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A novel reinforced concrete structural member comprising ultrahigh-performance concrete (UHPC) and engineered cementitious composite (ECC) was proposed for use in underground structures, such as prefabricated utility tunnels. In this composite member, UHPC was used as the exterior (i.e., soil side) protective layer to prevent the penetration of water and corrosive agents, and ECC was used as the interior (i.e., service environment side) protective layer to improve the member's crack-control capacity and fire resistance. Ordinary concrete was filled between the UHPC and ECC layers. The composite member was reinforced with perforated steel plates to achieve strong mechanical interaction between the composite materials and to ensure structural integrity. The proposed composite was experimentally characterized as follows. The tension-stiffening effect in ECC panels with a perforated steel plate was evaluated using direct tensile tests. The flexural behavior of the reinforced UHPC–concrete–ECC members was characterized with four-point bending tests. The failure processes of the composite members were analyzed via the digital image correlation (DIC) method. Finally, a simplified theoretical method was developed to estimate the yield and ultimate load-carrying capacities of the composite member, and the estimated results showed good agreement with the test results. Overall, the proposed composite member had favorable mechanical performance and durability due to the strategic use of UHPC and ECC. [ABSTRACT FROM AUTHOR]

In this work, the influence of a naphthalene sulfonate–based superplasticizer on the initial corrosion of galvanized steel bars in the simulated concrete pore solution was studied. The initial corrosion process of the galvanized steel in solution with superplasticizer was first monitored. The inhibition effect of the protective layer formed by the superplasticizer and the protective property of the calcium hydroxyzincate (CHZ) passive layer were defined. Scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM/EDS) was used to observe the surface morphology of the CHZ passive layer. The results showed that the addition of superplasticizer delayed the initiation of the corrosion process of galvanized steel bar and inhibited the hydrogen evolution. The protective property of the CHZ layer on galvanized steel varied with the concentration of superplasticizer. The inhibition effects of the superplasticizer on the initial corrosion will be beneficial to the quality of the steel–concrete interface. [ABSTRACT FROM AUTHOR]

This paper presents a methodology for numerically simulating the chloride diffusivity in mortar subjected to corrosion‐induced cracking with the aid of X‐ray microcomputed tomography (X‐ray µCT). In the research, the mortar sample with embedded steel rod was subjected to accelerated corrosion and then scanned by X‐ray µCT at consecutive corrosion periods of 0, 1,000, and 2,000 minutes. Upon the scanned CT images, a meso‐scale model consisting of multiple material phases with their intrinsic structures was built up and implemented into a finite element method for diffusion simulation. In the numerical simulation, the effects of multiple phases (void, crack, aggregate, rust, and interfacial transition zone between cement and aggregate) on the chloride diffusion of mortar are studied. Since the X‐ray µCT facilitates accurate local material information in time‐dependent and in situ manner, the diffusivity of mortar under different corrosion time and its spatial distribution characteristics (e.g., at different locations from the mortar surface) can be evaluated. [ABSTRACT FROM AUTHOR]

• Extended Drucker-Prager for concrete under non-uniform passive confinement. • A new approach for defining flow rule only by cyclic tests. • Isolating the effect of friction angle from hardening/softening rule. • Proposing two styles for Drucker-Prager plasticity parameters. Extended linear Drucker–Prager (DP) model has been a proper choice to capture the stress-strain of cylindrical concrete columns confined by fiber reinforced polymers (FRPs). However, most existing DP models calibrated by uniform active/passive tests are unsuitable for concrete under non-uniform passive confinement. This paper aims to calibrate the key parameters i.e., friction angle, plastic dilation angle and hardening needed for the DP model based on tri-axial test results on concrete cube subjected to non-uniform confinement. The effect of friction angle parameter is isolated from the hardening parameter within the yield function while the friction angle is defined as a function of concrete grade and hardening as a function of lateral confinement stiffness ratio. Here, the plastic lateral strains of non-uniform confined concrete, which could be obtained in cyclic tests but were usually left aside, are used to define a function for plastic dilation angle which works compatibly with the yield parameters. The resultant extended DP model could capture the stress-strain behaviour of concrete under both uniform or non-uniform passive pressure as well as the variable flow rules and different load paths occurring across the section of FRP-confined non-circular concrete. [ABSTRACT FROM AUTHOR]

Abstract Optical fiber sensing technology has been widely used for thermal strain detection of civil structures subjected to the environmental temperature variations. Due to the fragile material nature, bare optical fibers are seldom used in practice, as they cannot resist the harsh service environment. A packaging measure is usually required to enhance the durability of optical fiber-based sensors for reliable long-term detection. However, the existence of packaging and bonding materials brings about strain transfer loss, which makes the optical fiber readings not accurately represent the actual strains of the host material. This paper presents a combined experimental and theoretical study on a four-layer optical fiber sensing composites (i.e., the host material, an adhesive bonding layer, the sensing fiber, and its protection layer) subjected to thermal loading to correct the above-mentioned strain transfer error. Aluminum and polypropylene (PP) plates with different thermal expansion coefficients were surface-bonded with fiber Bragg grating (FBG) sensors and then exposed to various temperatures (30–70 °C). Theoretical study was developed to investigate how the strain measurement error was influenced by the thermal deformation incompatibility of multiple contacted layers (i.e., the optical fiber, the protective layer, the adhesive layer and the hosting material) with different thermal expansion coefficients. The accuracy and effectiveness of the proposed strain transfer function was then validated through comparisons with the test results. Based on the theoretical analysis, the sensitivity of the relevant material and geometrical parameters on the strain transfer efficiency was discussed to instruct the design of FBG based sensors. The proposed error-modification formula can be used to effectively improve the strain measurement accuracy and instruct the optimum design of FBG based sensors applied to civil structures under thermal loading. [ABSTRACT FROM AUTHOR]

Abstract The precast concrete sandwich panel (PCSP) is a popular structural type used for prefabricated construction. In this study, three kinds of glass fiber reinforced polymer (GFRP) connector, including a flat plate, a corrugated plate, and a hexagonal tube connector, were investigated for potential use in PCSPs. In-plane direct shear tests were conducted to evaluate the performance of the connectors based on a typical push-out configuration (i.e., three concrete wythes linked with connectors and two rigid foam layers in between). Twenty-five specimens were tested, and the parameters investigated consisted of GFRP laminate thickness, projected length, and shear force direction. The shear force vs. relative slip relationships and failure modes of all types of connector were compared and are discussed in detail in this paper. Meanwhile, finite element (FE) analysis was conducted to reproduce the test results. All three types of GFRP connector exhibited a progressive failure manner in spite of the linear brittle nature of the FRP materials; the corrugated plate connector achieved higher deformability than the flat plate connector, particularly when the GFRP laminate thickness was small; and the hexagonal tube connector exhibited similar behavior along two orthogonal directions, making them well suited for use as two-way connectors. The developed FE model was able to well predict the peak shear load and stiffness of all types of connector based on comparisons with the test results. [ABSTRACT FROM AUTHOR]

Abstract The use of local aluminosilicates to serve the local engineering applications could resolve the issue of unpredicted properties of the geopolymer caused by the wide variance in aluminosilicates reactivity. The locally available aluminosilicate byproducts (e.g., fly ash [FA] and granulated ground blast-furnace slag [GGBS]) in Qingdao were used to synthesize geopolymer cement in this study. Six geopolymer cement mortars with different FA/GGBS ratios were compared with ordinary Portland cement (OPC) and magnesium potassium phosphate cement (MKPC) mortars in terms of workability, setting time, strength development, volume stability and chloride permeability. The use of high volume of GGBS equips the geopolymer mortars with fast setting and high early strength despite poor volume stability. Thus, GGBS geopolymer mortars could potentially replace the high-cost MKPC mortar for rapid rehabilitation in construction. The geopolymer cement mortar with a FA/GGBS ratio of 4 behaves similar to OPC mortars in terms of fluidity, setting time, strength development, volume stability and chloride permeability. Therefore, FA-based geopolymer mortar blended with 20% GGBS could be considered as a high-efficiency, low-cost, eco-friendly and sustainable replacement of OPC mortar. Overall, the geopolymer cement properties could be engineered based on the FA/GGBS ratio in order to serve local engineering applications for the maximum utilization in different scenarios. Highlights • Engineering-oriented properties of geopolymer mortars were characterized. • FA/GGBS ratio dependent geopolymer cement was compared with OPC and MKPC. • Application cost and prospect of geopolymer cement were analyzed. [ABSTRACT FROM AUTHOR]

Highlights • The influence of non-uniform corrosion on the bond strength of the steel reinforcement in concrete were investigated. • Analytical models were proposed to predict the bond degradation due to pitting corrosion and uniform corrosion. • The bond strength was improved when the corrosion degree was smaller than 0.8%. • The bond strength reached the peak value and about 1.6 times of the control value when the corrosion degree was 0.2%. Abstract This paper investigates the influence of non-uniform corrosion on the bond strength of the steel reinforcement in concrete. To achieve the non-uniform corrosion, partial surface of the steel reinforcement was coated by an epoxy resin which could prevent effectively that part of steel from corrosion. Thus, corrosion only happened to the bare (un-protected) zone of the surface. A series of reinforced concrete (RC) specimens were cast and stored in a chloride environment. An impressed current was applied to accelerate the corrosion of the steel reinforcement. Pull-out tests were conducted on the specimens with different corrosion degrees. The experimental results showed that the bond strength was higher than that of the control (non-corroded bar) when the corrosion degree was smaller than 0.8% and was the highest (1.6 times of the control) when the corrosion degree was about 0.2%. The bond strength started to decrease significantly with further increases in corrosion degree. By considering the corrosion of the steel reinforcement and the pressure induced by the expansion of the corrosion products, analytical models were proposed to predict the bond degradation due to pitting corrosion and uniform corrosion. The models were also validated by the test results from existing literature. [ABSTRACT FROM AUTHOR]

Abstract An aluminosilicate precursor, such as metakaolin, can be transformed into a cement-like geopolymer binder via a phosphate activation approach. This paper identifies the effect of the addition of aluminum species into the phosphate activating solution on the formation of such geopolymers, from the fresh to the hardened state. Activating solutions with Al/P molar ratios of 0, 0.1, and 0.3 were prepared by blending monoaluminum phosphate (MAP) and orthophosphoric acid (OPA). The rheological properties, fluidity, and setting times of the fresh geopolymer pastes and the compressive strength of the hardened geopolymer matrices were studied. Liquid-state 27Al and 31P nuclear magnetic resonance (NMR) measurements for the chemical environments of Al and P, and spectroscopic, thermal, and microscopic analyses revealed that the soluble aluminum in the phosphate activating solution played an important role during the geopolymerization process. Seeding of aluminum species through inclusion in the activating solution allowed a rapid sol/gel transition that improved the rheological properties and setting time of the fresh geopolymer pastes at ambient temperature. However, although the increased concentration of aluminum phosphate oligomers promoted by the soluble aluminum addition contributed to the formation of a compact matrix with high early strength, it hinders the ongoing reaction of metakaolin in the later period, which has a detrimental influence on ongoing strength development beyond 7 days of curing. [ABSTRACT FROM AUTHOR]

Abstract The direct tensile properties of ultra-high performance engineered cementitious composites (UHP-ECC) were investigated under a range of strain rates from 0.0001 to 0.05 s−1 in this study. Three kinds of polyethylene fibers with different fiber aspect ratios were used as the reinforced fiber in the UHP-ECC. The rate sensitivity of the UHP-ECC in tension was evaluated in terms of initial cracking stress, peak stress, strain capacity, energy absorption capacity and elastic modulus at different strain rates. The cracking patterns, including crack number, crack width and crack spacing, of the UHP-ECC specimens were also monitored. The test results showed a significant rate effect on the initial cracking stress and a moderate effect on the peak stress and strain capacity. The fiber aspect ratio significantly influenced the tensile properties and failure modes of the UHP-ECC. The matrix strength, single fiber tensile and pull-out tests were conducted at corresponding strain rates to further explain the test results. Finally, the micro-structure of the tensile specimens was observed using a scanning electron microscope. [ABSTRACT FROM AUTHOR]

This paper investigates the tensile behavior of the recently developed ambient-cured one-part engineered geopolymer composites (EGC) incorporating different hybrid combinations of steel (ST) and polyethylene (PE) fibers while maintaining the total fiber volume at 2%. Two ambient-cured geopolymer matrices were manufactured: the first was synthesized by activating slag (100%) while the second was a blended of 50% fly ash and 50% slag. The effects of using different precursor materials and hybridization content on the matrix and composite properties of EGC including workability, density, compressive strength, matrix fracture properties (elastic modulus, fracture toughness and crack tip toughness), tensile response and matrix microstructure were evaluated. The effect of 212 µm sand addition on the matrix and composite properties of the hybrid composite 1.5% PE and 0.5% ST was also assessed. It was found that the slag based EGCs exhibited a relatively better tensile response (i.e. strain hardening and multiple cracking behaviors) compared to the blended EGC composites although they achieved a comparable compressive strength. SEM observations revealed that the slag geopolymer matrix was relatively denser and more compacted compared to the blended geopolymer matrix. The sand addition impaired the strain hardening and multiple cracking behaviors of both slag and blended EGC yet increased the compressive strength and enhanced the fracture properties of the geopolymer matrices. [ABSTRACT FROM AUTHOR]

The addition of supplementary cementitious materials may affect the Ca 2+ concentration and pH value of the concrete pore solutions. This work studied the influence of the calcium ion on the passivation of the galvanized steel bars in simulated concrete pore solutions with relatively low pH value of 12.5 and 13.15. The results showed that continuous protective layers were formed on the galvanized coating in the solutions with saturated Ca(OH) 2 at pH 12.5 and solutions with either KOH or NaOH at pH 13.15. The passive film formed in the saturated Ca(OH) 2 solution showed better protective properties than those in other solutions. In the solutions with lower concentrations of Ca 2+ at pH 12.5, there was no obvious reaction and no protective layer was formed on the surface. [ABSTRACT FROM AUTHOR]

This work aimed at studying the depassivation mechanism of the passive film on galvanized steel bar in concrete pore solution with different concentrations of chloride ion. The results showed that in a saturated Ca(OH) 2 solution, the critical chloride values for the depassivation of the passive films formed on galvanized steel bars and zinc metal specimens were 0.2 M and 0.3 M respectively. When the Cl − concentration was lower than the critical value, the donor density of the passive film increased and thus the protective property reduced with increasing the concentration of chloride ion, which could be interpreted by the point defect model (PDM). The corrosion products formed on the galvanized steel surface could inhibit the further ingress of the chloride ions and enhance the protective property, while such effect was not obvious for the zinc metal. [ABSTRACT FROM AUTHOR]

Three-point bending (TPB) tests were conducted on notched beams to compare the fracture properties of alkali-activated slag (AAS) concrete and ordinary Portland cement (OPC) concrete at three different compressive strength levels (30, 50 and 70 MPa). Parallel comparisons were also conducted between AAS mortar (AASM) and OPC mortar (OPCM). Load vs. mid-span deflection ( P-δ ) curves, load vs. crack mouth opening displacement ( P -CMOD) curves and load vs. crack tip opening displacement ( P -CTOD) curves of the tested beams were obtained. The fracture energy G F and the characteristic length ( l ch ) of the AAS and OPC concrete and mortar were calculated and analyzed. It was found that the G F of AAS concrete was always higher than that of OPC concrete given the same compressive strength, due to their denser and stronger interfacial transition zones (ITZs). At a compressive strength of 30 MPa, the G F of AASM was also larger than its OPC counterparts. However, the G F of AASM became lower than that of OPCM when the compressive strengths were 50 and 70 MPa, as more initial micro-cracks were formed in the AASM matrix with strength increase. In addition, the l ch values of AAS concrete and mortar were all smaller than those of OPC, implying that the formers were more brittle given the same compressive strengths. The elastic modulus of AAS concrete and mortar were found to be always lower than those of OPC. Micro-structural observations were carried out to explain the above phenomena. [ABSTRACT FROM AUTHOR]

Inorganic binder-based stabilization/solidification (S/S) of Pb-contaminated soil is a commonly used remediation approach. This paper investigates the influences of soluble Pb species on the hydration process of two types of inorganic binders: ordinary Portland cement (OPC) and magnesium potassium phosphate cement (MKPC). The environmental leachability, compressive strength, and setting time of the cement products are assessed as the primary performance indicators. The mechanisms of Pb involved in the hydration process are analyzed through X-ray diffraction (XRD), hydration heat evolution, and thermogravimetric analyses. Results show that the presence of Pb imposes adverse impact on the compressive strength (decreased by 30.4%) and the final setting time (prolonged by 334.7%) of OPC, but it exerts much less influence on those of MKPC. The reduced strength and delayed setting are attributed to the retarded hydration reaction rate of OPC during the induction period. These results suggest that the OPC-based S/S of soluble Pb mainly depends on physical encapsulation by calcium-silicate-hydrate (C S H) gels. In contrast, in case of MKPC-based S/S process, chemical stabilization with residual phosphate (pyromorphite and lead phosphate precipitation) and physical fixation of cementitious struvite-K are the major mechanisms. Therefore, MKPC is a more efficient and chemically stable inorganic binder for the Pb S/S process. [ABSTRACT FROM AUTHOR]

Buckling of steel reinforcement usually causes a sudden loss of the load-carrying capacity and the ultimate state of conventional reinforced concrete (RC) cylinders. However, reinforcing bars behave differently in fiber-reinforced polymer (FRP)-confined RC cylinders due to the lateral confinement effect of FRP. This paper presents a theoretical study into the buckling behavior of longitudinal steel reinforcing bars embedded in FRP-confined concrete subjected to cyclic axial compression. An empirical monotonic compressive stress-strain model considering the buckling effects proposed previously for laterally supported reinforcing bars is extended to a cyclic model by combining the monotonic envelope and the Menegotto-Pinto model accounting for the cyclic loops. The cyclic stress-strain models for both laterally supported reinforcing bars and FRP-confined plain concrete are then implemented into the OpenSees software platform and validated through comparisons with compressive test results on cyclically loaded FRP-confined plain and RC cylinders. The proposed cyclic stress-strain model for laterally supported reinforcing bars is expected to serve as a fundamental model for predicting the seismic behavior of FRP-strengthened RC cylinders with widely-spaced transverse ties under cyclic axial compression, in which case the local buckling of reinforcing bars usually occurs between two adjacent transverse ties. [ABSTRACT FROM AUTHOR]

This investigation sought to develop a chemically bonded refractory castable based on the acid-base reaction between calcium aluminate cement (CAC) and polyphosphate-based reactant. The mechanical properties, phase evolution, and microstructure of the polyphosphate-modified CAC under normal and elevated temperatures were investigated. The results show that CAC paste had poor fluidity and presented compressive strength decay when submitted to air curing of 20 °C for 28 days compared with that at 7 days. In contrast, the addition of sodium hexametaphosphate (SHMP) significantly improved the fluidity of the resulting binder due to its favorable deflocculating effect for thorough hydration of CAC particles. SHMP-modified CAC paste also exhibited greater compressive strength than CAC paste, and no strength regression occurred after 28 days. Moreover, this superior mechanical performance was maintained even after exposure to elevated temperatures up to 1000 °C. The observed excellent performance in strength was attributed to the enhancement from the phases of calcium aluminate hydrate-based gel and calcium-phosphate, known as chemically bonded ceramics formed by the acid-base reaction between CAC and SHMP. The formed chemically bonded ceramics impeded to some extent the conversion of the hydrates CAH 10 or C 2 AH 8 to C 3 AH 6 at ambient temperature and retained a notable bonding effect at high temperatures. [ABSTRACT FROM AUTHOR]

Magnesium potassium phosphate cement (MKPC) mortar as a construction repair material has been studied and applied for several years. In this paper, reactive magnesia sand is employed to replace conventional inert quartz sand as the fine aggregate of MKPC mortar, to adjust the M/P (i.e. MgO-to-KH 2 PO 4 ) molar ratio of cement paste during mortar formation, as well as to improve the mortar performance though strengthening the bonding capacity of the interfacial transition zone (ITZ) (i.e. the paste to aggregate interface). Experimental results showed that the compressive strength of the MKPC mortar with reactive magnesia sands could achieve a mean value of 36.65 MPa at 12 h, and outperformed about 27.30% than that with inert quartz sand at 28-day. The interfacial transition zone (ITZ) development in the MKPC mortar with magnesia sands was investigated using the on-site X-ray computed tomography (CT). The failure modes of specimens subjected to uniaxial compression were analyzed and visualized by means of image processing technique. In addition, the chemical interaction between the paste and the aggregate was theoretically and experimentally analyzed, explaining well the excellent volume stability of the MKPC mortar with reactive magnesia sands. [ABSTRACT FROM AUTHOR]

• A fully coupled meso -scale electro-chemo-mechanical phase field model is proposed to simulate the corrosion mechanism in concrete structures; • The mass transport process of moisture, chloride ions and oxygen are well captured by the model; • The corrosion-induced damages in both mortar and interfacial transition zones are vividly simulated. • The model is validated by existing test data and used to reveal the effects of various structural and environmental parameters. Corrosion-induced concrete cover deterioration is a major factor affecting the serviceability of the reinforced concrete (RC) structures. The entire corrosion process, including mass transport, physical/chemical/electrochemical reactions, and cover cracking, occurs at the meso -scale. This paper developed a fully coupled meso -scale electro-chemo-mechanical phase field method to accurately simulate the corrosion mechanism in RC structures. The simulation begins with the mass transport process in the concrete, including the moisture, chloride ions and oxygen. When the chloride concentration at the rebar surface reaches a critical value, corrosion initiates and then propagates. A meso -scale phase field model is adopted for characterizing the corrosion-induced damage in both mortar and interfacial transition zones (ITZs). In addition, crack direction dependent diffusivity tensors are proposed to consider the influence of damage on the mass transport process. The proposed numerical method is verified by previously reported experimental results, showing its ability to conduct high-fidelity simulations of corrosion-induced fracture in RC structures. Parametric studies are carried out to investigate the effect of aggregate distribution, cover thickness, relative humidity, and temperature on the corrosion process. [ABSTRACT FROM AUTHOR]

An experimental study was conducted to investigate the influence of FRP confinement on the buckling behavior of longitudinal steel reinforcing bars (rebars) in RC columns subjected to monotonic and cyclic axial compression. Test results of 12 FRP-confined RC columns and 12 FRP-confined plain concrete columns (control specimens) are presented with particular attention to the evolution of strains in the longitudinal steel rebars and the FRP jacket as the load increases. For an FRP-confined RC column subjected to monotonic axial compression, the contribution of longitudinal steel rebars to the overall axial load carried by the column was deduced by subtracting the contribution of the FRP-confined plain concrete from the total axial load. The test results showed that the compressive stress-strain response of longitudinal steel rebars in FRP-confined RC columns follows that from tensile testing before the axial stress reduces gradually due to the onset of buckling deformation. Significant interactions were found to exist between FRP confinement and the buckling of longitudinal steel rebars. This interaction should be carefully considered when formulating constitutive laws for both the longitudinal steel rebars and the FRP-confined concrete for use in theoretical modeling. [ABSTRACT FROM AUTHOR]

This paper presents the development of a design approach for predicting the fire resistance of reinforced concrete (RC) beams exposed to design fire. The proposed method is based on the energy-based time equivalence method in combination with a correction factor that is formulated as a function of concrete cover depth and compartmental characteristics. In the proposed approach, the fire resistance period of an RC beam under design fire exposure can be deduced from that of the same RC beam under standard fire exposure; the latter can be predicted using a set of explicit equations previously developed by the authors. The fire resistance data of RC beams under design fire exposure, which were used to formulate and validate the proposed approach, were generated using a reliable finite-element approach. The proposed design approach is shown to be accurate in predicting the fire resistance of RC beams under design fire exposure. [ABSTRACT FROM AUTHOR]

Chloride transport in structural concrete has been found to be both stress- and time-dependent. Several models have been developed to take into account the time dependency of the chloride diffusivity of concrete. However, less is understood about the stress dependency and the interaction between the two dependencies. Experiments were conducted to investigate chloride transport in concrete subjected to different tensile stress levels and different periods of wet-dry cycling exposure (56, 112, 168 and 224 d). Two key parameters were investigated - surface chloride content ( Cs) and the apparent chloride diffusion coefficient ( Da). It was found that regardless of the tensile stress level, the values of Da and Cs respectively decreased and increased with time due to the continuous hydration of cement and the periodic surface wetting with chloride solution. However, both the decreasing and increasing rates were found to be dependent on the tensile stress levels. Both Da and Cs increased with the tensile stress level due to the opening of pores and the development of microcracks in concrete. Practical models were developed to reflect the above time and stress dependency of Da and Cs, as well as their interactions. The models were validated through comparisons of the experimental and predicted chloride profiles of all the tested specimens. [ABSTRACT FROM AUTHOR]

Application of X-ray computed tomography (CT) for pore-related characterization and simulation of cement mortar was explored in this paper. Based on visualization and quantitation of the three-dimensional (3D) morphological features (i.e., appearance, size, volume, shape, and distribution) of pores, the porosity, tortuosity-connectivity, permeability, and molecular diffusion of cement mortar were computed and simulated. The simulation of the permeability and molecular diffusion of cement mortar was carried out on the basis of Darcy's law and Fick's second law, respectively. The dynamical velocity field and concentration field were respectively visualized. It was found that the X-ray CT technique provided a promising non-destructive alternative for both qualitative and quantitative analysis of the pore-related characterizations of cementitious materials. Furthermore, it was possible to use the simulations to replace lab-intensive experimental tests to quickly obtain the permeability and diffusivity parameters of cement mortar for application in engineering design. [ABSTRACT FROM AUTHOR]

Chemosynthetic phosphate-based geopolymer is a new type of low carbon and eco-friendly construction material. This article aims to investigate the formation mechanism and thermal stability of this new type of geopolymer. In this study, three mixes of the phosphate-based geopolymer were designed and synthesized at ambient temperature with different molar ratios of Si, Al and P, which were supplied by the raw materials of silica fume, metakaolin and monoaluminum phosphate. No accelerating setting condition (e.g., metal oxides addition or high temperature curing) was applied. The experimental results showed that the 28-day compressive strength of the hardened geopolymer paste arrived at about 31 MPa when the Al/P molar ratio is equal to 1.0, which was ensured by the formation of an amorphous structure of SiO 2 ·Al 2 O 3 ·P 2 O 5 ·nH 2 O and a crystalline phase, aluminum hydrogen phosphate (AlH 3 (PO 4 ) 2 ·3H 2 O). At elevated temperatures, a phase transition of the phosphate-based geopolymer was activated from aluminum hydrogen phosphate to berlinite (AlPO 4 ) and the silicon-containing phase was rearranged. [ABSTRACT FROM AUTHOR]

Alkali-activated concretes (AACs) are attracting increasing attention due to their potential as alternatives to ordinary Portland cement concrete (OPCC). This paper is a holistic review of current research on the mechanical properties of AAC including research on its compressive strength, tensile strength, elastic modulus, Poisson’s ratio, stress–strain relationship under uniaxial compression, fracture properties, bond mechanism with steel reinforcement, dynamic mechanical properties, and high-temperature performance. Three types of AAC are reviewed: alkali-activated slag, alkali-activated fly ash, and alkali-activated slag-fly ash concretes. The applicability to AAC of design formulas found in codes of practice that were developed to estimate the basic mechanical performances of OPCC is also discussed. It is shown that, in general, AAC exhibits better bond performance with steel reinforcement and better strength performance after exposure to elevated temperatures than OPCC. For the other reviewed mechanical properties, the differences between AAC and OPCC largely depend on the proportions of raw materials in the concrete; specifically, the slag to fly ash ratio may be a very influential factor. As there is a trend to combine slag and fly ash in the production of AAC to achieve normal temperature curing and environmental friendliness, further research is deemed necessary to determine how the slag to fly ash ratio influences the fundamental mechanical properties of AAC and how this affects practical designs. [ABSTRACT FROM AUTHOR]

Concrete cracking usually is an unavoidable phenomenon in reinforced concrete (RC) structures and has a significant effect on chloride diffusion and thus the deterioration of the structures. However, actual concrete cracks have complicated characteristics, and the mechanisms in which they influence the chloride penetration have not yet been well clarified. This study presents a careful quantification of the geometrical parameters of actual concrete cracks, including density, orientation, tortuosity and width, and explores their correlation with the chloride diffusion properties of concrete. Uni-axial compression tests were conducted on concrete specimens to create various extents of cracking severity. Afterwards, a non-steady state migration method was employed to evaluate the diffusivity of sound and cracked concretes. The geometry parameters of concrete cracks were quantified by an image analysis technique after the migration tests. The test results suggest a linear relationship between the crack tortuosity and the degree of crack orientation. In addition, chloride diffusion into concrete depends greatly on the crack density and crack tortuosity in addition to the crack width. In particular, the crack tortuosity is a critical factor influencing chloride penetration when the crack width ranges from 150 to 370 μm. Considering the crack tortuosity and orientation, the effective crack width and effective crack density are proposed as correlated with the chloride diffusivity of cracked concrete. [ABSTRACT FROM AUTHOR]

The deflection of reinforced concrete (RC) beams with low span-effective depth ratios can be attributed equally to shear and flexural deformations. Although truss models can be used to evaluate shear deformation, their prediction accuracy is strongly dependent on how the diagonal crack angle is estimated. This paper presents the results from an experimental test of nine RC beams that failed in shear and had low span-effective depth ratios (i.e., a / h 0 < 3 ). It then proposes a multi-angle truss model for predicting ultimate shear deformation while considering the effect of the bending moment variation along the span on diagonal crack angles. An empirical formula is also presented to predict the shear stiffness degradation of RC beams up to the ultimate state through regression analyses. Numerical results show that the proposed multi-angle truss model can be used to predict the shear deformation of RC beams with low span-effective depth ratios with reasonable accuracy. [ABSTRACT FROM AUTHOR]

This paper presents a set of closed-form solutions for Mode II debonding process of fiber-reinforced polymer-to-substrate bonded joints subjected to combined thermal and mechanical loading. Six different bond–slip models are considered in deriving the closed-form solutions. For each bond–slip model, explicit expressions for the debonding load, effective bond length, interfacial shear stress, interfacial slip as well as the load–displacement response are presented. Provided the bond length is sufficiently long, the debonding load depends only on the interfacial fracture energy and the temperature variation. A temperature increase leads to an increase in both the debonding load and the effective bond length, and the rate of increase of the latter depends on the bond–slip model of the interface. [ABSTRACT FROM AUTHOR]

Fiber-reinforced polymer (FRP) rods have been increasingly used in grouted ground anchors due to their high strength-to-weight ratio, excellent corrosion resistance, and convenience in incorporating the fiber sensing technology. To establish their pull-out capacity, FRP rods are usually embedded within a grouted steel tube and then subjected to pull-out in the laboratory. The aim of this paper is to develop a numerical method for predicting the nonlinear pull-out response of FRP rods embedded in steel tubes filled with cement grout. In the method, the cement grout is assumed to be subject to simple shear, the local interfacial bond stress–slip model of the bar-to-grout interface is represented by a piece-wise curve comprising elastic, softening, and frictional stages, and the unloading effect is also taken into account. A set of two second-order ordinary differential equations are derived in terms of the displacements of the FRP rod and steel tube and solved analytically to formulate the element transfer matrix. When the thickness of the steel tube approaches infinity, this method can be applied to the problem of FRP rods embedded in rock. Based on the developed numerical method, the interfacial bond properties and snapback phenomenon are analyzed. After the method is validated by comparisons with four sets of experimental data, the effects of the radius and length of FRP rods, the local peak bond stress and the residual frictional strength on the maximum pull-out load are evaluated in a quantitative manner. [ABSTRACT FROM AUTHOR]

Reinforcement corrosion induced by chloride penetration is a major threat to reinforced concrete structures in marine environments. The use of surface treatments to improve the durability of reinforced concrete structures has drawn increasing attention in recent decades due to their cost-effectiveness and convenience in construction. However, current knowledge of a practical, durable design to protect surface-treated concrete structures from chloride-induced steel corrosion remains insufficient. On this account, a rapid numerical approach was developed in this study to analyse the chloride diffusion in bilayer materials. In this approach, the time-dependency was considered for the chloride diffusion coefficients of both the surface-treated layer and the underlying concrete substrate and for the surface chloride content. The validity of this approach was verified with the finite element method and the computation time was only one hundred-thousandth that of the latter. This proposed approach makes it feasible in practice to estimate the probability of corrosion initiation in surface-treated concrete structures via Monte Carlo simulation. [ABSTRACT FROM AUTHOR]

• The effect of delayed PCE addition on the performance of one-part AAMs is reported. • PCE delayed addition slightly enhanced the compressive strength of one-part AAMs. • The delayed addition of PCE reduced its retardation effect in one-part AAMs. • PCE delayed addition slightly improved the rheological performance of one-part AAMs. • The mechanisms that govern such observations are discussed in detail. The admixture addition method is one of the vital factors influencing the fresh performance of ordinary Portland cement (OPC) through directly affecting the adsorption, hydration, and rheology. However, such an issue is less researched in the one-part alkali-activated materials (AAMs), which have great potentials in the industry due to their simple operation, similar to OPC. This study reports and discusses the effects of delayed polycarboxylate ether (PCE) addition at different contents on adsorption, reaction kinetics, and rheology of one-part AAMs. It is found that the delayed PCE addition had minimized the competitive adsorption phenomenon and reduced the consumption rate of PCE surfactants, which in turn facilitates easier adsorption of silicate species onto the GGBS grains. This had decreased the retardation effect of PCE, slightly improved the compressive strength, and lowered the viscosity and yield stress of the one-part AAMs. The mechanisms that govern such observations are further discussed. [ABSTRACT FROM AUTHOR]

Highlights: [•] The pull-out response of grouted FRP rods in steel tubes is analytically modeled. [•] The interfacial bond-slip model is incorporated into the analysis. [•] Different boundary conditions are considered in the analysis. [•] The solutions can be applied to the problem of FRP rods embedded in rock. [Copyright &y& Elsevier]

This paper presents an analytical model for predicting the short and long-term prestress losses in reinforced concrete (RC) beams strengthened with prestressed carbon fiber reinforced polymer (CFRP) sheets/plates. The long-term prestress loss model is developed by treating the strengthened beam as two sub-elements: an RC beam and a prestressed CFRP sheet/plate, connected through an adhesive bonding layer. Through force equilibrium and displacement compatibility of the two sub-elements, the influences of concrete shrinkage, concrete creep as well the interlayer slip between the RC beam and the CFRP sheet/plate on the prestress losses of CFRP are modeled using an incremental model. For a post-tensioned system with strong end anchorages, closed-form solutions for the prestress losses due to creep and shrinkage of concrete are obtained. The reliability of the proposed analytical model is validated through comparisons with previous test results reported by the authors and others research groups. The maximum difference between the experimentally observed values and the predicted values for the long-term prestress losses was found to be only 2.2% of the initial prestress of the CFRP. [ABSTRACT FROM PUBLISHER]

Intermediate crack-induced debonding is often a dominant failure mode in fiber-reinforced polymer (FRP)-strengthened reinforced concrete (RC) beams in flexure. It has been extensively studied for RC beams externally strengthened with unstressed FRP laminates. However, very little work has been done on FRP debonding for RC beams strengthened with prestressed FRP. This article presents a sectional analysis model for predicting the flexural capacity of RC beams strengthened with prestressed FRP laminates with due consideration of different failure modes. The focus is placed on the effective strain in the prestressed FRP at the ultimate states of intermediate crack-induced debonding or rupture of debonded FRP. Through back-calculation analysis of 51 RC beams strengthened with post-tensioned FRP laminates, a model for predicting the effective strain of prestressed FRP for ultimate strength prediction based upon sectional analyses was developed and validated through comparisons with test results. [ABSTRACT FROM AUTHOR]

The interfacial characteristics of geopolymer binder to aggregate composites are poorly understood, especially at molecular level. Herein, molecular models are developed to study, for the first time, the geopolymer-aggregate interface. Chemically, various forms of interfacial bonding are characterized, including Al-O-Si bonding through condensation reactions, Na O and H-bonding. An atomic-level interfacial transition zone (ITZ) is identified, attributed to the concentration of –OH groups. Increasing the Si/Al ratio of geopolymer is found to decrease the ITZ density, but have limited effect on the ITZ width. A heterogeneous diffusion characteristic occurs in geopolymer, due to the weak interfacial interaction. Mechanically, lowering the Si/Al ratio promotes the interfacial strength due to the stronger interfacial interaction and higher cross-linking degree in geopolymer. Under loading the interfacial fracture undergoes three stages: crack propagation, chain bridging (including aluminosilicate and ionic bridging) and breakage. The above atomic-level findings may facilitate a better design of geopolymer concrete in engineering. [Display omitted] [ABSTRACT FROM AUTHOR]

Highlights: [•] Finite element (FE) modeling of RC beams exposed to fire was highlighted. [•] Particular attention was paid to the modeling of interfacial bond-slip behavior. [•] Comparisons between existing test data and numerical results were presented. [•] FE predictions provided a good understanding of the local behavior of RC beams under fire. [Copyright &y& Elsevier]

A self-stressed steel fiber reinforced concrete (SS-SFRC) moment connection is proposed to strengthen multi-span simply-supported T-section reinforced concrete (RC) girders. In the proposed moment connection technique, old concrete needs to be removed from adjacent girder ends and negative moment reinforcement is placed into cast-in-place SS-SFRC to develop continuity over the interior support. Two series of tests consisting of 15 girder specimens were conducted to investigate the reliability of the proposed SS-SFRC moment connection. An analytical model is presented to predict the load-carrying capacity of the resulting continuous composite girders. Test results show that SS-SFRC is excellent in preventing concrete cracking at the interior support. Both the load-carrying capacity and the serviceability of the strengthened girders are significantly improved. The analytical model is demonstrated to predict the load-carrying capacity of the resulting continuous composited girders with an acceptable degree of accuracy. [ABSTRACT FROM AUTHOR]

Allowing for the tension stiffening effects resulting from the bond between steel reinforcement and surrounding concrete leads to effective deformation analysis of reinforced concrete (RC) members when using a nonlinear finite element analysis modeled on the smeared crack concept. Nowadays, externally bonded fiber reinforced polymer (FRP) composites are widely used for strengthening existing RC structures. However, it remains unclear to what extent the tension stiffening of postcracking concrete is quantitatively influenced by the addition of FRP composites, as a result of the bond between the FRP and the concrete substrate. This article presents a discrete model, which is based on rigid body spring networks (RBSN), for investigating the tension stiffening behavior of concrete in FRP-strengthened RC tensile members. A two-parameter fracture energy-based model was deployed to represent the bond-slip behavior of the FRP-to-concrete interface. The reliability of the RBSN model was verified through comparisons with previous test results. Further parametric analysis indicates that the tension stiffening of concrete is hardly influenced by the addition of FRP composites before the yield of steel reinforcement has occurred although concrete crack patterns and crack widths may be influenced by the bond-slip behavior of the FRP-to-concrete interface. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a new shear strength model for reinforced concrete (RC) beam–column joints subjected to cyclic lateral loading. In the proposed model, the reinforced concrete in the joint panel is idealized as a homogenous material in a plane stress state. The contribution of the joint shear reinforcement (including both the transverse steel reinforcement and the intermediate longitudinal steel reinforcement of the column) is taken into account through the nominal tensile strength of the idealized material. The effect of tensile straining in the transverse direction on the compressive strength of the idealized material is accounted for using the Kupfer–Gerstle biaxial tension–compression failure envelope. Comparisons with the results of 106 existing tests of both exterior and interior beam–column joints, with and without transverse steel reinforcement, demonstrate the accuracy of the proposed model. These comparisons also illustrate the superior accuracy of the proposed model over existing models. A subsequent trend analysis using the test database confirms that all key parameters influencing the shear strength of beam–column joints have been appropriately considered in the proposed model. The proposed model is believed to be suitable for design use due to its simple form, wide applicability and accuracy. [Copyright &y& Elsevier]

Abstract: This paper presents an experimental study on the seismic retrofit of reinforced concrete (RC) square columns with polyethylene terephthalate (PET) fibre reinforced polymer (FRP) composites, which is a new type of FRP with a larger tensile capacity compared to conventional FRPs. Test results of six PET FRP jacketed specimens are presented and compared with those of a specimen with high strength aramid FRP (HS AFRP) and two reference specimens. Hysteretic responses and failure modes of the specimens under a constant axial load and cyclic lateral loads, as well as the responses of the internal steel reinforcement and the FRP jackets are discussed in detail. The contributions of shear deformation and fixed end rotation due to the slip of longitudinal steel bars to the lateral displacement of columns are also investigated. The results show that PET FRP is a promising alternative to conventional FRPs for the seismic retrofit of RC columns. It improves the displacement ductility of RC columns significantly and does not rupture at the ultimate limit state. Future studies should examine a wider range of geometry and material properties and they should also be concerned with a systematic and direct comparison between specimens jacketed with PET FRP and conventional FRP with equivalent jacket stiffness. [Copyright &y& Elsevier]

Abstract: A post-tensioned negative moment connection technique is proposed to strengthen old multiple span simply-supported T-section reinforced concrete (RC) girders. In this technique, old concrete is removed from girder ends and flanges, a diaphragm is placed between the two adjacent girder ends over the interior support and negative moment steel strands are installed in new cast-in-place concrete and post-tensioned to make continuity. A total of six two-span simply-supported T-section RC girders were cast and five of them strengthened using the provision of the proposed post-tensioned negative moment connections. Both laboratory tests and nonlinear finite element (FE) analysis were conducted to investigate the structural performance of the strengthened RC girders. The moment redistribution in the strengthened continuous RC girders was also measured and is discussed. It is shown that the proposed post-tensioned moment connection technique can reliably transfer the negative moment over the support and markedly increase member stiffness and the load-carrying capacity of existing multiple span simply-supported RC girders. The validity of the FE analysis was also demonstrated through comparisons with experimental results. [Copyright &y& Elsevier]

Abstract: The enhancement of long-term durability of marine structures is a matter of interest to many researchers. The study presented in this paper examines the effectiveness of a water reducer and chloride barrier surface impregnation of the concrete cover of reinforced concrete (RC) structures, exposed to a marine environment. Specific focuses is on how surface cracks created (1) before impregnation and (2) after impregnation, affect the effectiveness of the surface treatment. The experiments are conducted in an environment which is as close as possible to the real humid subtropical marine environment. A series of reinforced concrete (RC) prisms and concrete cylinders, each treated with various commercial surface impregnation agents, were exposed to cyclic sea water shower under an outdoor environment to accelerate the dry/wet cycles for 1 year. Six types of surface impregnation agents, including four types of silane-based water repellent agents and two types of sodium silicate-based pore blockers (water–glass) were applied. Three types of RC prisms were prepared to simulate the different cracking possibilities, which may occur in surface impregnated concrete structures, during their service life. No cracks were introduced in the first prism group, while cracks were introduced before and after surface impregnation, in the second and third groups, respectively. The time-dependent water absorption of all specimens was monitored during exposure to the dry/wet cycles. Finally the specimens were split open to measure the penetration depths of the surface impregnation agents and the chloride penetration profiles. The areas with corrosion evident in the steel reinforcement in the RC prisms were also measured. Sodium silicate-based pore blockers were found to be inefficient in preventing chloride penetration of concrete under simulated marine exposures. The long-term efficiency of water repellent agents used for surface impregnation was found to be highly dependent on the type of agent and whether impregnation was carried out before or after crack formation. [Copyright &y& Elsevier]

This paper presents the results from a comparative study on the residual bond performance of epoxy and cement-bonded carbon fiber-reinforced polymer (CFRP) reinforcement to concrete interfaces that experienced different levels of elevated temperature exposure. In total twelve groups of concrete beams strengthened with CFRP sheet and grids of the same tensile stiffness, through epoxy and cement-based adhesive bonding, respectively, were carefully prepared and exposed to different test temperature levels from 20 °C to 600 °C. Both polymer cement mortar (PCM) and ductile engineered cementitious composite (ECC) were adopted as the cement binder. The strengthened beams were tested under three-point monotonic bending after different levels of elevated temperature exposure. It was found that with increasing the exposure temperature, the failure modes of the strengthened beams shifted from debonding in a thin layer of concrete substrate to debonding near the fiber-adhesive interface and tearing off of CFRP grids from the ECC matrix. The peak loads of concrete beams strengthened with cement-bonded CFRP grids are at least 50% higher than their counterparts strengthened with epoxy-bonded CFRP sheets given the same temperature exposure. [ABSTRACT FROM AUTHOR]

Existing studies have proved that interfaces between the prefabricated layer and the cast‐in‐place layer significantly affect the static performance of prefabricated concrete structures using the wet connection method, but little has been known about the fatigue performance, especially at high load levels. This study investigated the fatigue behavior of prefabricated composite concrete beams under negative bending moment induced by the high‐stress level cyclic load. Fifteen composite T‐beams with three different designed sections were tested. The failure mode, cracking and deflection behavior, steel strain development, and the new‐old concrete interfacial deformation were recorded and analyzed. It was found that the stress redistribution occurred with the debonding cracks generated at the new‐old concrete interface, causing the degradation of the static and fatigue performance. Setting interfacial shear keys could prevent such debonding failure at static and low cyclic loading levels. Meanwhile, the corresponding mechanisms were verified through cross‐sectional analysis. [ABSTRACT FROM AUTHOR]

Passive daytime radiative cooling has emerged as a promising green technology for the thermal management of buildings, vehicles, textiles, and electronics. Typically, both high solar reflectance and high thermal emissivity are prerequisites to achieve sufficient daytime cooling. However, colored radiative cooling materials are facing the dilemma of introducing visible light absorption, leading to challenges in balancing cooling and aesthetic demands. Here, three colored bilayer radiative cooling coatings, each comprised of a white base layer and a colored top layer with fluorescence enhancement are fabricated. Three phosphors (Sr2Si5N8:Eu2+, Y3Al5O12:Ce3+, and (Ba,Sr)SiO4:Eu2+) are employed with respective photoluminescence quantum yields (PLQYs) of 81%, 95.8%, and 91.0% as the colored pigment in the top layer. To mitigate the contradiction between coloration and solar reflectance, SiO2 microspheres are introduced into the top layer and utilize their Mie‐resonance‐based multiple scattering to increase the photoluminescent (PL) properties of the phosphors, which jointly boosts the effective solar reflectance (ESR) of the top layer. As a result, the three bilayer coatings exhibit soft colors while achieving subambient cooling with temperature drops of up to 1.5 °C. This fluorescence‐enhancement strategy may pave the way for preparing highly efficient radiative cooling coatings with tunable colors. [ABSTRACT FROM AUTHOR]

To broaden the applications of fiber reinforced polymer (FRP) reinforced concrete structures under marine environments, it is crucial to gain a fundamental understanding of their short- and long-term performance. In this study, the effectiveness of the proposed multi-scale simulation platform, which incorporates the proposed degradation model of FRP bars, is quantitatively assessed via the comparison between the predicted results and experimental load-displacement curves under different marine conditions (continuous immersion, wet-dry cycling and subtropical marine atmosphere). The spatially physical, chemical and mechanical information, such as pH value, moisture, hydration degree, strength, porosity during the exposure was also revealed. Finally, the long-term structural performance of FRP-reinforced concrete beams exposed to the predefined environmental conditions over durations of 20, 50 and 100 years are assessed using the multi-scale simulation platform. After 50 years of exposure, the load-bearing capacity of GFRP-reinforced concrete beams was found to decrease by 35.8 % in immersion conditions and by 17.9 % in subtropical marine atmosphere. The degradation of the load-bearing capacity in the case of 100 years exposure to subtropical marine atmosphere was found to be less severe than that (23.1 %) in the case of 50-year immersion. These findings not only deepen our understanding of the material and structural properties of FRP-reinforced concrete beams under various environmental conditions, but also validate the effectiveness of the proposed multi-scale platform. • A degradation model for FRP bar was proposed and validated via experimental data. • A multi-scale simulation platform was developed for FRP reinforced concrete. • The short-/long-term performance under different marine conditions was studied. • The simulation platform revealed physical, chemical, and mechanical information. • Immersion led to more severe degradation than subtropical marine atmosphere. [ABSTRACT FROM AUTHOR]

• Temperature compensation is discussed by considering interfacial action between the monitored structure and bonded FBG sensors. • Modified temperature-compensation function is proposed to enhance measurement accuracy of FBG sensors. • Effectiveness of the proposed function is validated by experiments. • Improved temperature-compensation measures for FBG sensors applied to structures under different loading conditions are suggested. • Hysteretic response time of FBG sensors is discussed and proper packaging design is experimentally proved to be feasible for decreasing the response time of FBG under thermal loading. Fiber Bragg grating (FBG) based sensors have been extensively used to monitor the deformation of structures (i.e., aircrafts, ocean platforms, bridges, tunnels, pavements and high-speed railways), for the advantages of absolute measurement, anti-electromagnetic interference, high precision, multiplexing and integration of sensing network over other sensors. However, due to the temperature-sensitive characteristics of FBG, temperature compensation should be considered to remove the thermal effect, so as to accurately reflect the strain information of the monitored structure. Traditional temperature-compensation method ignores the influence of the interfacial interaction between the structure and the bonded FBG on the measured data, which may lead to the compensated value smaller than the actual value. For this reason, modified temperature-compensation function with the influence of interfacial action considered is proposed to enhance the measurement accuracy of FBG based sensors. Laboratory tests on two kinds of samples under temperature loading have been conducted to validate the effectiveness of the proposed function. Improved temperature-compensation measures for FBG based sensors applied to structures under different loading conditions (i.e., high and low temperature, static and dynamic mechanical load) are discussed, with the temperature hysteresis of FBG sensing elements considered. The study in this paper presents a scientific instruction for the better interpretation of testing data measured by FBG sensors and the accurate use of FBG sensors for the temperature and strain detection of structures in engineering [ABSTRACT FROM AUTHOR]

An experimental work on the shear strengthening of reinforced concrete (RC) beams completely wrapped by large rupture strain (LRS) FRP is presented in this paper. A total of 14 three-point bending tests were carried out on simply supported RC beams, with a focus on the impacts of the shear span-to-depth ratio (1.53, 2.25, and 3.01) and FRP reinforcement ratio (0.37 %, 0.75 %, and 1.12 %) on the shear behavior of RC beams strengthened with LRS FRP. For the beams with a median length, the ductility coefficients increased by 81 %, 154 %, and 335 % with the increase in FRP reinforcement ratio. For the long beams, the ductility coefficients increased by 116 %, 286 %, and 470 % as the FRP reinforcement ratio improved. The ductility markedly improved with the increase in shear span-to-depth ratio. PET FRP's fracture was not found at the ultimate state, and the improvement in mid-span deflection after the peak state was mainly contributed by the shear deformation. The strengthened specimens exhibited a ductile shear failure mode. For the short RC beams, the shear capacity was slightly enhanced, and the ductility was not improved after being strengthened. The strengthened short RC beams still showed a brittle diagonal compression failure mode. The experimental shear contribution of PETT FRP was compared with the prediction of the existing design guidelines. Finally, an effective strain model of PET FRP, including the shear span-to-depth ratio effect, was proposed and verified with the experimental results. • Shear performance was studied for shear-critical RC beams completely wrapped by LRS FRP. • Ducile shear failure of LRS FRP-strengthened RC beams was observed. • Effect of shear span to depth ratio on shear behavior was discussed. • Six design guidelines for FRP shear contribution were evaluated. [ABSTRACT FROM AUTHOR]

Conventional concrete structures often rely on labor-intensive and wasteful formwork systems. In contrast, 3D concrete printing (3DCP) technology offers a more efficient alternative for fabricating complex-shaped permanent formwork. This study fabricates innovative composite beams using 3D-printed ultra-high-performance strain-hardening cementitious composites (UHP-SHCC) as permanent formwork, followed by casting infill materials. The spatially tailored fiber dosage distribution in the 3D-printed UHP-SHCC permanent formwork enables the performance optimization of the composite beam. The flexural behavior of three such formed composite beams is evaluated using four-point bending tests. A reference beam with homogeneous fiber distribution in the UHP-SHCC formwork is also tested for comparison. The failure mode, overall load–displacement response, and cracking performance of four beams are comprehensively characterized and compared. The results reveal that employing graded fiber dosages in UHP-SHCC formwork enhances fiber efficiency and structural performance while reducing fiber usage and material costs. The use of 3D-printed UHP-SHCC formwork with graded fiber distribution, despite using 25% less fibers and reducing the material cost by 12.8%, leads to a 65.6% increase in strength and over 50% increase in energy absorption, or a 77.4% increase in ultimate displacement with comparable peak load to the reference beam with homogeneous fiber distribution. [ABSTRACT FROM AUTHOR]

Precast concrete sandwich panel (PCSP), consisting of two concrete wythes and a thermal insulation layer in between, can improve greatly the energy efficiency in buildings. This study aims to investigate the structural performance of a new type of PCSP, for which reinforced geopolymer concrete is used as two wythes and connectors made of fiber-reinforced polymer (FRP) composites are used to realize the composition action. Eight so-formed precast geopolymer concrete sandwich panels (PGCSP) were fabricated and evaluated their flexural performance through four points bending test. The experimental parameters included connector type (plate-type and hexagonal tubular), connector spacing (300 and 525 mm), reinforcement type (basalt FRP reinforcement and steel reinforcement) and reinforcement ratio (0.30% and 0.43%). The load vs. mid-span deflection relationships, crack patterns, failure modes, reinforcement strain development relationships, and degrees of composite action of the panels were investigated in details. In addition, two-dimensional (2D) finite element (FE) analysis was conducted to reproduce the test results and to facilitate a better understanding on the panel's performance particularly the connector's behavior at different loading stages. It was concluded that a higher degree of composite action in the panels can be achieved through the use of FRP tubular connectors, besides, all the FRP tubular connectors can retain their load transfer capacities at the ultimate limit state owning to their ductile failure behavior. [ABSTRACT FROM AUTHOR]