Effect of uncertainty in calibrated chloride diffusion coefficients on maintenance and life-cycle decisions of reinforced concrete structures

Chloride attack severely impacts the performance of reinforced concrete. The total and free chloride ion concentrations (CICs) of self-compacting concrete (SCC) prepared with three supplementary cementitious materials (SCMs) - fly ash (FA), blast furnace slag (BS), and silica fume (SF) – were measured through the accelerated salt immersion tests. The apparent chloride diffusion coefficients (CDCs) at any exposure time and erosion depth were calculated using the Boltzmann-Matano method. The influence of the type and content of SCMs, the water-binder ratio (W/B), and the type of salt solution on CICs and CDCs were investigated. Both introducing SCMs and reducing W/B effectively reduced the CIC. The SCM that most effectively reduced CIC was SF, followed by BS and then FA. Free CICs were reduced to a greater degree than total CICs in FA and BS concrete, but the opposite was true for SF concrete. Presence of calcium chloride in salt solution increased total CICs while reducing free CICs. Apparent free CDC dropped over exposure time and initially increased with erosion depth but eventually stabilized. A model of apparent free CDC considering the time-depth dependence was created, which shows that time reduction factors of CDC is larger in SCM-containing SCC than in control SCC.

Chloride induced corrosion is an important reason for the deterioration of reinforced concrete structures. The chloride transport properties of steel fibre reinforced concrete (SFRC) coupled with and without bending load (stress level of 0.5) were investigated through bulk diffusion test. A prediction model for chloride ingress in SFRC under bending load was established based on the Fick's second law. The time-dependent chloride diffusion coefficient was discussed and calculated. Apparent chloride diffusion coefficient was used to predict the corrosion initiation of SFRC structures. The experimental results showed that SFRC had better chloride resistance than plain concrete, especially when specimens were under bending load. The chloride diffusion coefficient of SFRC under tension was 30 ~ 38 % lower than that of plain concrete. For unstressed concrete, the calculated corrosion initiation of SFRC was 6 ~ 40 % longer compared with plain concrete. For concrete under bending load, the corrosion initiation of SFRC was 2.2 ~ 3.6 times of that for plain concrete, varying with fibres dosage and cover thickness.

Service life prediction and time-variant reliability of reinforced concrete structures in harsh marine environment considering multiple factors: A case study for Qingdao Bay Bridge

The diffusion mechanism for chloride ions in concrete with varying water/cement ratio (w/c) is studied by chloride ion spray testing. A new model of chloride diffusion coefficient, which varies with the ingress depth of chloride ion and w/c, is developed. A spatial time-dependent reliability model of time to corrosion initiation (TCI) for reinforced concrete (RC) structures is developed, considering spatial variability of geometric, material and environmental parameters. The test results show that the chloride diffusion coefficient increases as w/c increases, and chloride diffusion velocity decreases as penetration depth increases. It is found that the probability of corrosion initiation considering spatial variability is 13.1–18.5% greater than that without consideration of spatial variability. This indicates that neglecting spatial variations in evaluating probability of corrosion initiation time overestimates the structural reliability. It is also found that the probability of corrosion initiation for future climate change is 9–11% higher than that obtained from constant temperature. The mean value of TCI based on a time-dependent chloride diffusion coefficient is 16.7% less than that obtained for a constant chloride diffusion coefficient. This suggests that it is more appropriate if penetration depth and w/c is considered when determining the time-dependent chloride diffusion coefficient.

A time-variant model of chloride diffusion in prestressed concrete cylinder pipe (PCCP) considering the effects of curing age

The use of supplementary cementitious materials (SCMs) such as silica fume (SF), metakaolin (MK), fly ash (FA), and slag (SL) can significantly extend concrete life, especially for those exposed to coastal environments. An essential part of estimating the service life of the reinforced concrete (RC) structures is estimating the time when the corrosion begins. The diffusion coefficient that leads to the concentration of chloride ions in concrete is one of the most important parameters for evaluating the corrosion initiation time. This study determined the chloride diffusion coefficient by preparing samples containing different admixtures based on the Nord-test accelerated migration test. The samples are placed in two environmental conditions, submerged and dry and wet cycles for 180 days. The Charged System Search (CSS) meta-exploration algorithm based on the Hesofer-Lind-Rakowitz-Fissler (HL-RF) method evaluated corrosion and chloride diffusion coefficient initiation time. Test results and numerical models showed that in a sample with admixture, its corrosion resistance was almost two times that of the ordinary concrete and the initial corrosion time. If several types of admixture are used, the corrosion time becomes twice as long as when only one admixture is utilized. The best performance for the durability of concrete belonged to the samples with three admixtures. Using additives, MK, SF, and SL, with concrete results in the probability of corrosion to decrease by about 40% after 25 years, and at 50 years, this value becomes about 50%.

PurposeMaintenance is a critical business function with a great impact on economic, environmental and social aspects. However, maintenance decisions' planning has been driven by merely economic and technical measures with inadequate consideration of environmental and social dimensions. This paper presents a review of the literature pertaining to sustainable maintenance decision-making models supported by a bibliometric analysis that seeks to establish the evolution of this research over time and identify the main research clusters.Design/methodology/approachA systematic literature review, supported with a bibliometric and network analysis, of the extant studies is conducted. The relevant literature is categorized based on which sustainability pillar, or possibly multiple ones, is being considered with further classification outlining the application area, modeling approach and the specific peculiarities characterizing each area.FindingsThe review revealed that maintenance and sustainability modeling is an emerging area of research that has intensified in the last few years. This fertile area can be developed further in several directions. In particular, there is room for devising models that are implementable, based on reliable and timely data with proven tangible practical results. While the environmental aspect has been considered, there is a clear scarcity of works addressing the social dimension. One of the identified barriers to developing applicable models is the lack of the required, accurate and timely data.Originality/valueThis work contributes to the maintenance and sustainability modeling research area, provides insights not previously addressed and highlights several avenues for future research. To the best of the authors' knowledge, this is the first review that looks at the integration of sustainability issues in maintenance modeling and optimization.

Probability-based maintenance modeling and planning for reinforced concrete assets subjected to chloride ingress

Chloride penetration is among the main causes of corrosion initiation in reinforced concrete (RC) structures producing premature degradations. Weather and exposure conditions directly affect chloride ingress mechanisms and therefore the operational service life and safety of RC structures. Consequently, comprehensive chloride ingress models are useful tools to estimate corrosion initiation risks and minimize maintenance costs for RC structures placed under chloride-contaminated environments. This paper first presents a coupled thermo-hydro-chemical model for predicting chloride penetration into concrete that accounts for realistic weather conditions. This complete numerical model takes into account multiple factors affecting chloride ingress such as diffusion, convection, chloride binding, ionic interaction, and concrete aging. Since the complete model could be computationally expensive for long-term assessment, this study also proposes model simplifications in order to reduce the computational cost. Long-term chloride assessments of complete and reduced models are compared for three locations in France (Brest, Strasbourg and Nice) characterized by different weather and exposure conditions (tidal zone, de-icing salts and salt spray). The comparative study indicates that the reduced model is computationally efficient and accurate for long-term chloride ingress modeling in comparison to the complete one. Given that long-term assessment requires larger climate databases, this research also studies how climate models may affect chloride ingress assessment. The results indicate that the selection of climate models as well as the considered training periods introduce significant errors for mid- and long- term chloride ingress assessment.

Instantaneous chloride diffusion coefficient and its time dependency of concrete exposed to a marine tidal environment

In order to examine the effect of load-induced transverse cracks on the chloride penetration in flexural concrete beams, two different concretes, Portland cement concrete (PCC) and fly ash concrete (FAC), were tested with various crack widths. Total 14 reinforced concrete (RC) beams, ten of which were self-anchored in a three-point bending mode, were immersed into a 5% NaCl solution with the condition of dry-wet cycles. Then, the free chloride ion contents were determined by rapid chloride testing (RCT) method. Based on the proposed analytical models of chloride penetration in sound and cracked concrete subjected to dry-wet cycles, the apparent chloride diffusion coefficient and chloride diffusivity of concrete were discussed. It can be found that the performance of chloride diffusivity in both concretes will be improved with the increase of crack width, and that the influence of convection action will also be augmented. Based on the two samples obtained in sound concrete after 15 and 30 cycles, the time-exponent, m, for chloride diffusion coefficient was determined to be 0.58, 0.42, 0.62 and 0.77 for PCC1, PCC2, FAC1 and FAC2 specimens, respectively. Finally, two influencing factors of fly ash content and crack width on chloride diffusivity were obtained by regression analysis of test data, and it can be seen that factors kf and kw can be expressed with quadratic polynomial functions of fly ash content, f, and crack width, w, respectively.

The relatively long life cycle of railway infrastructure means that maintenance and renewal decisions significantly influence the total life cycle cost (LCC) associated with the infrastructure. A decision support tool such as life cycle costing assists infrastructure managers in making maintenance and renewal decisions. A shift from qualitative to quantitative decision making is possible using decision support tools and modelling approaches based on appropriate data. Most LCC maintenance models in the literature are deterministic in nature. However, there is inherent uncertainty present within the reliability and maintainability (R&M) parameters. The uncertainty within the R&M parameters can be characterised through appropriate statistical distributions or using bootstrapping in conjunction with available data. A maintenance modelling approach based on stochastic methods and Monte Carlo simulation is presented in this paper with specific attention to a model developed for the rail component. The proposed model allows quantification of inherent uncertainty within the calculated LCC which is coupled to the uncertainty within the input R&M parameters. This modelling approach is flexible in nature and supports the use of large input data sets, capturing variability within the real-world situation of maintenance management. The flexibility of the modelling approach is demonstrated using an example which incorporates risk to assist an infrastructure manager in deciding whether to use flash butt or alumino-thermic welding during rail maintenance.

Chloride induced corrosion of reinforcing steel is recognized as the most common deterioration mechanism affecting reinforced concrete structures. As such, it has been in focus of research for more than thirty years. Numerous studies of chloride ingress, corrosion initiation, and corrosion propagation have been conducted. Most studies of chloride ingress focused on sound (uncracked) concrete. In reality, however, concrete is almost never crack free. Cracks form either in the construction phase (early age cracks, for example shrinkage cracks), or during the use of a structure (e.g. cracks caused by mechanical loads). While these cracks are usually not detrimental to the load bearing capacity of a structure, they are potentially a threat to its durability. Cracks occurring in the concrete cover diminish its protective capabilities, and present fast routes for ingress of deleterious species (e.g. chloride ions). While national and international design codes provide guidelines and limits for maximum crack widths in aggressive environmental conditions, these are often empirical and based on rules of thumb. As a result, only the surface crack width is considered. However, recent findings seem to indicate that an even more important factor could be the zone of debonding which occurs at the steel/concrete interface due to cracking of the cover. In this thesis, an attempt is made to increase the body of knowledge related to chloride ingress in cracked concrete. Laboratory experiments and numerical simulations were used during the study. Experimental data available in the literature is, at the moment, inconclusive. For years researchers have been trying to find a so-called ''threshold'' crack width for chloride (or water) transport, below which concrete can be treated as sound (uncracked). In this quest, mostly plain concrete specimens were used. While this approach resulted in increased understanding of chloride transport in cracks, it failed to address an important mechanism which affects only reinforced concrete - debonding occurring at the steel/concrete interface. Only recently have researchers focused their attention on the effect of damage at the steel/concrete interface on transport behavior and corrosion of reinforcement in concrete. In this thesis, compact reinforced specimen geometry is adopted, which mimics (with respect to crack geometry) the behavior of reinforced concrete beams. Cracked specimens were subjected to weekly cycles of salt water wetting and drying for a prolonged period of time. After the exposure, two-dimensional chloride maps were obtained by means of LIBS (Laser Induced Breakdown Spectroscopy), in collaboration with BAM Federal Institute for Materials Research and Testing in Berlin, Germany. The results showed that, once damage occurs at the steel/concrete interface, chloride ions penetrate parallel to the reinforcement, which could possibly be very harmful with respect to reinforcement corrosion. It has been frequently reported in the literature that autogeneous healing of cracks can reduce chloride ingress in concrete. To examine this, experiments were performed to investigate the influence of the curing (healing) regimen on chloride penetration depth in cracked specimens. Two regimens (submerged in water and in 95% relative humidity) both enabled the tested specimens to reduce chloride ingress in cracks, compared to the control series. It was found that, under favorable conditions, autogeneous crack healing does have a positive effect on chloride penetration resistance. Corrosion induced cover cracking has been extensively studied in recent years. When reinforcing steel corrodes, it causes expansive pressure on the surrounding concrete. As a consequence, concrete cover cracks. In the thesis, cracking induced by accelerated corrosion was studied using X-ray computed tomography. Mechanical properties of the rust layer were determined using the nanoindentation technique. Valuable insights were obtained, especially for fine tuning of numerical models. Numerical models can be of use in understanding complex problems. In this thesis, a model for chloride ingress in cracked concrete, based on the lattice modeling approach, was developed. Concrete is discretized as a set of one dimensional pipe elements through which the transport takes place. The model is coupled to the lattice fracture model, which enabled simulating chloride penetration around cracks in concrete. The model was validated using experimental results from the literature. Numerous experiments make use of external electrical field to accelerate chloride ingress. Therefore, the developed model was extended to enable modeling chloride ingress in accelerated experiments. Based on the lattice modeling approach and utilizing the Characteristic Galerkin scheme, the approach enables using smaller elements compared to the conventional Galerkin approach. Apart from its computational efficiency, it showed that the, under accelerated conditions, chloride front ahead of a crack may be much more sharp compared to the natural (i.e. diffusion) conditions. Therefore, it is questionable if findings from accelerated experiments on cracked specimens can be directly applied to real exposure conditions. Cracking of the concrete cover caused by reinforcement corrosion is also frequently modeled. In this thesis, the two-dimensional Delft lattice model was used to model it. First, the developed model was validated using a set of well-documented experiments from the literature. It was found that penetration of corrosion products into pores and open cracks needs to be considered in order to obtain good match with the experiments. Furthermore, it was observed that the internal pressure which causes cracking is not a deterministic value, but that it depends on local mechanical properties inside the concrete. Also, two hypothetical pitting scenarios were tested and compared to then uniform corrosion case. It appears (according to the 2D model) that pitting is more detrimental for the concrete cover than uniform corrosion. This means that cover cracking will occur at lower internal pressure compared to the uniform corrosion case. Techniques employed in this thesis (such as LIBS, X-ray computed tomography, and nanoindentation) can be used in the same or similar way for studying other deterioration mechanisms. Also, the chloride transport model can be easily modified to model other transport processes, such as moisture transport, sulfate transport, or carbon dioxide ingress. Employing a staggered scheme used in this thesis, these transport processes can be coupled with the mechanical analysis to model the influence of combined actions on reinforced concrete structures.

Predicting the chloride diffusion coefficient and surface electrical resistivity of concrete using statistical regression-based models and its application in chloride-induced corrosion service life prediction of RC structures

This study presents an analysis of the chloride diffusion coefficient (D RCM ), obtained in electrically accelerated chloride migration tests. As demonstrated here, the obtained chloride diffusion coefficient does not represent the apparent one, as it is independent of chloride binding. This is accounted for the fact that the D RCM is calculated based on the average chloride penetration depth measured in concrete and binding at the position of the chloride penetration front is very limited. It is also demonstrated in this study that the apparent chloride diffusion coefficient in concrete is not constant within the sample, because it depends upon the local chloride concentrations.