Concrete Cover Depth Research Articles

Life performance of 40-year-old RC beams with different concrete covers and bar diameters in natural corrosion environments

This study entailed an experimental evaluation of the life performance of 40-year-old reinforced concrete (RC) beams with different concrete cover depths and tensile bar diameters. Beams that were naturally corroded in a marine environment were used in this study, and no artificial process was employed to accelerate the corrosion. Most existing evaluations of beams employ electrical acceleration, admixture, or exposure to designed chloride environments to aid corrosion. Given the paucity of studies on naturally corroded beams, this study is of significance regarding the design, service, and life assessment of RC structures. Two-point load bending test was employed to examine the residual mechanical performance. The corroded bars were extracted to assess the corrosion properties, while the mechanical properties were evaluated via a tensile test. The focus of this study was on evaluating the progress of deterioration and capacity loss in naturally aged beams. The results indicated that a concrete cover of 50 mm satisfies the requirements of a design service life of 50 years. Furthermore, the cross-sectional loss–ultimate capacity loss relationship is consistently linear for the RC beams. Thus, the estimation of ultimate capacity loss based on cross-sectional loss is acceptable in case of natural corrosion of the RC beams. Hence, the findings can contribute to the prediction of the life performance and maintenance of existing structures as well as to designing new structures.

Fusion of NDE Scanning and Photogrammetric Data for Concrete Bridge Decks and Girders Condition Assessment

The first case history would present 3-D results of Impact Echo Scanning and Ground Penetrating Radar (GPR) Scanning plus Spectral Analyses of Surface Waves tests of post-tensioned concrete bridge girders. The NDE results were overlaid on photogrammetric images of visible corrosion/cracking damage to the concrete bridge girders of an aging bridge due to de-icing salts and a marine environment. Cart-based scanning with GPR and Impact Echo and Spectral Analyses of Surface Waves NDE methods is commonly being conducted of both bare concrete decks and asphalt overlaid decks to map both top and bottom delaminations and depth of concrete cover. The data is being used in conjunction with chlorides and carbonation testing of concrete cores to recommend repairs and evaluate remaining service lives for bridges. Photogrammetric images of bare concrete in particular are being used to visibly compare areas of spalling, delamination and deck patching for improved interpretation, visualization and mapping of internal deck delaminations and other concrete damage with this data fusion approach to provide more accurate surface and full-depth images of bridge deck conditions.

NDT assessment of a thick-walled reinforced concrete mock-up of NPP concrete structures

Different materials within the Nuclear Power Plants (NPP) concrete structure can undergo ageing process which causes defects in the materials and can lose, partially or totally, their designed function. The typical defects include corrosion of reinforcement steel, delamination, cracks, malfunction of post-tensioning or steel composite systems etc. The testing methods to detect the defects of concrete structures are (i) destructive examinations and (ii) non-destructive testing (NDT). The challenges for assessing the performance of the NPP concrete structures using (NDT) methods include (i) the limited access time (only during the annual overhauls) and (ii) the accuracy and reliability of the available NDT testing devices. To overcome these challenges, a mock-up wall representing a section of the concrete containment of the NPP was built. Simulated defects and errors were embedded inside the wall such as dimensional errors, honeycombing, delamination, and voids in the post-tensioned grouted tendon ducts. Different NDT techniques and combinations were used for assessing the simulated defects of the mock-up wall. These techniques included basic NDT methods as rebound hammer, ultrasonic pulse velocity (UPV) and concrete cover meter and advanced NDT as Ground Penetrating Radar (GPR) and ultrasonic pulse echo (MIRA and MONOLITH). The results of basic NDT techniques as concrete cover meter, rebound hammer and UPV presented the basic characteristic properties of the concrete mock-up wall as concrete cover depth, compressive strength, and quality of concrete. The use of advanced NDT as GPR and MIRA and MONOLITH enabled us to detect and locate the embedded defect and the construction faults inside the mock-up wall. We conclude that the construction of the with simulated defects offers the possibility to investigate accuracy and reliably of the available NDT methods and increases the field experts’ skills providing also an important and very much needed educational platform for future NDE experts.

Investigation on mechanical behavior of steel angle frame reinforced concrete beams under torsion

SummarySteel angle frame reinforced concrete (SARC) beam is a kind of composite beam with encased steel angle frame. The torsion tests were carried out on six SARC beams and one reinforced concrete (RC) beam to investigate their torsional behavior. Test variables include space between steel plates, angle of steel plates, concrete cover depth, and concrete strength. The results showed that the reduction of space between steel plates causes specimens to crack in advance, but improves torsional behavior after cracking markedly. The energy dissipation coefficient reduces as concrete cover depth increases. Damage index can be effectively reduced by increasing concrete strength, reducing plates spacing, and increasing concrete cover depth. The overall torsional behavior of SARC beam can be improved by reducing the spacing between the plates, adopting staggered vertical and oblique steel plates, and increasing concrete strength, but the effect is adverse when concrete cover depth increases. Moreover, the space between steel plates should be greater than 100 mm, and concrete cover depth should be less than 35 mm for maximizing the torsional strength of steel angle frame. Considering the positive roles of steel angle frame and oblique steel plates, the calculation methods of cracking and ultimate torsional moment were proposed. This study can provide a theoretical basis for the engineering application of SARC beams subjected to torsion.

Bond behavior of recycled coarse aggregate concrete with rebar after freeze–thaw cycles: Finite element nonlinear analysis

Abstract In this article, the bond performance of recycled coarse aggregate concrete with rebar after freeze–thaw cycles is analyzed by the ABAQUS finite element method. The result shows that the finite element simulation value of the ultimate bond strength of recycled aggregate concrete specimens is in good agreement with the experimental value, while the agreement between the finite element simulation value of the peak bond slip of recycled aggregate concrete specimens and the experimental value is low; the bond strength between rebar and recycled aggregate concrete increased with the increase of concrete strength and concrete cover depth; the calculation formula for the bond strength between recycled aggregate concrete specimens with different concrete strengths and different concrete cover depth after freeze and thaw cycles are obtained.

An assessment of a practical implementation of the deemed-to-satisfy durability design and specification approach in the Swakopmund-Walvis Bay Freeway upgrade project in Namibia

Steel corrosion in reinforced concrete (RC) structures in the marine environment is a major concern for engineers and asset managers. This paper presents an assessment of the application of a deemed-to-satisfy durability design and specification approach on the newly constructed RC bridges on the Swakopmund-Walvis Bay Freeway upgrade in Namibia, with a desired service life of at least 100 years. The deemed-to-satisfy design and specification approach comprised a combination of South African recommendations for binder type, minimum binder content and w/b ratio, EN 206-1 recommendations for cover depth, and South African National Roads Agency Limited (SANRAL) durability index targets along with on-site concrete quality control guidelines on placement, compaction and curing. As-built concrete durability properties (i.e. water sorptivity, oxygen permeability and chloride conductivity) and cover depth were measured and used to check (using a Fickian-based service life model) if the deemed-to-satisfy specifications were adequate. The results showed that the as-built concrete durability properties surpassed the deemed-to-satisfy durability performance targets, while the on-site cover depth values showed significant inadequacies in some bridge elements. The service life estimations showed that the deemed-to-satisfy durability performance specifications were adequate. Considering the relatively high concrete quality achieved on site, it is clear that the 100-year service life could have been achieved more sustainably using a good balance of concrete quality and cover depth [e.g. a lower concrete quality (using less cement) or lesser coved depth] if a more rigorous performance-based durability design and specification approach had been used. Ultimately, the service life analyses using the as-built concrete model input parameters highlight not only the critical importance of both concrete quality and cover depth in achieving durable RC structures prone to steel corrosion, but also the inherent limitations of the deemed-to-satisfy durability design and specification approach.

A Hybrid Particle Swarm Optimization with Dragonfly for Adaptive ANFIS to Model the Corrosion Rate in Concrete Structures

The use of reinforced concrete is common in marine structures. Failure of reinforcement due to corrosion has detrimental impacts on nearly all of these structures. Hence, proposing an accurate and reliable model was imperative. The goal of this paper is to develop a new hybrid model by combining Particle Swarm Optimization (PSO) with Dragonfly Algorithm (DA) for Adaptive Neuro-Fuzzy Inference System (ANFIS) to predict the corrosion current density (C11) of marine reinforced concrete. The neuro-fuzzy-based methods have emerged as suitable techniques for encountering uncertainties associated with the corrosion phenomenon in marine structures. To the best of our knowledge, this is the first research that predicts the C11 through a model integrating fuzzy learning, neural learning rules, and meta-heuristics. 2460 data are collected from 37 regions in Persian Gulf. The input parameters are age, concrete repairing history, height above the sea level, distance from sea, concrete compressive strength, rebar diameter, concrete cover depth, concrete electrical resistivity, chloride ion concentration and pH. The proposed rules for the estimation of C11 based on collected dataset are assessed based on the several metrics such as R2, efficiency, mean absolute percentage error (MAPE), and median of absolute error (MEDAE). According to the results, ANFIS-PSO–DA enables to predict C11 by R2 (0.92), MAPE (1.67), MEDAE (0.14), and EF (0.97). The results of sensitivity analysis revealed that concrete compressive strength and pH are the most effective parameters on the corrosion current density of reinforced concrete.

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

Chloride ingress into concrete is the primary cause of steel corrosion that leads to the deterioration of reinforced concrete (RC) assets and eventual reduction of its service life. The maintenance of RC buildings and structures has become a major concern, and thus the development of an effective maintenance plan is required. This study focuses on deterministic and probabilistic analysis of service life (or repair application time) for RC assets repaired by cover rebuilding techniques in extreme marine environments exposed to chloride attack. A maintenance model is developed, based on the mechanism of chloride transport in repaired concrete elements, that simulates different cover rebuilding methods and strategies. Computationally, the probability-based maintenance model combines the Latin Hypercube technique with finite element and finite difference methods. Probabilistic data for five governing random variables is considered, including diffusion coefficient, surface chloride concentration, concrete cover depth, concrete replacement depth, and critical chloride value for corrosion initiation. Two maintenance strategies are introduced, which can be categorized as corrective and preventive repair strategies. The deterministic study shows that a deeper repair depth leads to fewer repairs and larger repair intervals. The results of the probabilistic assessment reveal that a preventive repair model with regularly scheduled repair intervals efficiently decreases corrosion initiation risks. It was also demonstrated how shorter repair intervals and deeper depths of concrete repair can achieve an acceptable probability of corrosion initiation.

Heterogeneous data fusion for the improved non-destructive detection of steel-reinforcement defects using echo state networks

The degradation of roads is an expensive problem: in the United Kingdom alone, £27 billion was spent on road repairs between 2013 and 2019. One potential cost-saver is the early, non-destructive detection of faults. There are many available techniques, each with its own benefits and drawbacks. This paper builds upon the successful processing of magnetic flux leakage (MFL) data by echo state networks (ESNs) for damage diagnostics, by augmenting ESNs with the depth of concrete cover as part of a data fusion approach. This fusion-based ESN outperformed a number of non-fusion ESN comparators and a previously used analytical technique. Additionally, the fusion ESN had an optimal threshold value whose standard deviation was three times smaller than that of the nearest alternative technique, potentially prompting a move towards automated defect detection in ‘real-world’ applications.

Probability-Based Crack Width Estimation Model for Flexural Members of Underground RC Box Culverts

Crack control for slabs and beams in current design practices in Korea are based on the Frosch’s model, which is adapted in ACI 318. It is more difficult to have consistent quality control in underground construction sites, such as the RC box culverts used for electric power distribution built below the ground level. There are more discrepancies between the as-built dimensions and the design dimensions provided in drawings in these structures. Due to this variability in construction error, the crack widths measured in such structures have higher potential to have more differences than the calculated values. Although crack control is a serviceability concern, if the owner chooses to have a target crack width that needs better control, crack width estimations can be improved by considering such construction variability. The probability-based crack width model suggested in this study will allow minimizing the discrepancies between the measured and calculated crack widths and provide reliable estimations of crack widths. Typical size of slabs and beams ranging between 300 mm (12 in.) to 500 mm (20 in.) used in underground RC box culverts in Korea were tested under the four-point bending test program. The thicker specimens had smaller bar spacings which created more cracks with smaller crack widths. However, with smaller crack widths generated in these specimens, there were more errors between the measurements and calculated values. From site investigations in Korea, the thickness of slabs in underground box culverts varied the most among all parameters. As a result, the bottom concrete covers had the highest variability. Bottom concrete covers and bar spacings are the two most important parameters in concrete crack control. A probability-based crack width estimation model for flexural members was developed in this study to consider this construction variability. Monte Carlo simulations were performed to evaluate the probabilistic characteristics of the design surface crack widths with a target width of either 0.3 mm (12 mils) or 0.5 mm (20 mils). The probabilistic models of design variables included in the crack width estimation model were generated based on field-collected information from construction sites in Korea. Because the surface crack widths in RC flexural members are sensitive to the construction errors of concrete cover depths, and since there are differences between the assumed and actual stress distribution closer to the reinforcing bars, the probability of having surface cracks of 0.3 mm width (12 mils) is found to be quite high, such as 89% at the positive moment region (mid-span, bottom surface) of the top slab in RC box culverts and 45% for the negative moment region (support area, top surface) of the top slab with current design practice. In order to ensure crack widths to be smaller than the design target width, probability-based crack width factors are recommended in this study to improve estimations depending on the selected target reliability levels.

Investigate the performance of the removal of a cover concrete over a reinforced concrete column

The thickness of a concrete cover is essential in the engineering design and construction process. If such concrete cover depth does not meet the design standards and drawing specifications, it will eventually lead to flaws, such as surface cracks on construction components. These flaws can potentially affect the structure's strength, durability, and other qualities. In this research, the strength criteria are considered with three specimens that have had the clear cover thickness gradually removed. (50–25-0 mm) of Various concrete grades were used to evaluate the flexure behaviour of an over reinforced concrete column. The outcomes of a laboratory experiment into the elimination of clear concrete are presented. The information presented involves load vs. deflection characteristics, crack width, and stiffness after 28 days of testing.

Limit values of accelerated carbonation resistance to meet EC2 durability requirements

Although accelerated carbonation resistance has been extensively tested, there are no recommendations for the application of test results in codes of practice. The main objective of this study was to determine the limit values of accelerated carbonation resistance to satisfy the required service life of reinforced concrete structures with concrete covers as prescribed in EN 1992-1-1. The service life of 50 years was considered, as well as all carbonation exposure classes (from XC1 to XC4). A full probabilistic analysis was conducted using the fib-Bulletin 34 carbonation prediction model. Using the limit state function and a defined reliability index, the upper limits of the inverse effective carbonation resistance (R-1 ACC) for all exposure classes and in a function of concrete cover depth were determined. The determined values of R-1 ACC presented in this study represent the upper limit of the average value, as well as the maximum deviation of one sample in relation to this average value. Thus, a simple assessment of concrete quality is allowed in terms of carbonation resistance based on the accelerated carbonation depth measurements.

Procedure for determining the remaining time to initiation of chloride induced reinforcement corrosion for existing concrete structures

For efficient asset management of reinforced concrete infrastructure, owners want to know when to expect corrosion in structures. At present, no widely accepted procedures for testing of existing structures for the remaining time to corrosion initiation are available. This paper describes such a procedure, based on the authors’ long-time experience. From about 20 years age, the concrete contains its mature response to actual environmental loads, e.g. chloride ingress. By measuring the mean and standard deviation of the actual cover depth, taking chloride profiles, assuming a few parameters and a simple, pragmatic model, the expected time to corrosion initiation for a particular test area can be predicted. Required numbers of cores and samples per core are given. Uncertainties are taken into account by subtracting a safety margin from the measured concrete cover depth. Results of six cores per test area are classified and interpretations are given. Because of large variability, the results are classified in three ranges of time to corrosion initiation: five years or less, five to fifteen years, or more than 15 years. The procedure has been approved by the relevant national Standards committees and was issued as a CUR Recommendation in 2018. It is illustrated based on a field case and the obtained results are discussed.

Assessing the impact of curing on chloride penetration resistance of the concrete cover using sorptivity profiling

The chloride penetration resistance of the near-surface concrete cover depth is greatly influenced by the type and extent of curing provided. Very short or ineffective curing regimes result in less cement hydration and a more porous surface due to early evaporation of capillary water. The resultant depth-dependent capillary pore structure results in a depth-dependent resistance to ingress of aggressive fluids, such as chloride solutions. Without reliable performance test methods for pre-qualification and quality assurance, minimum requirements for curing in specifications for concrete structures exposed to chlorides are typically prescriptive. In this paper, a series of modified rate of absorption tests is used to describe and quantify the effect of curing on the chloride penetration resistance of two concretes subjected to four different curing regimes, including accelerated moist curing. The proposed test method can be used to optimize curing for durability performance and allow credit for use of accelerated curing.

Bond strength of reinforcing bars in ultra-high performance concrete: Experimental study and fiber–matrix discrete model

The study provided an experimental and theoretical investigation on the bond behavior of reinforcing bars in ultra-high performance concrete (UHPC) via a series of pullout tests. Test parameters included depth of concrete cover, bar embedment length and bar diameter. Failure mode, crack pattern and load-slip response were recorded and discussed under different test parameters. Test results indicated that inadequate concrete cover depth led to splitting failure while short embedment length resulted in concrete cone failure, corresponding to lower bond strengths. The bond strength increased up to 229.8% and the pullout energy increased 401.4% as the concrete cover depth increased from 0.5d to 2d. The development lengths were recommended to be 6d and 8d corresponding to the concrete cover depth of 2d, 1.5d, respectively. A theoretical bond strength model using fiber–matrix discrete concept was proposed and validated by comparing the calculated values with the test results and 343 test data collected from literature. The average ratios of the calculated results to the test results and collected data were 0.996 and 1.039, with a standard deviation of 0.255 and 0.252. The proposed model established the connection between the meso-scale bond property of fibers and matrix and the macro-scale bond strength of steel bars and UHPC, offering an alternative perspective in understanding the bond performance of steel bars in UHPC.

Experimental study and suggested mathematical model for chloride-induced reinforcement corrosion rate

The service life of reinforced concrete structures is significantly affected by the chloride-induced corrosion of reinforcing steel. This paper presents a comprehensive study on the influence of the corrosion environment and other parameters on the corrosion rate of reinforcing steel in concrete. Comparative tests were carried out on concrete block and beam specimens subjected to accelerated corrosion in indoor and outdoor environments for a period of 1.5 years. The experimental variables considered included the chloride content, depth of concrete cover, type of reinforcing steel and water-to-cement (w/c) ratio. The test results indicated that the corrosion rate of the reinforcing steel increases with the increase in the w/c ratio, chloride content, temperature and relative humidity, while it is reduced when the thickness of the concrete cover increases. Furthermore, an empirical model for predicting the corrosion rate of reinforcing steel in concrete is proposed using the experimental data, which offers a good prediction of the corrosion rate.

INFLUENCE OF POLYPROPYLENE FIBER ON THE BOND STRENGTH BETWEEN STEEL BAR AND CONCRETE

Bonding between reinforcement and concrete play an important parameter in the behavior and strength of reinforced concrete. Many parameters influence the bond between reinforcement and concrete such as concrete cover depth, concrete conditions, overall thickness of the structural element and curing. In the present study, a total of sixty specimens as cube with four different percentages of polypropylene fiber (0, 0.5,1 and 1.5) % is used to evaluate the bond strength and stress-slip relationship. The compressive strength of concrete, rebar diameter, concrete cover, and embedded length of reinforcement are taken into account as variables. The test results show that the ultimate bond strength increases with increasing each of the compressive strength of concrete, concrete cover, polypropylene fiber content and decreasing the nominal diameter of the reinforcing bar. The best percentage that gave the maximum failure load, is 1.5% of polypropylene fiber.

The Cover Depth Effect on Corrosion-Induced Deterioration of Reinforced Concrete Focusing on Water Penetration: Field Survey and Laboratory Study.

Reinforced concrete bridges were visually surveyed in Japan, Thailand, and Vietnam to study the deterioration caused by internal steel corrosion under different climates, focusing on the concrete cover depth. Spalling or cracking arising from corrosion is likely where water is supplied. According to prior studies and our surveys, a concrete cover depth of more than 40 mm was found to prevent spalling, regardless of environmental conditions and structure age. Because water supply at steel is a key corrosion factor, it was hypothesised that under natural conditions, the water penetration in concrete would remain at a depth of approximately 40 mm. Our laboratory study examined water penetration under drying and wetting conditions. The results also suggested that under periodic rainfall conditions, the threshold of water penetration was not exceeded. The numerical study indicated maximum moisture evaporation to facilitate oxygen diffusion occurred at a depth of approximately 30–40 mm unless the concrete was exposed to continuous drying for more than one month. It was experimentally and numerically concluded that an adequate cover depth of greater than 40 mm could inhibit moisture and oxygen penetration at the steel, which supported the survey findings of cover depth effect on a high resistance to corrosion-induced deterioration despite an increase in service life.

Experimental investigation of crack width variation along the concrete cover depth in reinforced concrete specimens with ribbed bars and smooth bars

Crack width variation along the concrete cover depth has been studied from the past for better understanding of the cracking phenomenon in reinforced concrete (RC) structures. Previous studies have highlighted important cracking behaviors like internal cracks. The behavior of ‘slip’ between the reinforcement and concrete and the formation of a nonuniform crack face along the concrete cover depth are still not very clearly understood. An experimental program has been conducted to study the crack width variation along the cover depth in concrete prisms reinforced with a central ribbed bar and smooth bar, by varying the concrete cover depths. Both in specimens with smooth bars (SS) and specimens with ribbed bars (SR), crack width is larger on the concrete surface than at the steel bar surface. The crack width at the reinforcement is considerably larger in the SS than in the SR. In the SR, the crack width increases from the reinforcement along the cover depth bi-linearly, while, in the SS, it increases linearly. For the SR, the aforementioned behavior is due to the occurrence of internal cracks. In the SS, significant slip has been identified at the reinforcement and concrete interface, whereas negligible slip has been observed in the SR. A surface crack width calculation model has been developed, considering both the strain difference and the effect of the nonuniform crack face along the concrete cover depth. Its predictions showed good agreement with the experimental surface crack widths from the conducted study and with the results from the experiments in literature.

Performance of GGBS Cement Concrete under Natural Carbonation and Accelerated Carbonation Exposure

One of the primary problems related to reinforced concrete structures is carbonation of concrete. In many cases, depth of carbonation on reinforced concrete structures is used to evaluate concrete service life. Factors that can substantially affect carbonation resistance of concrete are temperature, relative humidity, cement composition, concentration of external aggressive agents, quality of concrete, and depth of concrete cover. This paper investigates the effect of varying the proportions of blended Portland cement (ordinary Portland cement (OPC) and ground granulated blast-furnace slag (GGBS)) on mechanical and microstructural properties of concrete exposed to two different CO2 exposure conditions. Concrete cubes cast with OPC, and various percentages of GGBS (0%, 30%, 50%, and 70%) were subjected to natural (indoor) and accelerated carbonation exposure. The aim of this paper is to present the research findings and authenticate the literature results of carbonation by using GGBS cement in partial replacement of OPC. The concretes with OPC are compared to concretes with various percentages of GGBS, to assess the carbonation depth as well as rate of carbonation of GGBS-based concretes, under both accelerated carbonation and natural carbonation exposure conditions. Even though GGBS cement increases the carbonation depth, the results are not the same with different GGBS replacement percentages. A correlation is made between concrete samples exposed to 15 ± 2% carbon dioxide (CO2) concentration and those exposed to natural CO2 concentration. The results reveal that the products formed by carbonation are similar under both exposure conditions. The experimental tests also revealed that GGBS cement concrete has a lower carbonation resistance than OPC concrete, due to the consumption of portlandite by the pozzolanic reaction. The combination of 70% OPC and 30% GGBS behaved well enough with respect to accelerated carbonation exposure, the depth of carbonation being roughly equivalent to that of control group (100% OPC). The results also show that rate of carbonation becomes more sensitive as the percentage of GGBS replacement increases (binder ratio), rather than duration of curing. Concretes exposed to natural carbonation (indoor) achieved lower carbonation rates than those exposed to accelerated carbonation.