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Korde Chaaruchandra, Matthew Cruickshank, Roger P. West
Magazine of Concrete Research, Volume 73, pp 1261-1269; https://doi.org/10.1680/jmacr.20.00102

Abstract:
The stringent early age strength requirements for prestressed hollowcore (HC) slabs have restricted the use of alternative cementitious materials, such as ground granulated blast furnace slag (GGBS), in their factory production. In the present paper, an experimental study conducted in a precast factory is described in which GGBS-concrete HC slabs with 30% and 50% GGBS are compared with concrete slabs made from control samples (using 100% CEM II A-L cement, as used in summer mixes where rapid hardening is not required). The use of a novel admixture, thermal curing and a new early age strength determination technique enables confirmation of the development of the required very early strength, such that cutting and lifting of the HC slabs can occur within 20 hours of pouring. The HC slabs are instrumented to study the internal temperatures in the different slabs, leading to early age strength development whereby the slabs so produced are tested for their adequacy for immediate transportability and their ultimate capacity. Further, the control and 30% GGBS slabs are tested for early age creep under a flexural test in the factory. At later ages, using the new technique for making cube specimens, cubes and cores cut from the actual HC slabs are shown to have similar 28 day strengths and density.
Lea Ghalieh, Elie Awwad, George Saad, Helmi Khatib, Mounir Mabsout
Magazine of Concrete Research, Volume 73, pp 1250-1260; https://doi.org/10.1680/jmacr.20.00103

Abstract:
This study investigates the use of natural hemp fabrics for strengthening existing concrete columns. The presented results are based on an experimental setup analyzing the efficiency of natural hemp fibers for the confinement of existing concrete columns. The considered variables are: number of hemp fabrics layers, column length/diameter ratio, and the presence of transverse reinforcement. Uniaxial compression tests were performed on 36 specimens. The axial stress-strain curves, ductility, and failure modes were analyzed. The research tests results indicate that the three variables considered have significant effects on the confined columns strength and post peak behavior.
Xian Li, Yu Xiao, Tao Zhou, Yu-Ming Xu, Chun-Hui Li
Magazine of Concrete Research, Volume 73, pp 1189-1204; https://doi.org/10.1680/jmacr.19.00467

Abstract:
To facilitate the construction and the on-site quality testing, a new type of panel joints using cast-in-place connecting beams was proposed and studied. The proposed connecting beams consisted of overlapped vertical U-shaped steel bars stretching out of the wall and the footing, longitudinal pre-stressed steel strands, L-shaped steels and infilled high-strength grout. One monolithic shear wall specimen and five precast specimens were tested under simulated seismic loads. The test parameters included the presence of L-shaped steel or not, sectional depths and locations of connecting beams as well as the locally un-bond U-shaped steel bars. The effects of these parameters on failure modes, hysteretic curves, stiffness, ductility and energy dissipation capacity were evaluated. The test results indicate that precast shear walls with proposed panel joints can achieve desirable seismic behavior comparable to or better than that of monolithic specimens, and thus can be safely applied in buildings in seismic zones. The use of L-shaped steel, pre-stressed steel strands and high-strength grout effectively delayed the concrete crushing of wall toes, and thus improved the loading capacity, deformation capacity, initial stiffness, ductility and energy dissipation capacity of the precast shear walls.
Andri Setiawan, Robert L. Vollum, , Bassam A. Izzuddin
Magazine of Concrete Research, Volume 73, pp 1205-1224; https://doi.org/10.1680/jmacr.19.00562

Abstract:
This paper utilises nonlinear finite element analysis with 3-D solid elements to gain insight into the role of shear reinforcement in increasing punching shear resistance at internal columns of flat slabs. The solid element analysis correctly captures the experimentally observed gradual decrease in concrete contribution to shear resistance with increasing slab rotation and failure mode but is very computationally demanding. As an alternative, the paper presents a novel approach in which 3-D joint elements are combined with nonlinear shell elements. Punching failure is modelled with joint elements positioned around a control perimeter located at 0.5d from the column face (where d is the slab effective depth). The joint elements connect the nodes of shell elements located to either side of the punching control perimeter. The punching resistance of the joints is related to the slab rotation using the failure criterion of the Critical Shear Crack Theory. The joint-shell punching model (JSPM) considers punching failure both within the shear-reinforced region and due to crushing of concrete struts near the support region. The JSPM is shown to accurately predict punching resistance whilst requiring significantly less computation time than 3-D solid element modelling.
Santosh Ashok Kadapure
Magazine of Concrete Research, Volume 73, pp 1241-1249; https://doi.org/10.1680/jmacr.20.00001

Abstract:
Biomineralization is a process of formation of calcite from supersaturated solution due to the presence of bacterial cells and biochemical activities. In this article, biological approach towards development of sustainable construction materials is studied. Evidence of bacterial precipitation of calcite has led several research groups to explore the possibility of adopting this process in the area of construction materials. The goal of this review paper is to highlight the application of biomineralization in concrete. The effect of bio-based agent, bacterial solution on various parameters of concrete is discussed. Additionally, the factors for choice of bacteria for biomineralization process are also elaborated. This green biotechnology concept is a promising, environmentally safe and is a best substitute for conventional and current remediation methods to mitigate environmental problems in multidisciplinary fields.
Zhiyuan Zhou, , Aocheng Zhong, Amin Shahpasandi, Marvel Cham Sarabia,
Magazine of Concrete Research, Volume 73, pp 1283-1295; https://doi.org/10.1680/jmacr.20.00118

Abstract:
Fly ash (FA) is used as supplementary cementitious material to Ordinary Portland cement (PC) to reduce environmental impact and improve long-term properties of concrete. However, the replacement level of PC by FA is currently limited to 15%-30% in the industry, mainly due to problems with early age strength development. This research used calcium formate (CF) as an admixture in high volume fly ash (HVFA) composites to find the optimal dosages of CF that improve the early age strength most. From very low dosage of 0.5%, up to high dosage of 9% of CF were tried on PC and HVFA pastes with 60% and 70% FA. Compressive strength testing was performed as a preliminary identification of optimal mix designs to give the highest strength. Isothermal calorimetry, thermogravimetric analysis (TGA), x-ray diffraction (XRD) and scanning electron microscopy (SEM) were conducted to further investigate the effects of CF regarding hydration and microstructural aspects. The results show that PC pastes reached the highest strength with 2% CF. Both HVFA pastes with 60% and 70% FA achieved the highest strength at the CF dosage of 3%. At the age of 28 days, adding of 3% CF in HVFA mixes lead to more consumption of FA as well as more formation of CH, C-S-H, CaCO3 and ettringite which contribute to the increase of strength. Adding of very high CF dosage at 9% increased the hydration of C3A but could hinder the hydration of C3S in both PC and HVFA pastes with 60% and 70% FA.
Kuan-Hung Lin, Chung-Chin Yang
Published: 25 November 2021
Magazine of Concrete Research pp 1-12; https://doi.org/10.1680/jmacr.21.00110

Abstract:
This study investigated the effects of changing the type and surface area per unit mass (Sa) of activated materials on the compressive strength of ground granulated blast-furnace slag (GGBS) cement (GGBS > 60%). This study is divided into two parts. The first part involved the selection of activated materials. Compressive strength results show that gypsum improves the early strength of GGBS cement more effectively than does sodium silicate or sodium hydroxide. The gypsum activators increase GGBS activity through sulfates, generating ettringites and C–S–H to increase compressive strength. Chemical analysis shows that the sulfite (SO3) in GGBS cement has more stable compressive strength at 6–8%. The second part of this study investigated the influence of Sa. Gypsum was used as a strength activator. The sulfite in GGBS cement was controlled at 6.5% ± 0.5%, and Sa was adjusted as the variable. Linear regression analysis shows that the change in GGBS cement Sa (Msa) is positively correlated with compressive strength. The value of Msa can be estimated according to the value of Sa and the quantity of ingredients used for the study results (cement, GGBS and gypsum). The compressive strengths of GGBS cement at various curing ages can be predicted using Msa.
Wen-Yao Lu, Tung-Ming Lee, Hsueh-Cheng Ko, Jui-Ting Tsai
Published: 25 November 2021
Magazine of Concrete Research pp 1-15; https://doi.org/10.1680/jmacr.21.00221

Abstract:
This paper reports the test results for eight high-strength concrete square pile caps. The main variables are the area of flexural steel bars and the configuration patterns of flexural steel bars. The test results show that the ultimate load on the pile cap increases as the total area of the flexural steel bars increases. The load–displacement curves are not significantly affected by an increase in the total area of the flexural steel bars. For a specific area of flexural steel bar, the ultimate load on the pile caps in a bunched square reinforcement pattern is no greater than that for pile caps in a grid reinforcement pattern. Pile caps in a bunched square reinforcement pattern exhibit no significant mechanical advantage over pile caps in a grid reinforcement pattern. Square pile caps in a grid reinforcement pattern perform better because this pattern is more conducive to concrete casting than a bunched square reinforcement pattern. The softened strut-and-tie model more accurately predicts the ultimate load for pile caps than the ACI sectional method for various shear span-to-depth ratios and compressive strengths of concrete. The ACI sectional method overestimates the flexural strength of the square pile cap.
Haotian Fan, Ravindra K. Dhir, Peter C. Hewlett
Published: 22 November 2021
Magazine of Concrete Research pp 1-41; https://doi.org/10.1680/jmacr.21.00009

Abstract:
This study, third in the series, following from ground limestone and Class F fly ash, evaluates, as a cement constituent, the effect of using ground granulated blast furnace slag (GGBS) on the strength development of concrete, and consequently its embodied carbon dioxide (CO2e). The paper has been built from systematically analysing, evaluating and modelling the extensive data-matrix developed, having 85,099 data points, from the information sourced from 663 studies published in English, during 1974 to 2020, by 1,672 authors, working in 718 institutions in 49 countries, globally. It is shown that, at a given water/cement ratio, in comparison to Portland cement (PC), the use of GGBS results in a reduction in 28-day concrete strength, which increases with GGBS content, at a rate determined by the strength of concrete, GGBS fineness, and curing of concrete. It is also shown that, as to achieve a 28-day design strength, a lower water/cement ratio is required with a PC/GGBS blended cement than PC, this will reduce the actual CO2e savings that can be realised with the use of GGBS as cement constituent in manufacturing concrete. Finally, it is shown that GGBS is more effective in lowering CO2e of concrete than FA and GLS.
Carla Driessen-Ohlenforst, Michael Raupach
Published: 22 November 2021
Magazine of Concrete Research pp 1-27; https://doi.org/10.1680/jmacr.21.00020

Abstract:
In the context of a joint research project, a system for monitoring, protection and strengthening of bridges by using a textile reinforced concrete interlayer has been developed which consists of two carbon layers with a spacing of 15 mm and a special mortar. This setup led to the idea to build up an electrical field between the carbon meshes, which suppresses the ingress of chlorides into the concrete. This paper focuses on the question which voltages and electrical field strengths are necessary to prevent critical chloride contents at the reinforcing steel. For this purpose, extensive laboratory tests have been performed, followed by a numerical simulation study. By applying an electrical field, the negatively charged chloride ions are forced to move to the upper carbon mesh that is polarized as an anode. It has been investigated whether the voltages to implement an electrochemical chloride barrier are smaller than they have to be for the common preventive cathodic protection. One advantage of this chloride barrier is that because of the lower current densities the anodic polarisation of the carbon meshes can be reduced. Therefore, different voltages, electrical field strengths, anode materials and anode arrangements were investigated.
Qian Feng, Yongping Zhang, Phillip Visintin, Rongqiao Xu
Magazine of Concrete Research, Volume 73, pp 1151-1166; https://doi.org/10.1680/jmacr.19.00531

Abstract:
Several bond-slip models with corrosion effects are already available in the literature to describe the change in the bond due to the corrosion of the longitudinal reinforcement. However, these models do not consider either the beneficial effect of stirrups or the corrosion of the stirrups. This is very important as the stirrups provide confinement effect enhancing the bond strength of reinforcement, but the effect also gets deteriorated with corrosion impacts. To address this limitation, a new bond-slip model is proposed in this paper that includes the influence of corrosion on both of the reinforcement and stirrups. More significantly, the model also provides a correlation between the corrosion in the stirrups and longitudinal reinforcement. This means the model could be applied with knowledge of the corrosion of either the longitudinal reinforcement or the stirrups. This is beneficial to the prediction of the corrosion level of longitudinal bars and to take a step further to the prediction of the mechanical behaviour of the structures as it is relatively easy to physically measure the corrosion level of stirrups. A parametric study is then presented based on the developed model.
Huizhong Xue, , , , Yi Li
Magazine of Concrete Research, Volume 73, pp 1135-1150; https://doi.org/10.1680/jmacr.19.00465

Abstract:
Punching and post-punching shear behaviours of reinforced concrete (RC) flat plate structures are often studied using a representative slab-column connection being isolated from the parent structure. A set of numerical modelling techniques is established in this study to create a competent 3D nonlinear model to simulate punching and post-punching shear behaviours of RC slab-column connections without shear reinforcement. With which, the punching shear failure featuring a critical punching shear surface and an abrupt drop of the applied force in the load-displacement response is able to be accurately reproduced. The post-punching shear behaviour, taking the form of an increased load-carrying capacity which is ceased by rebar fracture in the suspension stage, is also well captured. Using the proposed numerical model, typical punching and post-punching shear failure mechanisms are studied in some detail.
Ke Du, Jiulin Bai, Nan Teng, Deng Yan, Huiming Chen, Jingjiang Sun
Magazine of Concrete Research, Volume 73, pp 1081-1099; https://doi.org/10.1680/jmacr.19.00418

Abstract:
Floor systems constructed from slabs and beams are critical structural elements of reinforced concrete (RC) frame structures, allowing them to resist progressive collapse. To elucidate the complex effects of the slab and its thickness on the progressive-collapse resistance of RC spatial frame structures, three 1/3-scale 2 × 2 span substructure specimens, including one 3D skeletal frame substructure and two 3D frame–slab substructures with different slab thicknesses, were tested in a middle-column-removal scenario. The test results indicated that the frame–slab substructure exhibited two resisting progressive-collapse stages: the primary mechanism stage under small deformations, and the secondary mechanism stage under large deformations. The slab contributions were separated from the floor system and quantified by comparing the results for the skeletal frame substructure and frame–slab substructure. Although there are no in-plane confinements, the slab also significantly improved the resistance against progressive-collapse due to the compressive and tensile membrane action in the primary and secondary mechanism stage, respectively. Increasing the slab thickness increased the compressive membrane action of the slab, which significantly enhanced the progressive-collapse resistance under small deformations. However, under large deformations, the beam and slab did not work synergistically, which limited the development of secondary mechanisms against progressive collapse.
Liangjun Hu, , Tomoyuki Maeda
Magazine of Concrete Research, Volume 73, pp 1100-1112; https://doi.org/10.1680/jmacr.19.00359

Abstract:
Bugholes are usually considered an aesthetic problem rather than a durability problem. Surface bugholes often appear on the sidewalls of a tunnel's lining concrete, which can be mitigated but never eliminated. However, the impact of surface bugholes on concrete durability is still unidentified. In cold regions, the lining concrete of a road tunnel is exposed to de-icer solutions splashed by traffic, and it is additionally exposed to freeze-thaw (F-T) cycles. This study focuses on the durability to F-T cycles of tunnel lining concrete with surface bugholes. For the objectives, the study was carried out on a laboratory-scale F-T test using large concrete blocks. An image analysis based on the RGB values of each pixel in the coloured photographic image, was performed to detect and quantify deterioration of the concrete surface throughout the F-T cycles. Although severe scaling never occurred during the 300 F-T cycles, the concrete with bugholes indicated a significant local scaling at the edges of the bugholes. Some bugholes enlarged and some merged with existing bugholes, in accordance with such local scaling. The paper presents synergetic and negative effects of bugholes on the local scaling of tunnel lining concrete exposed to F-T cycles.
, Gyorgy L. Balazs
Magazine of Concrete Research, Volume 73, pp 1113-1124; https://doi.org/10.1680/jmacr.19.00479

Abstract:
This paper experimentally investigated the behaviour of concrete at elevated temperatures in terms of shear deterioration using steel fibres. Forty five specimens of push-off model were used in which three samples for each category of maximum temperatures. Five maximum temperatures have been investigated i.e. 20, 150, 300, 500 and 700 °C. The main objective of this research is to investigate the effect of steel fibres on the behaviour of concrete at different maximum temperatures in terms of shear failure. This study showed the enhanced ductility and strength of the shear at elevated temperatures. Three main categories have been tested: plain concrete, steel fibre contents of 40 kg/m³ or 80 kg/m³. In addition, both crack width and crack slip have been measured using means of linear variable differential transducers (LVDTs). Results showed that the content of steel fibres increases the shear strength at ambient temperature by 103% and 102% for both SFRC-40 and SFRC-80, respectively. Although this enhancement is less important at high temperatures, i.e., 500 and 700 °C, still the mixes that contain fibres have higher residual shear strengths than plain mixes. Moreover, steel fibres content is affecting ductility in both ambient and high temperatures.
, Maryangela G. Lima, , Luciene F. S. Wickzick
Magazine of Concrete Research, Volume 73, pp 1180-1188; https://doi.org/10.1680/jmacr.19.00451

Abstract:
Steel reinforcement corrosion due to chloride penetration into concrete is one of the main causes of degradation of marine structures, being the subject of many researches. Among the various factors that influence the chloride penetration into concrete are the environmental factors, such as temperature and exposure condition. In this context, the present study evaluated the chloride penetration in three different exposure regions (atmospheric, tidal variation and underwater) at the temperatures of 15°C, 20°C, 25°C, and 30°C over 18 months, collecting samples every 6 months, at depths of 1mm to 5mm. The environmental conditions were simulated in the laboratory. The results show a higher concentration of chlorides underwater. Moreover, the chloride concentration increases more intensively from 6 to 12 months and the temperature is more effective from 20°C. This article also shows that temperature effects are higher for advanced ages.
Siyao Wang, , Jinhui Yang, Yuquan Hu
Magazine of Concrete Research, Volume 73, pp 1167-1179; https://doi.org/10.1680/jmacr.19.00561

Abstract:
This paper studies the method of determining the fracture parameters of pre-notched concrete beams with horizontal pre-set cracks and the influence of fracture intersection on fracture characteristics of concrete based on the three-point bending test. Pre-notched concrete beams with horizontal pre-set cracks were designed as 13 groups according to the different positions, lengths and widths of horizontal pre-set cracks. The fracture toughness was calculated on the basis of the cohesive force model of concrete, and the crack opening displacement obtained by using the digital image correlation technique. The study's results indicate that the distance between the pre-set crack and the notch tip as well as the pre-set crack length and width have various influences on the fracture propagation of edge mode I crack, the bearing capacity and unstable fracture toughness of concrete beams owing to varying degrees of loss of cohesion in the fracture process zone (FPZ). The study also finds that a large lack of cohesion in the FPZ indicates a high possibility of a reduction in the bearing capacity and unstable fracture toughness of concrete beams.
Hugh D. Miller, , Sara Mesgari,
Magazine of Concrete Research pp 1-13; https://doi.org/10.1680/jmacr.20.00366

Abstract:
The ability of fibres to resist crack growth in fibre-reinforced concrete (FRC) can be significantly influenced by the fibre/matrix bond. This investigation reveals surface treatment of fibres as a viable technique to develop a uniform bond along the fibre/cement interface to resist growth of microcracks and thereby complement the physical restraint against pull-out provided by fibres’ shape and friction. Previous reports have shown effective chemical treatment of glass, carbon, and polypropylene fibres. However, research into chemical surface treatment processes for steel fibres, the most common in concrete, is scarce and focused on corrosion and dispersion, rather than the fibre/matrix bond. Here, a silane treatment technique is proposed to strengthen the steel fibre/cementitious matrix bond. Surface energy measurements and XPS demonstrate the effectiveness of this treatment. Fibre pull-out tests conducted on silane-treated fibres show an apparent increase in pull-out energy, accompanied by a delay in reaching the peak load, compared to untreated fibres, suggesting increased resistance to crack initiation and growth. Furthermore, the results indicate improved flexural strength and direct tensile strength of mortar reinforced with silane-treated fibres compared to untreated fibres. The improvements are further corroborated by results from restrained drying shrinkage and volume of permeable voids.
Jian Zhang, Yuefeng Ma, Haixin Zhao, Jiaping Liu, Zhangli Hu, Hua Li, Kangchen Wang, Lingzheng Wu
Magazine of Concrete Research pp 1-30; https://doi.org/10.1680/jmacr.21.00159

Abstract:
Plastic shrinkage cracking propagating at early ages inevitably impairs the performance of concrete structures. To mitigate it, using a recently developed shrinkage-reducing polycarboxylate superplasticizer (SR-PCA) with outstanding shrinkage-reducing and water-reducing effectiveness can be a promising approach. This present work aims at investigating the effect of the SR-PCA on the plastic shrinkage cracking and clarifying the underlying mechanisms. For comparison, a traditional polyether type shrinkage reducing admixture (SRA) and a polycarboxylate superplasticizer (PCA) were also used. Based on the investigations on horizontal shrinkage, settlement, bleeding, evaporation mass loss, cement hydration, surface tension of pore fluid and capillary pressure, the underlying working mechanisms of SR-PCA were identified. The results indicate that the SR-PCA is able to reduce the crack area, maximum crack width and average crack width by up to 55%, 48% and 50%, respectively. Furthermore, prolonging the bleeding duration and initial time of capillary pressure build-up caused by retarding the cement hydration, lowering the evaporation mass loss, and a lower development rate of capillary stresses and capillary pressure peak value induced by decreasing the surface tension of pore fluid are responsible for preventing the plastic shrinkage cracking of mortars containing SR-PCA.
Jessica Regina Camilo, Abrahão Bernardo Rohden,
Magazine of Concrete Research, Volume 73, pp 973-987; https://doi.org/10.1680/jmacr.19.00485

Abstract:
This paper investigates the physical, chemical, and mechanical properties of concrete produced with rejected recyclable plastic waste, the potential to mitigate heat-induced concrete spalling, and the effects of high temperature on the residual properties of concrete. Concrete compressive and tensile strength, Young's modulus, crack width, mass loss, absorption by capillarity, chemical composition, and the evidence of heat-induced concrete spalling have been monitored in concrete samples produced with plastic waste particles and compared to that of concrete produced with commercial polypropylene fibers, after exposure to 200oC, 400oC, and 600oC for 2 hours. The use of 0.125% by volume of polypropylene fibers and plastic waste particles have proved to improve the heat-induced concrete spalling performance, contributing to the release of internal pressure after melting by different mechanisms. Positive effects in the concrete properties evidence the technical potential of incorporating rejected recyclable plastic waste particles in the construction chain.
Xinru Li, Zhimin Wu, Jianjun Zheng, , Xiangming Zhou
Magazine of Concrete Research, Volume 73, pp 1033-1047; https://doi.org/10.1680/jmacr.19.00459

Abstract:
The bond between reinforcement and concrete is one of the main factors affecting the structural performance. In terms of the bond mechanism, the bond performance of plain round bars is especially sensitive to the stress state of surrounding concrete. Lateral tensions have been confirmed to significantly weaken the bond properties of plain round bars but the effect of loading rate on such a condition is still not well understood. In the present paper, an experiment involving 178 pull-out specimens with various strengths of concrete and bar diameters is performed to analyse the influence of loading rate on the bond behaviour of plain round bars embedded in concrete under uniaxial lateral tensions. The main bond parameters are evaluated quantitatively. The results show that the bond strength is independent of loading rate and the ratio between the residual strength and the bond strength keeps constant, but the slip at the peak bond stress decreases obviously with increasing the loading rate. Finally, an empirical constitutive model of bond stress-slip considering the strength of concrete, the bar diameter, uniaxial lateral tensions, and the loading rate is presented and validated with experimental results.
Chaaruchandra Korde, Matthew Cruickshank, Roger P. West
Magazine of Concrete Research, Volume 73, pp 1060-1070; https://doi.org/10.1680/jmacr.19.00378

Abstract:
The precast industry, due to its specific early age strength requirements for hollowcore (HC) slab products, has been reluctant to introduce alternative cementitious materials like ground granulated blast furnace slag (GGBS), due to the slower strength development of such materials. This paper reports on a study undertaken at a precast production facility to examine how to overcome the consequences of strength loss due to replacement of up to 50% of the CEM I (42·5R), as usually used in cold weather conditions, with GGBS, for environmental or durability reasons. The GGBS concrete slabs were activated using thermal, chemical and mechanical means to enhance early age strength gain. The results were compared at early ages in terms of temperature profile during curing, cube compressive strengths, load carrying capacities for transportability requirements, ultimate load and short term creep. In addition, 28 day densities and strengths were compared for in-situ cubes and cores extracted from HC slab samples. The experimental results showed a favourable potential for replacing CEM I (42·5R) cement with up to 50% GGBS in winter without compromising on factory productivity.
Hongyan Zeng, Shen Qu, Yinghong Qin
Magazine of Concrete Research, Volume 73, pp 1011-1024; https://doi.org/10.1680/jmacr.19.00558

Abstract:
Microstructure and transport properties of graphene oxide (GO)-cement mortars with similar consistency were experimentally investigated and compared with those of control mortars that were made using Portland cement. Polycarboxylate superplasticizer (PCs) was used to improve the consistency of GO-modified mortars and the dispersibility of GO nanosheets. Results indicate that the GO-incorporating mortars with similar consistency reduce the chloride migration coefficient and the initial rate of water absorption by 33.2% and 61.6% compared with the control samples, respectively. This improvement is due to the addition of GO particles in the mortars which inhibit crack growth, change the morphology of hydrated crystals and refine the pore structure. Meanwhile, very small amounts of GO additives can significantly decrease the macropore volume fraction and the critical as well as average pore diameters. To understand the correlation between the pore-size distribution and the macroscopic transport performance, both the critical pore diameter and the macropore volume fraction have been proved to be exponentially dependent on chloride migration coefficient. The average pore diameter is linearly related to the initial absorption rate of the mortars, but is independent of the secondary absorption rate.
Jinzhen Li, ,
Magazine of Concrete Research, Volume 73, pp 1025-1032; https://doi.org/10.1680/jmacr.19.00559

Abstract:
The Compressible Packing Model (CPM) is used to calculate the packing density of quartz aggregate, which is a new method for designing aggregate gradation. In this paper, the aggregate gradation of ultra high performance concrete (UHPC) was optimized by the CPM. The number of interfaces between the aggregate and cement was counted by Image Pro Plus software. The results show that the CPM was modified by the equation Y=0.60073X+0.20551 to accurately calculate the packing density of quartz aggregate. The compressive strength of UHPC was positively correlated with the packing density of the aggregate. By increasing the packing density, the compressive strength of UHPC was increased from 97.9 MPa to 124.1 MPa, an increasing of 26.7%. The flexural strengths of the materials was sensitive to the number of interfaces. The smaller the number of interfaces was, the greater the flexural strength. The specimen with an interface of 5.56mm/mm2 had the smallest flexural strength of 21.5 MPa, the specimen with an interface of 3.596 mm/mm2 had the largest flexural strength of 25.9 MPa. When designing aggregate gradation, a gradation with high density should be selected, and the proportion of small-size aggregates should not be too large.
Mohammad Sadegh Barkhordari, Mohsen Tehranizadeh, Michael H. Scott
Magazine of Concrete Research, Volume 73, pp 988-1010; https://doi.org/10.1680/jmacr.19.00542

Abstract:
Different macro models have been proposed in the past decades for simulating the behavior of concrete shear walls because of their significant role in the seismic performance of urban buildings. In this paper, the ability of Timoshenko's beam theory to evaluate the behavior of slender and moderate-aspect-ratio concrete shear walls is evaluated. For this purpose, a nonlinear displacement-based Timoshenko fiber element was first added to OpenSees software. A new constitutive model based on modified compression field theory was used for the stress-strain relationship of the materials. To validate the utilized element and materials, the results of the numerical model were compared with a set of laboratory tests of concrete shear walls subjected to cyclic and seismic loads (shaking-table test). Evaluating the numerical model and the experimental results revealed that the analysis procedure is presented to model the RC walls predicts the overall and local response of the wall with acceptable accuracy and thus can be used as a reliable tool for the analysis of slender and moderate-aspect-ratio concrete shear walls.
Xinjie Wang, Pinghua Zhu, Shuqi Yu, Hui Liu, Yanlong Dong, Xiaoyan Xu
Magazine of Concrete Research, Volume 73, pp 1071-1080; https://doi.org/10.1680/jmacr.19.00436

Abstract:
Self-compacting concrete (SCC) are widely used in tunnel linings that are subjected to the risks of fire accident. During tunnel fire, personnel and structural safety are commonly threatened by explosive spalling of SCC due to high strength and high moisture content of SCC. Recent studies have suggested that coating layer of fire-resistant material can efficiently prevent SCC from spalling in tunnel fire. In this study, SCC samples coated with 6 mm SiO2 aerogel-cement mortar (SiO2-ACM) were prepared in comparison with those without coating. The effects of different moisture contents (0%, 25%, 50%, 75%, and 100%) on the fire resistance of SCC were investigated. The results show that SiO2-ACM exhibited excellent mechanical and durable performances and low thermal conductivity. During the exposure to the mimetic tunnel fire, the degree of spalling of SCC increased as moisture content increased. The SCC samples with SiO2-ACM coating exhibited much less spalling than those without coating. After mimetic tunnel fire, bare SCC had almost completely failed in compressive strength, whereas residual compressive strength of SCC coated with SiO2-ACM were more than 68.6% even SCC with 100% of moisture content. Therefore, SiO2-ACM coating provides an attractive avenue to enhance the behavior of SCC in tunnel fire.
Walter Gabriel Bareiro, Elisa Dominguez Sotelino, Flávio De Andrade Silva
Magazine of Concrete Research, Volume 73, pp 1048-1059; https://doi.org/10.1680/jmacr.19.00371

Abstract:
Refractory concretes are widely applied for the lining of steel shell of industrial structures subjected to high temperatures and pressure. Working linings of steel ladles are made of high alumina concretes applied in layers that receive heat flow. Modeling the thermo-mechanical behavior of these layers under service conditions is of paramount importance to evaluate the failure or cracking that determine their useful life. In this work, simulations of the heating and holding process of linings composed of several layers of different types of refractory with different alumina contents (51, 71 and 90wt%), previously studied, were performed. The developed model employed axisymmetric finite elements and the thermo-mechanical analyses were carried out using the ABAQUS software package. The results of the thermo-mechanical response were compared and it was observed that the combination using the refractory concrete with 90 wt.% of alumina as an inner layer, combined with materials of 71wt.% and 51wt.% as the middle and outer layers, respectively, is more efficient in terms of the thermal gradient. Therefore, would potentially lead to minor damage to the lining throughout the process of use. This indicates the importance of selection of the coating materials for application in layers.
Rachel M. Homer, , Thomas C.K. Molyneaux
Published: 30 September 2021
Magazine of Concrete Research pp 1-9; https://doi.org/10.1680/jmacr.20.00254

Abstract:
The corrosion of steel due to chloride ingress is one of the major causes of deterioration for reinforced concrete structures. In order to ensure a structure achieves the design life specified predictive modelling is employed. To undertake this modelling details are required on a number of variables, however, each of these variables has a level of uncertainty which can affect the reliability of the model. To address these issues this paper reports the analysis of data taken from four distinct elements on a port structure, in Australia, constructed in three phases between 1926 and 1985, using both pre-cast and cast in situ techniques. The cover depth, surface chloride concentration and chloride diffusion coefficient were determined for different elements on the structure, with a total of 244 data points obtained. The data was analysed to identify their statistical distributions and probability density functions produced. A novel analytical model was developed based on the solutions to Fick's equation. The results showed that both surface chloride and the diffusion coefficient were positively skewed with no significant variation in the sensitivity. It was also found that controlling the diffusion coefficient has a more influential effect than increasing the cover.
Yong-Qian Zheng, Wang-Geng Liang, Xue-Li Zhang
Published: 9 September 2021
Magazine of Concrete Research pp 1-15; https://doi.org/10.1680/jmacr.20.00070

Abstract:
In the past decade, more and more attention has been paid to the fire performance of concrete-filled double-skin steel tubular (CFDST) columns. The columns in actual structures are not isolated members and they are affected by the adjacent structural members. However, little research has been conducted on the behaviour of restrained CFDST columns under fire. Finite element (FE) models were developed to analyse the fire performance of axially and rotationally restrained circular CFDST columns in this paper. The predicted temperature-time curves, internal axial force-time curves and fire resistance agreed well with the related test results. Based on the verified models, a parametric study was performed to investigate the effect on the internal axial force-time curves and fire resistance. The parameters included axial load level, axial restraint ratio, diameter of the outer tube, slenderness ratio, diameter-to-thickness ratios of the outer and inner steel tubes, dimension ratio of the inner tube to the outer tube, load eccentricity, yield strengths of the outer and inner tubes, compressive strength of the concrete, fire protection thickness and rotational restraint ratio. Through regression analysis, a design formula was proposed to calculate the fire resistance of restrained circular CFDST columns.
Hung-Wen Chung, Thanachart Subgranon, Mang Tia, Harvey DeFord, Jose Armenteros
Published: 1 September 2021
Magazine of Concrete Research, Volume 73, pp 958-972; https://doi.org/10.1680/jmacr.19.00541

Abstract:
A high cementitious content in Portland cement concrete can cause early cracking of concrete. This study investigated the effects of minimized paste volume in structural concrete incorporating Portland limestone cement and Blended Aggregates (BA) Technique. Comparisons were made on the effects of different types of cement and paste volume of concrete. The results indicate that Type IL cement which contains 14% limestone powder has comparable performance as that of the concrete made with Type I/II cement. Based on the results of laboratory testing and numerical modeling, the performance of concrete with different paste volumes was evaluated. The paste volume of concrete could be reduced to around 27% without affecting its properties. The paste volume of concrete incorporating BA technique could be further reduced to 24%. The concrete mixes incorporating BA technique were noted to have better potential performance as the reference concrete. Using Type IL cement and BA technique can effectively reduce the initial cost and carbon dioxide emission in structural concrete without loss of performance of the concrete.
, Seung-Hyeon Hwang, Ju-Hyun Mun
Published: 1 September 2021
Magazine of Concrete Research, Volume 73, pp 879-889; https://doi.org/10.1680/jmacr.19.00449

Abstract:
This study examines the flexural performance of reinforced concrete (RC) columns strengthened through a seismic jacket technique developed using a prefabricated steel bar unit that forms peripheral closed-hoops and supplementary V-ties, satisfying the design requirements of the ACI 318-14 seismic provisions. Three RC jacketed columns were prepared with a parameter of axial load levels and tested under a constant concentric axial load and cyclic lateral loads. Analytical approaches were also applied to examine the axial-load–moment interaction of the RC jacketed columns when considering the confinement effect provided by the jacket section and to investigate the applicability of the previous performance-based design models proposed for virgin seismic columns. The test results show that the developed jacket technique possesses a good potential in enhancing the flexural stiffness, strength, and ductility of deficient columns even under a high axial load. RC jacketed columns possess a higher ductility than that estimated using the ductility parameter model, regardless of the axial load level. Thus, the jacket section including the seismic details of a transverse reinforcement can be conservatively designed based on the axial-load–moment interaction and ductility parameter model for the moment and ductility enhancements required by deficient columns.
Valentina Loganina, Mikhail Frolov, Roman Fediuk
Published: 1 September 2021
Magazine of Concrete Research, Volume 73, pp 890-903; https://doi.org/10.1680/jmacr.19.00446

Abstract:
The paper presents new compositions of lime heat-insulating dry building mixtures, designed specifically for the finishing of aerated concrete, differing in the presence of a modifying additive based on aluminosilicates and calcium hydrosilicates as well as highly porous filler – microspheres. The beneficial effect of a modifying additive based on a mixture of aluminosilicates and calcium hydrosilicates with high pozzolanic activity on the structure formation of lime finishing compounds has been established, which is manifested in a decrease in the content of free lime to 28.6%, an increase in plastic strength, an increase in the softening coefficient 2.2 times. It has been revealed that when using limestone heat-insulating dry construction mixtures of hollow glass microspheres and ash aluminosilicate microspheres, the crack resistance of the resulting coatings increases by reducing shrinkage deformations during the hardening process, increasing the ultimate tensile properties and cohesive strength of the finishing coating, and water resistance increases due to the formation of a composite structure with a closed porosity. Coatings based on the developed dry construction mixture are distinguished by low density and thermal conductivity, high vapor permeability while possessing a sufficiently high crack resistance and weatherability.
Qinghe Wang, Jinsheng Yang, Yongze Liang, Yuzhuo Zhang, Yanfeng Fang
Published: 1 September 2021
Magazine of Concrete Research, Volume 73, pp 904-918; https://doi.org/10.1680/jmacr.19.00385

Abstract:
Thin-walled steel decks have been generally treated as permanent formwork in the routine design of steel-bars truss slabs, and their effects on the time-dependent behaviour of steel-bars truss slabs have not been quantified. The objective of this study was to propose design procedures for the time-dependent behaviour of steel-bars truss slabs accounting for the effects of soffit steel decks. For this purpose, two full-scale slab samples were tested for nine months, with one slab subjected to non-uniform shrinkage and sustained loading and the other slab subjected to only non-uniform shrinkage. Typical design equations for reinforced concrete slabs and composite steel-concrete slabs were benchmarked against available test results to verify their applicability. Nonlinear finite-element models were established to describe the time-dependent behaviour of steel-bars truss slabs, and simplified design procedures were proposed. It was found that the steel-bars truss sample subjected to non-uniform shrinkage had a mid-span deflection accounting for 52.4% of the total deflection; that the design methods in available composite structure standards (e.g. CECS, 2010; ANSI, 2011; and CEN, 2004 b) significantly underestimated the long-term deflections; and that the proposed design procedures well predicted the time-dependent deflections of steel-bars truss slabs by considering the influence of the shrinkage gradient.
D. Breslavsky, A. Chuprynin, K.V. Spiliopoulos
Published: 1 September 2021
Magazine of Concrete Research pp 1-31; https://doi.org/10.1680/jmacr.20.00387

Abstract:
Long-term strength investigations of construction elements made of reinforced concrete under creep conditions are considered. A calculation method that allows the determination of the load-bearing capacity and long-term strength of reinforced concrete thin-walled elements under short- and long-term loading is proposed. The method is based on a combination of the Finite Element Method for solving boundary value problems at each time step and the finite differences’ method for integrating the initial value problems. The numerical results are compared with the experimental data obtained in the "load-unloading" mode during long-term deformation. An example of creep and damage calculations in the reinforced concrete coating and recommendations for parameters selection are given.
Dawang Li, Penggui Sun, Pan Lv, Xi Chen, Ping Li, Feng Xing
Published: 1 September 2021
Magazine of Concrete Research, Volume 73, pp 929-935; https://doi.org/10.1680/jmacr.19.00469

Abstract:
In this paper, a washing-machine shaped experimental device is presented to investigate the effect of marine environments on the penetration of chlorides in concrete. The proposed experiment is used to simulate different flow speeds and wave patterns surrounding the concrete and examine their effects on the penetration of chlorides in the concrete. The obtained experimental results show that there is significant difference in chloride profiles between the specimens exposed to marine environment with flow and wave actions and those immersed in stationary solution. The difference is found to increase with exposure time, suggesting the importance to consider the marine environment when predicting the long-term durability of concrete exposed in marine environment.
, Linhua Jiang, LuGuang Song, Hongqiang Chu
Published: 1 September 2021
Magazine of Concrete Research, Volume 73, pp 919-928; https://doi.org/10.1680/jmacr.19.00463

Abstract:
High performance and ultra-high performance concrete have been increasingly used as building materials in modern constructions, and their superior properties are derived from a tailored microstructure. In this study, Rietveld quantitative X-ray diffraction analysis was adopted to investigate phase evolutions of four cementitious pastes with varied addition of fly ash or silica fume at very low water to binder ratios. The results indicate that the phase changes concerning the contents of the major clinker phases mainly happen within the first 7 days. The content of portlandite (CH) in pure cement paste increases monotonically while due to pozzolanic reaction, it decreases in blended pastes between 7 and 28 days. At early age, addition of silica fume accelerates the hydration of cement, while introduction of fly ash shows significant retardation especially at 1 day. The degree of hydration (DOH) of cement is mainly determined by the effective water to cement ratio of the paste, and the incorporation of fly ash which show little reaction initially results in a higher DOH of cement. With steam curing, the content of CH significantly decreases in the blended pastes, which indicates a promoted pozzolanic reaction.
Changchun Yuan, Qing Hu, Yuyin Wang, Changyong Liu
Published: 1 September 2021
Magazine of Concrete Research, Volume 73, pp 945-957; https://doi.org/10.1680/jmacr.19.00526

Abstract:
In recent years, concrete-filled steel tubular (CFST) arches have been widely used and the out-of-plane buckling of CFST arches is becoming a major concern. So far, few studies have investigated the out-of-plane behaviors of CFST arches, and no codes have given the design formula of the out-of-plane stability bearing capacity. Therefore, fixed CFST parabolic arches with the circular cross-section under uniformly distributed loads are investigated in this paper. Considering the effect of material and geometric nonlinearity, the distributions of the internal forces and deformations are observed, and on this basis, an equivalent column model is established to derive the out-of-plane effective length coefficient. In addition, based on the stability theory, the nominalized slenderness ratio of CFST arches is modified considering the influence of rise-to-span ratios and can be substituted into any available stability coefficient equations in current design codes to calculate the out-of-plane bearing capacity of CFST parabolic arches. The proposed equation which has a similar formula form as those available stability equations in existing designing codes for CFST or steel members and meanwhile has equal or higher accuracy as the equations in available literature would be easier for the designers to accept and use.
Published: 1 September 2021
Magazine of Concrete Research, Volume 73, pp 936-944; https://doi.org/10.1680/jmacr.19.00498

Abstract:
This study aims to evaluate the effect of elevated temperatures on the physical, mechanical, and microstructural properties of a hooked-end steel fiber for temperatures of 100, 350, 750, and 1000 °C. Results show that an oxidation layer is formed on the surface of the fibers exposed to temperatures of 750 °C and above, which leads to an increase in external diameter and mass. Reductions in tensile strength are directly proportional to the temperature increase, while rupture strain values significantly increase for temperatures above the recrystallization temperature of steel. The present study contributes to the understanding of the contribution of steel fibers to the overall behavior of the composite after temperature exposure and serve as input for recently developed numerical models.
Ramesh Gopal, Sethumadhavan Krishnachandran
Published: 1 September 2021
Magazine of Concrete Research, Volume 73, pp 865-878; https://doi.org/10.1680/jmacr.19.00285

Abstract:
Reinforced concrete in aggressive environments, has higher concrete clear cover provided to protect the reinforcement against corrosion. However, providing such higher cover in severe environments could result in increased crack widths for the structural member. Hence, using a combination of Fibre Reinforced Polymer (FRP) bars and steel reinforcement bars (hybrid reinforcement) has been considered as a suitable solution to develop the full flexural capacity of the section without significantly affecting the serviceability and ductility requirements. This includes utilizing the non-corrodible nature of FRP by adding a layer of longitudinal reinforcement made of FRP bars at the corner areas of the cross section and keeping the main steel reinforcement at sufficient cover depth.This paper studies the flexural behavior of hybrid GFRP beams consisting of continuous glass fiber reinforced polymer (GFRP) bars along with conventional steel reinforcement and comparing them with conventional reinforced concrete beams through experimental and analytical studies. The test results indicated that the hybrid GFRP beam possess better flexural strength and ductility compared to conventional Reinforced Concrete (RC) beams and could be considered as a suitable alternative to replace RC beams in aggressive environments.
Huai-Shuai Shang, Jun-Hao Zhou, Zong-Long Xie
Magazine of Concrete Research pp 1-33; https://doi.org/10.1680/jmacr.20.00389

Abstract:
The mechanical properties of four types of concrete conduced on the same test machine under multi-axial (biaxial tension–compression, triaxial tension–compression–compression) dynamic (strain rate is 10−5s−5, 10−5s−5, 10−5s−5, 10−5s−5) complex stress states with different stress ratios have been compared and analyzed. The current work is mainly to summarize the dynamic multiaxial strength of four different types of concrete and find that the mechanical behavior of four types of concrete has similarities to an extent. The multiaxial dynamic strength of different types of concrete increases with strain rate increases and multiaxial dynamic strength in any direction of four types of concrete is lower than that the concrete static strength under uniaxial tension and compression. The test data indicated that the influence of strain rate and stress ratio on multiaxial dynamic strength of concrete are compared and analyzed. The unified failure criteria expressed in octahedral stress space of various types of concrete under dynamic multiaxial tension and compression was proposed. This investigation provided a theoretical foundation for the design and checking computation of structures subjected to working conditions under multi-axis dynamic loads.
Thomas Alexander Harrison, Rod Jones, Sivakumar Kandasami, Moray Newlands
Magazine of Concrete Research pp 1-25; https://doi.org/10.1680/jmacr.20.00340

Abstract:
From first principles, criteria for the EN12390-10 outside protected test have been developed and compared with existing practise. These criteria align well with the UK recommendations but are more conservative than the current French and Spanish requirements. The EN12390-10 outside protected test is unsuitable for classification as this would require anybody using the procedure with identical concrete to get the same result. Unfortunately, current tests that are suitable for classification, whilst safe, do not reflect the real performance of slowly reacting cements. Improving the test for classification purposes will take time and meantime the use of the EN12390-10 outside protected test and the criteria given in this paper provides a procedure by which both sustainability and durability may be achieved. The downside of this test is its slowness and, therefore, it is only suitable for type testing or the determination of limiting values and other criteria to be used in specifications or production.
Shutong Yang, Miao Yu, Kun Dong, Yushan Yang
Magazine of Concrete Research pp 1-62; https://doi.org/10.1680/jmacr.21.00013

Abstract:
An analytical model is proposed in this paper to predict the local bond strength τf by incorporating the heterogeneity at interface regions for deformed reinforcing bars centrally anchored in concrete. The rib width on bar surface is introduced as an interfacial characteristic parameter G in the proposed model accounting for the heterogeneity. Both the τf and local interfacial fracture energy GIIf from each specimen are linked to the G and determined analytically if the maximum pull-out loads Fmax are given from the test. It is found that the predicted τf is larger than the maximum average bond stress τavg-max. The discrepancy between the two values is reduced with the increasing of L/G. Moreover, as the L/G increases, the predicted τf shows a certain decrease and the reduction becomes smaller with stronger interfacial homogeneity. The predicted GIIf is significantly increased because of the weaker boundary effect. The validity of the proposed model is verified upon the comparisons between the predicted Fmax using the determined τf and GIIf and the experimental ones with the only failure mode of bar pull-out. Moreover, the bars here can be steel or FRP (fiber-reinforced polymer) bars, and the concrete refers to all types of cementitious materials.
M. H. Lai, A. M. Griffith, L. Hanzic, Q. Wang, J. C. M. Ho
Magazine of Concrete Research, Volume 73, pp 828-842; https://doi.org/10.1680/jmacr.19.00435

Abstract:
By electrostatic repulsion and steric hindrance, polycarboxylate based superplasticizer (SP) decreases the water demand for cement hydration in mortar/concrete production. Subsequently, less water is needed for a prescribed flowability, which improves its concurrent design limits of strength and flowability. Nonetheless, SP also introduces unfavourable dilatancy (or shear thickening) in mortar caused by clustering of mobile SP polymers in the interstitial void and adsorbed polymers on cement. Interestingly, the dilatancy does not increase monotonically with SP. Reversal of dilatancy occurs when a threshold of SP dosage is reached, and subsequent addition of SP decreases dilatancy because the fine particles in mortar are better dispersed with adequate SP. The interstitial void substantially decreases due to deflocculation of fine particles that improves wet packing density of mortar. Based on this, it is believed that by blending the cement with fly ash, and/or using multi-sized sand, the dilatancy of mortar can be alleviated. Herein, a rheology test programme of superplasticized mortar was conducted via a co-axial viscometer to: (1) find out the threshold SP dosage when dilatancy reversal occurs; (2) study the effect on dilatancy of mortar by using multi-sized powder and sand; (3) correlate the dilatancy of mortar to its wet packing density.
Jianxin Peng, Huang Tang, Linfa Xiao, Xiaokang Cheng, Jianren Zhang
Magazine of Concrete Research, Volume 73, pp 757-770; https://doi.org/10.1680/jmacr.19.00276

Abstract:
The diffusion mechanism for chloride ion in concrete with different water-cement ratio (w/c) is studied by chloride ion spray test. A new model of chloride diffusion coefficient which changes as the ingress depth of chloride ion and w/c is developed. A spatial time-dependent reliability model of time to corrosion initiation (TCI) for reinforced concrete (RC) structures is developed considering spatial variability of geometrical, material and environmental parameters. The test results show that chloride diffusion coefficient increases as w/c increases, and chloride diffusion velocity decreases as the penetration depth enhances. It is found that the probability of corrosion initiation for spatial variability is 13.1%∼18.5% higher than that without consideration of spatial variability. This indicates that neglecting spatial variations for evaluating probability of corrosion initiation time overestimates the structural reliability. It is also found that the probability of corrosion initiation for future climate change is 9%∼11% higher than that obtained from constant temperature. The mean value of time for corrosion initiation based on time-dependent chloride diffusion coefficient is 16.7% earlier than that obtained from constant chloride diffusion coefficient. This suggests that it will be more appropriate if the effect of time-dependent chloride diffusion coefficient considers penetration depth and w/c.
Magazine of Concrete Research, Volume 73, pp 731-742; https://doi.org/10.1680/jmacr.19.00391

Abstract:
This study aims to develop a shear strength model for steel fiber reinforced-prestressed concrete (SFR-PSC) beams. The dual potential capacity model (DPCM) was adopted to consider the shear resistant mechanisms of the compression and cracked tension zones at the critical section, and its formulations were modified to address the synergistic effect of prestress and steel fibers. In the proposed model, the shear demands to be resisted by the cracked tension and intact compression zones, respectively, were estimated based on the equilibrium conditions, and their corresponding potential shear capacity curves were also defined. It is assumed in the proposed method that the shear strength of SFR-PSC member is dominated by a specific shear resistance mechanism whose shear demand curve reaches the limit state (i.e., corresponding potential shear capacity curve) earlier than another one. For the verification of the proposed method, experimental results collected from literature were compared with those estimated by the proposed model.
Nagavedu Jayakumar Yogalakshmi, Kanchi Balaji Rao
Magazine of Concrete Research, Volume 73, pp 714-730; https://doi.org/10.1680/jmacr.19.00367

Abstract:
The complexity in shear behaviour increases with the decrease in shear span to effective depth ratio (av/d) because of the increase in contribution of concrete to the shear resistance of beams. Various models such as numerical models and Strut and Tie models for estimating the shear capacity of non-slender reinforced concrete beams are proposed in the literature; each of them having their own advantages and limitations. In the present study two simpler mechanics-based shear capacity models, one for beams with 1≤av/d<2.5 and the other for beams with av/d<1, are considered. To quantify the modelling error associated with the shear capacity models considered, the predicted shear capacities are compared with the respective experimental shear capacities of test beams included in the database (created in this study). The randomness in shear capacity is quantified by performing probabilistic analyses of one set of nominally similar beams for each shear span to effective depth range. Knowing the probability density function of shear capacity and the mean and standard deviation of shear capacity obtained using first order approximation, characteristic shear capacity equation (Eq. 5) is proposed for the design of beams with 1≤av/d<2.5 and av/d<1.
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