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This paper presents analytical models for evaluating effective hydraulic conductivity, diffusivity and sorptivity of concrete considering the properties of the Interfacial Transition Zone (ITZ) and the aggregate size distribution. Results of the proposed models compare well with the experimental results and those obtained from rigorous numerical Finite Element (FE) analysis. It is also found that a change of Interfacial Transition Zone (ITZ) properties has more influence on the effective hydraulic conductivity than it has on the effective hydraulic sorptivity. The significance of the proposed models is that they can estimate the hydraulic sorptivity and diffusivity of concrete by only knowing the hydraulic properties of mortar, effectively extricating the need for sophisticated and time-consuming FE analysis required for estimation of the effective hydraulic conductivity of concrete at the meso-scale. Furthermore, these models offer scientific insight into the effect of different components of concrete on its effective hydraulic properties.

The use of recycled concrete is a significant step to reduce the need for natural resources and the demand on landfill sites for disposing the waste. Previous studies have mainly focused on its behaviours under ambient temperature although the characteristics of recycled concrete after high temperature exposures is important in many practical applications such as fire resistance. Also, the effect of elevated temperature on the mechanical and porosity behaviours of recycled concrete aggregate (RCA) containing rubber for pavement base/subbase applications has never been studied. In this study, the effects of high-temperature environment on the mechanical properties of recycled concrete aggregate (RCA) mixed with crumb rubber (CR) in a range of 0.5–2% were investigated both experimentally and theoretically. Unconfined compression strength (UCS) and X-ray micro-tomograph tests were undertaken on the mixed samples which were heated to different temperatures ranging from 20 °C to 600 °C. It was observed from the experimental results that UCS increased under a higher temperature up to 300 °C. At an ambient temperature of 20 °C, RCA samples without any addition of CR exhibited the highest UCS value while under high temperature, RCA samples with 1% CR had the highest UCS value. The strength of RCA mixed with crumb rubber showed an increasing trend under a temperature higher than 150 °C, which can be attributed to the melting processes of the crumb rubber. According to the X-ray micro-tomograph tests, the inclusion of rubber led to a decreasing porosity. It is interesting to note that porosity rises when temperature increased from 20 °C to 150 °C. This may be due to the restructuring of the shape of rubber particles into spheres as well as the excessive pore water forces generated from water evaporation. Also, when the temperature increased from 150 °C to 450 °C, the porosity decreased since the melted rubber particles flowed into the voids and filled more pores. The outcomes of this study can provide practical guidance on the application of recycling concrete.

This paper presents the discovery of the most severe form of corrosion induced degradation of steel – delamination. The causes and effects of preferred corrosion of steel are quantitatively determined through a comprehensive experimental program and detailed analysis of steel microstructure. A large number of simulated corrosion tests are conducted by immersing steel in acidic solutions to acquire new understanding of the corrosion process of continuously cast steel. Three microstructural characteristics that affect the corrosion resistance of steel are quantified: grain size, phase composition and distribution of impurities. The results and their analyses presented in the paper indicate that preferred corrosion occurs at locations of steel where the grain size is larger; there is less cementite and more impurities. The analysis of causes and effects of the preferred corrosion provides insight into the prevention of preferred corrosion from the steel making process through to the delamination of steel in structures. This information could be used to predict, mitigate or avoid catastrophic failures of structures made of steel, potentially saving lives and resources, whilst advancing the knowledge in corrosion science of steel and improving the manufacturing process of steel.

Research on quantification of the size and porosity of the interface between the reinforcing steel and concrete is limited in the current published literature. This paper presents a reliable and accurate method for quantification of steel-concrete interface using the technique of Backscattered Electron (BSE) imaging. Details of sample preparation procedure designed to minimize the damage to the steel-concrete interface due to grinding and polishing are presented. A new thresholding method to separate porous band at the interface from steel and cement pastes is proposed. It is found that there is a relatively wide porous band with large pores and voids near the steel-concrete interface. It is also found that the size of porous band around the steel bar is not uniform, and that the water to cement ratio can significantly affect the size and distribution of porous band at the interface. Quantification of steel-concrete interface as proposed in this paper is of importance in crack prediction of corrosion-affected reinforced concrete structures.

Concrete permeability is usually determined by testing unstressed, non-damaged specimens in laboratories as a constant. In reality, however, concrete is subjected to various operational loads in the time span of its service life. Therefore, the permeability of stressed concrete can be time-variant and completely different from that of unstressed conditions. This paper intends to develop a new methodology to determine the permeability of stressed concrete over time, based on the damage mechanics of concrete meso-structure in conjunction with Monte Carlo simulation and finite element analysis. It is found in this study that the permeability of concrete starts to increase when the applied stress in concrete reaches a certain threshold value for the given aggregate fraction. It is also found that the aggregate fraction is the most influential factor for permeability degradation of concrete. The significance of the developed methodology is that it can determine the permeability degradation of concrete with different mechanical properties and geometries of constituent materials under various applied loads over time, which would be otherwise impractical experimentally.

This paper investigates the effect of corrosion on degradation of mechanical properties of G250 mild steel. Corrosion tests are conducted by immersing steel into hydrochloride acid solutions, with acidity ranging from 0.00001 M to 3 M. Both macro analysis and micro analysis are undertaken to investigate the mechanism of corrosion induced degradation of mechanical properties. Intergranular corrosion is found for mild steel during corrosion. Results indicate that there is considerable reduction in mechanical properties due to corrosion, which is attributed to a combined effect of hydrogen embrittlement, changes in element composition, grain size and phase composition, and the residual stress concentration at corrosion pits.

Corrosion of steel in reinforced concrete structures has been a major worldwide problem. The time to cover cracking plays a key role in assessment of serviceability of reinforced concrete structures subjected to corrosion. A large number of analytical, numerical, and empirical models have been developed to predict the time to time to cover cracking. In addition, extensive experiments have been conducted in order to verify the developed models. In this paper, an overview of the existing models is presented. A large experimental database of reported data on time to cover cracking is collated. Performance of four analytical models taken from the available literature is then examined using the established experimental database. Sensitivity analysis followed by a probabilistic study is carried out to identify the factors affecting time to cover cracking as a service life indicator by means of the selected models. The results from sensitivity analysis show that the porous zone and the properties of rust are the most influential factors for time to cover cracking. It is also shown that there is high uncertainty in predicting service life of reinforced concrete structures based on time to cover cracking.

This study presents a new dual time-variant chloride diffusion model considering maximum phenomenon for concrete structures exposed to marine environment. The model is developed from the assumption that the concentration gradient is the sole or dominant driving force for chloride ingress in concrete, and both the chloride diffusion coefficient and peak chloride content are considered as time-variant. The applicability and accuracy of the developed model are verified by comparing the fitted and predicted chloride profiles with field data. Based on the developed model, time-variant probabilistic evaluation of corrosion initiation in a typical prestressed box girder located near coast is carried out. Results reveal that the maximum phenomenon has a significant effect on the prediction of the time to corrosion initiation. The results of sensitivity analysis indicate that the critical chloride content is the decisive parameter in the prediction of the corrosion initiation time, followed by the relative humidity, water-to-binder ratio, temperature, thickness of concrete cover, etc. Besides, the effects of the correlation between random variables and the distance from the coast on the time-variant probability of corrosion initiation are investigated. It is found that the correlation among random variables should be considered, and the effect of distance from the coast on the time-variant probability of corrosion initiation is sharply decreasing at a very short distance from the coast.

Although extensive research on steel corrosion in concrete has been undertaken it is more on longitudinal reinforcing bars than stirrups as suggested in the review of published literature. The effect of corrosion of steel stirrups on structural behaviour can be significant for shear failure mode. This paper aims to investigate the degradation of mechanical behaviour of corroded stirrups. Details of a comprehensive experiment designed to examine the mechanical behaviour of corroded stirrups in concrete structural members are presented. A micromechanical damage model for failure mechanism of corroded stirrups is proposed and a model for damage factor is derived. Based on these models, a constitutive model for corroded stirrups is developed and verified with test results. It is found in the experiment that both the strength and ductility of corroded stirrups decrease with the increase of corrosion and that the hemispherical model for the pit shape is more appropriate for the prediction of strength reduction of corroded stirrups. The constitutive model developed in the paper can be used to predict the mechanical behaviour of corroded stirrups with reasonable accuracy, paving the way for the assessment of corrosion induced shear failure of reinforced concrete structures.

The large uncertainty of the value of the deformation modulus of rust in the literature and the granular property of rust make it difficult to correctly simulate the interaction between the corroded reinforcement and concrete, including rust expansion and bond slip behavior. To fill the gap, this study thereby quantifies the deformation modulus of rust in concrete under different levels of confinement, and takes into consideration the effect of the corrosion rate through an in-situ testing method. To obtain a more accurate deformation modulus of rust, the chemical composition and quantity of rust are examined by using X-ray diffraction. Moreover, the effect of diffusion of rust into concrete porous zone and crack was incorporated into the compressive deformation of rust. The results show new empirical relationships between the deformation modulus of rust and the confinement pressure together with the corrosion rate are obtained. These relationships are further validated by the test results in the literature. The present work can contribute to narrow the uncertainties in the deformation modulus of rust for simulating and predicting the deterioration of concrete structures due to reinforcement corrosion.

The expansive nature of corrosion products at the steel-concrete interface directly leads to the cracking and spalling of steel reinforced concrete, and its premature failure. In this paper, fundamental causes that affect the distribution and growth of corrosion products at the steel-concrete interface are quantitatively examined. The thickness of the layer of corrosion products and its growth with time are measured accurately using the technique of backscattered electron imaging and with a statistically significant number of samples, which are then verified with Faraday’s law. A model for the distribution and growth of the non-uniform corrosion layer is developed. It is found in the paper that the corrosion products are non-uniform around the steel-concrete interface, with the maximum at the bottom side of steel bar with respect to casting direction. It is also found that the amount of corrosion products increases with the increase of water to cement ratio of concrete. The significance of this research is that it has quantitatively examined the effect of microstructure of the steel-concrete interface, a fundamental factor, on the distribution and growth of corrosion products. Findings presented in the paper can contribute to the accurate prediction of corrosion-induced cracking of steel reinforced concrete and hence prevent its premature failure.

A review of research literature suggests that steel corrosion in concrete can accelerate if the corroding steel is under high levels of stress but this important phenomenon has not been investigated thoroughly. Both laboratory tests and field surveys indicate that the effects of corrosion on structural behaviour in reinforced and prestressed concrete structures are different, with the latter having less concrete cracking but more serious structural collapses. The intention of this paper is to investigate the corrosion propagation of prestressing steel strands in chloride-laden environment. A long term experimental program is presented in which the corrosion of twelve prestressing steel strands embedded in concrete was examined. The effects of the level of stresses in the steel and type of steels on corrosion rate are studied. It is found that the corrosion rate of prestressing steel strands in concrete increases with the increase of the level of stresses applied and that the pitting corrosion is the main form of prestressing steel corrosion in chloride-laden environment. The paper concludes that the corrosion of prestressing steel in concrete structures poses higher risk to the structure than that of reinforcing steel in terms of structural collapse.