the cement hydration reaction relative to FA, leading to the formation of more hydration products such as calcium silicate hydrate (C-S-H), ettringite (AFt), and calcium carbonate (CaCO3), and refining the pore structure by reducing harmful large pores. Consequently, SSA-blended samples achieved higher long-term compressive strengths than their FA counterparts at equivalent replacement levels. Notably, a 60% SSA cement paste attained over 30 MPa compressive strength at 56 days, about a 116% increase compared to the 60% FA mixture. These findings demonstrate the feasibility of SSA as a high-volume SCM, offering a viable and sustainable alternative to FA in concrete applications.

Deep beams are widely utilised in high-rise buildings and bridge structures; however, their failure mechanism is complex. Among various models, the strut-and-tie model (STM) serves as the most widely used design tool. Nevertheless, conventional STM typically focuses on the load-bearing capacity of the compressive zone, often underestimating the contribution of longitudinal reinforcement in the tensile zone, which is considered to be relatively insignificant.,This research reveals that the contribution of reinforcement in the tensile zone to the bearing capacity of deep beams without stirrups is actually significant. To verify the role of tensile zone materials, this paper proposes deep beams utilising Engineered Cementitious Composites (ECC) in the tensile zone. This approach not only effectively protects the internal steel reinforcement but also improves the shear capacity of deep beams. Through experimental research, this study validates the mechanical performance of the aforementioned ECC-enhanced concrete beams. It establishes, based on segmental analysis, a closed-form solution that considers both the size effect of compressive concrete and the slip between longitudinal reinforcement and concrete. The results demonstrate that this proposed method can accurately predict the ultimate bearing capacity of ECC-enhanced concrete beams. This finding further confirms that tensile zone materials, including not only steel reinforcement but also ECC-enhanced materials, can significantly enhance the shear bearing capacity of deep beams. Consequently, neglecting the shear performance of materials in the tensile zone (including ECC) may lead to inaccurate design results.

This paper aims to investigate the mechanism of the effect of recycled carbon fibers on the fracture toughness of cement mortar and assess their environmental impact. The physical and interfacial properties of RCF were first characterized and found comparable to those of virgin carbon fibers (VCF). Three-point bending tests revealed that RCF significantly enhances the fracture energy, ductility index, crack initiation toughness, unstable toughness and cohesive toughness of cement mortar by up to 446.84 %, 238.71 %, 104.22 %, 137.12 % and 169.09 %, respectively. Additionally, the bilinear softening constitutive curve for recycled carbon fibers reinforced cement mortar was applied to compute the cohesive toughness which was validated by experiments and the related research on virgin carbon fibers reinforced cement mortar was conducted for comparison. Finally, a strength-based life cycle assessment showed that the global warming potential and primary energy depletion indicators of RCF-reinforced mortar were reduced by 38.47 % and 13.79 %, respectively, compared to VCF-reinforced mortar. These results confirm the engineering feasibility and sustainability of using RCF as a substitute for VCF in cement mortar.

Engineered cementitious composites (ECCs) exhibit strain hardening and multiple cracking behavior, which significantly influences the flexural performance of ECC beams. In this study, the size effect on the axial compressive stress-strain behavior of ECCs is investigated. Axially loaded cubes and prisms with different mix proportions, sizes, and aspect ratios are tested to examine the size effect on the ductility of ECCs. The axial strain of ECC specimens under axial loading is separated into a size-independent elastic strain and a size-dependent plastic strain allowing for the size effect. A simplified axial compressive stress-strain model is proposed and the tensile properties of ECCs are regressed from the experimental data of a self-conducted tension test for flexural analyses, which accounts for the size effect on the formation of hinges. Through comparisons with experimental results, the validity of the proposed approach is verified.

Concrete production accounts for about 8 % of global CO2 emissions, yet its carbonation potential offers a pathway toward carbon neutrality. This paper presents an analytical model that describes the diffusion of CO2 and the subsequent processes of dissolution and carbonation in concrete. The aim of the research is to provide an analytical method to calculate the time-dependent CO2 uptake by concrete during the diffusion and subsequent carbonation processes. Additionally, the impact of environmental CO2 concentration and concrete composition on carbonation depth and corresponding CO2 consumption is examined. Findings indicate that higher calcium oxide, magnesium oxide, sodium oxide, and potassium oxide content, along with increased CO2 exposure, enhance carbonation. Under natural conditions, ordinary Portland cement (OPC) concrete absorbs similar to 5.14 % of its weight in CO2, while geopolymer (GP) concrete absorbs similar to 3.38 %. With 50 % CO2 exposure, carbonation depths in OPC and GP concretes after 10 years are estimated at similar to 20 mm and similar to 25 mm, respectively. These insights guide concrete design to maximize CO2 sequestration in construction materials.

Concrete beams reinforced with hybrid longitudinal reinforcements (a combination of steel and FRP rebars) exhibit superior performance in ductility, stiffness and durability compared with conventional concrete beams. However, most available approaches rely on some empirical assumptions and ignore the slip between longitudinal rebars and adjacent concrete and the confinement effect of steel stirrups on concrete. The purpose of this paper is to propose a generic analytical approach to overcome these limitations. The main advantages of the proposed analytical approach are that it requires only the shear-friction and bond-slip material properties of concrete beams and can reasonably predict the whole loading process of concrete beams reinforced with longitudinal steel rebars, FRP rebars or a combination of both. Finally, a parametric study is conducted to investigate the effects of stirrups and longitudinal reinforcement on the behaviour of concrete beams. It is found that stirrups have a significant effect on the strength and ductility of concrete beams and hence cannot be ignored in the design of beams.

Large rupture strain fiber-reinforced polymer (LRS-FRP) composites exhibit a large rupture strain exceeding 5%, thereby enhancing the ductility and strength of concrete as a confinement reinforcement with notable energy dissipation capability. Nevertheless, available approaches are empirically derived from small-sized specimens, which renders them unsuitable for practical structures with larger sizes or aspect ratios. In this study, a passive stress-strain model for concrete wrapped by LRS-FRPs under concentric loading is proposed by incorporating partial interaction shear friction and bond-slip mechanisms. This model accommodates the size effect and a corresponding design equation is proposed to ensure the strain-hardening behavior of LRS-FRP-wrapped concrete prisms. The proposed passive stress-strain model is then employed in the segmental analysis to simulate the behavior of LRS-FRP-wrapped concrete beams considering the size effect and the confinement effect. Finally, a parametric study is conducted to investigate the effects of LRS-FRP thickness and longitudinal reinforcement on the behavior of beams.

Alkali-activated materials (AAM), recognized as low-carbon construction materials, exhibit superior compressive strength and erosion resistance. However, their practical adoption is hindered by rapid setting and poor workability. While using AAM in precast permanent formwork systems can mitigate these limitations and enhance structural durability, high drying shrinkage remains a barrier to widespread implementation.This study develops an alkali-activated materials permanent formwork (AAMPF) with low drying shrinkage and enhanced workability for reinforcing concrete columns (PFNC). Experimental investigations optimized the water-to-binder ratio, precursor composition, and additives to achieve these properties. The uniaxial compressive performance of PFNC and carbon fiber woven mesh (CFWM)-confined composite columns was then quantitatively evaluated. Results demonstrate that AAMPF significantly increases structural load-bearing capacity, while CFWM confinement further improves axial strength and ductility. Additionally, interface shear-friction analysis of PFNC revealed size-independent elastic deformation and size-dependent plastic deformation in AAM under axial stress. Based on these findings, an axial stress-strain model incorporating size effects is proposed for confined composite columns, providing theoretical foundations and design guidelines for AAMPF applications.

Steel tubes can significantly enhance the strength and ductility of concrete-filled steel tube (CFST) and steel tube confined concrete (STCC) columns. Existing research work have mainly focused on the former type while the later type of members not only avoid longitudinal steel bulking but also provide better ductility performance to resist seismic loads which is further investigated in this paper. Based on partial-interaction shear-friction mechanics, a passive stress-strain model for short steel tube confined concrete columns is proposed. The main benefit of the proposed model is that it is based on mechanics for rectangular and circular cross-sections which could allow for the size effect. Besides, this model also gives the ultimate strength of steel tube confined concrete columns and is further simplified as a design-oriented approach. The proposed passive stress-strain model and the simplified model for the ultimate strength of concrete columns show good correlation with experimental data. Compared with three other existing approaches, the simplified model could enhance the computing efficiency to a new level. Then a parametric study is conducted to investigate the effect of specimen size on the behaviour of steel tube confined concrete columns.

The mechanical and durability properties of ultra-high-performance fiber-reinforced concrete (UHPFRC) are superior to conventional are relatively complicated and cannot be applied to the analytical analysis of loaded beams for the ultimate and serviceability limit states. In this paper, a piecewise linear axial stress-strain relationship is proposed. The stress-strain relationship is further simplified as a rectangular stress block, and the stress of concrete during the whole loading process is accordingly evaluated. The development of the beam hinge at the midspan is described in detail, and it is then incorporated into the concrete stress blocks to derive an analytical approach and a closed-form solution for modeling the whole loading process of UHPFRC beams. Through comparisons with experimental results collected from the literature, it is validated that the proposed approaches can reasonably predict the whole loading process, including the ultimate strength, flexural rigidity, and ductility of UHPFRC beams, which only require material properties without any experimental calibration.

Thin-walled steel short columns often suffer from local buckling and limited load-carrying capacity. To address this issue, this study uses high-ductility concrete to reinforce thin-walled steel tubes, aiming to enhance both their load-carrying capacity and ductility. However, the challenge lies in the low bond strength between steel and high-ductility concrete. Therefore, the paper aims to propose an appropriate bond interface treatment to ensure a reliable bond between these two materials. Subsequently, axial compression tests were conducted for twelve short thin-walled steel tubes, comprising three unreinforced steel tube columns and nine columns reinforced with high-ductility concrete. The effect of the following parameters on both unreinforced and reinforced thin-walled steel tubes was investigated: length-to-slenderness ratio, width-to-thickness ratio and the thickness of reinforced high-ductility concrete. The ultimate load-carrying capacity, failure modes and local buckling behavior of steel tubes are evaluated and high-ductility concrete can enhance the mechanical performance of thin-walled steel tubes. A design equation is proposed to predict the ultimate strength of short thin-walled steel tubes reinforced with high-ductility concrete, which provides design guidelines for the application of high-ductility concrete strengthening existing steel tube structures.

Tests have shown that providing passive confinement to concrete, either through the use of internal stirrups, external fibre-reinforced polymer (FRP) wraps, FRP tubes or steel tubes, can increase the concrete strength and, in particular, the concrete ductility, thereby allowing greater absorption of energy and consequently ductile failure. The problem of including the benefits of passive confinement in design is in generalising the effect of passive confinement because it varies with the member size, the configuration of the confining reinforcement and material properties. In this paper, all aspects of the complex fundamental mechanics of passive concrete confinement are explained both qualitatively and quantitatively through the use of shear–friction and bond–slip mechanics. The mechanics model was found to have a good correlation with test results. An analysis-oriented procedure is described for quantifying the passive stress–strain of concrete for rectangular sections and it is envisaged that this can be used to develop simplified rules for design, in particular for new types of members and those with new materials.

There are innumerable tests on small stocky circular cylinders with either internal passive encasement of the concrete with circular stirrups or spirals, or external passive encasement through fibre-reinforced polymer (FRP) wrapping or through the use of FRP tubes or steel tubes. These tests have shown that passive confinement can increase the strength but, in particular, can substantially increase the ductility of concrete cylinders. Because of these important benefits, the effect of passive confinement on the concrete stress/strain in a particular cylinder section is invariably derived from tests such that substantial member testing is required. In this paper, it is shown how the passive stress/strain of the concrete for a particular circular member can theoretically be determined directly from their partial-interaction shear-friction and partial-interaction bond-slip material properties for any reinforcement arrangement and geometry of the circular cylinder. This procedure provides tools for designing for the benefits of passive confinement directly without the need for member testing.

The strength and ductility of concrete prisms can be significantly enhanced by confinement reinforcement, such as stirrups, wraps, and tubes. Recently, a combination of confinement reinforcement is applied to concrete prisms for their better ductility or a specific use. However, most existing approaches only give the ultimate strength of confined concrete prisms and do not consider the size effect. As a result, these approaches cannot provide structural engineers with a simple tool for the ductility design of concrete prisms with size effect. In this paper, based on the fundamental partial interaction bond-slip and shear-friction mechanisms, a passive stress-strain model of concrete confined by a combination of confinement reinforcement is proposed. The main advantages of the proposed model are that the size effect is considered and it can be applied to concrete members of rectangular or circular cross-section reinforced by novel combinations of confinement reinforcement. After the proposed model is verified with test results of concrete prisms reinforced by FRP wrapping and stirrups/steel tube, it is concluded that this model can be applied in the ductility design of FRP wrapped concrete prisms.

Confinement reinforcement, such as tubes, stirrups, and wraps, can significantly improve the strength and, more importantly, ductility of eccentrically loaded concrete prisms. However, the confinement effect in flexural analyses is sometimes not considered in design codes because there is no simple approach for quantifying it. Although the finite-element method has been proposed to analyze laterally confined concrete prisms under eccentric loads, the complicated computational program may hinder its wide applications. In this paper, based on fundamental partial-interaction shear-friction and bond-slip material properties, a passive stress-strain model of concrete with the confinement effect is introduced and reduced to a rectangular stress block. A closed-form solution is then derived for the ductility of fiber-reinforced polymer (FRP)-wrapped concrete prisms under eccentric loads. The main advantage of the solution is that the size effect is considered and no structural tests are required. After the validity of the solution is verified with experimental results, a parametric study is conducted to quantify the effects of various key factors on the ductility of FRP-wrapped concrete prisms under eccentric loads.

The ductility of a beam is important in reinforced concrete member design at the ultimate limit state, especially in resisting dynamic loads such as those from earthquakes or blasts. Concrete confinement reinforcement, such as stirrups or tubes, are widely used in structures and can significantly enhance the ductility of concrete beams. However, this confinement effect is normally ignored in current design standards, limiting the ability to design specifically for ductility or to estimate the ductility of existing structures. In this paper, a novel concrete passive stress/strain relationship based on the application of partial interaction and shear friction theories is simplified to a rectangular stress block for flexural analyses. This confined concrete stress block is then applied to quantify the moment/rotation of a hinge where both the confinement of the concrete and hinge lengths are quantified through mechanics. The aim of this paper is to provide a mechanics based approach for quantifying the ductility of RC beams that can be used to develop simple design rules without the need for large amounts of member testing. (c) 2020 American Society of Civil Engineers.

Bonding of carbon fiber reinforced polymer (CFRP) plates to a concrete member is a widely used strengthening method. CFRP plates used in construction degrade due to harsh environmental conditions such as high temperature or alkaline solution seepage from concrete. However, the adhesive between CFRP plates and concrete may have a positive effect on the durability performance of CFRP plates. In this paper, the long-term performance of both naked and adhesive coated CFRP pultruded plates subjected to different-temperature water or alkaline solution (20, 40 and 60 °C) are investigated to evaluate the protective effect of adhesive on CFRP plates. It is found that the adhesive coating can slow the deterioration of mechanical properties especially the tensile properties and fiber-matrix interfacial properties. The water absorption mechanism of CFRP plates was also investigated.

本文研究了东丽T700与国产碳纤维增强环氧树脂基拉挤杆在20℃和60℃蒸馏水浸泡下的水吸收与剪切性能变化.结果表明,随着温度升高,水扩散速率提高,且沿纤维方向的水扩散系数高于垂直方向.浸泡7d后,碳纤维拉挤杆的剪切强度均发生明显下降,但随着浸泡时间延长,20℃水浸泡试样的剪切强度反而有所上升,而60℃水浸泡试样的剪切强度继续缓慢下降.比较而言,国产碳纤维增强拉挤杆的剪切强度相对较高,界面粘结强度大,但耐水性能相对较差,浸泡后期的强度下降更为明显.