Search

High-belite cement (HBC) is increasingly used in hydraulic concrete, of which the frost resistance is an important index in cold climatic areas. In the present study, the mass loss, dynamic modulus, cube compressive strength, micro-morphology and pore characteristic parameters of HBC concrete and reference specimens were investigated under freezing and thawing, and their internal frost damages were quantified by micro-Vickers hardness and ultrasonic theory. Test results showed that the frost resistance of HBC concrete was similar to that of ordinary Portland cement (OPC) concrete under the same proportioning, and that their freeze–thaw failure was a continuous and asynchronous process of both surface-to-interior damage and overall deterioration caused by the test mechanism and specimen size. Service life of concrete could be calculated through the thickness of the damaged layer obtained by ultrasonic theory, and models of Vickers hardness distribution and compressive strength prediction based on equivalent Vickers hardness were proposed to evaluate the performance of frozen concrete with high accuracy. This potentially provides a better understanding of freeze–thaw damage and a reference for the popularisation and application of HBC concrete.

In the present work, cohesive and overlapping crack models are integrated in a comprehensive numerical algorithm in order to investigate both tensile and compressive failures in plain or steel-bar reinforced concrete (RC) structural elements. These two fracture mechanics models offer a high capability in the investigation of non-linear phenomena occurring in the loading process of plain concrete or RC beams subjected to bending. Based on the crack-length control scheme, these non-linear models are able to describe snap-back and snap-through instabilities, crack formation/propagation, steel yielding/slippage, concrete crushing and scale effects on the structural brittleness. In the present work, some parametric studies are carried out on plain concrete or RC beams in order to highlight how the abovementioned structural phenomena, which are observed in engineering practice, can be effectively captured by means of a non-linear fracture mechanics approach. Moreover, dimensional analysis is adopted to confirm how the behaviour of lightly reinforced concrete beams can be effectively described by means of two non-dimensional brittleness numbers, leading to a scale effect on the minimum reinforcement percentage, ρmin, proportional to beam depth raised to −0.15.

This study models the flexural strength of concrete modified with cashew nutshell ash (CNA) using reactivity index concepts. Cashew nutshell was valorised and its ash was used at 5, 10, 15 and 20 wt.% of cement via mix proportions of concrete grades C 25–40 targeted at 25–40 MPa at 28 days. The reactivity indexes (RIs) were evaluated based on the oxide compositions and chemical moduli of both the cement and CNA. The design mix parameters, water/binder and binder/aggregate ratios, and RIs were applied to model the flexural strength at 7–90 days’ curing. The results revealed that, at 28 and 90 days’ curing, the developed models yielded high precisions at 91 and 84% R2, respectively, hence indicating a good agreement. Moreover, there was a good relationship between these developed models and previous studies. Therefore, the flexural strength of concrete incorporating supplementary cementitious materials (SCMs) can be efficiently predicted via these developed models; this would save both time and money when carrying out the experimental works.

The combined use of coal bottom ash (CBA) and ground CBA (GCBA) as partial replacement of fine natural aggregate (FNA) and Portland cement, respectively, was investigated for the production of concrete mixtures. Five concrete mixtures incorporating GCBA at 20 wt% replacement of binder and CBA at 0, 25, 50, 75 and 100 wt% replacement of FNA were produced and tested for strength, microstructural characteristics and carbonation resistance. A base concrete mixture made of conventional concrete was also produced as a reference concrete mixture. Test results showed that the combined use of GCBA and CBA in concrete mixtures results in enhancement of the microstructure, carbonation resistance and compressive strength of the resulting concrete mixtures. Scanning electron microscopy and X-ray diffraction analyses pointed at the occurrence of pozzolanic reaction and pore-filling effect of GCBA and CBA in the resulting concrete mixtures. It was observed that 25 wt% replacement of FNA with CBA is an optimum dosage considering its effects on the later age strength and durability characteristics of the resulting concrete mixtures.

Both obsidian and perlite belong to acid volcanic rock and have a similar structure and chemical composition, but the insulation material prepared from expanded obsidian cannot reach the performance level of that prepared from expanded perlite. To date, obsidian cannot be used in large quantities, although its resource is abundant. In order to comprehensively utilise this resource, in the present work the authors developed a new type of lightweight thermal insulation material by using obsidian as a main raw material which is energy-efficient and environmentally friendly. The effects of raw materials such as obsidian, sodium silicate, hydrogen peroxide and cetyltrimethylammonium bromide on physical properties including thermal conductivity, porosity, density and compressive strength were studied in detail. The optimised thermal insulation material exhibits low thermal conductivity (0.051–0.86 W m−1 K−1), low density (0.1–0.24 g/cm3) and adjustable compressive strength (0.09–1.52 MPa). Compared to other inorganic insulation materials, this new thermal insulation material is very light and has a better thermal insulation performance. Therefore, this work proposes a new energy-efficient way to prepare a promising noncombustible thermal insulation material, and also provides a way to use obsidian resources in large quantities.

Modelling the expansion and damage generated by alkali–aggregate reaction (AAR) in reinforced concrete structures is quite complex, yet necessary to obtain accurate predictions of the structural response of distressed members. Several AAR models have been developed to predict expansion and damage at the material (microscopic) or the structural (macroscopic) scales. However, those models tend to either neglect or overemphasise the critical physicochemical parameters of the reaction, which limits their applicability. Therefore, a new simple yet reliable finite-element approach is proposed to fill this gap. It accounts for the most important parameters affecting AAR through an engineering approach, without the need for non-technical guesses or to ‘fit’ model parameters. The proposed model is validated through the computational simulation of reinforced concrete specimens cast and monitored in the laboratory. Results show that AAR expansion was accurately simulated by accounting for the anisotropic (stress state dependent) nature of the reaction, mechanical properties deterioration and an analytical equation capable of representing AAR's free expansion. Next steps include validating the approach by simulating real structures and incorporating phenomena like leaching and combined distress mechanisms.

Besides strength and elastic modulus properties of concrete, creep and shrinkage are also important in designing structures as they can affect seriously their integrity in the long term. It is shown that design codes internationally, as well as researchers in general, have omitted to consider the influence of aggregate, in particular its stiffness, in the models developed for estimating creep and shrinkage of concrete. This paper shows that the American ACI, Australian AS 3600, European Eurocode 2, Japanese JSCE and B4 models are mostly found to estimate creep coefficient and shrinkage strain of concrete inaccurately, irrespective of the type of aggregate used – natural, recycled or secondary. It is proposed that the stiffness of aggregate can be represented reasonably by its pore structure and in turn, for ease of measurement, by its water absorption. Two empirical models are proposed for estimating creep coefficient and shrinkage strain of concrete covering a wide range of natural, recycled and secondary aggregates alone or in any combination of these, as well as the cement effect and its proportion effect in the form of aggregate/cement ratio.

This paper deals with an evaluation of the generalised partial safety factor for resistance properties by using a statistical determination of a resistance model for predicting the punching capacity. The factor takes the material, model and geometric uncertainties into account. The Eurocode 2 design model for the assessment of the punching resistance without shear reinforcement was statistically analysed using an extensive database of experimental tests. The correction factor b of the model and the coefficient of variation of the model errors were evaluated according to Annex D of EN 1990. The statistical parameters of the basic variables in the resistance functions were assumed according to the recommendations of the Joint Committee on Structural Safety Probabilistic Model Code. The comparison of the safety factors obtained with the material safety factor currently used reveals that the current factor 1.50 underestimates the values obtained for the design model analysed.

Seismic assessment of reinforced concrete (RC) buildings heavily relies on the deformation capacity predictions of members. Similarly, the prediction of the deformation capacity of retrofitted RC columns has the utmost importance for seismic retrofit design. In this paper, a new unified drift model is proposed in order to estimate the rotation capacity of RC columns retrofitted with concrete jacketing, steel jacketing and fibre-reinforced polymer (FRP) wrapping. In order to generate the model, an extensive database was compiled consisting of 269 columns composed of 39 concrete-jacketed, 91 steel-jacketed and 139 FRP-wrapped RC columns failing in flexure mode. In the proposed drift model, the key model parameters are the confinement stress ratio, the axial load ratio and the lap splice length. The accuracy and scatter of the estimations with regard to the previous models were studied. For all of the column retrofitting options, the proposed unified model was observed to have rotation predictions better than the previous models in terms of the mean of the predicted to experimental rotation capacity ratios and with a reduced standard deviation.

The durability of ultra-high-performance concrete (UHPC) exposed to freezing–thawing (F–T) is closely associated with its micromechanical properties. In this study, the nanoindentation technique is employed to characterise the micromechanical properties of UHPC at different F–T cycles. It is observed that the indentation moduli of low-density calcium silicate hydrate, high-density calcium silicate hydrate, and calcium hydroxide in the studied UHPC decrease moderately with the decreasing ranges of 11.58, 14.56 and 3.3%, respectively, after undergoing 1500 F–T cycles. More micropores gradually form, converge and connect, and the volume fraction of micropores increases up to about 120%. The interfacial transition zone (ITZ) between steel fibre and cement paste is more prone to act as the weakest zone than that between fine sand and cement paste. The thickness of ITZ enlarges from 22 μm of the unconditioned stage to 60 μm at 1500 F–T cycles. Based on the micromechanical properties obtained from the nanoindentation test, the modulus of elasticity values of UHPC under different F–T cycles are predicted using the multiscale homogenisation model. It is proved that the combined micromechanical characterisation and multiscale homogenisation technique is capable of predicting the macromechanical properties of UHPC and their changes under F–T action.

Concern has been expressed in the recent decades about the corrosion prediction of concrete structures in a chloride environment. This paper summarises the corrosion prediction models developed for chloride-contaminated concrete, including both the empirical and electrochemical corrosion models. The definition methods of the steel de-passivation process for electrochemical models, such as anode–cathode ratio (Aa/Ac), critical chloride concentration (CCl,crit), modification of anodic Tafel slope (βa) or passive film resistance (Rf), are considered essential and discussed in detail. Furthermore, electrochemical parameters regarding the definition and selection in electrochemical models and experimental measurements are discussed. This study confirms that Aa/Ac and CCl,crit have been generally used in recent years, and that the modifications of βa and Rf related to free chloride concentration are technically feasible. In addition, the discreteness of electrochemical parameters in electrochemical models and experimental measurements are observed, and time-dependent characteristics of electrochemical parameters are scarcely presented in the existing literature.

This paper presents the results of an experimental evaluation of masonry grout made with lime-activated slag–glass powder blend as a binder. The purpose of this study is to develop a grout mixture that is low-carbon dioxide (‘low-carbon’) and has the potential to replace the traditional grout material used in load-bearing concrete block masonry construction. Traditional grout material uses Portland cement, production of which is responsible for a large proportion of carbon dioxide emissions. Thus, elimination or even reduction in the use of Portland cement will minimise environmental impacts. Three grout mixtures incorporating different proportions of glass powder as a partial replacement of slag as an aluminosilicate precursor were investigated to determine the optimum mix proportion that yields the desired properties of the grout material used in concrete block masonry construction. The workability, compressive strength and permeability properties of the mixtures were evaluated. Results from this study show that the use of glass powder at 25% replacement of slag as a precursor in the binder system provides the best performance, and that this mixture can be successfully used as an alternative and low-carbon grout material for concrete block masonry construction.

The potential of utilising alkaline industrial waste with weak or no cementitious properties for carbon dioxide (CO2) capture and for new binders by accelerated carbonation was studied; steel slag (SS), calcium carbide residue (CCR) and waste hydrated cement (WHC) were compared. The carbon dioxide uptake and strength development of SS, CCR and WHC were studied and the relationship of strength development with carbon dioxide uptake and characteristics of carbonation products was analysed. The results indicate that carbon dioxide uptake of SS, CCR and WHC strongly depends on the mineral composition: calcium hydroxide (Ca(OH)2) and calcium silicate hydrate (C–S–H) show relatively higher carbonation activity; dicalcium silicate (Ca2SiO4) absorbs less carbon dioxide than calcium hydroxide and C–S–H. The carbon dioxide uptake of SS, CCR and WHC are 6.1, 23.2 and 17.9%, respectively, after 2 h carbonation curing. Compacted SS, CCR and WHC specimens displayed a compressive strength of 74.9, 20.5 and 28.6 MPa after carbonation for 2 h. Compressive strength development depends on carbon dioxide uptake and mechanical properties of raw materials and carbonation products. Calcium carbonate (CaCO3) and amorphous products fill the pore structure, making the matrix denser; improvement of pore structure relates positively to carbon dioxide uptake. The highest compressive strength of compacted SS is mainly caused by the high elastic modulus of SS and formation of C–S–H gel in the carbonation process.

Prestressed hollowcore slabs are potentially critical segments in the construction sequence of a precast concrete structure, in which very early age strength is vital for ensuring factory productivity. Importantly, the construction industry is striving to reduce concrete's carbon footprint by using Portland cement substitutes, particularly using ground granulated blastfurnace slag (GGBS) in Ireland. This paper investigates the early age strength enhancement methods for slag-concrete mixes, in order to address the well-known retarding effects arising from slag's inclusion. The strength enhancement methods, including temperature curing and a novel accelerating admixture, are used to activate the slag in the concrete mixes, with replacement rates up to 50% GGBS. The efficacy of these methods is investigated in relation to establishing the development of compressive strength and modulus of elasticity over time (being relevant to stress transfer), particularly at very early and at later ages for strength compliance. The study demonstrates the potential for using up to 50% GGBS mixes for slab production while still maintaining the strengths sufficient to achieve existing productivity targets in precast concrete production.

In this study, shear resistance of ultra-high-performance fibre-reinforced concrete (UHPFRCs) containing different types (smooth, hooked and twisted fibre), fibre aspect ratios (l/d = 100, 95 and 65) and volume contents (0.0, 0.5 and 1.5 vol.%) of steel fibre was investigated using a new shear test method. Shear resistance of UHPFRCs strongly depended on the fibre type, fibre aspect ratio and fibre volume content. The twisted fibre exhibited the highest resistance in terms of shear strength, shear strain capacity and shear peak toughness, the smooth was the next and the hooked fibres produced the lowest response. The bigger the aspect ratio of fibre added to the mixture, the higher the shear resistance of UHPFRCs can be observed. Besides, the shear resistance of UHPFRCs was significantly enhanced with the increase of fibre volume content. The compressive strength and flowability of all mixtures were also checked. The compressive strength increased from 184 MPa for ultra-high-performance concrete to 210 MPa for UHPFRCs, whereas the flowability of mixtures varied from 203 mm to 250 mm. Finally, a theoretical model to predict the shear resistance of UHPFRC beams without the stirrup bar was proposed and verified.

A modified design method of a novel large-span floor system called an open-web sandwich slab (OWSS) is discussed. By considering some crucial parameters ignored in current design procedures, a new design procedure based on experiments and simulations is proposed. Considering the dimensions of the shear keys, the linear stiffness of the chords and the linear stiffness ratio between the chords and the shear keys, four specimens were tested and the failure modes and plastic behaviour of the specimens were studied. Based on the experimental results, a series of finite-element simulations was conducted and a crucial integrality coefficient was proposed. Based on the tests and simulations, a modified design procedure was then developed, with recommended values of some crucial coefficients. Finally, a case study was examined to show the efficiency of the new design procedure and the advantage of the OWSS; the results showed that the newly designed OWSS had advantages compared with a traditional solid beam.

In order to improve the performance of wet joints in prefabricated ultra-high-performance concrete (UHPC) bridges, a comparative study was conducted to investigate the flexural behaviour of UHPC joint beams. Experimental tests were implemented on a complete beam and on five further specimens with, respectively, vertical joint, rhombus joint, vertical joint with strips, tied rhombus joint with strips, and welded rhombus joint with strips. The specimens’ failure modes, ultimate flexural capacities, strain distributions and load–deflection curves were investigated. The test results show that the flexural capacity of the joint beams is lower than that of the complete beam. Among the five specimens with wet joints, the welded rhombus joint with top and bottom strips had the greatest stiffness, the highest cracking resistance capacity, and the best flexural performance. From the best to the worst performing, the remainder of the wet joint specimens took the following order: the unwelded rhombus joint with strips, the vertical joint with strips, the rhombus joint and the vertical joint. In accordance with the test results and the related theoretical analysis, this paper proposes formulas to calculate the flexural capacity of the beam with UHPC joints.

In cold regions, concrete practitioners face challenges when trying to achieve quality results with concrete produced under low temperatures. The addition of nano-silica, which has vigorous reactivity, to concrete can produce mixtures with a dense microstructure and improved hardened properties under cold temperatures. Thus, this research focused on gaining a fundamental understanding of the performance of nano-modified concrete which was mixed, cast and cured at a temperature of −5°C, without any method of heating or insulation. This study adopted the response surface method as a statistical modelling approach to assess the effect of different parameters on the performance of 28 mixtures. Four factors were implemented in this model – water/binder ratio, fly ash content (0–25%), nano-silica dosage (0–4%) and type of antifreeze admixtures – followed by optimisation scenarios. The mixtures’ performance was assessed based on multiple responses: initial and final setting times, early- and late-age compressive strengths and resistance to freezing–thawing cycles. In addition, mercury intrusion porosimetry, thermogravimetry and backscattered scanning electron microscopy were conducted to capture the microstructural evolution of the mixtures. Nano-modified mixtures with and without fly ash, especially with a low water/binder ratio (0·32) and high calcium nitrite content, showed promising performance when cast under cold weather conditions without any protection method.

The corrosion resistance of concrete strengthened with three types of fibre-reinforced polymer (FRP) sheets (unidirectional carbon-FRP (CFRP), unidirectional basalt-FRP (BFRP) and bidirectional BFRP) was evaluated under chloride attack and the combined attack of chloride and ultraviolet radiation. After different corrosion durations, powder samples were collected from various depths of the concrete specimens to measure the total chloride ion concentrations (CICs). Scanning electron microscopy (SEM) was conducted to analyse the microscale morphology changes of the concrete surface layer, FRP fibre layer and resin matrix layer before and after combined attack. The results showed that all three types of FRP sheet provided significant chloride ion resistance, with the barrier effect of CFRP being better than that of BFRP under the same conditions. The total CIC of the FRP-strengthened concretes was higher under the combined attack than under chloride attack alone. The SEM observations showed that the resin matrix and fibre filaments of the FRP layer under combined attack were damaged, resulting in an increase in the permeability of the FRP-strengthened concretes. Finally, a time-varying model of CIC inside the FRP-strengthened concrete under chloride attack was established.

Based on the structural characteristics of pervious concrete, an improved mix design method for pervious concrete was developed based on permeability, compressive strength and slurry layer thickness relationships. The results of validation experiments showed that when the slurry layer thickness of the pervious concrete is about 0.35–0.55 mm, the effective porosity is about 22–36%, the permeability coefficient is about 4.88–8.90 mm/s and the compressive strength is about 5.9–18.5 MPa. With an increase in the slurry layer thickness of the pervious concrete, the compressive strength increased, but the effective porosity and permeability coefficient decreased, and vice versa. The results of this study proved that pervious concrete can be formulated to satisfy permeability and compressive strength requirements by using the improved mix design method.

This study investigated the effect of silicate modulus (M = silicon dioxide (SiO2)/sodium oxide (Na2O) by mass) and activator content S (mass proportion of sodium oxide and silicon dioxide to ground granulated blast-furnace slag (GGBS)) of sodium silicate (SS) solution on the compressive strength of alkali-activated GGBS (AAS) mortars. At a given solid content, the specimens activated by SS with modulus of 1.0 and 1.5 had the highest compressive strength. In the case of the same modulus of SS, the compressive strength of the AAS mortars rose linearly and then remained unchanged or declined slightly with the increase in S. The mechanism of the action of SS on AAS was studied using backscattered electron (BSE) and energy-dispersive spectroscopy (EDS). The BSE image of AAS shows that GGBS particles are tightly bonded to the surrounding hardened, jelly-like slurry, and that the structure of the GGBS particles in the AAS slurry is intact without an apparent hydrating layer around it. EDS analysis revealed that only active materials in the GGBS, mainly calcium, are dissolved to participate in reaction, and that dissolving these materials does not damage the GGBS structure. S is solidified by these active materials, which embed the GGBS particles therein to form a whole.

The stringent early-age strength requirements for production of prestressed hollowcore (HC) slabs have restricted the use of alternative cementitious materials, such as ground granulated blast furnace slag (GGBS). 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). 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 h of pouring. The HC slabs are instrumented to study the internal temperatures in the different slabs, leading to early-age strength development whereby these slabs are tested for immediate transportability and 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.

The use of natural hemp fabric for strengthening existing concrete columns was investigated in this work. The results presented in this paper were based on an experimental setup analysing the efficiency of natural hemp fibres for the confinement of existing concrete columns. The variables considered were the number of hemp fabric layers, the 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 analysed. The test results indicate that the three variables considered had significant effects on the confined columns’ strength and post-peak behaviour.

Biomineralisation is a process of formation of calcite from supersaturated solution due to the presence of bacterial cells and biochemical activities. In this article, a biological approach towards the 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 biomineralisation in concrete. The effect of a bio-based agent and bacterial solution on various properties of concrete is discussed. Additionally, the factors in the choice of bacteria for the biomineralisation process are also elaborated. This green biotechnology concept is a promising, environmentally safe substitute for conventional and current remediation methods to mitigate environmental problems in multidisciplinary fields.

Replacement of Portland cement (PC) by fly ash (FA) is currently limited to 15–30% by mass, mainly due to low early age strength development of concrete. This research uses calcium formate (Ca(HCO2)2; CF) as an admixture to high-volume FA (HVFA) composites to improve its strength properties. HVFA represents 60–70% of cement replaced by FA and dosage of CF varies from 0.5% up to high dosage of 9% of cement content. Compressive strength, isothermal calorimetry, thermogravimetric analysis, X-ray diffraction and scanning electron microscopy were conducted to investigate the effects of CF on hydration and microstructural aspects. The results show that 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 3% CF to HVFA mixes led to higher consumption of FA as well as higher formation of calcium hydroxide (Ca(OH)2), calcium silicate hydrates, calcium carbonate (CaCO3) and ettringite, which contribute to the increase of strength. The addition of very high dosages of CF at 9% increased the hydration of tricalcium aluminate but could hinder the hydration of tricalcium silicate in both PC and HVFA pastes with 60% and 70% FA.

This paper utilises non-linear finite-element analysis with three-dimensional (3D) 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 the failure mode but is very computationally demanding. As an alternative, the paper presents a novel approach, in which 3D joint elements are combined with non-linear 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 while requiring significantly less computation time than 3D solid element modelling.

To facilitate construction and on-site quality testing, a new type of panel joint using cast-in-place connecting beams was developed and studied. The proposed connecting beams consist of overlapping vertical U-shaped steel bars stretching out of the wall and the footing, longitudinal prestressed 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 studied included the presence or absence of L-shaped steel, sectional depths and the locations of connecting beams and locally non-bonded U-shaped steel bars. The effects of these parameters on the failure modes, hysteresis curves, stiffness, ductility and energy dissipation capacity were evaluated. The test results showed that the precast shear walls with the proposed panel joint achieved desirable seismic behaviour – comparable to or better than that of monolithic specimens – and can thus be safely applied in buildings in seismic zones. The use of L-shaped steel, prestressed steel strands and high-strength grout effectively delayed concrete crushing at the wall toes and thus improved the loading capacity, deformation capacity, initial stiffness, ductility and energy dissipation capacity of the precast shear walls.

This paper investigates and compares the effect of steel fibres obtained through recycling waste tyres (known as RTSF), and a commercially available hooked steel fibre (HSF), on plastic shrinkage cracking in self-compacting concrete. The volumetric deformations of the specimens, bleeding, and the mass loss have been quantified. Mixtures containing 2.5, 5, 7.5 and 10 kg/m3 of RTSF, and 5 and 7.5 kg/m3 of HSF have been tested. The results show that an almost similar reduction of the crack area can be attained if HSF is replaced by a slightly higher amount of RTSF. However, the former seems to be more effective in restraining plastic shrinkage. Both fibres decreased the volumetric shrinkage and the bleeding capacity of the specimens.

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.

Steel reinforcement corrosion due to chloride penetration into concrete is one of the main causes of the degradation of marine structures and has been the subject of much research. Among the various factors that influence chloride penetration into concrete are environmental factors such as temperature and exposure conditions. In this context, the present study evaluated chloride penetration for three different exposure conditions (atmospheric, tidal variation and submerged) at temperatures of 15°C, 20°C, 25°C and 30°C over 18 months; samples were collected every 6 months, at depths of 1–5 mm. The environmental conditions were simulated in the laboratory. The results showed a higher concentration of chlorides in the submerged specimens. Moreover, the chloride concentration increased more intensively from 6 to 12 months and temperature was more influential above 20°C. The temperature effects were also found to be higher at advanced ages.

Several bond–slip models considering corrosion effects are available in the literature to describe the change in bond due to corrosion of longitudinal reinforcement. However, these models do not consider either the beneficial effect of stirrups or corrosion of the stirrups. This is very important because stirrups provide a confinement effect that enhances the bond strength of reinforcement, but the effect deteriorates with corrosion impacts. To address this limitation, a new bond–slip model was developed that includes the influence of corrosion of both the reinforcement and stirrups. More significantly, the model also provides a correlation between the corrosion in stirrups and longitudinal reinforcement. This means that the model can be applied with knowledge of the corrosion of either the longitudinal reinforcement or the stirrups. This is beneficial for the prediction of the corrosion level of longitudinal bars and, to take a step further, for the prediction of the mechanical behaviour of structures as it is relatively easy to measure the corrosion level of stirrups physically. The new model is presented in this paper along with a parametric study based on the developed model.

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.

The behaviour of concrete reinforced with steel fibres at elevated temperatures was investigated in terms of shear deterioration. Using three different steel fibre contents (0, 40 and 80 kg/m3), 45 push-off specimens were tested at five maximum temperatures (20, 150, 300, 500 and 700°C). The main objective of the research was to investigate the effect of steel fibres on the behaviour of concrete at different maximum temperatures in terms of shear failure. The results revealed enhanced ductility and shear strength at elevated temperatures. In addition, the crack widths and crack slips were measured using linear variable differential transducers. It was found that the use of steel fibres increased the shear strength of the concrete at ambient temperature by 103% and 102% for steel fibre contents of 40 and 80 kg/m3, respectively. Although this enhancement was reduced at the high temperatures (500 and 700°C), the mixes containing fibres still had higher residual shear strengths than the plain concrete mix. The steel fibre content also affected the ductility at both ambient and high temperatures.

The punching and post-punching shear behaviours of reinforced concrete (RC) flat plate structures are often studied by using a representative slab–column connection isolated from the parent structure. In this study, a set of numerical modelling techniques is established to create a competent three-dimensional non-linear model to simulate punching and post-punching shear behaviours of RC slab–column connections without shear reinforcement, through the use of 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.

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 three-dimensional (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: a primary mechanism stage, with small deformations, and a secondary mechanism stage, with 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 were no in-plane confinements, the slab also significantly improved the resistance against progressive collapse, owing to the compressive and tensile membrane action in the primary and secondary mechanism stages, 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.

This paper examines the effect of heating rate on the microstructure of cement-based systems exposed to sustained elevated temperatures. Besides a conventional mortar designed to attain a compressive strength of 30 MPa at 28 d, a high-strength mortar was also designed to achieve a corresponding strength of 90 MPa. Samples were extracted for testing under three different heating rates, ranging from 1°C/min to 10°C/min, to a sustained elevated temperature up to 400°C. This study finds that the microstructure of high-strength mortar is more sensitive to the heating rate in comparison with that of the normal-strength mortar. The porosity of the mortar mixtures uniformly increases with an increase in the temperature of exposure. However, there was a decrease in the porosity, at any sustained elevated temperature, with an increase in the heating rate. It is seen that the fractal dimension of the pores increases with an increase in the sustained temperature. Additionally, this fractal dimension was seen to increase with an increase in the heating rate. However, this trend was more pronounced at lower temperatures of exposure, so that the rate of heating appears to become less conspicuous at higher temperatures of soaking.

In this technical note, a consistent finite element formulation of the Hyperstatic Reaction Method (HRM) for tunnel linings design is proposed by introducing a variational consistently linearized formulation. It permits to consider a nonlinear interaction between a lining structure and the surrounding ground. Recent advances of the HRM in regard to the consideration of the nonlinear response of the segmented tunnel lining exposed to design loads use an iterative algorithm for solving the nonlinear system of equations. In the proposed Variationally consistent Hyperstatic Reaction Method (VHRM), a distributed nonlinear spring model representing the interaction between the lining and the ground soils is considered in a variationally consistent format. Computing the tangential spring stiffness via consistent linearization, and using Newton-Raphson iteration, requires significantly smaller number of iterations as compared to the original HRM model based on nodal springs. Furthermore, the method is applicable for simulations using solid finite elements (2D and 3D), as well as beam or finite shell elements, respectively.

Analytical modeling is conducted to examine the quasistatic response of Miura-ori-based metamaterials under compression in two principal directions. For the $$x_{3}$$ direction (out-of-plane) compression, the analytical results agree well with experiments and finite-element (FE) simulations. For compression in the $$x_{1}$$ direction (in-plane), the analytical model can predict the initial force. Additionally, the strain-hardening effect of the cell wall material of Miura-ori metamaterial is also taken into consideration, and verified by the FE simulations. The ratio of the two forces corresponding to compression in the two respective directions is also obtained, offering a convenient way of assessing material properties. The energy absorption efficiency in different directions is compared. This study demonstrates that the performance of origami metamaterials can be tuned merely by changing the geometric parameters of the origami unit. The work should also provide theoretical guidance for designing metamaterials at small-scale unit cells.

The compressible packing model (CPM), which is a method for designing aggregate gradation, was used to calculate the packing density of quartz aggregate. The aggregate gradation of ultra-high-performance concrete (UHPC) was optimised using the CPM. The number of interfaces between the aggregates and cement was determined using Image-Pro Plus software. The CPM modified using the equation Y = 0·60073X + 0·20551 was found to calculate the packing density of quartz aggregate accurately. The compressive strength of the UHPC was found to be positively correlated with the packing density of the aggregate: an increase in the packing density led to a compressive strength increase from 97·9 MPa to 124·1 MPa, an increase of 26·7%. The flexural strengths of the material was found to be sensitive to the number of interfaces: the smaller the number of interfaces, the greater the flexural strength. The specimen with an interface length of 5·56 mm/mm2 had the smallest flexural strength (21·5 MPa) and the specimen with an interface length of 3·596 mm/mm2 had the largest flexural strength (25·9 MPa). It is recommended that, when designing aggregate gradation, a gradation with high density should be selected and the proportion of small-size aggregates should not be too large.

The microstructure and transport properties of mortars made with graphene oxide (GO) and cement and with similar consistency were experimentally investigated and compared with those of control mortars made using Portland cement. Polycarboxylate superplasticiser was used to improve the consistency of GO-modified mortars and the dispersibility of GO nanosheets. Compared with the control samples, the GO-modified mortars with similar consistency showed a 33·2% reduction in the chloride migration coefficient and a 61·6% reduction in the initial rate of water absorption. These improvements are 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. Very small amounts of GO additives were found to significantly decrease the macropore volume fraction and the critical and average pore diameters. In an attempt to understand the correlation between the pore size distribution and the macroscopic transport performance, both the critical pore diameter and the macropore volume fraction were found to be exponentially dependent on the chloride migration coefficient. The average pore diameter was found to be linearly related to the initial absorption rate of the mortars, but was independent of the secondary absorption rate.

In recent decades, different macro-models have been proposed for simulating the behaviour of concrete shear walls because of their significant role in the seismic performance of urban buildings. The ability of Timoshenko's beam theory to evaluate the behaviour of slender and moderate-aspect-ratio concrete shear walls was evaluated in this work. For this purpose, a non-linear displacement-based Timoshenko fibre 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 element and the materials used, 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 tests). Evaluation of the numerical model and the experimental results revealed that the analysis procedure used to model the reinforced concrete walls predicted their overall and local responses with acceptable accuracy. It can thus be used as a reliable tool for the analysis of slender and moderate-aspect-ratio concrete shear walls.

Self-compacting concrete (SCC) is widely used in tunnel linings that are subject to fire risks. During tunnel fires, personnel and structural safety are commonly threatened by the explosive spalling of SCC due to its high strength and high moisture content. Recent studies have suggested that a coating of fire-resistant material can efficiently prevent SCC from spalling during a tunnel fire. In this study, SCC samples coated with a 6 mm thick layer of silicon dioxide aerogel-cement mortar (SiO2-ACM) were prepared and compared with uncoated samples. 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 durability performances as well as low thermal conductivity. During exposure to mimetic tunnel fire, the degree of spalling of SCC increased as moisture content increased. The SCC samples with the SiO2-ACM coating exhibited much less spalling than those without such coating. After the mimetic tunnel fire, the uncoated SCC samples had almost completely failed in terms of compressive strength, whereas the residual compressive strength of the coated SCC samples was more than 68·6% even for the SCC with 100% moisture content. Therefore, a SiO2-ACM coating provides an attractive means by which to enhance the behaviour of SCC during a tunnel fire.

The physical, chemical and mechanical properties of concrete incorporating rejected recyclable plastic waste (RRPW), the potential for the mitigation of heat-induced concrete spalling and the effects of high temperature on the residual properties of concrete were investigated. Concrete compressive and tensile strengths, Young's modulus, crack width, mass loss, water absorption by capillarity, chemical composition and evidence of heat-induced concrete spalling were monitored in concrete samples made with RRPW particles and compared with those of concrete samples made with commercial polypropylene (PP) fibres, after exposure to 200°C, 400°C and 600°C for 2 h. The use of 0·125% by volume of PP fibres and plastic waste particles was found to improve the heat-induced concrete spalling performance, due to the release of internal pressure after melting by different mechanisms. The positive effects on concrete properties show the technical potential of incorporating RRPW particles in construction materials.

Refractory concretes are widely used for the linings of the steel shells of industrial structures subjected to high temperature and pressure. The working linings of steel ladles are made of high-alumina concretes applied in layers that receive heat flow. Modelling the thermo-mechanical behaviour of these layers under service conditions is of paramount importance to evaluate failure or cracking in order to determine their useful life. In this work, the previously studied heating and holding process of linings comprising several layers of different types of refractory with different alumina contents (51, 71 and 90 wt%) was simulated. The developed model employed axisymmetric finite elements and the thermo-mechanical analyses were carried out using the software package Abaqus. The results of the thermo-mechanical responses were compared. It was found that the combination of refractory concrete with 90 wt% of alumina as the inner layer and materials with 71 wt% and 51 wt% alumina as the middle and outer layers, respectively, was the most efficient in terms of the thermal gradient. Therefore, this combination could potentially lead to minor damage to the lining throughout its use. This work highlights the importance of selection of coating materials for layered applications.

The bond between reinforcement and concrete is one of the main factors affecting structural performance. In terms of the bond mechanism, the bond performance of plain round bars is especially sensitive to the stress state of the 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. Experiments involving 178 pull-out specimens with various concrete strengths and bar diameters were performed to analyse the influence of loading rate on the bond behaviour of plain round bars embedded in concrete under uniaxial lateral tension. The main bond parameters were evaluated quantitatively. The results showed that bond strength was independent of loading rate and the ratio of residual strength to bond strength remained constant, but the slip at the peak bond stress decreased obviously with an increase in loading rate. An empirical constitutive model of bond stress–slip considering the concrete strength, bar diameter, uniaxial lateral tension and loading rate was developed and validated with experimental results.

The mixing order of silica sand, clay (kaolinite), and water controls the microstructure of resulting artificial soil samples. Most homogeneous microstructures can be achieved by applying the mixing order “sand-water-clay.” The following methods were used to validate this statement: (1) optical observation, (2) X-ray tomography, (3) scanning electron microscopy, and (4) Mercury intrusion porosimetry. For all samples, clays are mainly organized in a homogeneous matrix but are also dispersed heterogeneously in micrometer-sized layers surrounding sand particles, particularly where sand grains show a greater roughness. At water contents ≥1.5 w L , the microstructures are visually similar from the mm to μm scale whatever mixing order is used. However, for water contents lower than 1.5 w L , the mixing order controls the distribution of the clay particles. This paper proposes a motivated choice of a preparation protocol of artificial clayey materials to be used in laboratory experiments. It might contribute to better understanding and modeling grain movements and arrangements in artificial muds, used for instance in underground mining, foundation settlement, hydraulic containment, road construction, soil stabilization, and in natural soils in the occurrence of soil liquefaction, industrial brick manufacturing, and in studying shear processes in tectonic fault zones.

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 9 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. Non-linear 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 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.

In recent years, concrete-filled steel tubular (CFST) arches have been widely used and the out-of-plane buckling of CFST arches has become a major concern. So far, few studies have investigated the out-of-plane behaviours of CFST arches, and no codes have given the design formula of the out-of-plane stability bearing capacity. Therefore, in this paper fixed CFST parabolic arches with a circular cross-section under uniformly distributed loads are investigated. Considering the effect of material and geometric non-linearity, 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 nominalised 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 form of formula similar to the available stability equations in existing designing codes for CFST or steel members and which has an accuracy equal to or greater than the equations in the available literature – would be easier for the designers to accept and use.

The paper presents new compositions of lime heat-insulating dry building mixtures, designed specifically for the finishing of aerated concrete, characterised by 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, and is manifest in a decrease in the content of free lime to 28·6%, an increase in plastic strength, and a 2·2 times increase in the softening coefficient. 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 increasing water resistance by 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 and high vapour permeability, while possessing a sufficiently high crack resistance and weatherability.