Embedded Steel Reinforcement Research Articles

Structural Assessment & Repair Techniques

Structures are meticulously designed to withstand predetermined loads throughout their lifespan. However, reinforced concrete (RC) structures, which have been widely employed worldwide for the past 50-60 years, are susceptible to a range of issues leading to varying degrees of damage. Factors such as material deterioration, construction techniques, workmanship quality, overloading, aggressive environments, and the fatigue and corrosion of embedded steel reinforcement contribute to the natural degradation of RCC. This deterioration is now evident in numerous concrete structures. In the current landscape of building research, the significance of repair and rehabilitation cannot be overstated. This is particularly crucial in developed countries where extensive rehabilitation efforts, especially for heritage buildings, are imperative due to their cultural and historical significance. Dealing with these challenges requires a systematic approach, encompassing thorough assessments, innovative repair strategies, and a commitment to sustainable solutions to ensure the longevity and resilience of our built environment.

Passivation of Steel Reinforcement in Low Carbon Concrete

Both the high CO2 emissions associated with cement production and the increasing demand for concrete call for the use of binder types that can be produced in a more climate-friendly way than that of ordinary Portland cement. To ensure that these binders can also be used in reinforced concrete structures, their influence on the corrosion behavior of embedded steel reinforcement must be investigated. In the study presented here, the passivation behavior of steel in mortars made from various new types of binders is investigated. In addition to alkali-activated materials with high and low calcium contents, a calcium sulfoaluminate cement and a binder produced from calcium silicate hydrate (C-S-H) phases, synthesized in an autoclave, were investigated. While the steel clearly passivated in the alkali-activated slag and the C-S-H binder, the calcium sulfoaluminate cement showed the lowest open circuit potentials and polarization resistances, indicating a less effective level of passivation. The metakaolin geopolymer with a potassium-based activator showed an onset of passivation that was dependent on the environment of the specimens at an early age, whereas the alkali-activated fly ash with a sodium-based activator showed a delay in passivation that was not influenced by the environment of the specimens at an early age.

Effects of SCMs on chloride binding capacity of ultra-low water-to-binder ratio cement paste with internally introduced chloride

Chloride binding is a critical factor affecting the chloride-induced corrosion of concrete with embedded steel reinforcement. The supplementary cementitious materials (SCMs) play an important role in chloride binding including chemical binding and physical adsorption. However, limited knowledge is available on effects of SCMs and their chemical components on chloride binding of cement systems with ultra-low water-to-binder ratio (w/b). This study investigated the chloride binding mechanisms of cement paste with ultra-low w/b ratio incorporating three types of SCMs with different chemical compositions. The chloride was introduced internally through the mixing water. The free chloride content was measured using pore solution expression and titration. The characterization techniques, including XRD and TGA, Zeta potential, SEM/EDS, and FTIR were conducted to evaluate the physically and chemically bound chlorides. The results showed that the content of total bound chloride was a function of the content of chemical bound chloride and physical adsorbed chloride that were governed by CaO and Al2O3 in SCMs. The CaO had an instrumental effect on the physical chloride binding capacity, while Al2O3 was the main factor in the chemical chloride binding.

Data Analysis and Model Development for Chloride Attack Strength Loss Index in Chikoko Blended Concrete

This study investigates the resistance of Chikoko blended concrete to chloride attack, which is critical for ensuring the durability and longevity of concrete structures. Chloride attack is a major concern for concrete structures, as it can lead to corrosion of embedded steel reinforcement, undermining structural integrity. The study employs the chloride attack strength loss index (CASLI) to gauge performance, examining the influence of various factors, including Chikoko concentration, calcination temperature, and curing age. A comprehensive analysis demonstrates a quadratic relationship between these factors and compressive strength. The optimal outcome occurs at a 10% Chikoko concentration and calcination temperatures ranging from 400°C to 800°C. These conditions yield a concrete with high resistance to chloride attack and robust compressive strength. A quadratic model was developed using design Expert 13 in other to predict CASLI derived from the data, providing a reliable tool for forecasting the durability of Chikoko blended concrete under chloride exposure. The study also identifies key independent variables for further optimization, enabling concrete with both high chloride resistance and enhanced compressive strength. This approach shows considerable improvements over traditional concrete made with natural sand, suggesting Chikoko blended concrete as a viable alternative for high-performance applications. Overall, the results indicate that Chikoko blended concrete has significant potential for use in construction, combining durability with high strength. This makes it a promising material for a variety of construction projects, particularly where chloride exposure is a concern, such as in coastal environments or de-icing salt-exposed structures.

Coarse-Grained Monte Carlo Simulations with Octree Cells for Geopolymer Nucleation at Different pH Values.

Geopolymers offer a potential alternative to ordinary Portland cement owing to their performance in mechanical and thermal properties, as well as environmental benefits stemming from a reduced carbon footprint. This paper endeavors to build upon prior atomistic computational work delving deeper into the intricate relationship between pH levels and the resulting material's properties, including pore size distribution, geopolymer nucleate cluster dimensions, total system energy, and monomer poly-condensation behavior. Coarse-grained Monte Carlo (CGMC) simulation inputs include tetrahedral geometry and binding energy parameters derived from DFT simulations for aluminate and silicate monomers. Elevated pH values may can alter reactivity and phase stability, or, in the structural concrete application, may passivate the embedded steel reinforcement. Thus, we examine the effects of pH values set at 11, 12, and 13 (based on silicate speciation chemistry), investigating their respective contributions to the nucleation of geopolymers. To simulate a larger system to obtain representative results, we propose the numerical implementation of an Octree cell. Finally, we further digitize the resulting expanded structure to ascertain pore size distribution, facilitating a comparative analysis. The novelty of this study is underscored by its expansion in both system size, more accurate monomer representation, and pH range when compared to previous CGMC simulation approaches. The results unveil a discernible correlation between the number of clusters and pores under specific pH levels. This links geopolymerization mechanisms under varying pH conditions to the resulting chemical properties and final structural state.

Bacterial Viability in Self-Healing Concrete: A Case Study of Non-Ureolytic Bacillus Species.

Cracking is an inevitable feature of concrete, typically leading to corrosion of the embedded steel reinforcement and massive deterioration because of the freezing-thawing cycles. Different means have been proposed to increase the serviceability performance of cracked concrete structures. This case study deals with bacteria encapsulated in cementitious materials to "heal" cracks. Such a biological self-healing system requires preserving the bacteria's viability in the cement matrix. Many embedded bacterial spores are damaged during concrete curing, drastically reducing efficiency. This study investigates the viability of commonly used non-ureolytic bacterial spores when immobilized in calcium alginate microcapsules within self-healing cementitious composites. Three Bacillus species were used in this study, i.e., B. pseudofirmus, B. cohnii, and B. halodurans. B. pseudofirmus demonstrated the best mineralization activity; a sufficient number of bacterial spores remained viable after the encapsulation. B. pseudofirmus and B. halodurans spores retained the highest viability after incorporating the microcapsules into the cement paste, while B. halodurans spores retained the highest viability in the mortar. Cracks with a width of about 0.13 mm were filled with bacterial calcium carbonate within 14 to 28 days, depending on the type of bacteria. Larger cracks were not healed entirely. B. pseudofirmus had the highest efficiency, with a healing coefficient of 0.497 after 56 days. This study also revealed the essential role of the cement hydration temperature on bacterial viability. Thus, further studies should optimize the content of bacteria and nutrients in the microcapsule structure.

Parametric Iterative Method for Addressing an Embedded-Steel Constitutive Model with Multiple Roots

In this paper, an iterative procedure to find the solution of a nonlinear constitutive model for embedded steel reinforcement is introduced. The model presents different multiplicities, where parameters are randomly selected within a solvability region. To achieve this, a class of multipoint fixed-point iterative schemes for single roots is modified to find multiple roots, achieving the fourth order of convergence. Complex discrete dynamics techniques are employed to select the members with the most stable performance. The mechanical problem referred to earlier, as well as some academic problems involving multiple roots, are solved numerically to verify the theoretical analysis, robustness, and applicability of the proposed scheme.

Feasibility of incorporating SO42--ions in zeolite-like matrices based on alkaline aluminosilicate binders

Factors determining minimization of the influence of sulfate environments on concrete and risk of corrosion of embedded steel reinforcement have been analyzed. Coatings based on alkaline aluminosilicate binders were proposed to prevent transport of the sulfate ions into concrete. The formation of water resistant zeolite-like matrices on the alkaline aluminosilicate binder of the Na2O-K2O-Al2O3-SiO2-H2O system at 20 ± 2 °C can be provided by the use of calcium-containing modifying additives. Optimized molar ratios between oxides of binder constituent were SiO2/Al2O3 = 4–5 and K2O/(Na2O + K2O) = 0.15–0.30. The results of the study show that the speed of incorporating sulfate ions in cement matrix of binder from the sulfate environment is dependent upon a cation and ranks as follows: (NH4)2SO4 > MgSO4 > Na2SO4 > CaSO4. Complete protection of concrete against the penetration of sulfates can be reached when the developed coating is applied in a thickness of 3 mm.

Flexural strengthening of reinforced concrete beams using hybrid fibre reinforced engineered cementitious composite

In this study, flexural strengthening of reinforced concrete (RC) beams using steel and polyvinyl-alcohol hybrid fibre reinforced engineered cementitious composite (SPH-ECC) with embedded steel reinforcement bars is proposed. The effectiveness of the strengthening was investigated by experimental and numerical studies. The flexural behaviours of one unstrengthened 3500 mm long, 200 mm wide, and 325 mm deep RC beam and three RC beams strengthened with different configurations of 50 mm thick SPH-ECC layer(s) were studied by conducting four-point bending tests. Detailed flexural behaviours in terms of peak load, failure mode, load-deflection curve, cracking patterns, interfacial bond-slip, strain distribution and ductility of the tested beams were studied and compared. Experimental results showed that both the flexural strength of strengthened beams, which were in the range of 125% to 210% of the unstrengthened control beam, and the interfacial bond-slip behaviours between concrete and SPH-ECC was highly depended to the strengthening configuration used. Crack width control ability of the beams was also improved by using SPH-ECC. A finite element (FE) procedure using surface-to-surface cohesive model was also developed to model the flexural behaviours of the strengthened beams. Comparison with experimental results demonstrated that the proposed FE model could accurately predict the flexural behaviours including interfacial bond-slip between the SPH-ECC layers and the RC beam part of the strengthened beams.

Computational Intelligence-Based Structural Health Monitoring of Corroded and Eccentrically Loaded Reinforced Concrete Columns

Corrosion of embedded steel reinforcement is the prime influencing factor that deteriorates the structural performance and reduces the serviceability of reinforced concrete (RC) structures, especially during earthquakes. In structural elements, RC columns play a vital role in transferring the superstructure’s load to the substructure. The deterioration of RC columns can affect the structures’ overall performance. Hence, it becomes essential to estimate the remaining life of deteriorated RC columns. In the literature, only limited analytical models are available to calculate the remaining life of corroded and eccentrically loaded RC columns. As the number of dependent parameters increases, assessing the residual life of the structural elements and providing a practically applicable suitable model become very complex. Machine learning (ML)-based prediction models are beneficial in dealing with such complex databases. In this article, an ML-based artificial neural network (ANN), Gaussian process regression (GPR), and support vector machine (SVM) algorithms have been applied to estimate the residual strength of corroded and eccentrically loaded RC columns. The performance of the analytical and ML models is accessed using commonly used performance indices, namely, the coefficient of determination (R2), root mean square error (RMSE), mean absolute error (MAE), mean absolute percentage error (MAPE), a-20 index, and Nash–Sutcliffe (NS). The results of the proposed ANN model have been compared with the existing analytical model to identify the suitability of the best model. Based on performance analysis, the precision of the GPR and SVM models is lower than that of the ANN model. The processed results revealed that the R2 value of the ANN model for training, testing, and validation datasets is 0.9908, 0.9757, and 0.9855, respectively. The MAPE, MAE, RMSE, NS, and a-20 index for all the datasets are 8.31%, 48.35 kN, 72.53 kN, 0.9886, and 0.8978, respectively. The precision of the ANN model in terms of the coefficient of determination is 225.77% higher than that of the analytical model. The sensitivity analysis demonstrates that the compressive strength of concrete plays the most significant role in the load-carrying capacity of corroded and eccentrically loaded RC columns. The proposed ANN model is reliable, accurate, fast, and cost effective. This model can also be used as a structural health-monitoring tool to detect the early damages in the RC columns.

Corrosion Behavior in RC Member with Different Cover Depths under Cyclic Chloride Ingress Conditions for 2 Years

Concrete structures are considered as durable construction material, but corrosion of the embedded steel reinforcement occurs under chloride exposure as concrete has porous properties. Herein, a cyclic drying–wetting test was performed for two years using saltwater to accelerate steel corrosion in a reinforced concrete (RC) member. The open-circuit potential (OCP) was measured using a newly developed and replaceable agar sensor. The corrosion potential was measured considering the chloride concentration, water-to-cement (w/c) ratio, and cover depth at three levels. Furthermore, its relationships with influential parameters were evaluated using averaged OCP results. The measured OCP showed a linear relationship with the cover depth, and this tendency was more distinct with increasing retention period and higher chloride concentration. For the highest w/c ratio (0.6), values below −100 mV were monitored after only six months regardless of the cover depth, and values below the critical potential level (−450 mV) were evaluated at lower cover depths (30 and 45 mm). The results of regression analysis considering the exposure environment showed a clear relationship in the case of high chloride concentration (7.0%). A linear relationship between cover depth and OCP was derived with a reasonable determination coefficient ranging from 0.614 to 0.771.

Experimental investigation on the flexural behaviour of stainless steel reinforced concrete beams

The durability of reinforced concrete (RC) structures and infrastructure has been the subject of significant attention from the engineering research community in recent years, mainly owing to the deterioration of RC elements due to corrosion of the embedded steel reinforcement. In this context, stainless steel reinforcement can provide an efficient solution to enhance the expected lifetime of concrete structures, reducing the damage due to corrosion of the reinforcement and carbonation and deterioration of the concrete. However, current international design standards for reinforced concrete structures do not include appropriate guidance for stainless steel reinforced concrete (SSRC). In order to investigate the behaviour of stainless steel RC beams, a series of six beam tests was conducted and is discussed herein. The key performance measures for RC beams such as load-deflection response, cracking behaviour and deflections at service load are assessed. The validity and applicability of existing design rules, which were developed for carbon steel RC, are also examined for stainless steel reinforced concrete members. Other recently developed design procedures, based on the Continuous Strength Method and including an accurate material model for the stainless steel bars, are also examined.

Steel Corrosion Behavior of Reinforced Calcium Aluminate Cement-Mineral Additions Modified Mortar.

Mineral additions can eliminate the conversion in calcium aluminate hydrates and thus inhibit the future strength retraction of calcium aluminate cement (CAC). However, the impacts of these additions on the protection capacity of CAC concrete in relation to the corrosion of embedded steel reinforcement remains unclear. This paper focused on the corrosion behavior of steel reinforcement in slag, limestone powder, or calcium nitrate-modified CAC mortars via XRD and electrochemical methods (corrosion potential, electrochemical impedance, and linear polarization evaluation). The results indicate that strätlingite (C2ASH8), which is formed in slag-modified CAC, has poor chloride-binding ability, leading to decline in corrosion resistance of the steel reinforcement. The electrochemical parameters of specimens immersed in NaCl solution suddenly drop at 14 days, which is 28 days earlier than that of the references. In contrast, the Ca2[Al(OH)6]20.5CO3OH·H2O (CaAl·CO32−-LDH) and 3CaO·Al2O3·Ca(NO3)2·12H2O (NO3-AFm) in limestone powder and calcium nitrate-modified CAC mortar show great chloride-binding ability, thereby improving the corrosion resistance of the steel reinforcement. The electrochemical parameters of specimens modified with calcium nitrate maintain a slow decreasing trend within 90 days.

Using artificial neural network and non‐destructive test for crack detection in concrete surrounding the embedded steel reinforcement

AbstractBond between steel and concrete is one of the key aspects of structural design and its performance evaluation. In the past much research work has been focused on understanding bond deterioration owing to corrosion of reinforcement, however, there exists no nondestructive method to access the bond condition. In this regard, the presented experimental research work details the development of a nondestructive testing method to estimate the crack condition of concrete surrounding the steel reinforced by using ultrasonic pulse velocity test. In addition, a multilayer feedforward back propagation perceptron artificial neural network (ANN) is developed in order to avoid simplification assumptions for developing models to predict the cracking, owing to the nonlinear complex stress distribution at the steel‐concrete interface. The ANN is used to predict the crack width and to conduct sensitivity analysis of the various factors influencing the bond deterioration. A high accuracy level is achieved between the predicted and the experimental values with R2 of 0.97 and the most influential parameter is highlighted.

Chloride Ingress Control and Promotion of Internal Curing in Concrete Using Superabsorbent Polymer

Chloride ingress into concrete from the surrounding environment can result in the corrosion of the embedded steel reinforcement and cause damage to the concrete. Superabsorbent polymer (SAP) with fine particle size was incorporated into the structure of concrete for controlling the chloride ingress and improving its compressive strength via promotion of internal curing. The SAP used in this study was evaluated for its absorbency property when exposed to cementitious environment such as aqueous solution of Ca (OH)2 and cement slurry. The results were compared to that in sodium chloride solution, the environment where absorbency of most of the SAP found in the market are well studied. Results showed that although SAP absorbency decreased with increasing concentration of Ca (OH)2 and cement, the results suggest that water containing cementitious materials are able to be absorbed by SAP. Chloride ingress into 28-day cured concrete specimens were determined using Rapid Chloride Penetration Test (RCPT) method employing 60V DC driving force. Concrete samples with size of 50 mm height x 100 mm diameter were prepared using a M25 mix design with 0.4 and 0.45 water to cement ratios and different percentages of SAP such as 0.05%, 0.1% and 0.15% with respect to cement mass. Results showed that concrete with 0.15% SAP gave the best result with 14% less chloride permeability than concrete with no SAP for a 0.4 water to cement ratio. Concrete samples for compressive strength tests with size of 200 mm height x 100 mm diameter were prepared using the same mix design and percentages of SAP and cured for 28 days. Results showed that the best results for compressive strength was found at 0.1% SAP at a 0.4 water to cement ratio which can be attributed to internal curing provided by SAP.

Assessment and Evaluation of Blended Cement Using Bamboo Leaf Ash BLASH Against Corrosion

Concrete provides a high degree of protection against corrosion of embedded steel reinforcement. Owing to the harsh environmental conditions and the presence of aggressive elements from the marine environment, deteriorating corrosion affects the durability of reinforced concrete structures. This study evaluated the effectiveness of bamboo leaf ash BLASH as a supplementary cementing material or admixture with Portland cement to improve the durability of reinforced concrete structures. Specimens of 0, 10, 15, and 20% BLASH mixtures were prepared using 16, 20, and 25 mm steel reinforcements. A total of 100 cylindrical specimens were cast and used in this study. The specimens were accelerated by corrosion using impressed current techniques and a galvanostatic method in a simulated environment. The results show that specimens with a BLASH content of 10% exhibited superior performance and exhibited longer corrosion initiation and propagation times. It has a higher resistance to acid penetration and lower corrosion rates. The crack parameters of the specimen with BLASH admixtures, such as the crack width and crack frequency, were negligible. The use of BLASH as an admixture strengthens its durability and improves its residual strength and serviceability. Doi: 10.28991/cej-2021-03091707 Full Text: PDF

Corrosion of Steel Rebars in Anoxic Environments. Part I: Electrochemical Measurements.

The number of reinforced concrete structures subject to anoxic conditions such as offshore platforms and geological storage facilities is growing steadily. This study explored the behaviour of embedded steel reinforcement corrosion under anoxic conditions in the presence of different chloride concentrations. Corrosion rate values were obtained by three electrochemical techniques: Linear polarization resistance, electrochemical impedance spectroscopy, and chronopotenciometry. The corrosion rate ceiling observed was 0.98 µA/cm2, irrespective of the chloride content in the concrete. By means of an Evans diagram, it was possible to estimate the value of the cathodic Tafel constant (bc) to be 180 mV dec−1, and the current limit yielded an ilim value of 0.98 µA/cm2. On the other hand, the corrosion potential would lie most likely in the −900 mVAg/AgCl to −1000 mVAg/AgCl range, whilst the bounds for the most probable corrosion rate were 0.61 µA/cm2 to 0.22 µA/cm2. The experiments conducted revealed clear evidence of corrosion-induced pitting that will be assessed in subsequent research.

Use of Soft Computing Techniques to Predict the Bond to Reinforcing Bars of Underwater Concrete

The current knowledge and building code recommendations for bond behavior between concrete and embedded steel reinforcement become unwarranted when casting takes place in humid or fully submerged water conditions. The main objective of this paper is to assess the effect of underwater casting on the concrete mechanical and bond properties, including the development of regression models that simplify prediction of washout loss and corresponding bond stress-slip properties. Washout loss was determined using the standard CRD C61 test method and air-pressurized tube that simulates higher hydrostatic water pressure. Particular care was placed to reuse the same washed concrete samples for determining the residual compressive strength and bond properties to embedded steel bars. Test results showed that the washout loss and hydrostatic casting depth are directly affected by the concrete composition. Hence, for example, the decrease in water-to-cement ratio and/or addition of anti-washout admixture or silica fume improved concrete cohesiveness and resistance to washout. The stiffness and confinement of steel bars normally achieved in dry conditions curtailed with underwater casting, which led to reduced ductility and ultimate bond strength at failure. The experimental bond stress-slip data are compared with the CEB-FIP Code models and design strengths specified by ACI 318-19 and European Code EC-2. Regression analysis and predictive charts are established to facilitate assessing the effect of concrete composition on washout characteristics and residual strengths following underwater casting.

The ability of UHPC prepared with unpurified sea sand and seawater to protect embedded steel against corrosion

Preparing concrete with unpurified sea sand and seawater is of great significance for the construction in coastal areas short of river sand and pure water. However, unpurified sea sand and seawater contain a lot of chloride ions which may cause steel corrosion problems. In this paper, unpurified sea sand and seawater are used to prepare ultra-high performance concrete (UHPC), and the potential differences between the steel bars with protective layer thickness of 5mm and 15mm and the Cu/CuSO4 reference electrode under two types of dry-wet cycle and the chloride ion concentrations at different depths from UHPC specimen surface after 40 dry-wet cycles are tested, the UHPC specimens are broken to observe corrosion of embedded steel bars. The results show that UHPC prepared with unpurified sea sand and seawater has good ability to protect embedded steel reinforcement against corrosion under dry-wet cycle conditions in lab.

Evaluation of corrosion resistance of corrosion inhibitors in concrete structures by Impressed voltage test

There are several commercially marketed corrosions inhibiting admixtures for concrete, presently not tested adequately and lack long term performance results. In this study laboratory studies were conducted on two commercially available Corrosion inhibitors to determine the effectiveness and their corrosion resistance parameters of concrete with OPC and PPC. This study examined the durability parameters in concretes mixed with Anodic Inhibitor (Calcium Nitrite based) and Bipolar Corrosion inhibitor (amino alcohol-based). The Accelerated corrosion testing (Impressed voltage test) was conducted to determine the efficacy of inhibitors. For PPC mixes, the Calcium Nitrite Inhibitor has shown better results in reducing the rate of corrosion of the embedded steel reinforcement. For OPC mixes both the inhibitors have given better results compared to control mix in reducing the corrosion rate of embedded steel.