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In this paper, the cracking of concrete cover caused by steel bar corrosion was discussed and studied based on the three-period model of steel bar corrosion in concrete. In the analysis, the contour of the corrosion layer was simplified into a semi-ellipse according to the non-uniformity of steel bars in reinforced concrete under a natural environment. The steel bar corrosion was divided into three different periods by two corrosion points, namely, the void to be filled was exactly filled and the concrete cover was exactly cracked. Different assumptions are made for each corrosion period. Before the concrete cover cracks, it is developed into an elastic plate. Based on the theory of elastic mechanics, the maximum corrosion depth of steel bar when concrete cover cracks is predicted. After the concrete cover cracks, a model of the relationship between the crack width of the concrete cover and the maximum corrosion depth of steel bar was established by considering the filling of cracks by corrosion products. The rationality of the model was tested by employing experimental data from the literature, and it was found that the calculated value of the model was in good agreement with the experimental data in the literature.

The mechanical properties of four types of concrete, tested under multiaxial (biaxial tension–compression, triaxial tension–compression–compression) dynamic (strain rates of 10−5, 10−4, 10−3 and 10−2 s−1) complex stress states with different stress ratios, were compared and analysed. The same test apparatus was used in order to avoid test errors caused by differences of the triaxial testing machine. The aim of this work was mainly to determine the dynamic multiaxial (DM) strengths of the four concrete types. It was found that the mechanical behaviour of the four concretes was similar to a certain extent. The DM strengths of the concretes increased with an increase in strain rate and their DM strengths in any direction were lower than the concrete static strength under uniaxial tension and compression. The influence of strain rate and stress ratio on the DM strengths of the concretes were compared and analysed. A unified failure criterion was developed, expressed by the octahedral stress space criterion, of the various concrete types under DM tension and compression. This investigation provides a theoretical foundation for the design and checking of structures subjected to DM load conditions.

Reinforced-concrete (RC) structures are inevitably subjected to sustained loads during their service life. In the natural environment, the exposed surfaces of RC members often experience dry–wet cycles due to rainfall and tides. An important parameter in structural design is the the bond behaviour between steel bars and concrete, and this is affected by the coupled effect of sustained loads and dry–wet cycles. In this study, 32 RC beam specimens were subjected to different levels of sustained load (25%, 50% and 75% of ultimate load) and dry–wet cycles for a period of 120 d. The immediate slip and time-dependent slip (TDS) between the steel bar and concrete during testing were recorded and analysed. It was found that the TDS increased with an increase in the duration of sustained loading and greater sustained load levels resulted in larger TDSs. After long-term testing, the bond strength and bond stress–strain were investigated using a flexural-bond test. The results showed that bond strength increased due to the coupled effects of sustained load and dry–wet cycles. Based on the test results, an empirical model for steel bar–concrete TDS considering the effect of sustained load and dry–wet cycles was developed.

The movement of water and ions in the calcium silicate hydrate (C-S-H) gel pores determines the durability for the material. In this study, molecular dynamics was utilized to study aqueous NaCl solution capillary transport through the C-S-H gel pore with pore size of 3.5 nm, 2.5 nm, 1.5 nm and 1 nm. The penetration depth for the solution advancing frontier with menisci shape follows a parabolic relation as the function of time, matching well with classic LW capillary adsorption theory. The progressively reduction for the solution/C-S-H contact angle reflects the hydrophilic nature of the C-S-H surface. With decreasing of C-S-H gel pore size, the ions are filtered by the small C-S-H gel pore, with water invading deeply in the gel pore, and chloride and sodium ions remaining in the region of gel pore channel entry. The ultra-slow transport mechanism for ions in the nanometer channel has been further explained by the local structure and dynamics of ions hydrated ultra-confined in the gel pore. While the silicate chains in C-S-H surface can provide non-bridging oxygen sites to associate with the sodium ions by Na-Os connection, the courter calcium ions in C-S-H surface can capture the chloride ions, forming the ionic pair. Both Na-Os and Ca-Cl pairs have longer resident time that the unstable H-bond connection between water and solid oxygen. Furthermore, the transition region with slow mobility is formed at the channel entry region due to the coupling immobilization effect by both surfaces normal and parallel to the transport pathway. The presence of transition zones results in the necking of channel entry, which inhibits the ions with larger hydration shell from penetration. Additionally, at channel with size less than 2 nm, the Ca-Cl and Na-Cl ionic pairs accumulate to cluster and are highly concentrated in the center of the gel pore, blocking transport pathway for water molecules and ions.

Pull-out test was conducted to investigate the bond behaviour between recycled aggregate concrete and steel bar after rapid freezing-thawing cycles in fresh water and saline water. The test variables were number of freezing-thawing cycles (0, 25, 50 and 75), diameter of steel bar (14, 18 mm) and freezing-thawing medium (fresh water, saline water). According to the test results, the effect of freezing-thawing cycles, diameter of steel bar and freezing-thawing medium on the bond behaviour was analysed. It was found that the bond strength between steel bar and recycled aggregate concrete decreases as the freezing-thawing cycles increased, and after the same number of rapid freezing-thawing cycles, the loss of bond strength in saline water was greater than that in fresh water. Moreover, the loss of compressive strength was greater than the loss of bond strength between steel bar and recycled aggregate concrete after the same number of rapid freezing-thawing cycles in saline water.

The hydration and dissolution of tri-calcium silicate mineral is of great importance for guiding the material design. In order to understand the hydration mechanism of C3S mineral in sodium sulfate solution, molecular dynamics was employed to construct three interface models of C3S and Na2SO4 solution of 0 mol/L, 0.79 mol/L and 1.58 mol/L and the structure, dynamics and dissolution properties of the interfacial species were investigated. Due to hydrophilic nature of C3S surface, the water molecule in hydrate layer deeply tessellated on the vacancy surface region shows high intensity packing, ordered arrangement, enlargement of the dipolar moment value and orientation preference. With progressively increasing simulation time, the hydroxyl groups and calcium atoms in the high reactive surface gradually escape from the substrate bonding restriction and diffuse away to the solution region. Furthermore, both ions prefer to adsorbing on the surface of tri-calcium silicate mineral: the highly solvated surface calcium ions associate with the oxygen sites in the sulfate ions, forming the Ca SO4 ionic pairs in the interfacial region; the sodium ions are attracted by the non-bridging oxygen in the silicate monomers, forming Na O connection. The surface adsorbed water molecules, sulfate and sodium ions, strongly associated with the calcium atoms and hydroxyl groups, form a ionic and hydrate layer, stabilizing the tri-calcium silicate surface and inhibiting the dissolution of ions in C3S. With increasing ions’ concentration, the sulfate and sodium ions in the solution are more probable to form cation-anion pairs and further accumulate to the ionic cluster.

The degradation of bond behavior between corroded steel bars and concrete under sustained load is among the main issue for the durability of structures. In this study, the impressed current method was used to achieve target corrosion levels of tensile steel bar (0%, 1%, 3%, and 5%), while specimens bearing the sustained load (0%, 25%, 45%, and 65% ultimate load). The sustained load lasted for 60 days, during which the instantaneous slip and time-dependent slip were recorded. Finally, the bond strength of all kinds of specimens was measured by the flexural-bond test. The test results show that corrosion and load significantly affect the slip and bond strength between steel bar and concrete. Slight corrosion (less than 1%) leads to higher bond strength, up to 1.22 times that of the control group, while severe corrosion (more than 2.42%) leads to lower. Additionally, the strength reduction caused by the coupling of load and corrosion is more obvious than the single, by up to 45.66%. In addition, based on the test results, the time-dependent slip and bond strength prediction model were proposed, which can be used for predicting the service life of structures in the erosive environment.

This work presented an experimental investigation on bond behavior of corroded steel bars embedded in concrete under the action of three different levels of reciprocating loads (25%, 45% and 65% of ultimate load) and sustained chloride attack using 51 RC beam specimens with dimensions of 100 mm × 150 mm × 550 mm. In the test, the steel is corroded by the method of energized accelerated corrosion (theoretical corrosion ratio of steel is set as 2%, 4% and 6%). Test results show that the effect of reciprocating load will cause the bond strength between uncorroded steel bars and concrete to decrease in the chloride environment, but the effect is not significant. Under the coupled effect of reciprocating loads and energized accelerated corrosion, when the corrosion ratio of longitudinal reinforcement is constant, as the increase of the reciprocating load level, the bond strength between the steel bar and concrete of the specimen decreases. When the theoretical corrosion ratio of steel bars is small, the bond strength between steel bars and concrete gradually increases with the increase of the corrosion ratio; as the corrosion ratio of steel bars further increases, the bond strength shows a downward trend. The theoretical corrosion ratio of steel bars reaches a peak of about 2%. In this paper, the mathematical model of bond strength-slip has been proposed. This work provided an important theoretical basis for correctly assessing the durability of buildings or structures subjected to reciprocating loads in coastal areas.

Early-age cement-based materials undergo significant physical and chemical changes and have a complex mutual relationship among their temperature, humidity and mechanical behavior. As the second part of this study, this paper builds a multi-field coupling equation model taking into account the staged characteristics of the deformation for early-age cement-based materials based on the Hybrid Mixture Theory, and the accuracy of calculation results are verified with the test results obtained in the first part of this study. Moreover, the multi-field coupling model has been adopted to simulate the evolution of the different types of deformation for early-age cement-based materials. The results show that the multi-field coupling model established in this paper and the staged deformation calculation model established for early-age cement-based materials in the first part of this study can be used to better describe the development of their early-age temperature, humidity and deformation. Moreover, thermal strain is the earliest deformation that may cause cracking of cement-based materials, and chemical shrinkage has a certain compensation effect on thermal strain. In addition, the coefficient of thermal expansion (CTE) and the coefficient of stiffness influence (CSI) as well as their effects on thermal and chemical deformations have been analyzed. The results of this study can provide an important reference for analyzing the performance evolution of early-age cement-based materials.

Steel bars and concrete in practical engineering is served under a complex stress state, the bond behavior of steel bar embedded in concrete, inevitably, was affected by the stress state. So, the center pull out tests were conducted to investigate the bond strength and the bond stress-slip curves of steel bar embedded in recycled coarse aggregate concrete under the action of lateral compression load. For recycled coarse aggregate concrete, the replacement percentage of recycled coarse aggregate to the sum of natural coarse aggregates and recycled coarse aggregates was 30%. The variables of two types of steel bars (deformed steel bars with diameter 14 mm, 18 mm, 22 mm and plain steel bars with diameter 12 mm), four levels of lateral compressive load (0 kN, 200 kN, 320 kN and 450 kN) were considered in the experimental study. The failure modes of center pull out specimen were observed and recorded. Based on the test results, the influences of lateral compressive stress, type of steel bars and diameter of deformed steel bars on the bond behavior of steel bar embedded in recycled aggregate concrete were studied and analysed. It was found that the pull out force and bond strength for deformed steel bar are larger than that for plain steel bar under the same lateral compressive load, the bond strength increases with the increase of lateral compressive load.

Recycled WPC product using in construction industry occasionally falls short of expectation owing to lack of sufficient ductility and damping capacity, ground waste rubber tire (WTR) with substantial elasticity can be a proper candidate to meet the gap when adding appropriately. Here, WTR after alkali activation was blended in waste wood flour (WWF) treated with compatibilizer, the mixture after oven-dried was subsequently filled into thermoplastic recycled polyethylene (RPE) or polypropylene (RPP) base (WTR/WPC) through hot-press molding process. Mechanical properties (tensile strength ft, flexural strength fb, impact strength fa) of RPE-based and RPP-based WTR/WPC strips with 4 different WTR fractions were characterized. Its damping properties (loss factor β) were measured and identified by ultrasonic agitation and half-power bandwidth identification method, respectively. The coarse WTR has balanced but stable effect on the improvement of ft, fb, fa, and β of WTR/WPC with rigid RPP base than that with flexible RPE, which attributes to the synthetic effects of good deformation capacity of coarse WTR themselves, strong bridging capacity of WWF, and sufficient stick-slip intercalation between WWF or WTR and rigid RPP. The ft, fb and fa, β with 5% coarse WTR of RPP-based WTR/WPC reaches 12.32 MPa, 15.74 MPa; 2.58 MPa and 0.0853, respectively.

The 27 reinforced concrete beam test specimens of 550 mm × 150 mm × 100 mm with initial cracks were used to investigate the bond properties of steel bars embedded in concrete under the action of three different levels of sustained loads (25%, 45% and 65% of ultimate load) alone, chloride attack (NaCl solution with a concentration of 5%) alone and coupling of sustained loads and chloride attack for 180 d. Based on the test results, the influence of three working conditions on bond behavior between steel bars and concrete has been compared and analyzed. Subjected to the same load level, the bond stress between steel bars and concrete under chloride attack was higher than that without chloride attack, the bond strength of the specimen subjected to the coupled effect of sustained load and chloride ion attack was higher than that under the single factor. As the load level increased, the bond strength of steel bars embedded in concrete gradually decreased whether there was chloride ion attack or not. When the corrosion ratio of the longitudinal bars was small (corrosion ratio ≤ 0.14 without chloride ion corrosion and corrosion ratio ≤ 0.28 with chloride ion corrosion), the bond strength between steel and concrete gradually increased with the increase of the load level; with the further improvement of the corrosion ratio of steel bars, the bond strength showed a decreasing trend. The free end slip of specimens is smaller than the load end. Based on the test results, the reinforced concrete bond-slip curves and its constitutive relationships under different conditions have been proposed.

An aqueous epoxy resin with four different dosages was employed to enhance ordinary Portland cement (OPC) mortar for the first time used as a polymer-cement repairing material. Experimental results showed that the flexural toughness of the epoxy resin modified OPC mortar was obviously improved without losing too much compressive strength. In addition, three different interface bonding strengths used for evaluation of the interface bonding properties were all enhanced with certain dosages of epoxy resin. For instance, the interfacial flexural bonding strength, direct interface shear strength and interface tensile strength of the OPC mortar with epoxy resin dosage at 5% was increased by approximately 16.7%, 29.8% and 6.9% respectively compared to the control mortar without epoxy resin. It is assumed that the epoxy resin formed as polymer films which could improve the microstructure of OPC mortars as evidenced by scanning electron microscopy analysis via integrating different components in binder matrix and crack bridging effect is the main reason for the related improvements. Thus, this newly developed OPC mortar reinforced by suitable contents of aqueous epoxy resin is a promising repairing material for mitigating deteriorations of concrete which awaits further applications.

Water repellent treatment has been considered an effective preventive method against water and aggressive ions penetration into concrete and consequently can improve the durability of concrete structures. In reality, many concrete structures are exposed to conditions with high risk of carbonation. In this contribution, one type of ordinary concrete had been prepared and surface impregnated by 400 g/m2silane cream and 100 g/m2and 400 g/m2silane gel. In addition, integral water repellent concrete was produced by adding 2% silane emulsion. Then, the specimens were exposed to accelerated carbonation for 7, 28, and 72 days. The effect of water repellent treatment on carbonation of concrete has been investigated. The results indicate that surface impregnation reduced carbonation depth of concrete under RH 70%, but integral water repellent concrete increased carbonation. Carbonation reaction started behind the hydrophobic layer in the surface-impregnated concrete. The coefficient of carbonation can be described better by a hyperbolic function of time. Treatment by 400 g/m2silane gel and silane cream showed better efficiency on reducing carbonation than usage of 100 g/m2. Coefficient of water capillary suction was decreased significantly by both surface impregnation and integral water repellent treatment. It is an effective method to protect concrete from water penetration into the material.

In many cases, service life of reinforced concrete structures is severely limited by chloride penetration until the steel reinforcement or by carbonation of the covercrete. Water repellent treatment on the surfaces of cement-based materials has often been considered to protect concrete from these deteriorations. In this paper, three types of water repellent agents have been applied on the surface of concrete specimens. Penetration profiles of silicon resin in treated concrete have been determined by FT-IR spectroscopy. Water capillary suction, chloride penetration, carbonation, and reinforcement corrosion in both surface impregnated and untreated specimens have been measured. Results indicate that surface impregnation reduced the coefficient of capillary suction of concrete substantially. An efficient chloride barrier can be established by deep impregnation. Water repellent surface impregnation by silanes also can make the process of carbonation action slow. In addition, it also has been concluded that surface impregnation can provide effective corrosion protection to reinforcing steel in concrete with migrating chloride. The improvement of durability and extension of service life for reinforced concrete structures, therefore, can be expected through the applications of appropriate water repellent surface impregnation.

The experiment study on the air-entrained concrete of 100 mm cubes under triaxial compression with different intermediate stress ratioα2=σ2D : σ3Dwas carried out using a hydraulic-servo testing system. The influence of rapid freeze-thaw cycles and intermediate stress ratio on the triaxial compressive strengthσ3Dwas analyzed according to the experimental results, respectively. The experimental results of air-entrained concrete obtained from the study in this paper and the triaxial compression experimental results of plain concrete got through the same triaxial-testing-system were compared and analyzed. The conclusion was that the triaxial compressive strength is greater than the biaxial and uniaxial compressive strength after the same rapid freeze-thaw cycles, and the increased percentage of triaxial compressive strength over biaxial compressive strength or uniaxial compressive strength is dependent on the middle stress. The experimental data is useful for precise analysis of concrete member or concrete structure under the action complex stress state.

The experimental study of air-entrained concrete specimens subjected to different cycles of freeze-thaw was completed. The dynamic modulus of elasticity, weight loss, the cubic compressive strength, compressive strength, tensile strength and cleavage strength of air-entrained concrete were measured after 0, 100, 200, 300, 400 cycles of freeze-thaw. The experimental results showed that the dynamic modulus of elasticity and strength decreased as the freeze-thaw was repeated. The influences of freeze-thaw cycles on the mechanical properties, the dynamic modulus of elasticity and weight loss were analyzed according to the experimental results. It can serve as a reference for the maintenance, design and the life prediction of dams, hydraulic structures, offshore structures, concrete roads and bridges in northern cold regions.

The high water-cement ratio concrete specimens under biaxial compression that completed in a triaxial testing machine were experimentally studied. Strength and deformations of plain concrete specimens in two loading direction under biaxial compression with stress ratio of α=0, 0.25, 0.5, 0.75, 1.0 were obtained after 0, 25, 50 cycles of freeze-thaw. Influences of freeze-thaw cycles and stress ratio on the peak stress and deformation of this point were analyzed according to the experimental results. Based on the test data, the failure criterion expressed in terms of principal stress after different cycles of freeze-thaw, and the failure criterion with consideration of the influence of freeze-thaw cycle and stress ratio were proposed respectively.