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Ling-Yun Feng, Ai-Jiu Chen, Han-Dong Liu
International Journal of Concrete Structures and Materials, Volume 15, pp 1-14; https://doi.org/10.1186/s40069-021-00475-8

Abstract:
Rubberized concrete is an environmentally friendly building material that mixes rubber particles from old automobile tires into normal concrete in place of fine aggregate. The addition of rubber particles can improve the abrasion resistance of normal concrete observably. It has a good application prospect in hydraulic engineering, especially in the concrete building parts with high abrasion resistance. However, there are few experimental studies on the abrasion resistance of rubberized concrete, and the influence law and mechanism of rubber particles on the abrasion resistance of concrete are not understood. In this paper, the abrasion resistance of rubberized concrete is studied using the underwater-steel-ball method. The results show that rubber particles increase the slump of concrete mixtures. The abrasion resistance of rubberized concrete increases significantly with increasing rubber particle content, whereas the compressive strength decreases linearly. For the same rubber particle size and content, the abrasion resistance of rubberized concrete positively correlates with compressive strength and larger rubber particles significantly improve the abrasion resistance. Rubber particle content is the factor that most strongly affects abrasion resistance of rubberized concrete, followed by the compressive strength. Rubber particle pretreatment methods of NaOH + KH570 can significantly improve the abrasion resistance of rubberized concrete.
Yong Zhou, Yuxuan Yang, Bigya Gyawali, Weiping Zhang
International Journal of Concrete Structures and Materials, Volume 15, pp 1-15; https://doi.org/10.1186/s40069-021-00468-7

Abstract:
This paper presents the simulation of the permeation of saturated cement paste based on a novel pore network model. First, a 2D hydration model of cement particles was developed by extending the work of Zheng et al. 2005 to provide the background for the network construction. Secondly, the establishment of the pore network model and simulation of permeation of saturated cement paste were carried out. The irregular pores between any two hydrated cement particles were linearized with clear distances as the diameters of pores. The straight tubular pores were interconnected with one another to form the network model. During this process, the weighted Voronoi diagram was employed to operate on the graphical expression of the hydrated cement particles. Water permeation in saturated cement paste was simulated to verify the pore network model. Finally, the factors including water–cement ratio, reaction temperature, reaction time and cement particle size that would influence water permeation were numerically investigated.
Ninghui Liang, Lianxi Ren, Shuo Tian, Xinrong Liu, Zuliang Zhong, Zhiyun Deng, Ru Yan
International Journal of Concrete Structures and Materials, Volume 15, pp 1-23; https://doi.org/10.1186/s40069-021-00472-x

Abstract:
To study the hybrid effects of polypropylene fiber and basalt fiber on the fracture toughness of concrete, 13 groups of notched concrete beam specimens with different fiber contents and mass ratios were prepared for the three-point bending test. Based on acoustic emission monitoring data, the initiation cracking load and instability load of each group of specimens were obtained, and the fracture toughness parameters were calculated according to the double-K fracture criterion. The test results show that the basalt fiber-reinforced concrete has a greater increase in initial fracture toughness, and the toughness of coarse polypropylene fiber-reinforced concrete is more unstable. Moreover, after the coarse polypropylene fiber content reaches 6 kg/m3 and the basalt fiber content reaches 3 kg/m3, increasing the content will not significantly improve the fracture toughness of the concrete. The polypropylene–basalt fiber will produce positive and negative effects when mixed, and the mass ratio of 2:1 was optimal. Finally, the fitting analysis revealed that the fracture process of polypropylene–basalt fiber-reinforced concrete (PBFRC) can be objectively described by the bilinear softening constitutive curve improved by Xu and Reinhardt.
, Tieng Cheewaket, Chai Jaturapitakkul
International Journal of Concrete Structures and Materials, Volume 15, pp 1-11; https://doi.org/10.1186/s40069-021-00473-w

Abstract:
This research aimed to create value of construction and demolition waste to be able used as a recycled coarse aggregate (RCA) in durable concrete, based on 7-year field investigation in marine site. Fly ash was used to substitute Portland cement type I in RCA concrete varied from 0 to 50% by weight of binder with three W/B ratios and comparing to natural aggregate (NA) concrete. Cubical concrete specimens were cast having round steel bars embedded with various concrete coverings to evaluate the durability performances. After 28-day curing, the specimens were placed at a tidal zone in the gulf of Thailand and investigated both mechanical and durability performances at 7-year exposed period. Based on site monitoring, 15–25% fly ash RCA concrete with W/B ratio of 0.40 would be advantaged to resist destruction due to the marine attack when compared with NA concrete with the same water-to-binder ratio.
Yibo Yang, , Weizhen Zeng, Yanjun Li, Qiaohui Chen, Wenying Guo, Hengchang Wang, Yingqin Chen
International Journal of Concrete Structures and Materials, Volume 15, pp 1-11; https://doi.org/10.1186/s40069-021-00470-z

Abstract:
To reduce the cost of lightweight concrete (LWC) partition panels and to address recycling concrete waste, this work utilized completely recycled fine aggregate (CRFA) to replace the natural fine aggregate and ceramsite in the preparation of LWC and LWC partition panels. To this end, an autoclave-free curing process and an air-entraining agent were used to prepare the CRFA-LWC. The workability, compressive strength, drying shrinkage, and pore structure of the CRFA-LWC and the performance of the CRFA-LWC partition panels were then investigated. The results show that the optimal ratio of the CRFA to the cement is 2.2 for the lightweight concrete, and the optimal panel cross section is a rounded rectangular one. All the pores in the CRFA-LWC have a diameter of smaller than 0.17 mm, and the diameter of 89% of them is less than 0.05 mm. In order to satisfy the drying shrinkage requirements stipulated by Chinese code JC/T 169-2016, the CRFA-LWC should be cured for at least 10 days. The economic analysis concludes that the material cost of CRFA-LWC is 40% lower than that of the autoclaved ceramsite concrete. In addition, utilizing CRFA in lightweight concrete can ease the shortage of natural aggregate.
Wei-Jian Yi, Fan Yi,
International Journal of Concrete Structures and Materials, Volume 15, pp 1-23; https://doi.org/10.1186/s40069-021-00469-6

Abstract:
In the recent two decades, the progressive collapse of reinforced concrete (RC) frame structures attracted unprecedented research interests in the structural engineering community. Experiments are regarded as an essential method in this field since actual cases can barely provide sufficient and effective data to support rigorous research. In this paper, prevailing experimental assumptions and configurations among over 100 series of experiments are quantitatively revealed by a bibliometric collection based on systematic search in an academic database. Since numerous experiments have been reported on the progressive collapse of RC frame structures, this paper subsequently presents a state-of-the-art review summarizing both experimental consensuses and controversies constituted by three main aspects: (a) static mechanisms, (b) dynamic behavior, and (c) threat-dependent research. The significance of secondary mechanisms, existing problems of dynamic effects, and potential flaws of the threat-independent assumption are discussed in detail with experimental findings. Future needs are emphasized on research targets, correlations between experiments and design, dynamic effects, threat-dependent issues, and retrofitting. These recommendations might help researchers or designers realize a more reliable and realistic progressive collapse design of RC frame structures in the future.
SangWoo Park, Jangwoon Beak, Kukjoo Kim, Young-Jun Park
International Journal of Concrete Structures and Materials, Volume 15, pp 1-20; https://doi.org/10.1186/s40069-021-00467-8

Abstract:
With the increasing installation cases of underground explosive facilities (e.g., ammunition magazines, hydrogen tanks, etc.) in urban areas in recent years, the risk of internal explosions is also increasing. However, few studies on the measures for reducing damage by the ground vibration have been conducted except for maintaining safety distance. In this study, a method for attenuating the vibration propagated outward by installing a blast-proof panel was numerically and experimentally investigated. Two cubical reinforced concrete structures were manufactured according to the concrete strength and a blast-proof panel was installed on only one side of the structure. Then, acceleration sensors were installed on the external surface to evaluate the propagation of vibration outward depending on the installation of a blast-proof panel. Before a field experiment, a preliminary numerical simulation was performed. The results showed that the acceleration propagated outward could be effectively reduced by installing a blast-proof panel. Even though the performance of a blast-proof panel on vibration reduction was also investigated in the field experiment, significantly larger absolute accelerations were estimated due to the different experimental conditions. Finally, the vibration reduction effect of the blast-proof panel was numerically evaluated according to its thickness and the internal explosion load. A blast-proof panel more effectively reduced the acceleration propagated outward as its thickness increased and the explosion load decreased.
Correction
, Irem Şanal, Jaime Marian
International Journal of Concrete Structures and Materials, Volume 15, pp 1-1; https://doi.org/10.1186/s40069-021-00466-9

Abstract:
An amendment to this paper has been published and can be accessed via the original article.
Aocheng Zhong, , Elisa Lumantarna, Zhiyuan Zhou, Priyan Mendis
International Journal of Concrete Structures and Materials, Volume 15, pp 1-12; https://doi.org/10.1186/s40069-021-00461-0

Abstract:
Steel fibre (SF) reinforcement has been shown to improve the ductility of high strength concrete (HSC), which is known to be brittle. Research conducted to date on steel fibre reinforced concrete and its effects have emphasised post-failure performance and cracking mechanism. The difficulty in predicting the behaviour of fibres is due to the randomly distributed nature of the material within the matrix leading to a probability distribution of results. Published literature has shown a benefit of adding steel fibres in terms of the ductility performance of structures. Clearly, there is a potential for such material as replacement of conventional steel reinforcement. This study proposes a theoretical model of evaluating the potential of using steel fibres as a replacement material to conventional steel reinforcement bars based on the case study, laboratory and theoretical methodologies. The compressive strength of the concrete at key dates, the effective fibre cross-sectional were measured, and a prediction model was created based on the measurement parameters. The use of four-point flexural testing, standard compressive testing and software image modelling provided the study with relevant data used to analyse and compare to the prediction. Greater ductility performance and toughness were observed with increased fibre volumes, confirming proposed predictions and conclusion drawn from published literature. No consistent or conclusive correlations between fibre volumes and the compressive strength of concrete were found. A relationship between fibre volumes and predicted moment capacities of steel fibre reinforced concrete beams was found based on the proposed theoretical flexural analysis method.
Jin Zhou, Zhelun Zhang, Tessa Williams, Sashi K. Kunnath
International Journal of Concrete Structures and Materials, Volume 15, pp 1-15; https://doi.org/10.1186/s40069-021-00463-y

Abstract:
The development of fragility functions that express the probability of collapse of a building as a function of some ground motion intensity measure is an effective tool to assess seismic vulnerability of structures. However, a number of factors ranging from ground motion selection to modeling decisions can influence the quantification of collapse probability. A methodical investigation was carried out to examine the effects of component modeling and ground motion selection in establishing demand and collapse risk of a typical reinforced concrete frame building. The primary system considered in this study is a modern 6-story RC moment frame building that was designed to current code provisions in a seismically active region. Both concentrated and distributed plasticity beam–column elements were used to model the building frame and several options were considered in constitutive modeling for both options. Incremental dynamic analyses (IDA) were carried out using two suites of ground motions—the first set comprised site-dependent ground motions, while the second set was a compilation of hazard-consistent motions using the conditional scenario spectra approach. Findings from the study highlight the influence of modeling decisions and ground motion selection in the development of seismic collapse fragility functions and the characterization of risk for various demand levels.
Liang-Dong Zhuang, Hong-Bing Chen, Yuan Ma, Ran Ding
International Journal of Concrete Structures and Materials, Volume 15, pp 1-25; https://doi.org/10.1186/s40069-021-00464-x

Abstract:
The headed studs have been widely applied in steel–concrete composite structures as shear connectors. However, the tensile performance of headed studs is also key to the structural performance in many cases such as the semi-rigid composite joints including steel beam–concrete wall joint and steel column–base joint. Therefore, this study presents experimental and analytical study on the whole-process tensile behavior of headed studs. Tests on a total of 33 pullout specimens are first conducted. The tensile capacity and load–deformation behavior of the anchorage concrete, which dominates the structural performance of headed studs, are thoroughly analyzed. In addition, test data in the literature are collected for quantitatively evaluating the influence of embedment depth, bearing area, boundary conditions, and concrete strength on the tensile behavior of the anchorage concrete. On the basis of the influence evaluation, an analytical model represented by a piecewise function is proposed to describe the whole-process load–deformation behavior of the anchorage concrete and validated through the comparison between the predicted curves and all collected experimental results. Then the proposed model is applied to simulate the rotational behavior of the typical semi-rigid joint anchored by headed studs, which takes the contribution of the anchorage concrete into consideration, and is verified by experimental results. The research findings indicate that tensile behavior of anchorage concrete is crucial to the structural performance of semi-rigid joints, even for headed studs with large embedment depth and bearing area.
, Huaming Tang, Shenyao Han
International Journal of Concrete Structures and Materials, Volume 15, pp 1-16; https://doi.org/10.1186/s40069-021-00465-w

Abstract:
In this paper, polyvinyl chloride (PVC) coarse aggregate with different mixing contents is used to solve the problems of plastic pollution, low energy absorption capacity and poor damage integrity, which provides an important reference for PVC plastic concrete used in the initial support structures of highway tunnels and coal mine roadway. At the same time, the energy absorption characteristics and their relationship under different impact loads are studied, which provides an important reference for predicting the energy absorption characteristics of concrete under other PVC aggregate content or higher impact speed. This study replaced natural coarse aggregate in concrete with different contents and equal volume of well-graded flaky PVC particles obtained by crushing PVC soft board. Also, slump, compression, and splitting strength tests, a free falling low-speed impact test of steel balls and a high-speed impact compression test of split Hopkinson pressure bar (SHPB) were carried out. Results demonstrate that the static and dynamic compressive strength decreases substantially, and the elastic modulus and slump decrease slowly with the increase of the mixing amount of PVC aggregate (0–30%). However, the energy absorption rate under low-speed impact and the specific energy absorption per MPa under high-speed impact increase obviously, indicating that the energy absorption capacity is significantly enhanced. Regardless of the mixing amount of PVC aggregate, greater strain rate can significantly enhance the dynamic compressive strength and the specific energy absorption per MPa. After the uniaxial compression test or the SHPB impact test, the relative integrity of the specimen is positively correlated with the mixing amount of PVC aggregate. In addition, the specimens are seriously damaged with the increase of the impact strain rate. When the PVC aggregate content is 20%, the compressive strength and splitting strength of concrete are 33.8 MPa and 3.26 MPa, respectively, the slump is 165 mm, the energy absorption rate under low-speed impact is 89.5%, the dynamic compressive strength under 0.65 Mpa impact air pressure is 58.77 mpa, and the specific energy absorption value per MPa is 13.33, which meets the requirements of shotcrete used in tunnel, roadway support and other impact loads. There is a linear relationship between the energy absorption characteristics under low-speed impact and high-speed impact. The greater the impact pressure, the larger the slope of the fitting straight line. The slope and intercept of the fitting line also show a good linear relationship with the increase of impact pressure. The conclusions can be used to predict the energy absorption characteristics under different PVC aggregate content or higher-speed impact pressure, which can provide important reference for safer, more economical, and environmental protection engineering structure design.
, Irem Şanal, Jaime Marian
International Journal of Concrete Structures and Materials, Volume 15, pp 1-9; https://doi.org/10.1186/s40069-021-00462-z

Abstract:
Super absorbent polymers (SAP) are the recent promising chemical admixtures with the potential for reducing the shrinkage, cracking, freeze/thaw and increasing the durability of the concrete. These polymers are classified as hydrogels when cross-linked and can retain exceptionally high amount of liquid solutions of their own weight. In this paper, the flowability of the concrete is quantified by means of developing a percolation-based image processing method and the transient behavior of the viscosity of the SAP-contained mortar mixture is characterized via numerical solution of Navier–Stokes relationship. In addition, rheological measurements and the analytical development has been carried out for complementary verification of the viscosity trends. Controlling the flow within such relatively short period of time is essential for tuning the functionality of concrete during the construction as well as it’s respective resilience during the extended period of application.
Dapeng Yang, Bei Zhang, Guojun Liu
International Journal of Concrete Structures and Materials, Volume 15, pp 1-22; https://doi.org/10.1186/s40069-021-00459-8

Abstract:
Steel-fiber reinforced concrete slabs have good blast and spall resistance. In this study, compression and splitting tensile experiments were carried out to obtain the basic quasi-static mechanical properties of the steel-fiber concrete specimens and the influence of steel-fiber parameters was revealed. In-field explosion experiments were performed to study the dynamic responses and damage modes of the steel-fiber reinforced concrete slabs. Five typical spall damage modes were observed, the distribution law of the spalling fragments was obtained, and the influence of steel-fiber shape, length, length–diameter ratio and volume percentage on the spall performance were revealed. These results will provide a basis for the application of steel-fiber reinforced concrete slabs in protective structures.
, , , Eleftherios Eleftheriou, , Rogiros Illampas
International Journal of Concrete Structures and Materials, Volume 15, pp 1-17; https://doi.org/10.1186/s40069-021-00460-1

Abstract:
Fabric reinforced cementitious matrix (FRCM) composites, also known as textile reinforced mortars (TRM), an inorganic matrix constituting fibre fabrics and cement-based mortar, are becoming a widely used composite material in Europe for upgrading the seismic resistance of existing reinforced concrete (RC) frame buildings. One way of providing seismic resistance upgrading is through the application of the proposed FRCM system on existing masonry infill walls to increase their stiffness and integrity. To examine the effectiveness of this application, the bond characteristics achieved between (a) the matrix and the masonry substrate and (b) the fabric and the matrix need to be determined. A series of experiments including 23 material performance tests, 15 direct tensile tests of dry fabric and composites, and 30 shear bond tests between the matrix and brick masonry, were carried out to investigate the fabric-to-matrix and matrix-to-substrate bond behaviour. In addition, different arrangements of extruded polystyrene (XPS) plates were applied to the FRCM to test the shear bond capacity of this insulation system when used on a large-scale wall.
In-Deok Han, Dongyeop Han
International Journal of Concrete Structures and Materials, Volume 15, pp 1-15; https://doi.org/10.1186/s40069-021-00458-9

Abstract:
The aim of this research is to provide a quantitative method for evaluating concrete segregation. Because of various conditions of concrete materials, mix proportions, and delivery, concrete can be segregated. The acquisition inspection executed in construction field for supplied ready-mixed concrete is an important quality control process for concrete. Among the inspections conducted at the project site, segregation of concrete mixture should be evaluated before placing the concrete mixture, currently a qualitative inspection on concrete segregation was conducted. For a normal concrete mixture with slumping behavior, shear slump or collapse slump often occur as an indication of segregation. The suggested evaluation index of segregation for normal concrete (EISN) was induced from the shape of the concrete slumping: relation between the maximum distance of flow and the minimum distance of flow. To evaluate the feasibility of EISN, two different concrete mixture conditions were tested. The recommended EISN parameter of segregation is 1.09 using the three grades of concrete quality. This new quantitative method of evaluating segregation of the concrete mixture is expected to contribute to a more efficient quality control in concrete construction.
S. M. Iqbal S. Zainal, , Raizal S. M. Rashid
International Journal of Concrete Structures and Materials, Volume 15, pp 1-28; https://doi.org/10.1186/s40069-021-00457-w

Abstract:
The knee beam–column joint is a critical location in a Reinforced Concrete (RC) structure particularly when subjected to earthquake vibrations. The current structural design codes dictate the use of high amounts of steel reinforcements in the frame joint to manage large strain demands in seismic-prone regions. However, these codes could result in the congestion of steel reinforcements in the limited joint area which can consequently produce numerous construction complications. This study aims to improve the structural performance of Knee Joint (KJ) by reducing the load induced to the embedded steel reinforcements during seismic vibrations. Hence, this study attempted to develop a Hybrid Fiber Reinforced Concrete (HyFRC) by combining multiple synthetic fibers to be introduced onto KJ. Six KJ specimens were cast using five developed HyFRC materials and one Control specimen to be experimentally tested under lateral cyclic loading. The results indicated significant improvements for the HyFRC KJ specimens particularly in energy dissipation capacity, stiffness degradation rate, displacement ductility toughness, steel reinforcement strain and hysteretic behavior. A total of six Finite Element (FE) KJ models were developed using the HyFRC materials to verify the results from the experimental testing. The accuracy of the proposed FE models resulted in average percentage differences of 25.89% for peak load, 3.45% for peak load displacement and 0.18% for maximum displacements from the experimental data. In conclusion, this study developed HyFRC materials that are beneficial in providing cost-efficient alternatives to Reinforced Concrete (RC) KJ structures in areas with low to moderate level of seismic risks.
Bolanle Deborah Ikotun, Akeem Ayinde Raheem
International Journal of Concrete Structures and Materials, Volume 15, pp 1-9; https://doi.org/10.1186/s40069-021-00456-x

Abstract:
This paper presents the findings of an investigation into the influence of green-synthesized nano-TiO2 on the characteristics of wood ash (WA) cement mortar. Mortar specimens were prepared by partial replacement of cement with WA (10% by weight) and addition of 1, 2 and 3% nano-TiO2 by weight of binder; using constant water-to-binder ratio (w/b) for all mixtures. The properties evaluated are setting time of the binder and flexural and compressive strength with water absorption of the mortar. The results indicated that addition of 1 and 2% nano-TiO2 reduced setting times of WA cement paste. Also, the flexural and compressive strength of WA cement mortar were higher with the incorporation of up to 2% nano-TiO2. The water absorption of WA cement mortar was reduced when nano-TiO2 was added with 2% incorporation having the best result. The incorporation of NT in WA cement mortar improved its workability and strength characteristics.
Tianxiang Pi, Zhongheng Du, Huchen Zhang, Sicheng Wang
International Journal of Concrete Structures and Materials, Volume 15, pp 1-18; https://doi.org/10.1186/s40069-021-00455-y

Abstract:
Autoclaved aerated concrete (AAC) block masonry has been widely used for bearing walls of multi-story buildings or non-bearing walls of high-rise buildings because of its unique advantages, such as lightweight, low pollution output, and excellent thermal insulation performance. However, traditional AAC block masonry has the disadvantages of high water absorption, poor adhesion to mortar, and low construction efficiency. In order to improve the performance of traditional AAC masonry, this paper proposed a new kind of mortar-free AAC block masonry with concrete core-columns. Fundamental mechanical properties of compression and shear were studied. We divided a total of 16 compression specimens into four groups according to different hollow ratios and strength grades of the block, and eight shear specimens into two groups based on different hollow ratios. Each specimen consists of three-layer blocks with two core columns at the point of quadri-section. The diameters of columns were, respectively, 100 mm and 80 mm. The specimens were loaded at a constant speed to evaluate their bearing capacity, displacement response, crack development, and damage state. The formula of the average values and design values of the compressive and shear strength of masonry were obtained statistically. The stress–strain constitutive relation of masonry expressed by a three-stage curve was presented. Furthermore, the result of numerical analysis using the ABAQUS finite element program aligned well with the experimental results. The compressive strength and shear strength of the new type of masonry are no less than traditional AAC masonry, and new masonry has higher construction efficiency and more stable strength.
Yuliia Khmurovska, Petr Štemberk, Svyatoslav Sikorin, Jiří Němeček, Daria Jóźwiak-Niedźwiedzka, Magdaléna Doleželová, Yuliya Kaladkevich, Eryk Pavalanski, Viktar Fatseyeu
International Journal of Concrete Structures and Materials, Volume 15, pp 1-14; https://doi.org/10.1186/s40069-020-00452-7

Abstract:
The effect of gamma-ray irradiation on cement mortar properties is investigated in this study in order to understand the mechanism behind the strength and stiffness reduction, which may be significant according to the available researches. 60Co irradiation facility with the generating dose rate of 0.1–10 Gy/s and the total activity of 4.4·1015 Bq (120 kCi) was used to perform the irradiation, so that the total observed dose of the irradiated samples reached the values ranging from 12.0 to 15.0 MGy. An identical set of control samples was placed in the same laboratory conditions away from gamma radiation. The results of nanoindentation, X-ray diffraction analysis and mercury intrusion porosimetry of the irradiated and the control samples are shown and explained in detail in this study. The nanoindentation creep compliance and the nanoindentation elastic modulus of the irradiated and the control samples do not show any significant difference. The mineral composition obtained using the X-ray diffraction analysis of the irradiated and the control samples is also similar. The pore structure rearrangement and microcrack occurrence, which were evidenced by the mercury intrusion porosimetry and scanning electron microscopy, led to the porosity increase and may be attributed to the significant decrease of compressive strength.
Eskinder Desta Shumuye, , Zike Wang
International Journal of Concrete Structures and Materials, Volume 15, pp 1-20; https://doi.org/10.1186/s40069-020-00437-6

Abstract:
In this study, the effect of curing temperature on the properties of slag cement concrete after high-temperature exposure was studied, and elevated curing temperature (45 ± 2 °C and 95% relative humidity (RH)) was selected to compare with the standard curing temperature (20 ± 2 °C and 95%RH). Four different concrete mixes with the same mix proportion, except for different slag replacement ratios, were used: 0% (reference), 30% (slag), 50% (slag), and 70% (slag). After high-temperature exposure at 200, 400, 600, and 800 °C, the effect of slag replacement, high temperature, and curing temperature on the compressive strength and mineralogical and microstructural properties of slag cement concrete were studied. Test results indicated that the compressive strength of concrete cured for 7 d at elevated temperatures increased by 28.2, 20.7, 28.8, and 14.7% compared with that cured at the standard curing condition at slag percentages of 0, 70, 50, and 30%, respectively. X-ray diffraction (XRD) and Scanning electron microscope (SEM) results revealed that concrete cured at elevated temperatures exhibited a more condensed phase and contained a higher percentage of hydrates than that cured for 7 d in the standard curing condition. However, after 56 d of curing, concrete in the standard curing condition exhibited a more stable phase and a higher concentration of hydrates.
Mohammad Reza Hamidian,
International Journal of Concrete Structures and Materials, Volume 15, pp 1-16; https://doi.org/10.1186/s40069-020-00453-6

Abstract:
Oil palm shell (OPS) concrete filled steel tube (CFT) columns are acknowledged to be a new type of sustainable composite column. In this type of column, the conventional coarse aggregate was partially replaced with OPS lightweight aggregate to provide a green composite column. This type of CFT column showed higher energy absorption and flexibility compared to CFT columns with normal weight concrete. This research studied the effect of the strength of OPS concrete on the axial compressive behaviour of CFT columns for two grades of OPS concretes. The behaviour was comparable to that of CFT columns with two grades of normal concrete. The results showed that the CFT columns with OPS concrete achieved a new post-peak behaviour. The experimental results of the axial compressive load were compared with the estimation of two international standards. EC4-1994 and ACI318-14 showed a reliable and conservative estimation of the axial load capacity of CFT columns, respectively.
Olaniyi Arowojolu, , Abdullah Almakrab, Nicholas Saras, Richard Nielsen
International Journal of Concrete Structures and Materials, Volume 15, pp 1-12; https://doi.org/10.1186/s40069-020-00444-7

Abstract:
The shear span-to-effective depth ratio (a/d) is one of the factors governing the shear behavior of reinforced concrete (RC) beams, with or without shear reinforcement. In high-strength concrete (HSC), cracks may propagate between the aggregate particles and result in a brittle failure which is against the philosophy of most design guidelines. The experimental results of six HSC beams, with and without shear reinforcement, tested under four-point bending with a/d ranged from 2.0 to 3.0 are presented and compared with different model equations in design codes. The a/d ratio has higher influence on the shear strength of reinforced HSC beams without shear reinforcement than beams with shear reinforcement. Most of the shear resistance prediction models underestimate the concrete shear strength of the beams but overpredict shear resistance of beams with shear reinforcement. However, the fib Model code 2010 accurately predicted the shear resistance for all the beams within an appropriate level of approximation (LoA).
Gian Felice Giaccu, Davide Solinas, , Luigi Fenu
International Journal of Concrete Structures and Materials, Volume 15, pp 1-21; https://doi.org/10.1186/s40069-020-00445-6

Abstract:
Prestressed segmentally constructed balanced cantilever bridges are often subjected to larger deflections than those predicted by calculations, especially for long-term effects. In this paper, the case of modular balanced cantilever bridges, which are prestressed segmental bridges obtained through a repetition of the same double cantilever, is investigated. The considered bridges are two typical cases of modular balanced cantilever both subjected to large deformations during their lifetime. In this case, due to the unusual employed static scheme, creep deflections indefinitely evolve over time particularly at the end of the cantilevers and in correspondence with the central joint. These remarkable deflections cause discomfort for vehicular traffic and in certain cases can lead to the bridge collapse. Important extraordinary maintenance interventions were necessary to restore the viability of the bridges and to replace the viaduct design configuration. To this aim, the static schemes of the structures were varied, introducing new constraints, new tendons, and carbon fiber reinforcements. In the present work, time analysis was performed to compare the time-dependent behavior of the bridge according to two different creep models, the CEB-FIP Model Code 2010 and the RILEM Model B3, with the real-time-dependent behavior of the bridge observed during its lifetime. The two different employed models exhibit different behaviors in terms of displacements and bending moments acting on the bridge. Interesting considerations are made on their reliability in simulating the long-term creep effects that evolve indefinitely over time. Moreover, retrofitting techniques have been proposed and modeled to predict their effectiveness in reducing time-dependent deflections.
Han-Seung Lee,
International Journal of Concrete Structures and Materials, Volume 15, pp 1-15; https://doi.org/10.1186/s40069-020-00438-5

Abstract:
Calcined hwangtoh is a pozzolanic material that is increasingly being used as a mineral admixture in the concrete industry. This study shows a hydration model for cement–hwangtoh blends and evaluates the various properties of hwangtoh-blended concrete using reaction degrees of binders. First, a kinetic reaction model is proposed for analyzing the pozzolanic reaction of hwangtoh. The reaction of hwangtoh includes three processes: the initial dormant period, boundary reaction process, and diffusion process. The mutual interactions between the binary reactions of cement and hwangtoh are thought to be in line with the items in capillary water and calcium hydroxide. Second, the reaction degrees of cement and hwangtoh are determined based on a blended hydration model. Furthermore, the chemical (chemically combined water and calcium hydroxide contents), mechanical (compressive strength), thermal (hydration heat), and durability aspects (carbonation depth) of hwangtoh-blended concrete are systematically predicted. The results show good agreement with experimental results.
International Journal of Concrete Structures and Materials, Volume 15, pp 1-17; https://doi.org/10.1186/s40069-020-00450-9

Abstract:
The purpose of this study is to examine the effects of the seismic wave velocity on vertical displacement of a cable-stayed bridge’s deck under asynchronous excitation. The Quincy Bayview Bridge located in Illinois, USA, and four other generic bridges are selected for the study. Ten records obtained from earthquakes in US, Japan, and Taiwan are used as input for the seismic excitation in the time-history analysis. Two equations are proposed in this study to determine a critical seismic wave velocity that would produce the greatest vertical deck displacement. The critical wave velocity depends on the total length of the bridge, the fundamental period of the bridge, and the C-factor. The C-factor in this study is 0.72, which is based on analyzed results from the five selected bridges. The two equations and the C-factor are verified through application on two 3-span cable-stayed bridges studied previously by Nazmy and Abdel-Ghaffar. The proposed C-factor of 0.72 is recommended for use for typical 3-span cable-stayed bridges with a side-to-main span ratio of about 0.48. The methodology developed in the study, however, can be applied to any specific bridge to examine the excitation of the deck vertical displacement under the longitudinal seismic ground motion.
Jae-Wook Jung, Jung-Wuk Hong
International Journal of Concrete Structures and Materials, Volume 15, pp 1-16; https://doi.org/10.1186/s40069-020-00442-9

Abstract:
Structural behavior against the blast load is evaluated for a structure reinforced by high-performance fiber-reinforced cementitious composites (HPFRCC). The Alfred P. Murrah Federal Building, which experienced a terrorist attack in 1995 is taken into consideration for the blast analysis using the finite element method. The continuous surface cap model (CSCM) is used to simulate the behavior of normal concrete and HPFRCC. By reinforcing normal concrete with HPFRCC, damage, and deformation of the structure are significantly reduced. This study presents an efficient reinforcement method by performing an explosion simulation on the structure using HPFRCC and evaluating the behavior according to various reinforcement methods. Specific reinforcements according to the types of members are required to enhance the efficiency of reinforcement. With the optimized reinforcement using high-performance fiber-reinforced cementitious composites (HPFRCC), the resistance to blast load is significantly improved.
Seung-Jae Lee, Tae-Sung Eom, Eunjong Yu
International Journal of Concrete Structures and Materials, Volume 15, pp 1-14; https://doi.org/10.1186/s40069-020-00440-x

Abstract:
This study analytically investigated the behavior of reinforced concrete frames with masonry infills. For the analysis, VecTor2, a nonlinear finite element analysis program that implements the Modified Compression Field Theory and Disturbed Stress Field Model, was used. To account for the slip behavior at the mortar joints in the masonry element, the hyperbolic Mohr–Coulomb yield criterion, defined as a function of cohesion and friction angle, was used. The analysis results showed that the lateral resistance and failure mode of the infilled frames were significantly affected by the thickness of the masonry infill, cohesion on the mortar joint–brick interface, and poor mortar filling (or gap) on the masonry boundary under the beam. Diagonal strut actions developed along two or three load paths on the mortar infill, including the backstay actions near the tension column and push-down actions near the compression columns. Such backstay and push-down actions increased the axial and shear forces of columns, and ultimately affect the strength, ductility, and failure mode of the infilled frames.
, Jong-Wook Lim, Su-Hyun Jeong
International Journal of Concrete Structures and Materials, Volume 15, pp 1-24; https://doi.org/10.1186/s40069-020-00448-3

Abstract:
To figure out the change in the reinforcing effect of FRP system used for the retrofit of RC beam when it is exposed to high temperature, it is required to evaluate not only the behavior of the entire beam, but also the bond performance at anchorage zone through a bond test according to the increase of external temperature. Moreover, the study to find various fire-protection methods is necessary to prevent the epoxy from reaching the critical temperature during an exposure to high temperature. In this manner, the fire-resistance performances of externally bonded (EB) FRP and near-surface-mounted (NSM) FRP to concrete block were evaluated by high-temperature exposure tests after performing a fire-protection on the surface in this paper. Board-type insulation with mortar was considered for the fire-protection of FRP system. After the fire-protection of the FRPs bonded to concrete blocks, an increasing exposure temperature was applied to the specimens with keeping a constant shear bond stress between concrete and the FRP. Based on the result, the temperature when the bond strength of the FRP disappears was evaluated. In addition, a finite element analysis was performed to find a proper method for predicting the temperature variation of the epoxy which is fire-protected with board-type insulation during the increase of external temperature. As a result of the test, despite the same fire-protection, NSM specimens were able to resist 1.54–2.08 times higher temperature than EB specimens. In the design of fire-protection of FRP system with the board-type insulation, it is necessary to consider the transfer from sides as well as the face with FRP. If there is no insulation of FP boards on the sides, the epoxy easily reaches its critical temperature by the heat penetrated to the sides, and increasing the thickness of the FP board alone for the face with FRP does not increase the fire-resistance capacity. As a result of the FE analysis, the temperature variation at epoxy can be predicted using the analytical approach with the proper thermal properties of FP mortar and board.
International Journal of Concrete Structures and Materials, Volume 15, pp 1-15; https://doi.org/10.1186/s40069-020-00449-2

Abstract:
Concrete structures undergo internal damage; this usually starts at the atomic level with defects that then grow and form cracks, which can propagate through the material. Here, a method of preparation of poly(methyl methacrylate) (PMMA) nanocapsules adhesive system via miniemulsion polymerization technique is reported, where MMA + DMA (resin + accelerator) and BPO (hardener) components are separately encapsulated by PMMA shells. The crack-healing potential of these nanocapsules was then investigated by embedding them into the mortar matrix. The prepared PMMA core–shell self-healing nanostructures survived the mixing and hardening processes, and the hardened mortar alkaline environment. The stress fields associated with propagating cracks (load‐induced cracking) broke the brittle/weak inert shell of these core–shell structures, resulted in releasing the healing agents to bridge the nascent and early-stage fractures (< 10 µm) in a short time. Long-term healing was achieved through the formation of polymorph calcite crystals in the presence of moisture and CO2, which improved the durability of mortar by filling the gaps. Formulation design (addition of chemical admixtures) and process parameters (blade design and mixing speed) were found to directly impact the uniform distribution of nanocapsules, the survival rate of nanocapsules, and the overall strength of the hardened concrete. The stepwise approach to formulate and fabricate a novel high-strength self-healing concrete system unlocks unique opportunities to design nanomaterials that safeguard the integrity of concrete structures.
, Min Jae Kim
International Journal of Concrete Structures and Materials, Volume 15, pp 1-11; https://doi.org/10.1186/s40069-020-00447-4

Abstract:
The present study investigated the resistance of concrete blended with ground granulated blast furnace slag (GGBS) and ferronickel slag (FNS) to cycles of freeze and thaw. The replacement ratio of the binders was 0%, 50 wt% of GGBS and 30 wt% of GGBS + 20 wt% of FNS for O100, OG50 and OG30F20, respectively. Specimens consisted of cement paste and concrete kept at 0.45 water/binder ratio. After 28 days of curing, specimens were subjected to freeze and thaw cycles (300) for measuring the variation of strength, weight loss and fundamental transverse frequency. Simultaneously mercury intrusion porosimetry was performed to examine the pore structure modifications at 28 days. The hydration products for cement paste cured at each determined age were characterized by X-ray diffraction and the content of CH and CSH was obtained from thermogravimetric analysis (TGA). As a result, the ternary blended concrete specimens showed lower deterioration degree when subjected to the freeze and thaw cycles. This may be due to a latent hydraulic and/or pozzolanic reaction producing more CSH in the matrix, which in turn increases the volume of small pores. The increased content of C–S–H gel for OG30F20 was confirmed by TGA, accounting for 69.9%. However, the binder system consisting of ordinary Portland cement and GGBS did not exhibit higher resistance to the given deleterious environment, presumably due to a delayed hydration process.
Diego L. Castañeda-Saldarriaga, Joham Alvarez-Montoya, Vladimir Martínez-Tejada,
International Journal of Concrete Structures and Materials, Volume 15, pp 1-18; https://doi.org/10.1186/s40069-020-00451-8

Abstract:
Self-sensing concrete materials, also known as smart concretes, are emerging as a promising technological development for the construction industry, where novel materials with the capability of providing information about the structural integrity while operating as a structural material are required. Despite progress in the field, there are issues related to the integration of these composites in full-scale structural members that need to be addressed before broad practical implementations. This article reports the manufacturing and multipurpose experimental characterization of a cement-based matrix (CBM) composite with carbon nanotube (CNT) inclusions and its integration inside a representative structural member. Methodologies based on current–voltage (I–V) curves, direct current (DC), and biphasic direct current (BDC) were used to study and characterize the electric resistance of the CNT/CBM composite. Their self-sensing behavior was studied using a compression test, while electric resistance measures were taken. To evaluate the damage detection capability, a CNT/CBM parallelepiped was embedded into a reinforced-concrete beam (RC beam) and tested under three-point bending. Principal finding includes the validation of the material’s piezoresistivity behavior and its suitability to be used as strain sensor. Also, test results showed that manufactured composites exhibit an Ohmic response. The embedded CNT/CBM material exhibited a dominant linear proportionality between electrical resistance values, load magnitude, and strain changes into the RC beam. Finally, a change in the global stiffness (associated with a damage occurrence on the beam) was successfully self-sensed using the manufactured sensor by means of the variation in the electrical resistance. These results demonstrate the potential of CNT/CBM composites to be used in real-world structural health monitoring (SHM) applications for damage detection by identifying changes in stiffness of the monitored structural member.
Chang Seok Lee,
International Journal of Concrete Structures and Materials, Volume 15, pp 1-16; https://doi.org/10.1186/s40069-020-00446-5

Abstract:
In older reinforced concrete (RC) buildings, columns are fragile elements that can induce collapse of entire buildings during earthquakes. An accurate assessment of the seismic vulnerability of RC buildings using nonlinear response history analyses requires an accurate numerical model. The peak-oriented hysteretic rule is often used in existing numerical models to simulate the hysteretic behavior of RC members, with predefined backbone curves and cyclic deterioration. A monotonic backbone curve is commonly constructed from a cyclic envelope. Because cyclic envelope varies according to loading protocols, particularly in a softening branch, it is difficult to obtain a unique backbone curve irrespective of loading protocols. In addition, cyclic deterioration parameters irrespective of loading protocols cannot be found because these parameters are estimated with respect to the backbone curves. Modeling parameters of existing numerical models can also vary with respect to loading protocol. The objective of this study is to propose a loading protocol-independent numerical model that does not require estimates of modeling parameters specifically tuned for a certain loading protocol. The accuracy of the proposed model is verified by comparing the simulated and measured cyclic curves of different sets of identical RC column specimens under various loading protocols.
, Jiandong Lu, Di Wang,
International Journal of Concrete Structures and Materials, Volume 15, pp 1-19; https://doi.org/10.1186/s40069-020-00441-w

Abstract:
Prefabricated construction is becoming increasingly prevalent, however, it is rarely applied in underground constructions, except for tunnel linings, due to the difficulties that arise in jointing various prefabricated components in underground conditions. To solve the vertical location problem of embedded mechanical couplers during the construction of wall–beam–strut joints for a prefabricated metro station, a new connection using welded steel plates is proposed. In this paper, four full-scale specimens of wall–beam–strut joints connected using welded steel plates and mechanical couplers were experimentally tested under monotonic and low-reversed cyclic loading conditions. The testing results were analysed in terms of the ultimate bearing capacity, failure mode, hysteresis, skeleton curve, stiffness degradation, energy dissipation and strain of the reinforcement bars. Notably, the two kinds of joints had similar ultimate bearing capacities and failure modes, but the crack distributions on the tops of the waler beams were different. For the specimens with the welded steel plate connection, tensile horizontal cracks first appeared on the top surface of the beam, where the welded steel plate was located, and then coalesced gradually; however, this cracking pattern was not observed during the experimental test of the specimens connected with the mechanical couplers. Furthermore, it was determined that the energy dissipation and ductility of the welded steel plate connection were better than those of the mechanical coupler connected joint, because the steel plate could redistribute the internal force in the joint and increase the stiffness. It was concluded that the proposed welded steel plate connection could be more favourable than the mechanical coupler connection in the construction of a prefabricated metro station in Guangzhou. Moreover, the results obtained from these experiments could provide guidelines for the corresponding connections employed in underground-prefabricated structures.
Correction
Xudong Chen, Dandan Shi, Nan Shen, Shengtao Li, Saisai Liu
International Journal of Concrete Structures and Materials, Volume 15, pp 1-1; https://doi.org/10.1186/s40069-020-00454-5

Abstract:
An amendment to this paper has been published and can be accessed via the original article.
Arash Sedaghatdoost, Kiachehr Behfarnia, Hamidreza Moosaei, Mohammad Bayati, Mohammad Sadegh Vaezi
International Journal of Concrete Structures and Materials, Volume 15, pp 1-9; https://doi.org/10.1186/s40069-020-00434-9

Abstract:
The properties of mortars containing waste glass powder (WGP) as a cement substitute for sustainable construction at various high temperatures were investigated. For this purpose, specimens from four mixtures with WGP at various percentage levels of 0, 5, 10 and 15% were prepared and exposed to the specified temperatures. After that, the compressive and flexural strength were determined at high temperatures. The mass loss was also measured by weighing the samples before and after exposing to the high temperatures. The microstructure of mortars was analyzed by petrographic examination. Based on the obtained results, incorporation of WGP as partial replacement of cement could improve strength characteristics of the mortars at the elevated temperatures up to 17%. Also, the optimum ratio of cement replacement level was found to be 10%. In addition, the petrographic images of the mortars showed that at the same time with the strength loss of specimens, the red discoloration of WGP occurred that is attributed to the oxidation of iron compounds that starts at temperatures above 200 °C.
Eunmi Ryu, Heesun Kim, Yeongsoo Shin
International Journal of Concrete Structures and Materials, Volume 14, pp 1-18; https://doi.org/10.1186/s40069-020-00443-8

Abstract:
The purpose of this study was to investigate the thermal and cyclic behaviors of fire-damaged walls designed with different failure modes, aspect ratios and heated areas. These cyclic behaviors include temperature distribution, maximum lateral load, stiffness, ductility, and energy dissipations, etc. Toward this goal, the concrete wall specimens were exposed to heat following an ISO 834 standard time–temperature curve and the cyclic loading was applied to the fire-damaged walls. The test results showed that exposure to fire significantly reduced the cyclic performance of the RC walls. Especially, it was observed that heated area, designed failure mode, and aspect ratio have influences on maximum lateral loads, stiffness, and ductility of the fire-damaged walls, while almost no effects of the heated area, designed failure mode, and aspect ratio on temperature distribution and energy dissipation were found.
Jang-Woon Baek, Su-Min Kang, Tae-Ho Kim, Jin-Yong Kim
International Journal of Concrete Structures and Materials, Volume 14, pp 1-22; https://doi.org/10.1186/s40069-020-00439-4

Abstract:
Recently, as a new precast concrete (PC) construction method for increasing economy and constructability, the PC double-beam system has been developed for factories or logistic centers, where construction duration is particularly important. In this study, half-scaled PC double beam–column connection was tested under gravity loading and cyclic lateral loading. The major test parameters included the use of the spliced PC column and the addition of reinforcement at the beam–column joint. In the gravity loading test, the flexural behavior of the PC double beam was investigated. The test results showed satisfactory flexural capacity at the PC double-beam section, validating the composite action between the PC and RC members. In the cyclic lateral loading test, the seismic performance of the PC double beam–column connection was investigated. Based on the test results, the failure mode, load-carrying capacity, deformation capacity, energy dissipation capacity, secant stiffness, and shear strength of the PC double-beam system were evaluated and compared with those of a conventional RC double beam–column connection. According to the test results, the structural performance of the PC double beam–column connection was comparable to that of the RC double beam–column connection and satisfied the acceptance criteria of moment frame in the ACI 374.1-05 provision.
S. M. Samindi M. K. Samarakoon, Bjarte Hodne
International Journal of Concrete Structures and Materials, Volume 14, pp 1-12; https://doi.org/10.1186/s40069-020-00433-w

Abstract:
Post-tensioned unbonded tendons are widely used in flat slabs/plates when there is a demand for large span lengths, durable tendons and a reduction in the weight of structure. For post-tensioned flat slab/plates, different tendon layouts have been discussed in the literature. It is vital to compare the structural response (i.e., deflection and stresses) and the clashing of tendons of the proposed tendon layouts in the literature to select an appropriate layout. Hence, this study focuses on the analysis of three different six-panel flat plates (i.e., panel sizes: 6 m × 6 m, 9 m × 6 m and 11 m × 6 m) with five different tendon layouts, using computer programs ADAPT-Floor Pro and FEM-Design 17, based on linear finite element (FE) analysis. Short-term/long-term deflection and stress due to service load obtained from the computer programs has also been compared, to highlight the differences. Ultimate bending moment of resistance was calculated theoretically for different layouts and compared. Results from the analysis show that, when a higher portion of tendons is concentrated instead of distributed, stresses caused by other structural loads are counteracted best. The layout with all tendons concentrated also has the best results in terms of deflections.
, Andrea Saccani, Luca Baldazzi, Isabella Lancellotti
International Journal of Concrete Structures and Materials, Volume 14; https://doi.org/10.1186/s40069-020-00435-8

Abstract:
Alkali-activated materials are a promising type of binder candidate as a substitute to Portland cement. Fly ashes can be used as binder precursors giving higher environmental benefits. In the present research, fly ashes (Type F) containing different amounts of unburned carbonaceous matter have been used to formulate mortars. Serious problems concerning the workability in the fresh state have been found when high carbon content are reached. An attempt to avoid the preliminary treatments used to eliminate the unburned matter is carried out by exploiting different mix-design receipts obtained by changing the water/binder ratio, the ratio of the alkaline activators and using different types of superplasticizer additives. Data so far collected underline that a high amount of unburned carbonaceous matter can not only compromise the mechanical properties of the materials, but also the rheological ones and underline the necessity to develop ad hoc additives for this type of binders.
Abudushalamu Aili, Ippei Maruyama
International Journal of Concrete Structures and Materials, Volume 14; https://doi.org/10.1186/s40069-020-00431-y

Abstract:
Mechanical properties and durability of cement-based materials are largely affected by pore structures. This paper provides an overview of several experimental techniques to characterize pore size distribution and specific surface area, with focus on pores in calcium silicate hydrates. The reviewed experimental techniques are nitrogen and water vapor sorption isotherm, proton nuclear magnetic resonance (1H-NMR) and small-angle scattering (SAS). Different pretreatment methods are compared for sorption measurements. Pore size distribution and specific surface area are analyzed using data from different methods to understand difference and consistency of these methods. It is found that pore size distribution calculated from sorption isotherm is very sensitive to adsorption model. Though specific surface areas from different techniques are quite different from each other, they are all able to detect the microstructural alteration due to long-term drying.
Tahmina Tasnim Nahar, Anh-Tuan Cao,
International Journal of Concrete Structures and Materials, Volume 14, pp 1-17; https://doi.org/10.1186/s40069-020-00430-z

Abstract:
This paper proposes an approach to assess and predict the seismic risk of existing concrete gravity dams (CGDs) considering the ageing effect. The combination of fragility function and cumulative absolute velocity (CAV) depending on two failure states has been used in the analysis. It represents the time-variant degradation of the concrete structure and the conditional change of structural vulnerability in the case of the seismic excitation. Therefore, the seismic risk assessment captures here the nonlinear dynamic behavior of a concrete gravity dam through the fragility analysis. Incremental dynamic analysis for the fragility curves is adopted to state the performance of the dam in terms of different intensity measures. To assess the capacity of the aged concrete gravity dam, this research introduces a way to estimate the CAVlimitof CGDs with varying time. For a case study, an existing concrete gravity dam in Korea has been taken into consideration to apply this approach. The numerical finite element model is validated by optimizing the recorded field data. The proposed approach and its findings will be helpful to CGDs operators to ensure whether a dam needs to stop after a specific time using the extracted mathematical model. Furthermore, as this mathematical model is the function of time, the operator can get an idea about dam conditions at any specific time and can take necessary steps.
International Journal of Concrete Structures and Materials, Volume 14, pp 1-17; https://doi.org/10.1186/s40069-020-00432-x

Abstract:
The aim of this work was to provide further confirmation of the possible use of non-linear ultrasonic techniques for detecting the cracking due to corrosion of steel reinforcements in concrete. To this end accelerated steel corrosion tests have been conducted on model reinforced cement mortar specimens, while monitoring the appearance and width evolution of visible surface cracks, and performing non-linear ultrasonic measurements based on the phenomena of harmonic distortion and intermodulation. A new parameter, based on the difference between the amplitude of the fundamental frequency and the sum of the amplitudes of all the first-order and second-order intermodulation products, has been proposed in this work. The results confirm that the appearance of visible surface micro-cracks are preceded and accompanied by the observation of strong non-linear features in the received signal. Furthermore, the new parameter proposed in this work is as efficient as the relative non-linearity parameters, classically used in harmonic distortion non-linear ultrasonic studies, for detecting the non-linear features associated with the critical events of the cracking of cement mortar due to embedded steel corrosion. A hypothesis has been developed considering the possible effect of the filling of the void space by liquid containing rust products after the formation of new cracks or the enlargement of its width. This filling process, which might be particularly enhanced by net convective transport of liquid, would explain the evolution of the values of all the parameters used for putting in evidence the non-linear elastic features after the critical events of the cracking process.
Samer Ghosn, Nour Cherkawi, Bilal Hamad
International Journal of Concrete Structures and Materials, Volume 14, pp 1-17; https://doi.org/10.1186/s40069-020-00429-6

Abstract:
This paper reports on the first phase of a multi-phase research program conducted at the American University of Beirut (AUB) on “Hemp and Recycled Aggregates Concrete” (HRAC). HRAC is a new sustainable concrete material where hemp fibers are incorporated in the mix, the coarse aggregate content is reduced by 20% of the concrete volume, and 50% of the natural coarse aggregates (NCA) are replaced by recycled concrete aggregates (RCA), thus saving on natural resources and addressing the problem of waste material disposal. The effect of the new material on concrete consistency and hardened mechanical properties was studied. Also, few durability tests were conducted. Variables included percentage replacement of NCA by RCA (0 or 50%), maximum size aggregate (10 or 20 mm), hemp fiber length (20 or 30 mm), and hemp fiber treatment (alkali or silane or acetyl). Fiber characterization tests were conducted including morphology, crystallinity, and thermal analysis. The tests indicated that alkali and acetyl fiber treatments were better than the silane treatment in removing impurities on the fiber surface. Also, alkali and acetyl treatments have increased the crystallinity of the fibers while silane treatment decreased it. Results of mechanical properties tests showed that while HRAC has considerable lower compressive strength and modulus of elasticity than plain concrete, the flexural strength and splitting tensile strength are not significantly affected. The flexural stress–strain behavior of HRAC is ductile as compared to the brittle behavior of the plain concrete beams indicating positive impact on toughness and energy dissipation. The durability tests indicated that whereas HRAC mixes have higher absorption than plain concrete, they have better thermal properties and their resistance to freeze–thaw cycles is comparable to plain concrete. All test results were not significantly affected by fiber length or fiber treatment.
Bin Cai, Bo Li,
International Journal of Concrete Structures and Materials, Volume 14, pp 1-17; https://doi.org/10.1186/s40069-020-00428-7

Abstract:
Fire tests and subsequent bending tests of four reinforced concrete (RC) beams were performed. Based on these tests, the post-fire performance of RC beams was further studied using finite-element simulation through reasonable selection of suitable thermal and thermodynamic parameters of steel and concrete materials. A thermodynamic model of RC beams with three sides under fire was built using finite-element analysis (FEA) software ABAQUS. The FEA model was validated with the results of fire tests. Different factors were taken into account for further parametric studies in fire using the propsed FE model. The results show that the main factors affecting the fire resistance of the beams are the thickness of the concrete cover, reinforcement ratio of longitudinal steel, the fire exposure time and the fire exposure sides. Based on the strength reduction formula at high temperature of steel and concrete and four test results, an improved section method was proposed to develop a calculation formula to calculate the flexural capacity of RC beams after fire. The theoretical calculation method proposed in this paper shows good agreement with FEA results, which can be used to calculate the flexural capacity of RC beams after fire.
Aniket B. Bhosale,
International Journal of Concrete Structures and Materials, Volume 14, pp 1-19; https://doi.org/10.1186/s40069-020-00427-8

Abstract:
Improvement in fracture behaviour of fibre-reinforced concrete (FRC) due to the inclusion of various types and combinations of fibres is widely reported. The fracture behaviour of FRC needs to be fully understood for the optimum use of these fibres in structural elements. Fracture behaviours of synthetic fibre-reinforced concrete (SynFRC), hybrid fibre-reinforced concrete (HFRC) and steel fibre-reinforced concrete (SFRC) are investigated in this study using digital image correlation (DIC) technique. This work focuses on improvement in the structural performance of FRC through a comprehensive study of the change in the crack length, crack opening and fracture process zone (FPZ) due to different fibres addition and their combinations. Three distinct fibre dosages of 0.50%, 0.75%, and 1.00%, of macro-polyolefin fibres, hooked end steel fibres and their hybrid combination are regarded as research parameters. Test outcomes indicate that HFRC offers higher post-cracking resistance when compared to SynFRC. SFRC showcases superior fracture performance than that of HFRC and SynFRC. Full-field strain measurements from DIC are used to measure the crack openings at different load levels during the fracture tests. Results of DIC analysis show good agreement with experimental measurements. Continuous monitoring of strain contours using DIC reveals the effective engagement of fibres along the depth at higher dosages for HFRC when compared to that of SynFRC. Also, HFRC had longer cracks than SFRC at a particular load.
Mohammed Ibrahim, Aftab Ahmad, Mohammed S. Barry, Luai M. Alhems, A. C. Mohamed Suhoothi
International Journal of Concrete Structures and Materials, Volume 14, pp 1-15; https://doi.org/10.1186/s40069-020-00425-w

Abstract:
This study focuses on the development of durable structural lightweight concrete (LWC) by incorporating expanded perlite aggregate (EPA) in the range of 0 to 20% by weight. In order to ensure its durability when exposed to chloride environment, concrete was produced with low water-to-cement ratio and ordinary Portland cement (OPC) was replaced with 50% and 7% ground granulated blast furnace slag (GGBFS) and silica fume (SF), respectively. The mechanical properties and durability of concrete were assessed by determining the unit weight, compressive strength, flexural strength, drying shrinkage, chloride permeability and migration, as well as resistance of concrete to corrosion of reinforcing steel. Very importantly, thermal insulation properties were determined using a hot guarded plate. In addition, a finite element model (FEM) was prepared to study the behavior of EPA-modified concrete under seismic loading. The results showed that the unit weight of concrete was reduced by 20% to 30% when compared with the normal weight concrete (NWC). The compressive strength of the developed LWC was sufficient to be used as structural concrete, particularly of those mixtures containing 10% and 15% perlite aggregate. The durability of LWC was comparable to NWC in terms of chloride diffusion and resistance of concrete to corrosion of reinforcing steel. The tangible outcomes also include the superior thermal insulation properties of LWC compared to NWC. The greater incorporation of EPA in the concrete resulted in better behavior under seismic loading.
Jie Huang, Jun Shi, Hengheng Xiao, Jiyang Shen, Baisong Yang
International Journal of Concrete Structures and Materials, Volume 14, pp 1-22; https://doi.org/10.1186/s40069-020-00417-w

Abstract:
This paper investigated the working behavior characteristics of six reinforcement concrete (RC) beams subjected to bending based on the numerical shape function (NSF) method and structural stressing state theory. Firstly, the structural stressing state mode is expressed based on the generalized strain energy density (GSED) derived from the measured strain data. Then, one of the Carbon Fiber Reinforced Plastic (CFRP)-strengthened RC beams is taken as an example and the leap characteristics of RC beam’s stressing state are detected by applying the Mann–Kendall (M–K) criterion, updating the existing definition of the structural failure load. Accordingly, the stressing state modes and strain fields of the CFRP-strengthened RC beam are proposed to reveal their leap characteristics. Furthermore, through comparing the working performance of six RC beams, the effects of different strengths and different reinforcement ratios on CFRP strengthening performance are investigated. Finally, the NSF method is applied to reasonably interpolate the limited strain data for further revealing the stressing state characteristics of the RC beams. The research results explore a new analysis method to conduct an accurate estimation of the structural failure load and provide a reference for the future design of CFRP-strengthened RC beams.
International Journal of Concrete Structures and Materials, Volume 14, pp 1-16; https://doi.org/10.1186/s40069-020-00424-x

Abstract:
This study presents a detailed experimental program for reinforced concrete T-beams strengthened in shear with near-surface mounted (NSM) basalt fiber-reinforced polymer (BFRP) bars. This paper aims to introduce and evaluate a nonmechanical anchorage technique for shear strengthening using NSM-BFRP bars. T-beams were strengthened using manually manufactured closed or U-shaped hybrid BFRP stirrups (BFRP bars and BFRP sheets). The experimental program was developed to study the effects of these anchorage techniques. The results showed that the shear capacity increased by 8%–46% for beams strengthened with NSM-BFRP bars without anchorage. However, the presence of the proposed anchorage system increased the shear capacity of the strengthened beams by 39.6%–81.6%. Moreover, the maximum strains induced in the BFRP bars ranged from 27 to 59% of their ultimate strains according to the spacing between the NSM and the presence of the anchorage. The proposed anchorage technique prevented the premature debonding of the NSM-BFRP bars.
Ruiyuan Huang, Shichao Li, Long Meng, Dong Jiang, Ping Li
International Journal of Concrete Structures and Materials, Volume 14, pp 1-15; https://doi.org/10.1186/s40069-020-00423-y

Abstract:
The dynamic mechanical properties of steel fiber-reinforced concrete (SFRC) under high temperature and high strain rate were studied using a split Hopkinson pressure bar (SHPB) of 74 mm in diameter. As it is difficult to achieve constant strain rate loading in SHPB experiments with high temperature and high strain rate, this paper first presents a method for determining the strain rate under non-constant strain rate loading conditions. This method is proposed to deal with experimental data under non-constant strain rate loading conditions. Then, the influences of temperature on the ultimate compressive strength, peak strain, and failure modes of SFRC under different strain rates were analyzed and the results show that SFRC has a strain rate hardening effect. This paper also points out that there is a strain rate threshold for SFRC. If the strain rate is less than the strain rate threshold, there is a temperature softening effect. Conversely, if the strain rate is greater than the strain rate threshold, there is a temperature hardening effect. Finally, the relationship between the ultimate compressive strength and fiber volume fraction, strain rate, and temperature is presented and the prediction results are consistent with the experimental data.
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