Column Mode Research Articles

Recycling biochar derived from tannery liming sludge for chromium adsorption in static and dynamic conditions

In tannery industry, unhairing and liming (termed as liming) are indispensable but it is a very polluting part of process when producing quality leather. Expelled wastewater from liming generates significant amounts of liming sludge. In this study, efficacies of liming sludge biochar derived for adsorption of chromium from tannery wastewater are described. The biochar was characterized by FTIR, SEM, EDX, and BET technologies, and pHzpc. Surface area and responsible functional groups of biochar were as follows: 9.2 m2/g and C-H, C-N, C-O, C=C, N-H, S=O, and O-H. EDX analysis revealed chromium adsorption and SEM images led to changes in surface morphology. pHzpc for dry liming sludge and biochar were 4.1 and 6.1, respectively. Batch adsorption was verified by assorted parameters including biochar dose, shaking speed, contact time, and dilution factor. Column study results confirm the highest chromium adsorption efficiencies of 408.12 and 533.41 mg/g for 3 cm and 5 cm bed height, respectively. Regression coefficient suggested that adsorption obeys the Freundlich isotherm and pseudo-second order kinetics in batch mode, while it follows the Yoon–Nelson model in column mode. The physicochemical parameters of tannery wastewater prior and post-adsorption revealed good removal efficiencies for chloride content (Cl), chemical oxygen demand (COD), and biochemical oxygen demand (BOD) were, respectively, 50.5%, 80.1%, and 85.5%. Desorption process recovers 69.37 mg/g Cr from adsorbed biochar which could be reused in the tanning process. Consequently, an innovative state-of-the-art tannery liming sludge biochar can satisfactorily use in tannery wastewater treatment by following wealth-from-waste principle.

Experimental and numerical study on eccentric compression properties of laminated bamboo columns with a chamfered section

The eccentric compression behavior of laminated bamboo lumber (LBL) columns with a chamfered section was investigated using eccentricity of 30 mm, 60 mm, 90 mm and 120 mm. The effect of eccentricity ratio on the ultimate bearing capacity, ultimate strain and failure mode was analyzed through eccentric compression tests. The failure modes of LBL columns with different eccentricities were basically same, which belonged to brittle tension failure. With the increase in eccentricity ratio, the ultimate bearing capacity and ultimate strain gradually decreased. The cross-section strain of the specimen was linearly distributed along the height direction, which conformed to the plane section assumption. The lateral deflection curves had similar characteristics under different load levels and could be expressed by sine half-wave curves. Based on the Hill failure criterion, a 3D finite element model was developed to calculate the ultimate bearing capacity of LBL columns with different dimensions under eccentric compression. According to the simulation results and test results, a general empirical formula was proposed considering both the influence of slenderness ratio and eccentricity ratio. The reliability of the proposed formula was verified by comparing the calculated results with the results in the existing literature.

Controller and sensor placement for a 3D irregular building based on Hankel norm

Placing controllers and sensors properly is important in structural health monitoring and control. Many optimization methods require much computation efforts. This paper used Hankel norms to develop the placement rules, because they involve the input and output gains and thus could be shaped by the locations. In modal form, their computations are relatively simple. The location and mode influences on norms were arranged in rows and columns, respectively, to form a matrix, and was normalized by the column (mode) root mean square. The optimization goal is to choose locations with higher index values and lower correlations to ensure higher controllability and observability, and with less effort to be compensated for by gains. Hankel norm is compatible with the LQR control objectives in that they are both 2-norm, so the methodology is appropriate to be applied to the base isolation benchmark building for structural control, which is an eight-story irregular building with ninety-two candidate locations for controllers and thirty-six locations for sensors. Following the method, ten controller locations and eighteen sensor locations were determined. Earthquake time history analysis using LQG technique validated the effectiveness of thus determined subset of locations by comparing with other subset of locations.

Optimization of Crystal Violet Dye Removal in Fixed Bed Column Using Eucalyptus camaldulensis as a Low-Cost Adsorbent

Adsorption through waste adsorbents is one of the developing technologies used for treating textile wastewater. The present study explores the possible outcome of Eucalyptus camaldulensis biomass as an adsorbent for removing crystal violet dye from aqueous solutions. Eucalyptus camaldulensis biomass was used as such and used in fixed bed column mode to testify its potential at different parameters. Effect of different constraints like bed height (cm), flow rate (mlmin-1), initial dye concentration (mgL-1), and pH were studied along with breakthrough curve and exhaust time. Maximum breakthrough curve and exhaust time and utilization of mass transfer zone were observed at bed height of 20 cm. However, the promising results are obtained at higher dye concentration (50 mgL-1), lower flow rate (1 mlmin-1), and at lower pH of 5. This study reveals promising results at acidic pH. This study reflects that adsorption capacity and breakthrough curve favor lower acidic pH. The adsorption data in batch mode follow the Langmuir isotherm and best fit to pseudo-second-order reaction kinetics. The breakthrough curve and mass transfer zone are individually testified, and the breakthrough curve obeys the assumptions of the Thomas model, and R2 (0.933-0.997) values confirm the data that its best fit with the Thomas model.

Coupling Effect of Eccentricity and Slenderness Ratios on RCFST Column Instability Modes

The instability damage modes of rectangle concrete-filled steel tube (RCFST) columns that are subjected to eccentric compression can be divided into two types based on the modified Jezek analytical procedure, namely, the eccentricity ratio (γ) and the slenderness ratio (λ) coupling effect. The RCFST columns have unilateral compression yield failure mode when γ is small. However, it has compressive and tensile mode on both sides when the value of γ is large. In this work, parametric analyses were performed to test 32 RCFST long columns by varying γ with different λ combinations. From the analysis, it was found that the results of the theoretical analysis of the load-tension strain (P-ε) responses and the instability modes of the RCFST long columns are similar to the experimental results. Further, the proposed analytical method aids in better understanding the effects of γ and λ coupling on the stability behavior of the RCFST columns.

Experimental evaluation on characteristics of a falling-film flow on the horizontal tube subject to ultrasonic field

The falling-film flow characteristics significantly influence the heat and mass transfer process in the horizontal tube absorber. This study experimentally investigates the effect of ultrasonic field on the flow mode transition and thickness distribution of the liquid film on the horizontal tube. The water-calcium chloride (CaCl2/water) was selected as the working fluid. The effects of the factors, such as the tube diameter, tube spacing, solution concentration, ultrasonic power, sound source distance and ultrasonic incidence angle, on the hydrodynamics of the liquid flow were analyzed. The results show that the mode transition Reynolds number increases with the increasing ultrasonic power, but reduces comparing to that without ultrasound. Moreover, in the column mode, the fluctuation range of the film thickness decreases with the increase of the ultrasound power; and the ultrasonic field help increase the average film thickness outside the tube and enhance the uniformity of liquid film.

Biodegradation of 4-chlorophenol in batch and continuous packed bed reactor by isolated Bacillus subtilis

In present work, biodegradation of 4-Chlorophenol (4-CP) has been successfully achieved using bacteria i.e. Bacillus subtilis (MF447841.1), which was isolated from the wastewater of a nearby drain of Hyundai Motor Company service centre, Agartala, Tripura (India). Geonomic identification was carried out by 16 S rDNA technique and phylogenetic processes. Both, batch and column mode of experiments were performed to optimize various parameters (initial concentration, contact time, dosages etc.) involved in the significant biodegradation of 4-CP. Based on R2 value (0.9789), the Levenspiel's model was found to be best fit than others. The kinetic parameters; specific growth rate (μ), yield of cell mass (YX/S), and saturation constant (KS), were obtained as 0.6383 (h−1), 0.35 (g/g), and 0.006884 (g/L), respectively. The isolated strain has shown the ability of degrading 4-CP up to 1000 mg/L initial concentration within 40 h. Bacterial strain was immobilized via developing calcium alginate beads along by optimizing weight proportion of calcium chloride and sodium alginate and size of the bead for further experiments. Various process parameters i.e. initial feed concentration, bed height, rate of flow of were optimized during packed bed reactor (PBR) study. Maximum biodegradation efficiency of 4-CP was observed as 45.39% at initial concentration of 500 mg/L within 105 min, using 2 mm size of immobilized beads which were formed using 3.5% w/v of both calcium chloride and sodium alginate within. Thus, Bacillus subtilis (MF447841.1) could be used for biological remediation of 4-CP pollutant present in wastewater. Moreover, because of affordable and eco-friendly nature of water treatment, relatively it has the better scope of commercialization.

Reducing energy costs during solvent regeneration at the tar deasphalting unit

For propane tar deasphalting units, a rather high energy intensity of the technological process is characteristic. In order to reduce it, an assessment was made of the possibility of reducing the steam consumption during the regeneration of propane from asphalt solution. The study was carried out using a Honeywell UniSim Design modeling system, in which a model of a propane tar deasphalting unit was formed. The Peng-Robinson method was used as a mathematical package for calculating the thermodynamic properties of the components of the fractions. The component composition of the feedstock is represented by oil fractions with boiling points from 405 to 616 °C. When carrying out a computational experiment, the following values of technological parameters were used: the tar consumption was 38.9 t / h, the ratio (propane: raw material) was (6.4: 1), the yield of deasphalted oil was about 30 wt%. The performed analysis of a typical scheme for the regeneration of propane from asphalt solution showed that in the process stream supplied to the feed of the stripping column K-6, there is already a sufficiently large amount of a vapor phase consisting practically of propane and traces of oil fractions. To efficiently use the energy of the flow without attracting additional energy costs, it is advisable to separate the gas and liquid phases before they enter the column K-6, that is, to include an additional separator in the technological scheme before it. The performed computational experiment showed that in the proposed version of the technological scheme, the steam consumption required for the release of propane decreases by 17.5%, which, accordingly, for the subsequent devices of the scheme, reduces the amount of water discharged into the sewage system. Optimization of technological modes of the stripping column K-6 provides a clear separation of propane, in the flow of which the content of bitumen fractions is 0.03 mol%, which makes it possible in industrial conditions to return the flow of propane to the feed of the extraction column. The proposed technological solution for propane recovery can be used in the processes of one- and two-stage tar deasphalting.

The most adverse fire scenario research of steel frame structure fire-resistant design based on structural vulnerability analysis

The complex responses of the building under fire condition lead to difficult realization of structural fire-resistance design. By identifying the weakness areas, the defined most adverse scenario of the structural fire-resistance design has practical significance. In this paper, the characteristics of fire load, steel at elevated temperature and failure modes of steel beam and column in fire test, were discussed to determine that the damage consequences of steel components under fire were equivalent to component fracture. Meanwhile, the calculation method of joint well-formedness after component failure was given to recognize the failure mode of steel frame structure under fire condition based on structural vulnerability theory (SVT). Then, the most adverse fire scenario of the steel frame structure fire-resistance design under the conditions of single component failure and component failure continuously were analysed. The calculation method of the most adverse fire scenario based on the structural SVT was proposed. According to the failure mode identified by the SVT, the most adverse fire scenario was that the component with the minimum damage pattern subjected to fire in the condition of a single component failure occurred. When the component failed continuously, the components induced the maximum damage pattern (the vulnerability index was the largest) were the key components and such components exposed to fire was the most adverse fire scenario. Finally, two calculation models-6-story 4-span planar steel frame and 3D CFS composite shear wall structures-were selected to verify the proposed calculation of the most adverse fire scenarios. It was concluded that the proposed calculation method could reduce the number of fire scenarios that need to be calculated for design and was suitable for application to performance-based fire resistance design.

Peak strength prediction of reinforced concrete columns in different failure modes based on gene expression programming

The peak strength of reinforced concrete (RC) columns plays an important role in the appraisal of inelastic seismic performance. It depends on various parameters related to the geometry, reinforcement detail, material property, confinement effect, and loading condition. In applications, it is usually a prior condition to classify the failure modes of RC columns for predicting the peak strength accurately. Yet, classifying the failure modes of RC columns in an accurate way is a difficult task due to the complexity of the shear transfer mechanism. Thus, there is a need to develop a peak strength prediction model for RC columns failing in different modes directly. In this study, an attempt has been made by implementing the gene expression programming (GEP) method to realize this purpose. The experimental data required for the implementation of the GEP method are based on extensive results of RC columns tested in quasi-static cyclic loading. To validate the efficiency of the developed model, a detailed comparison against existing equations is conducted. The comparative results indicate that the developed model produces a rational prediction for the peak strength of RC columns in various failure modes. Based on the developed model, the peak strength can be predicted in a unified way for both ductile and non-ductile RC columns, which is beneficial for the seismic evaluation of existing structures.

Batch and column mode adsorption studies of reactive red 195 dye using Adenanthera paronina L seed activated carbon

The adsorptive removal of textile dye (Reactive Red 195) was investigated using a novel activated adsorbent prepared from the seeds of Adenanthera paronina L. Five different varieties of activated carbon were prepared by altering the conditions for the process. The surface characteristics of the five carbon materials were analysed using SEM, FTIR and other physico-chemical parameters. The BET surface area of the AC's varied between 446 and 948 m2/g. One adsorbent (APAC5) with a BET surface area of 948 m2/g was selected for the adsorption studies under batch and column mode. The adsorbent APAC5 has the ability to remove 64.42 and 82.35 mg/g of RR195 dye (initial concentration of 75 mg/L) under batch and column mode respectively. The nonlinear form of kinetic and isotherm models give better results than the linearized models. The nonlinear pseudo-first order kinetic model best suited with high r2adj value (0.806 < r2adj < 0.993) and low Sd (0.140 < Sd < 5.954). The prepared carbon is undoubtedly good adsorbent for removal of textile dyes from its effluents.

Cast-in-place and prefabricated UHPC jackets for retrofitting shear-deficient RC columns with different axial load levels

Ultra-high performance concrete (UHPC) is a new class of concrete materials that features high strength, ductility, and damage tolerance. This study experimentally evaluated three new UHPC jacketing strategies for retrofitting shear-deficient reinforced concrete (RC) columns, including cast-in-place UHPC jackets with or without steel bar mesh and prefabricated UHPC panels with steel bar mesh. The steel mesh had a high nominal yielding strength of 550 MPa. The impact of different axial load levels on the seismic performance of the retrofitted columns was also explored. Two control columns and five retrofitted columns were tested under lateral displacement reversals. Results showed that while all retrofitting methods enhanced the strength capacity of the shear-deficient column, the UHPC jacketing without steel mesh failed to improve the shear-critical behavior and drift capacity of the column. With the inclusion of high-strength steel mesh in the UHPC jacket, the control columns’ brittle shear failure was significantly improved. The proposed novel UHPC jacketing method with on-site installation of prefabricated UHPC panels showed comparable seismic retrofitting performance to the cast-in-place UHPC jacketing. In addition to the experimental study, suitable strength models, which can reasonably evaluate the strength and failure mode of the RC columns retrofitted using the proposed UHPC jackets, were suggested.

Composite of polypyrrole with sugarcane bagasse cellulosic biomass and adsorption efficiency for 2,4-dicholrophonxy acetic acid in column mode

The 2,4-Dicholorophenoxyacetic acid (2,4-D) is a herbicide used against broad leave herbs and it is carcinogenic and mutagenic in nature, which need to be remediated to avoid negative impact on the living organisms. In view of promising adsorption efficiency of composite materials, the present investigation is focused on the preparation of polypyrrole (PPY) and sugarcane bagasse (SB) based composites. The composites were characterized for surface properties and thermal stability. The prepared composite was utilized for the sequestration of 2,4-D from aqueous solution in column mode as function of bed height, flow rate and 2,4-D concentration. Thomas model was applied to the rate of adsorption at different 2,4-D concentrations, flow rates and bed heights to predict the breakthrough curves. The BDST (bed depth service time) model was also applied to predict the breakthrough curves as function of bed heights. The biosorption capacities of SB and PPY/SB were 6.1 and 8.63 (mg/g), respectively. The results indicated that sugarcane bagasse composite with PPY is efficient adsorbent for the remediation of 2,4-D and could have potential for the removal of herbicides from effluents.

Functionalization of walnut shell by grafting amine groups to enhance the adsorption of Congo red from water in batch and fixed-bed column modes

Several pollutants found in industrial wastewater, such as Congo red dyes (CR), have been shown to be toxic to organisms. To protect human health and the environment, industrial effluents must be purified before being released into bodies of water. In order to effectively remove CR from the solution, amino functionalized walnut shells (WNS) were synthesized. Batch and fixed-bed column systems were used in this research. The Langmuir isotherm model fits the experimental findings well in these two systems, whereas the pseudo-second-order and Thomas kinetic models characterize batch and column CR adsorption, respectively. The adsorption capacity of ACWNS toward CR was to 224.4 mg g−1 at 303 K from Langmuir model. Electrostatic interactions and hydrogen bonding may be the mechanisms. The column research showed that adsorption was more favorable at low inlet solution concentration, low flow rate, or high column depth. The column mass transfer capacity parameter (B) increased with increasing concentration (0.461, 1.59, and 2.47 for 50, 100, and 150 mg L−1, respectively) owing to a greater driving force for CR adsorption onto ACWNS, while adsorbate–adsorbent affinity (β) exhibited a reverse trend. The regeneration property of CR-loaded ACWNS was excellent using NaOH solution. The thermodynamic study revealed that CR removal was both spontaneous and endothermic. ACWNS is promising to remove CR from the solution.

Shear strength model and prediction of failure mode for multi-spiral column

The damage of reinforced concrete columns due to shear is often serious, so this type of failure should be avoided from the design. In this paper, a model derived based on the discrete computational method is proposed to calculate the shear strength of column carried by multi-spiral transverse reinforcement, accounting for the effect of compression depth. Furthermore, based on this model, a method is proposed to predict the failure mode of multi-spiral columns. The test database of multi-spiral columns from previous studies is used to validate both the shear strength and failure mode predictions. The proposed model with a crack angle of 40 degrees gives the best estimation of the shear strength of multi-spiral columns, and the proposed method predicts well the failure mode of these columns. To avoid shear failure, the ratio of the minimum shear strength calculated from the proposed model with a crack angle of 40 degrees to the shear force based on the moment-curvature analysis is suggested to be larger than 1.2.

Vanadium removal from mining ditch water using commercial iron products and ferric groundwater treatment residual-based materials

Removal of vanadium from liquid waste streams protects the environment from toxic vanadium species and promotes the recovery of the valuable metal. In this study, real mining ditch water was sampled from a closed vanadium mine (V–Fe–Ti oxide deposit, Finland) and used in sorption experiments at prevailing vanadium concentration (4.66–6.85 mg/L) and pH conditions (7.02–7.83). The high concentration of vanadium in the water represents a potential health concern according to the initial risk assessment carried out in this study. Vanadium was efficiently removed using four different iron sorbents: ferric oxyhydroxide with some goethite (CFH-12), poorly crystallized akaganéite (GEH 101), ferric groundwater treatment residual (GWTR), and GWTR-modified peat (GWTR-Peat). Higher dosage (6 g/L with 24 h contact time) and longer contact time (72 h using 1 g/L dosage) resulted in removal efficiencies of higher than 85%. Kinetic data were well represented by the Elovich model while intra-particle diffusion and Boyd models suggested that the sorption process in a real water matrix was significantly controlled by both film diffusion and intra-particle diffusion. Column studies with CFH-12, GEH 101, and GWTR-Peat showed that the breakthrough started earlier with the mining ditch water compared to a synthetic vanadium solution (investigated only with CFH-12), whereas GEH 101 proved to have the best performance in column mode. The Thomas and Yoon-Nelson column models were found to agree with the experimental data fairly well with the 50% breakthrough time being close to the experimental value for all the studied sorbents.

Highly efficient plutonium scavenging by an extraction chromatography resin containing a tetraaza-12-crown-4 ligand tethered with four diglycolamide pendent arms

An efficient extraction chromatography resin, containing tetraaza-12-crown-4 functionalized with four diglycolamide moieties, was evaluated for the separation of plutonium. This chromatography resin yielded very large distribution coefficients for Pu4+ (>105) in 0.5 – 6 M HNO3 feed solutions. Various physicochemical properties such as sorption kinetics, Pu4+ sorption mechanism, and its sorption capacity were investigated. The sorption kinetics, following a pseudo-second-order model, showed that about 10 minutes of equilibration was sufficient for >99.9% sorption of Pu4+. The sorption of Pu4+ on the resin followed the Langmuir monolayer model, which was confirmed by a theoretical calculation based on the kinetic model. The Pu4+ sorption on the resin was driven by a large exothermic enthalpy change (ΔH = −31.4±2.2 kJ/mol) and a positive entropy change (ΔS = 224±15 J/mol/L). The resin could sorb a maximum of 12.1±0.8 mg of Pu per gram of resin, which is equivalent to 1:2 metal/ligand complex on the resin. The Pu4+ from the resin phase was completely stripped with 0.5 M oxalic acid. A possible application of this resin for the separation / pre-concentration of Pu4+ was successfully demonstrated in the column mode.

Research on nano-concrete-filled steel tubular columns with end plates after lateral impact

Abstract This paper presents thirteen square columns to study the behavior of nano-concrete-filled steel tubular columns with end plates after lateral impact. The failure modes of the square columns subjected to lateral impact damage or not subjected to lateral impact damage were compared. The lateral impact loading height, steel tubular thickness, and column height were set as the test parameters in these tests. The effects of test parameters on the ultimate capacity, initial stiffness, and ductility of columns are discussed in this paper. The bearing capacity of square columns is decreased because of the lateral impact loading which can also be concluded from the test results. And with the steel tube thickness increasing, the bearing capacity and initial stiffness of columns are increased and ductility has no obvious change. However, with the column height increasing, the bearing capacity and stiffness of columns are decreased and ductility is increased. Furthermore, the strain development of the columns under axial compressive loading is also discussed in the paper. The results indicated that the corner of the square column is more easily damaged under compressive loading. According to the test results, the calculated formula is proposed to predict the ultimate capacity of nano-concrete-filled steel tubular columns with end plates after lateral impact. The calculated results have a good agreement with the test results.

Behaviour and failure modes of steel parking structure column under transverse impact

The columns of steel parking structures are prone to collision while parking vehicles. The current design codes offer limited guidance for designing built-in columns with sufficient impact resistance to transverse impacts. In this study, we performed a series of numerical studies to comprehensively understand the behaviour and failure modes of the impacted column. First, a simplified analysis method was proposed to simplify the overall structural model. The simplified analysis models (SAMs) and overall analysis models (OAMs) were compared, and the results demonstrated that the proposed method can reasonably predict the behaviour of the impacted column. The behaviour of the column was estimated under common loading conditions, and the effects of the impact velocity, axial loading, and impact height were studied. Furthermore, two structural configurations, SAM-1 and SAM-2, were compared and evaluated. SAM-2 had better impact resistance than SAM-1 owing to the presence of a back beam, which reduced the effect of the additional moment due to deformation, thereby decreasing the external work and total input energy. The failure modes were classified into three categories considering different impact velocities, axial loads, and impact heights. The robustness of the structure under impact can be improved by designing a reasonable axial loading level for the column to avoid failure mode 3.

Behavior of Quarry Rock Dust, Fly Ash and Slag Based Geopolymer Concrete Columns Reinforced with Steel Fibers under Eccentric Loading

Geopolymer concrete, also known as an earth-friendly concrete, has been under continuous study due to its environmental benefits and a sustainable alternative to conventional concrete construction. The supplies of many source materials, such as fly ash (FA) or slag (SG), to produce geopolymer concrete (GPC) may be limited; however, quarry rock dust (QRD) wastes (limestone, dolomite, or silica powders) formed by crushing rocks appear virtually endless. Although significant experimental research has been carried out on GPC, with a major focus on the mix design development, rheological, durability, and mechanical properties of the GPC mixes; still the information available on the structural behavior of GPC is rather limited. This has implications in extending GPC application from a laboratory-based technology to an at-site product. This study investigates the structural behavior of quarry-rock-dust-incorporated fiber-reinforced GPC columns under concentric and eccentric loading. In this study, a total of 20 columns with 200 mm square cross-section and 1000 mm height were tested. The FA and SG were used as source materials to produce GPC mixtures. The QRD was incorporated as a partial replacement (20%) of SG. The conventional concrete (CC) columns were prepared as the reference specimens. The effect of incorporating quarry rock dust as a replacement of SG, steel fibers, and loading conditions (concentric and eccentric loading) on the structural behavior of GPC columns were studied. The test results revealed that quarry rock dust is an adequate material that can be used as a source material in GPC to manufacture structural concrete members with satisfactory performance. The general performance of the GPC columns incorporating QRD (20%) is observed to be similar to that of GPC columns (without QRD) and CC columns. The addition of steel fibers considerably improves the loading capacity, ductility, and axial load–displacement behavior of the tested columns. The load capacities of fiber-reinforced GPC columns were about 5–7% greater in comparison to the CC columns. The spalling of concrete cover at failure was detected in all plain GPC columns, whereas the failure mode of all fiber-reinforced GPC columns is characterized with surface cracking leading to disintegration of concrete cover.