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A strut-and-tie model (STM) for unsymmetrically-loaded deep beams is proposed in this paper through simplifying the unsymmetrical problem into symmetrical and anti-symmetrical problems. This analytical solution starts with derivations of principal compressive stress fields based on the theory of elasticity. Furthermore, a minimum-strain-energy criterion is adopted to define the optimal STM geometry from the stress field, including the dimensions of struts and nodal zones, and strut angles. The proposed STM is validated against 140 test results, including 14 unsymmetrically-loaded deep beams and 126 symmetric deep and short beams, and finite element models (FEMs) for the 14 unsymmetrical deep beams. The FEM is also used to simulate the load-displacement relationships of unsymmetrical deep beams that could not be modelled by STM. In addition, a worked example is presented to demonstrate in detail the procedure of analysing an unsymmetrically-loaded deep beam using the proposed solution. In summary, the STM predictions are in good agreement with experimental results and FEM predictions. Therefore, the proposed STM is capable of giving accurate and consistent predictions for both symmetrical and unsymmetrical deep and short beams. Ministry of National Development (MND) National Research Foundation (NRF) The authors wish to acknowledge the first authors scholarship from the Singapore Ministry of National Development and National Research Foundation under L2 NIC Award No. L2NICCFP1-2013-4.

Scanning electron microscopy (SEM) is a widely used method for the analysis of concrete micro structure. To quantitatively analyze the SEM images with high efficiency and accuracy, an automatic segmentation framework is proposed in this paper. The deep segmentation algorithm is purposely optimized from PointRend based on the characteristic of SEM images to improve prediction accuracy, especially the performance around boundaries. Moreover, the SEM images can be segmented without additional treatment. Cement paste samples with 0.2 and 0.4 water-to-cement ratios are prepared and cured for 1, 3, 7, 14, and 28 days. Totally SEM images with 2267 labeled cement particles are included to build the dataset. From the results of intersection over union and pixel accuracy, the proposed algorithm outperforms the trainable waikato environment for knowledge analysis (WEKA) segmentation, Fully Convolutional Networks (FCN), and the original PointRend method. The segmentation results are used to calculate the hydration degree of two cement paste samples. Good agreement is obtained with the hydration degree calculated by using nonevaporable water in the samples for the 5 curing durations. At last, the shape of the cement particles is analyzed. Irregularity and roundness of the cement particles do not change significantly with an increase in curing duration. Ministry of National Development (MND) National Research Foundation (NRF) This work was partially supported by the National Key Research and Development Program of China (Grant No. 2021YFF0500801), the National Research Foundation, Singapore, and Ministry of National Development, Singapore, under its Cities of Tomorrow R&D Programme (CoT Award No. COT-V2-2019-1), Shenzhen Science and Technology Program, China (Grant No. RCYX20200714114525013) and Open Funding of State Key Laboratory of High Performance Civil Engineering Materials (Grant No. 2021CEM006).

Corrosion damage in reinforcing steel bars has been a major cause of cracking and spalling of reinforced concrete. To extend the service life of concrete structures, non-destructive testing methods are necessary to assess the corrosion status in order to conduct a timely repair. At the early stage of corrosion, rust grows from the reinforcing bar, subsequently generates cracks towards the surface of the concrete. Ultrasonic methods have been widely used to detect cracks in concrete. However, it is challenging to characterise them due to the heterogeneous material properties of the concrete. In this paper, ultrasonic imaging technique based on diffuse coda wave has been explored to inspect and characterise corrosion-induced cracks. In this method, scattering cross-section of the crack is reconstructed with the Locadiff imaging technique. Based on the assumption that both crack tips have the same scattering cross-section, the size of the crack can be estimated when the location of the reinforcing bar is known. Numerical simulations were carried out to image straight and curved cracks, showing excellent accuracy. Experiments were designed subsequently on concrete samples with accelerated corrosion. The induced cracks were characterised by the proposed ultrasonic method, and compared with X-ray CT results, showing very good agreement.

This paper focuses on the condition where a reinforced concrete (RC) deep beam is subjected to two concentrated loads (P1 and P2), which are positioned at respective distances a and c from bottom supports. In general, equal shear spans (a = c) and equal point loads (P1 = P2) are commonly assumed in most experimental programmes. Nevertheless, such cases are rare in practice. In contrast, due to the fact that deep beam floor normally subjects to irregular layout of column grid, deep beams are usually subjected to unequal load magnitude (load inequality or P1 ≠ P2) and unsymmetrical load positions (load asymmetry or a ≠ c). Although previous research has demonstrated the significant influences of load asymmetry and inequality on the structural performance of deep beams at room temperature, no research programme has been performed to study their influences on the fire performance of deep beams. As a result, this study is proposed to investigate the shear behaviour of unsymmetrically-loaded deep beams at fire condition, analytically and experimentally. The current research first derives a strut-and-tie-model (STM) to predict the shear capacity of RC deep beams under unequal/unsymmetrical loading conditions and at elevated temperatures. The STM takes into account the effects of unsymmetrical loading configurations, contribution of steel reinforcement on strut capacity, and thermal-induced gradient. Additionally, a test programme on six RC deep beams under fire condition is conducted for verification of the proposed model. Observations from this experimental programme show that load asymmetry and inequality have different effects on the fire performance of deep beams. To be more specific, an increase in load inequality leads to an alteration in failure mode of the deep beams, whist load asymmetry does not affect the failure mode as much. On the other hand, increasing load inequality can result in a significantly increased mid-span deflection at failure and a slightly increased failure time. Besides, effect of load inequality on deflection and failure duration depends on the loading positions. Furthermore, the proposed STM yields good agreement between test data and model predictions for unequally- and/or unsymmetrically-loaded deep beams, at both room and high temperatures. Ministry of National Development (MND) National Research Foundation (NRF) This research work is supported by the Singapore Ministry of National Development and National Research Foundation under L2 NIC Award No. L2NICCFP1-2013-4.

The scanning electron microscopy (SEM) is widely applied to analyze the microstructure of concrete. SEM results are generally analyzed by human experts with different levels of expertise, and some tasks are extremely time consuming. In this study, a dataset consisting of 3600 SEM images was first built. Then, a deep-learning framework based on a convolutional neural network (CNN) was implemented for classifying cement paste mixtures with different water-to-cement ratios and different amounts of added silica fume. The accuracy of the classification reaches a high level of 94%. To improve the generality and efficiency of the proposed method, transfer learning technology with three transfer configurations was implemented and tested on a dataset of mortar samples. The result indicated that transfer learning enabled the new model to achieve higher accuracy and generality than training a network with randomly initialized parameters. The model accuracy increases with an increasing number of free convolutional layers, although the training time becomes longer. Finally, the critical features that greatly influence the classification were identified via visualization of the CNN model. Relatively small unhydrated cement particles have higher influence on mixtures with lower water-to-binder ratios, whereas hydration products are more influential in the case of mixtures with higher amounts of water or without silica fume. Ministry of National Development (MND) This work was partially supported by the Ministry of National Development, Singapore, under its Cities of Tomorrow R&D Programme (CoT Award No. COT-V2-2019-1), the National Natural Science Foundation of China (No. 52008136), the Shenzhen Science and Technology Program (No. GXWD20201230155427003-20200823110420001), and the Foundation of Guangdong Key Laboratory of Oceanic Civil Engineering (No. LMCE202104).

The behaviour of composite beam-slab structural system under fire condition has been investigated in the past few decades. However, previous research works mainly focused on isolated beam-slab sub-assemblages. In this paper, the boundary continuity effect on structural behaviour of composite beam-slab sub-assemblages subject to fire is investigated, where three different boundary continuities are considered, namely, internal, edge, and corner composite slab. Six specimens with a single unprotected interior beam were tested to failure, and two aspect ratios of 1 and 1.5 for each boundary condition were included. A total vertical load of 100 kN was applied to each specimen, which was heated by an electrical heating furnace from ambient temperature to 1000 °C within 1 h and maintained at 1000 °C until failure of the specimen occurred. The internal composite slab panel with an aspect ratio of 1 had the longest failure time and the edge composite slab panel with an aspect ratio of 1.5 sustained the longest duration. Finite Element (FE) models of the specimens validated by the experimental results were adopted to study membrane forces in the beam-slab sub-assemblage. Numerical simulations showed that different boundary continuities resulted in different membrane forces, and both the axial and rotational stiffness values in the direction of continuous edges were much greater than those of discontinuous edges for typical composite slabs. Moreover, the authors proposed minor amendments to Bailey-BRE method for calculating the enhanced load capacity at ultimate limit state and showed that the predictions were conservative when compared with test results. Agency for Science, Technology and Research (A*STAR) The research presented was funded by Agency for Science, Technology and Research (A*Star Singapore) and cooperated with Singapore Civil Defence Force (SCDF). The financial support of A*STAR (M4070013.030) is gratefully acknowledged.

The block tearing behaviour and resistance of S700 high strength steel (HSS) bolted connections in tension have been investigated through tests and numerical simulations and reported in this paper. The experiments were performed on sixteen S700 high strength steel bolted connections, including seven 2-bolt connections (with each comprising two bolts arranged perpendicular to the loading direction) and nine 4-bolt connections (with each comprising four bolts arranged in two rows). All specimens were carefully designed, with a series of geometric parameters, including the end and edge distances as well as the longitudinal and transverse pitches, varied and studied. The experimental setup and procedures, together with the key observed results, including the failure loads, the load–elongation curves and the block tearing failure modes, are fully reported and discussed. The experiments were supplemented by numerical simulations; finite element models were firstly developed to replicate the experimental behaviour and then employed to perform parametric studies to generate further numerical data on S700 high strength steel bolted connections susceptible to block tearing over a wide range of geometric dimensions. On the basis of the test and numerical 2 data, the existing design methods for high strength steel bolted connections susceptible to block tearing, as given in the European code, American Specification and Australian Standard, were evaluated. The evaluation results revealed that all three considered design codes lead to consistent and safe-sided but conservative predictions of the failure loads, due principally to the lack of appropriate consideration of the location and area of the shear failure plane. The proposals of Teh and Uz [1], developed for normal strength steel bolted connections, were then investigated and shown to result in substantial improvements of block tearing resistance predictions, although some of the resistances are unsafe. Finally, the reliability of the proposal of Teh and Uz [1] when used for S700 high strength steel bolted connections was demonstrated by means of statistical analyses.

Concrete-Encased Concrete-Filled Steel Tube (CECFST) columns are a new type of composite columns that possess great strength, stiffness, ductility, corrosion and fire resistance. To fully realise their benefits, an accurate design approach of calculating the strength of these members is necessary. In this paper, the plastic stress distribution (PSD) and load contour (LC) methods in the design code of predicting the cross-section strength of CECFST columns is first evaluated through numerical and published experimental data. Considering the shortcomings of conventional design methods, a unified design approach is then developed. Compared to the conventional PSD method, the developed method can yield conservative resistance predictions for uniaxial eccentric loaded (UEL) CECFST columns. In addition, instead of using linear moment interaction curves to predict the strength of biaxial eccentric loaded (BEL) CECFST columns, elliptical moment interaction curves are adopted in the proposed design approach. Most importantly, a normalised 3D axial-load–biaxial-moment (n–my–mz) interaction surface can be generated through the proposed method, which only requires determining 6 critical points on the interaction surface. Unlike the conventional design approach, in which the UEL and BEL composite columns need to be respectively designed through PSD and LC methods, a unified design procedure can be achieved through the proposed approach. Besides, the confinement effect from both the steel tube and the stirrups can also be considered in the proposed approach and much higher accuracy and consistency can be achieved in predicting the cross-section strength of CECFST columns. The authors gratefully acknowledge the funding entitled as ‘‘The behaviour and design of innovative composite concrete-steel structural system’’ provided by Jurong Town Corporation in Singapore

Five simple beam-column joints with composite slabs were tested under a middle column removal scenario. Fin plate connection was used in all the joints. The influences of joint type, slab thickness, number of shear studs and bolt holes were investigated. Load-resisting mechanism of the composite joints against the applied ‘push-down’ load was investigated and the respective contribution of flexural action, compressive arch action and catenary action was quantified. Structural behaviour of the joints in terms of axial force, bending moment, failure mode and energy absorption was presented. To evaluate design values provided by current design guidelines and building codes, test results of the joints were compared with design values. It was found that the composite slab was beneficial to flexural resistance of the joints. Although tying capacity was reduced due to bending moment, tie force requirement could be met for conventional composite joints with 75 mm thick slab. A proposed novel fin plate connection with slotted bolt holes had better performance than conventional connection in terms of energy absorption and tying resistance. The test results provided a better quantitive understanding of FP connections under progressive collapse scenario and were able to serve as benchmark for further simplified numerical models.

Protection of structures from fire is of extreme importance. Geopolymer is a novel material that has wide-ranging applications, and this review article focuses on assessing the potential of geopolymers towards enhancing the structural fire resistance by critically reviewing its properties subjected to elevated temperature exposure. The properties of geopolymers are categorized into three scales, namely, micro-scale, meso-scale and macro-scale, and are discussed at length. It is noted that geopolymers are chemically stable and do not undergo breakdown of chemical structure in contrary to OPC hydration products. Thermal deformations occurring in geopolymers, which cause macro-cracking, are discussed. Compressive strength of geopolymers is observed to be affected by microstructural changes (including crack formation, pore structure changes, densification, sintering, and melting) and phase composition changes (such as growth or destruction of crystals and transformations in geopolymer paste). Geopolymer-based binders show inherently superior fire resistance as compared to Portland cement-based binders. However, it requires careful mix design, to achieve substantial chemical stability, low volume changes, strength endurance, and spalling resistance. Factors such as choice of precursor, use of aggregates, total alkali content in geopolymer, water content, etc. are critical and should be controlled. The influence of these factors is discussed at length in this article. The current applications of geopolymers for heat and fire resistance have also been briefly presented.

Anchorage of steel bars is critical in reinforced concrete design to ensure composite action between concrete and rebars. The force–slip relationship of rebars is another factor governing the failure mode and rotation of structural joints. These two aspects can be predicted by bond–slip models and depend on bond behaviour at the reinforcement–concrete interface. However, current bond–slip models show either low accuracy and limited applications due to simplified assumptions or complicated computation due to nested iteration loops. This paper proposes analytical models derived from control field equations, which are characterised by low computational cost, high accuracy and broad application. The control field equations describe force equilibrium of embedded rebars by second-order differential equations and give accurate predictions by incorporating the local bond stress–slip relationship. The proposed models are appropriate for adequate and inadequate embedment lengths of rebars and can describe the evolution of the bond–slip distribution along embedment length. Closed-form solutions can be obtained for perfect anchorage and obviate iterative nested computations. If closed-form solutions are unavailable, owing to free-end slip for short embedment length, a mathematical function in Matlab is used to solve non-linear control field equations and complicated nested iteration is avoided. Finally, the proposed models show good agreement with published experimental results.

Perimeter beams in reinforced-concrete buildings are subjected to considerable torsion from slab negative bending moments. In the event of progressive collapse caused by column removals, torsion effects on such beams become critical, owing to redistribution of loads. The aim of this study is to explore the effects of slab corners on the torsional behaviour of perimeter beams in reinforced concrete beam–slab buildings, particularly in the case of column loss. A series of torsional tests on peripheral beams was conducted, comparing cases with and without slabs. Thereafter, physics-based numerical models were developed, based on the test results, to investigate the effect of varying the slab's parameters, such as thickness and reinforcement ratio, on torsional capacity. The models showed that the presence of reinforced concrete slabs could increase the torsional strength of perimeter beams by as much as 97%. The enhancement came first from the slab corner, which acted as a set of compressive struts. This effect increased with increasing slab thickness. It was also observed that the slab corners tend to shift torsional failure (which occurred at the twisted beam near the support) to about one beam depth away. The contribution of negative bending moment from slab reinforcement along the edges could also enhance overall structural capacity.

This paper presents an experimental study on the effects of elevated temperature and water quenching on compressive strength and microstructure of ultra-high performance concrete (UHPC) containing polypropylene (PP) fiber, steel fiber, and larger aggregates. UHPC samples were exposed to elevated temperature up to 900 °C. The residual strengths of the samples were measured after furnace cooling or immediate quenching in water. Microstructures and phase change of UHPCs were investigated by using X-ray Diffraction (XRD) and Field Emission Scanning Electron Microscope (FESEM). Test results show that the compressive strength increased significantly after exposure to 300 °C due to further hydration of unhydrated cement clinkers but start to decrease sharply after 600 °C exposure due to decomposition of hydration products. Steel fiber had beneficial effect, but PP fiber and larger aggregate reduced the compressive strength of UHPC because they promoted damage at micro scale. Water quenching decreased compressive strength significantly for all exposure temperatures. This reduction is mainly attributed to the formation of microcracks caused by high thermal stress. National Research Foundation (NRF) This experimental program is based on research/work supported by the Land and Livability National Innovation Challenge under L2 NIC, Singapore. Award No. L2NICCFP1-2013-4.

In this study, an investigation on the deformation profiles near the plastic hinge regions of propped cantilever beams is carried out by both experiment and numerical simulation. In the experiment,...

The flexural-torsional buckling behaviour of fixed-ended cold-formed stainless steel equal-leg angle section columns was experimentally and numerically investigated in this paper. The testing programme was conducted on two press-braked austenitic stainless steel equal-leg angle sections, and involved material testing, initial imperfection measurements and sixteen fixed-ended column tests. The test setup and procedure, together with the key experimental results, load–mid-height torsional rotation histories and flexural-torsional buckling failure modes, were fully reported. Finite element models were then developed to simulate the fixed-ended cold-formed stainless steel equal-leg angle section column tests, and the numerical structural responses and failure modes were shown to match well with the corresponding experimental observations. The validated finite element models were then utilised to carry out parametric studies to generate an extensive pool of numerical data on fixed-ended cold-formed stainless steel equal-leg angle section columns susceptible to flexural-torsional buckling. The load-carrying capacities derived from both the experiments and FE modelling were compared with the resistance predictions from the established design standards in Europe, America, and Australia/New Zealand and a recently proposed direct strength method. The comparison results generally indicated that the existing design codes unduly underestimate the flexural-torsional buckling resistances of fixed-ended cold-formed stainless steel equal-leg angle section columns, while the direct strength method substantially improves the design accuracy on average, but with many overpredicted resistances, indicating that further improvements are generally required. The authors are grateful to Outokumpu Oyj for providing stainless steel angle section specimens and to Mr. Subasanran Chelladurai, Mr. Cheng Hoon Tui and Mr. Siew Pheng Choi for their assistances in the tests. The financial support from NTU Research Scholarship is also acknowledged.

This paper investigated the effect of geometry and fraction of polypropylene (PP) fiber and the induced microcracks on permeability of ultra-high performance concrete (UHPC) subjected to elevated temperature. Residual permeability and microcrack networks of fifteen UHPC mixes were characterized and an analytical model correlating residual permeability of UHPC with fiber fraction and geometry was proposed. Results showed that increasing fiber length and dosage had much stronger effect than increasing fiber diameter on enhancing permeability due to greater enhancement on percolation of fiber tunnels. It was found that permeability of UHPC is positively correlated with both the aspect ratio and dosage of PP fibers. However, at low fiber aspect ratio, increased fiber dosage does not increase the permeability of UHPC. Similarly, at low fiber dosage, solely increasing fiber aspect ratio does not contribute much to increasing permeability of UHPC. The proposed model thus provides insight for PP fiber selection and optimization to prevent explosive spalling of concrete. NRF (Natl Research Foundation, S’pore)

The feasibility of using engineered cementitious composites (ECC) in joint cores of reinforced-concrete (RC) beam–column sub-assemblages as a means to enhance seismic behaviour is evaluated. Four RC beam–column sub-assemblages are constructed and tested under lateral cyclic loading. One RC beam–column sub-assemblage with normal concrete in the joint and one RC sub-assemblage with ECC in the joint are designed to gravity loads; neither type has transverse reinforcement in the joint core. Similarly, one RC sub-assemblage with normal concrete in the joint and one with ECC in the joint are designed for seismic provisions, but the latter has no transverse reinforcement in the joint. The test programme thereby allows direct comparison of the structural performance of ECC joints with concrete joints for beam–column sub-assemblages for both gravity and seismic design situations. Results show that the use of ECC joints significantly changes the behaviour of the joint from brittle to ductile. Both ECC specimens exhibit superior damage tolerance, with limited shear distortion and multiple fine cracks in the joints, even though no transverse reinforcement is provided in the joint. Moreover, specimens with ECC joints demonstrate improved bond behaviour in early loading cycles.