In order to investigate the bond and anchorage characteristics of steel bars embedded in ultra-high-performance concrete (UHPC), Tepfers’ (1979) thick-walled cylinder theory and elastic mechanics theory were used to establish a theoretical calculation model of bond strength between deformed steel bars and UHPC under the conditions of splitting failure and pullout failure, respectively. The proposed calculation model was verified with a large number of test data from the literature and experiments conducted by the authors. Finally, the anchorage length of deformed steel bars in UHPC was calculated by the centre point method, the check point method and the design specification of normal concrete structure; recommended values of anchorage length are given based on anchorage reliability theory. The research results can provide design basis for design engineers, and are also a useful guideline for the popularisation and application of UHPC materials.

The high specific density of concrete significantly increases the dead load in buildings. Preferring the aggregate used in concrete as pumice, which is a volcanic material, decreases the concrete's density. However, pumice causes a decrease in the mechanical properties of concrete due to its pores structure. The improvement of the mechanical properties of concrete provides material savings by reducing the cross-sections of the structural elements to be used in buildings. Ultra-high-performance concrete, which has been an important subject of studies in recent years, is an important construction material for civil engineering. In this study, fresh concrete, physical and mechanical properties of lightweight ultra-high-performance concrete (LW-UHPC) with pumice additive were investigated. Nano carbon black was added to the mixture as 5, 10 and 15% of the cement weight. Significant increases were observed in the mechanical properties of nano carbon black added concrete. The addition of nanocarbon black to concrete at a certain ratio increased the compressive strength and flexural strength of concrete by 9.9% and 10.6%, respectively. In addition, it was observed that the sulphate resistance increased in direct proportion to the increase in the amount of nanocarbon black.

The effect of a recently developed socket-type shear connector (SSC) on the flexural behaviour of a composite basement wall (CBW) was examined in this work. To this end, 12 CBWs composed of cast-in-place (CIP) piles fabricated with H-shaped steel beams or reinforcing steel plates were prepared by varying the arrangement and amount of SSCs. Two-point loading was applied to simply supported CBW specimens. The CBW specimens with more SSCs had a higher effective stiffness in the elastic state and higher moment capacity in the ultimate state, irrespective of the cross-sectional details of the CIP piles. These trends were particularly prominent when a reinforcing steel plate was used in the SSCs. The post-peak behaviour of the CBW specimens subjected to a simulated load with a negative external moment tended to be more ductile. Consequently, a higher degree of composite action was fully exerted on the CBWs with greater numbers of SSCs. Using established equations for the sectional details of CBWs with SSCs, the nominal partially composite to full composite flexural capacity ratios of the CBW specimens subjected to simulated loads with positive and negative external moments were calculated to be, respectively, 0.83 and 0.91 for 0.5ηsc and 0.79 and 0.90 for 0.75ηsc, where ηsc is the normalised shear connector capacity specified in ANSI/AISC 360-16.

The use of slag-reinforced concrete is necessary due to environmental concerns around the world, but the decrease in concrete durability caused by chloride-ion rich environments requires more advanced concrete-mix designs, especially in coastal areas. This study aims to assess the influence of slag addition on chloride-ion transport performance in reinforced concrete beams under sustained bending moment, considering two conditions of environmental chlorine exposure: drying-wetting cycles and soaking. Results show that the chloride diffusion coefficient increases in tension zones and decreases in compression zones when the sustained bending load in concrete beams increases. However, the addition of slag can significantly reduce the chloride diffusion depth and diffusion coefficient values for both environmental conditions, with a more significant effect on the drying-wetting cycle. The chloride concentration on the surface of slag concrete (SC) in compression zones is greater than that in SC tension zones and that in ordinary concrete (OC) in both tension and compression zones. The study established a prediction model describing the chloride-penetration characteristics of concrete beams subjected to sustained bending moment. The proposed model is a reasonable approach for predicting chloride distribution in reinforced concrete beams with slag addition subjected to bending moments.

The lack of information regarding the shear behaviour of high-strength longitudinally reinforced concrete beams without shear reinforcement (HRCBW) impedes design engineers from using the full yield strength of the material. In this work, current shear design provisions for HRCBW were re-examined based on the probability density function (PDF), confidence interval and confidence level. Based on the principal shear mechanism of beam and arch actions, a probabilistic shear capacity model for HRCBW was developed based on Bayesian theory and the Markov chain Monte Carlo method, taking into account both aleatory and epistemic uncertainties. Statistical characteristics (e.g. mean, standard deviation, distribution type) of the shear capacity of HRCBW were determined by the Kolmogorov–Smirnov test and statistical analysis. The accuracy and applicability of three major shear design provisions for HRCBW (ACI 318-19, BS EN 1992-1-1:2004 and fib Model Code 2010) were re-examined based on the PDF, confidence interval and confidence level.

The bond behaviour between fibre-reinforced polymer (FRP) composites and concrete is a complex problem that is influenced by material properties, joint geometric configuration and the surrounding environment. The three fundamental bond performance indicators of concern are interfacial fracture energy, bond strength and debonding strain. This paper introduces a new semi-empirical model for predicting interfacial fracture energy (Gf) between externally bonded FRP and intact concrete in terms of the most influential material and geometric parameters, namely concrete's compressive strength and maximum aggregate size, stiffness of FRP composites and bond width and length of FRP composites relative to the dimensions of the concrete member. The prediction of Gf helped generate accurate predictions for the other two bond performance indicators. The present model was developed using non-linear regression analysis before validation using almost one-third of the total database consisting of 425 points, collected from credible publications. The accuracy of the present model of Gf outperformed those of well-known literature. The excellent agreement in trend behaviour of the present model with those reported in related literature works postulated further the model's validity. The estimation of debonding strain and bond strength in terms of fracture energy demonstrated superior accuracy over those provided by different relevant literature models.

Mobilisation of alternate load path (ALP) mechanisms in three-dimensional (3D) beam–slab systems is a key factor in designing structures against progressive collapse. Existing analytical methods on 3D beam–slab systems focusing on limited load-resisting mechanisms often lead to uneconomical and unrealistic design, while finite element models with 3D solid elements are too complicated and time-consuming for 3D beam–slab systems. To address these shortcomings, this paper aims to provide structural engineers with two simple but effective and reliable approaches to predict the structural behaviour of 3D beam–slab systems. They include (i) an analytical method and (ii) a simplified finite element model based on strip method and grillage analysis. Both approaches are validated against published test results for 3D beam–slab systems. Compared to existing approaches on 3D beam–slab systems, these two proposed methods incorporate all the load-resisting mechanisms in both the beams and slabs, giving more accurate and realistic predictions of load–displacement curves for the sub-structures considered. In addition, parametric studies on the analytical approach are presented to shed light on the role of boundary conditions and the contribution of slabs to the load resistance capacity of 3D beam–slab structures against progressive collapse.

In response to the shortcomings of traditional concrete crack repair materials, a new generation of repair materials was developed – a microbial repair material (MRM) based on sodium alginate modification. A brushing technique is used to fix microorganisms on the cracks to be repaired so that they can deposit calcium carbonate (CaCO3) in situ to repair the cracks. The fundamental properties of the MRM were studied, along with the macroscopic morphology and surface water absorption of specimens before and after the repair of mortar cracks. The material changes and microstructure of the repair products were also analysed. The results showed that the calcium carbonate content, water absorption ratio and heating shrinkage rate of the MRM with modified sodium alginate were better than those of the repair material without microorganisms. It was also found that the microorganisms were fixed in cracks on the surface of the mortar using the brushing technique with sodium alginate as the carrier. They adhered tightly to the cracks after only two repairs and produced repair products (mainly calcium alginate and calcium carbonate) and the surface water absorption ratio was reduced by about 65% compared with that before repair.

Strengthening of reinforced concrete (RC) structures may be required for several reasons and flexural strengthening of normal concrete (NC) beams is widely carried out using laminated fibres. However, the incompatibility of the concrete cover can lead to fibre debonding or concrete cover dislocation. These are some of the critical setbacks in common practice and the cost of fibre can be high. A novel flexural strengthening technique for NC beams is thus proposed: durable strips of stainless steel plate (SSP) bonded over a engineered cementitious composite (ECC), which is in complete contact with the flexural reinforcement of a slave beam through shear connectors. The flexural behaviour of NC beams strengthened with ECC and SSP strips was investigated experimentally and numerically. Three variables were investigated: the thickness of the SSP, the ECC layer with or without deformed steel bars and three different strengthening techniques. The experimental results showed that hybrid ECC–SSP strengthening improved the ultimate capacity of the NC beams by as much as 156%. A three-dimensional finite-element model (FEM) was developed and validated using the experimental results. The FEM accurately predicted the experimentally observed flexural behaviour of the NC beams with hybrid ECC–SSP strengthening. An analytical model to predict the ultimate load of NC beams with hybrid ECC–SSP strengthening was also developed.

The issue of low compressive strength in concrete is primarily caused by the anti-washout admixture (AWA) added to cement-based cementitious materials (CBCMs). In this paper, the AWA was added to alkali-activated cementitious materials (AACM) to prepare underwater non-dispersible concrete (UNDC). The effect of different AWA contents on the compressive strength of AACM was studied by measuring the compressive strength, hydration process, pore structure, microscopic appearance, and chemical composition. The results show that the compressive strength of AACM-UNDC moulded underwater can reach higher than 80% of the compressive strength of AACM-UNDC moulded on land. The polymerization and decomposition reaction of AACM can lead to the formation of strong covalent bonds, which can not only interweave with the long chain structure of AWA to form a dense network structure, but also can avoid the AWA wrapping on the surface of the activated material through the charge effect, and hinder the hydration reaction process of AASM.

In this study, the uniaxial compression and splitting tensile properties of high-ductility concrete (HDC) at different temperatures were investigated. The uniaxial compression strength and splitting tensile strength of HDC under standard curing (20 ± 2°C; ≥95% humidity), water curing (15 ± 2°C; 100% humidity) and −20°C curing (−20 ± 2°C, 75–85% humidity) conditions were investigated. The variation of stress–strain response, elastic modulus and peak stress of HDC under uniaxial compression at different temperatures and the evolution of splitting tensile strength and energy were analysed. The results revealed the strength variation mechanism of HDC under different curing conditions. From the test data, the relationships between the uniaxial compressive strength, splitting tensile strength of HDC and splitting tensile energy and the curing temperature were obtained. A constitutive model of the uniaxial compression of HDC under environments of different temperatures was established. With increasing curing temperature, the compressive strength, splitting tensile strength, elastic modulus and energy of HDC specimens increased gradually, while the peak strain decreased gradually. The effect of curing temperature on compressive strength was significantly more pronounced than that on splitting tensile strength.

For concrete materials, durability is closely related to mass transport property. The tortuosity of concrete material is an important factor to account for transport property. In this paper, tortuosity of concrete is studied by experiments of MIP and permeation test to consider the exist of aggregate which induce in the interfacial transition zone. Tortuosity gained through corrugated pore structure model (CPSM) and Katz-Thompson model is compared and analyzed to quantify the tortuosity of fly ash concrete. Interfacial transition zone (ITZ) was observed by scanning electron microscope. The results show that the effect of aggregate should be considered when studying the tortuosity of concrete materials. With aggregate value fraction of 43.8%, when considering ITZ around coarse aggregate, concrete tortuosity is 1.29-16.18 times higher than the mortar matrix.

The issue of low compressive strength in concrete is primarily caused by the antiwashout admixture (AWA) added to cement-based cementitious material (CBCM). In this work, the AWA was added to alkali-activated cementitious material (AACM) to prepare underwater non-dispersible concrete (UNDC). The effect of different AWA contents on the compressive strength of AACM was studied by measuring the compressive strength, hydration process, pore structure, microscopic appearance and chemical composition. The results show that the compressive strength of AACM-UNDC moulded underwater can reach higher than 80% of the compressive strength of AACM-UNDC moulded on land. The polymerisation and decomposition reaction of AACM can lead to the formation of strong covalent bonds, which can not only interweave with the long chain structure of AWA to form a dense network structure but also avoid the AWA wrapping on the surface of the activated material through the charge effect, and hinder the hydration reaction process of AACM.

The beam–column joint (BCJ) is a critical region in a framed structure because during such events as earthquakes it is susceptible to earlier failure than adjacent members, leading to shear failure, and will endanger building users if not designed properly. BCJs designed using preseismic code provisions follow the non-ductile approach and might not resist postelastic rotation without enduring greater damage. Retrofitting techniques offer great opportunities for strengthening damaged BCJs. In this study, the effectiveness of a novel retrofitting scheme based on carbon fibre reinforced polymer (CFRP) confined ultrahigh-performance fibre-reinforced concrete (UHPFRC) in rehabilitating initially damaged BCJ specimens was assessed. Three retrofitting schemes using UHPFRC with and without confinement are proposed: (1) in situ casting of 25 mm thick UHPFRC jackets; (2) in situ casting of steel wire mesh-confined UHPFRC and (3) in situ casting of CFRP-confined UHPFRC. The confining action was achieved by sandwiching wire or CFRP mesh between two layers of UHPFRC. The results of this study indicate that BCJ specimens retrofitted with confined UHPFRC had improved overall seismic response, compared with specimens retrofitted only with UHPFRC. Further, the wire mesh-based retrofitting scheme proved to be more efficient than the CFRP mesh-based scheme.

This study investigates the role of slag substitution (0%, 30%, and 60%), and chloride concentration (1.5% and 3.5% NaCl) on microstructural changes during strength development between 28 and 360 days, rebar corrosion up to 600 days, and chloride binding behaviour in chloride-rich geopolymer concrete (GC). The microstructural changes of GC were evaluated through field-emission-scanning-electron-microscopy (FESEM), energy-dispersive-X-ray-spectroscopy (EDS), X-ray-diffraction (XRD), and Fourier-transform-infrared-spectroscopy (FTIR) analyses. The obtained results indicated that strength enhancement was higher for fly ash-GC (F-GC) mixes. The presence of chloride in GC mixes caused strength reduction at all ages, however, fly ash/slag-GC (F/S-GC) mixes made with higher slag mostly showed lower strength reduction than other mixes. Further, F/S-GC mixes made with higher slag exhibited less negative corrosion potential (Ecor) and lower corrosion current density (Icor) than other mixes, indicating better resistance against rebar corrosion. Chloride binding capacity was mostly higher for GC mixes made with higher slag content. Higher amount of Ca-bearing gels and higher atomic Ca/Si ratio in F/S-GC mixes were responsible for reducing the influence of chloride in strength reduction and rebar corrosion, when compared with F-GC mix. The shifting of Si-O-Si(Al) bond to lower wavenumber indicated more binding gel formation, thereby denser microstructure in F/S-GC mixes.

Natural pumice concrete (NPC) is a building material with the advantage of lightweight, high thermal resistance. In cold regions, NPC has to face the damage from freeze-thaw cycles. Freeze-thaw damage is closely related to changes in the pore structure of concrete. Therefore, it is meaningful to investigate evolution characteristics and damage threshold of pore structure for NPC under freeze-thaw cycles. In this study, freeze-thaw cycles tests, nuclear magnetic resonance (NMR) tests were designed. The characteristics of the evolution of the pore structure during freeze-thaw cycles were discussed. The results showed that the porosity in NPC specimens increases with the number of freeze-thaw cycles, and the main evolution of the pores showed the degradation of fine capillary pores (10nm1000 nm). After freeze-thaw cycles, the proportion of coarse capillary pores and non-capillary pores increased by 4.83%-10.59%. This evolutionary feature will directly lead to the degradation of the mechanical properties of NPC. Additionally, a pore damage model was established, and the pore damage threshold was also calculated based on the experimental results. The obtained damage threshold of pore structure can provide the theoretical foundation for the application of NPC in cold regions.

Deep penetrating sealer (DPS) is a general term for a class of waterproofing agent that can fill the internal pores of concrete; it is produced by mixing an alkali metal silicate solution as the base material with catalysts and additives. The objective of this study was to investigate the characteristics and mechanism of the effect of different amounts of DPS sprayed on concrete. The water absorption, hydrophilicity, permeability, microhardness, abrasion erosion resistance and pore structure of concrete were tested at different water/cement ratios and amounts of sprayed DPS. The test and analysis results showed that, compared with a control concrete, concrete absorbed less water and exhibited less permeability after being sprayed with DPS. Moreover, the hydrophilicity, microhardness and abrasion erosion resistance improved, while the pore volume significantly decreased. The effect was more evident when the amount of sprayed DPS was higher. This study provides a reference for practical engineering applications by demonstrating the variation in concrete surface properties after spraying different amounts of DPS.

Capacitance-based self-sensing properties of cement pastes at different curing times were studied in this work. Cement paste samples that did not require any functional materials were tested at 7, 14 and 28 days. Continuous and discontinuous loading cycles (minimum load of 21 kPa and maximum load of 98 kPa) were applied to the cement pastes. It was found that capacitance-based self-sensing is superior to resistance-based self-sensing owing to its greater dependence on stress and reversibility. The capacitance change was found to be irreversible for the 7-day cement paste sample owing to irreversible deformation. Over time, capacitive self-sensing became more effective. The stress sensitivity (fractional increase in capacitance divided by stress) for the 28-day cement paste sample was 2.54 × 10−8/Pa. It was also found that, with an increase in hydration time, the capacitance decreased, indicating the strength increase in cement paste can also be sensed with capacitance measurements.

In reinforced concrete (RC) structural systems, the use of ultrahigh-performance fibre RC (UHPFRC) as an alternative to ordinary concrete is promising, especially in critical locations, such as wet joints between prefabricated members. To better understand and guide the construction practice of reinforced UHPFRC members, 28 pull-out specimens were tested to investigate the bond performance of steel bars embedded in UHPFRC. The influences of embedment length and bar diameter were analysed and discussed. Owing to the high cracking resistance of UHPFRC, no crack formation or splitting failure was found during the test. It is concluded that the bond development and deterioration process of steel bars in UHPFRC are fundamentally similar to those observed in ordinary concrete except for the higher initial bond stiffness and peak bond strength. Moreover, formulae for calculating normalised bond strength and residual bond strength are proposed, and an analytical model for bond stress–slip response has been developed accordingly, based on a modification of the model recommended by fib Model Code 2010. Finally, suitable anchorage lengths of deformed steel bars in UHPFRC are discussed and suggested.

The establishment of an aggregate model that better matches real situations is one of the prerequisites to studying the mechanical properties of concrete. Previous models have focused on aggregates with regular shapes; however, this differs from the morphology of real aggregates, particularly recycled aggregate (RA). Due to the presence of adhered mortar, RA has more complex structural characteristics than natural aggregate (NA). It is therefore difficult to model RA, especially the distributions of irregular angles and sharp corners. A new modelling method based on the compression of circles and spheres is proposed in order to obtain circular, elliptical and convex polygonal aggregates in two-dimensional (2D) models and spherical, ellipsoidal and convex polyhedral aggregates in three-dimensional (3D) models. The compression method has excellent scalability and applies to both NA and RA in both 2D and 3D models. Using the proposed compression modelling method, the aspect ratios, sharp corners, flakes, edges and needles of RA and NA can be characterised. Random aggregate models showed that the compression modelling method was able to construct 2D and 3D geometric models of concrete made with NA and RA with desirable aggregate distributions and aggregate morphological characteristics.