Offshore Concrete Structures Research Articles

Mechanical and durability performance of polyvinyl alcohol fiber hybrid geopolymer-portland cement concrete under freeze–thaw cycles

Cold winter temperatures at high latitudes in coastal regions lead to prolonged exposure of offshore concrete structures to freeze–thaw (F–T) damage, which significantly reduces the mechanical and durability properties of concrete. To improve the durability of concrete under F–T damage, this study investigated the combined effects of fly ash (FA) and bentonite on the frost resistance of polyvinyl alcohol fiber reinforced geopolymer concrete (PFRGC). The study comprehensively analyzed the effect of fly ash and bentonite content on the rate of mass change and compressive strength under various F–T damage conditions. In addition, uniaxial compressive tests were carried out at different stages of F–T damage and the resulting stress–strain curves and compressive properties were analyzed. Correlations between peak stress, peak strain, modulus of elasticity and deterioration time of the concrete were developed. SEM microscopy tests were also used to investigate the evolution of the internal microstructure and the morphological characteristics of the erosion products under freeze–thaw conditions. The results indicated that the change in concrete mass with a growing number of F–T cycles can be divided into two periods: a gentle increase and a faster increase. A continuous increase in the rate of mass increase was observed for the specimens containing fly ash and bentonite, while the compressive strength of these specimens continued to decrease. During F–T damage, the maximum stress decreased slightly and the maximum strain increased gradually as the volume of bentonite and fly ash increased. This study provides a theoretical basis and technical basis for the development of fly ash and bentonite to improve the frost durability of offshore structures.

Assessment of impact resistance recovery in Ultra High-Performance Concrete through stimulated autogenous self-healing in various healing environments

Ultra High-Performance Concrete (UHPC) is widely acknowledged for its remarkable mechanical properties, owing to its compact microstructure. The response of UHPC to impact forces plays a vital role in ensuring the safety and longevity of structures, specifically in protective buildings, high-performance pavements and offshore concrete structures. In this context, this paper reports on an experimental investigation aimed at assessing the effects of stimulated autogenous self-healing of UHPC on the recovery of its performance under impact loadings. Drop weight tests were performed on UHPC slabs, with a 10 kg heavy impactor dropped from the height of 1 m on the centre of the specimens. Specimens were pre-cracked by repeated impacts up to 40% of their predetermined capacity. Pre-cracked specimens were exposed to different healing conditions, water submersion, 95% ± 5% RH, and wet/dry cycling (12/12 h) either in water or in a NaCl solution. Self-healing was evaluated through rebound height, elastic stiffness recovery, natural frequency, and laser displacement measurements. High-speed cameras and Digital Image Correlation were used to capture rebound height and crack formation. Performance was assessed at time 0, pre-damaging, 1, 2, and 4 months. After the healing period, all specimens were tested to failure. Specimens exhibited an increasing healing efficiency when moving from 95% ± 5% RH, over wet/dry cycling, to submerged conditions. Specimens healed continuously under submerged conditions exhibited a complete closure of surface cracks (50–150 μm) and an 80% recovery in natural frequency. Furthermore, they showed a more than 10% increase in stiffness and energy dissipation capacity after four months of healing.

Exploring capillary adsorption of corrosive medium in epoxy resin-modified calcium-silicate-hydrate channels

In coastal environment development and island construction, cement-based materials are crucial, but they are exposed to high temperatures, humidity, salt, and weathering in marine environments, leading to the deterioration of offshore concrete structures' durability. To mitigate these effects, epoxy resin, a polymer material with excellent adhesion with cement-based materials and chemical corrosion resistance, can slow down harmful ion erosion and improve concrete structure durability. This paper proposes a molecular dynamics simulation model to investigate the transport of pure water and NaCl solution in calcium-silicate-hydrate (C-S-H) gel pore channels with and without epoxy coating. The study demonstrates that epoxy resin can effectively slow down water molecule and ion transport in the C-S-H channels. When the solution passes through the C-S-H gel pore channel, the water molecules occupy the adsorption sites of the epoxy molecules, resulting in partial exfoliation of the epoxy resin. The exfoliated epoxy resin forms an enhanced hydrogen bonding network with the water molecules and forms ion pairs with the ions, which induces the ions and water molecules to gather around the epoxy molecules and form a larger cluster structure, thus slowing down the migration of ions and water molecules in the C-S-H pore channel.

Experimental investigation of shear-extension coupling effect in anisotropic reinforced concrete membrane elements

Performance based analysis under seismic loads using the finite element method for wall-type reinforced concrete (RC) members in buildings and in important structures like liquid retaining structures, nuclear containment structures, offshore concrete gravity structures etc., necessitates the understanding of the non-linear behaviour of the constituent membrane elements. The current orthotropic formulation of the softened membrane model (SMM) can be strictly used only when the reinforcement is symmetric to the principal axes of applied stresses. When the reinforcement is asymmetric, shear strain is generated due to the normal stresses in the principal axes of applied stresses, which is referred to as shear-extension coupling. An anisotropic formulation is required to capture the generated shear strain. The current study quantifies the shear strain due to asymmetry in reinforcement, by testing panels under biaxial tension-compression using a large-scale panel testing facility. A model for the shear strain is proposed based on the tests data. The paper presents the experimental programme, important test results and the modelling of shear strain. Expression developed for the shear strain can be incorporated in the solution algorithm of the SMM for improved prediction of the shear behaviour of a membrane element. This further aids in accurate prediction of the seismic performance of the important structures mentioned earlier.

Degradation of concrete in marine environment under coupled chloride and sulfate attack: A numerical and experimental study

The corrosion of offshore reinforced concrete structures under combined chloride and sulfate ions attack is a complex corrosion phenomenon. The purpose of this study is to develop numerical models in conjunction with experimental studies for long-term durability assessment of offshore concrete structures. A numerical model was developed to describe the simultaneously transport of chloride and sulfate ions considering the competitive binding by hardened cement pastes effect. The model was validated by a series of experimental studies. The validated model was implemented to predict the service life of the offshore RC structures under various chloride and sulfate solutions. The results show that chloride ions induced corrosion of steel bars in offshore RC structures is highly influenced by the concentration of sulfate ions. The sulfate ions induced concrete expansion and cracking from ettringite formation could potentially accelerate chloride ions induced corrosion of steel bars in concrete, ultimately the premature failure of the offshore RC structures.

Neural-Network-Based Ultrasonic Inspection of Offshore Coated Concrete Specimens

A thin layer of protective coating material is applied on the surface of offshore concrete structures to prevent its degradation, thereby extending the useful life of the structures. The main reasons for the reduction in the protective capability of coating layers are loss of adhesion to concrete and flattening of the coating layer wall. Usually, the state of the coating layer is monitored in the setting of water immersion using ultrasonic inspection methods, and the method of inspection still needs improvement in terms of speed and accuracy. In this study, the ultrasonic pulse echo method was used in a water immersion test of the coated specimens, and continuous wavelet transform (CWT) with complex Morlet wavelets was implemented to define the received waveforms’ time of flight and instantaneous center frequency. These allow one to evaluate the thickness of the coating layer during water immersion. Furthermore, phases of reflected echoes at CWT local peaks were computed using a combination of Hilbert transforms (HT) and wave parameters derived from CWT. In addition, three relative wave parameters of echoes were also used to train deep neural networks (DNN), including instantaneous center frequency ratio, CWT magnitude ratio, and phase difference. With the use of three relative waveform parameters of the DNN, the debonded layer detection accuracy of our method was 100%.

Experimental evaluation of the effect of cement type and seawater salinity on concrete offshore structures

Although seawater exposure is relatively common, there are relatively few studies that couple the microscopic characterization to the mechanical/ macroscopic behavior of cement-based materials, by using a reformulation of seawater with all its multiple salts in the laboratory. The experimental study includes the effect of salinity, cement type, and specimens geometry on the degradation phenomenon. The seawater solution was recomposed in the laboratory by dissolving salts in deionized water. A stable brucite layer and calcium carbonate were formed at the surface of the exposed specimen. Continuous leaching of calcium hydroxide and formation of Friedel’s salt was also observed. The mechanical properties of the attacked specimens remain equal or slightly lower than the control specimens even if significant changes could be observed in the microstructure. Due to seawater exposure, the hollow cylinders recorded the highest mass and volume gain thus the largest amount of products formed. These evolutions were accelerated with the increase of salinity contrariwise to expansion that was not affected by salinity increase and remained minor. Ordinary Portland cement recorded higher volume and weight gain than seawater resisting cement.

Chemo-mechanical coupling model of off-shore concrete structures

Seawater represents a complex phenomenon due to the presence of multiple ions. Previous researches aimed to estimate the resistance losses due to sulphate and chloride attacks based on probabilistic approaches. Few studies have focused on the complex chemo-mechanical modeling by coupling all the ions present in seawater and in concrete. In this study, the model proposed is based on the coupling between two codes CemPP- for the microstructure generation of the cementitious material, the evolution of the hydration and chemical reactions - and Cast3M - for the mechanical calculations and damage modeling. Results highlight the importance of considering each effect of cement paste’s ion reacting with seawater and their role on the overall mechanical behaviour of the structure affected by the chemical phases present at equilibrium.

Verification Test of Restoration Effect of Offshore Concrete Structure Using Permanent Formwork Panels and Underwater Inseparable Mortar

In recent years, the deterioration of harbor structures has become a matter for concern, and the establishment of effective repair methods has become urgent. In this study, a repair method for installing permanent formwork panels and filling them with underwater inseparable mortar is proposed as an effective section-restoring method for deteriorated concrete caisson sidewalls. To investigate the integrity between existing concrete and repaired parts as well as the effect of thickness increase due to panel installation, static bending tests of a flat-reinforced concrete (RC)-beam-modeled side wall used in caisson structures are conducted. The results obtained from this study are as follows: (1) by attaching the permanent formwork panels to the flat RC beam, the bending crack occurring load was improved by a factor of three, and the rebar yielding load and maximum load were doubled. This tendency was the same even when the RC beam had a cross-sectional defect. (2) The RC beam with the permanent formwork panels finally failed with diagonal cracks opening after the maximum load was reached. The ductility ratio at this time was approximately 6. To improve the deformation performance, it is necessary to apply shear reinforcement. (3) The analysis results by the fiber model assuming the complete adhesion of the permanent formwork panels, underwater inseparable mortar, and RC beam were performed, and the results were compared with the experimental results. As a result, it was clarified that the adhesion of each constituent material was retained until the yield of the main reinforcing bar. After that, pulling out of the main reinforcing bar from the concrete and damage to the permanent formwork panel became apparent. (4) The curvature of the cross-section at the yield of the main reinforcing bar was φyc = 0.007 m−1, and it was confirmed that the assumption of plane conservation was valid in the cross-section with a curvature less than that. This research is expected to promote the efficient maintenance of existing concrete caisson structures. As the amount of cement used and the construction period can be reduced, it is assumed that the environmental load can be reduced as well.

An integrated framework for modelling time-dependent corrosion propagation in offshore concrete structures

Reinforcement corrosion is an enduring durability issue essential to the designs and maintenances of diverse offshore concrete structures. The corrosion propagation can be triggered and governed by a variety of environmental conditions, which is a complex process comprising the stages from time-dependent concrete decay during the initiation phase to the active corrosion phase. Misinterpretations and/or oversimplifications, due to difficulties in numerical modelling, on either phase would greatly impair the robustness of the estimation on corrosion rate. In this study, a novel numerical framework, integrating the chemo-physical–mechanical and electrochemical modelling techniques, is freshly developed to effectively address the conundrum in predicting time-dependent corrosion propagation. The developed framework is actively validated against a variety of experimental problems, where both long-term microcell and macrocell corrosions, as well as corrosions triggered under various sources are considered. The numerical studies thoroughly demonstrate the potential versatility of the proposed method in modern offshore civil engineering.

Probabilistic calibration of fatigue safety factors for offshore wind turbine concrete structures

Current fatigue design rules for offshore concrete structures were adopted from the oil and gas industry. When better models or more information are available, partial safety factors can be re-calibrated according to target reliability levels for offshore wind turbines. This paper describes a framework for reliability-based calibration of fatigue partial safety factors for offshore wind turbine concrete structures. Offshore wind turbine loads accounting for the statistical distribution of turbulence intensity are estimated using a fully-integrated aeroelastic model. Based on available experimental fatigue tests, a fatigue reliability model for concrete is formulated and applied in two numerical examples. Results indicate that the recommended material partial safety factor in the DNV standard for Offshore Concrete Structures can be lowered without compromising structural safety. The proposed modification can potentially contribute to structural design optimization and further cost reduction in offshore wind energy.

Influence of aggregate surface treatment on corrosion resistance of cement composite under chloride attack

The high porosity of interfacial transition zone (ITZ) could significantly influence the durability of offshore concrete structures. This study aims to investigate the effects of aggregate surface by coating slag and silica fume on the ITZ microstructure, ultimately the corrosion resistance of the cement composite materials exposed to chloride attack. First, a series of experimental studies on chloride transport were carried out. Then, an experimentally validated model was developed to further understand the fundamental reactive-transport behaviour of chlorides in concrete in relation to different aggregate surface coating methods. The experimental results show that aggregate surface coating using silica fume can significantly reduce the porosity of interfacial transition zone (e.g. more than 40% reduction of porosity 5 μm from aggregate surface). Most importantly, the numerical predictions indicate that the slag and silica fume coating could potentially prolong the initiation time of the steel bar corrosion in offshore structures by more than 60% and 170%, respectively.

Effects of a 70-year old artificial offshore structure on oxygen concentration and macrobenthos in the Gulf of Gdańsk (Baltic Sea)

Artificial structures with dense benthic assemblages on their surfaces are likely to affect the adjacent soft bottom and the communities that live there. In this study, we investigated environmental and benthic parameters in the vicinity of a 70-year old offshore concrete structure in the Gulf of Gdańsk, in the southern part of the Baltic Sea. Samples were collected at three distances from the structure in autumn of 2015, 2016 and 2017. Each year, temperature and dissolved oxygen loggers were deployed 4–5 months prior to the collection of benthic samples. The obtained results revealed a significant impact of the structure on several parameters. The average oxygen concentration at 1 m distance was 0.6 mg L−1 lower than at 7 m and 50 m distance from the structure. We registered hypoxic events that lasted up to 4 days, with the oxygen concentration dropping to 0.57 mg L−1, and this is the first report on hypoxia in the coastal zone (depth < 8 m) of the Gulf of Gdańsk. Despite the decrease in the oxygen concentration, no adverse effects on the benthos were observed. The highest values of the taxa richness, abundance and biomass were generally recorded at sampling sites located close to the structure.

Damage constitutive model of concrete under repeated load after seawater freeze-thaw cycles

Offshore concrete structures in cold climates inevitably experience seawater freeze-thaw cycles, which increase surface scaling and internally damage the concrete, thereby altering the mechanical properties. The constitutive relation and compression damage to the concrete after seawater freeze-thaw cycles are absent from existing studies of structural nonlinearity. Repeated uniaxial compression tests on prism specimens were conducted to investigate the stress-strain and the damage variable of the concrete after 0, 25, 50, 75, 100, and 125 seawater freeze-thaw cycles. Using plastic damage theory and experimental results, we developed a constitutive model of concrete subjected to seawater freeze-thaw cycles, which was input to ABAQUS for numerical simulation. The results showed that the reloading stiffness degradation trend of compression was basically the same for different numbers of freeze-thaw cycles. Compression damage could be calculated by uniform equations after the peak point. The modeling results matched well with the experimental results, indicating that the constitutive model can provide a theoretical basis for nonlinear analysis or design of offshore bridges in cold climates.

Innovative Techniques/Technologies to Enhance the Durability of Civil Infra Structures PART 2: Corrosion Protection Techniques/ Technologies

Background: The durability of civil infrastructures is a severe problem faced by Scientists, Engineers and Technologies of 21st Century, due to the increasing pollution of our environments. This paper deals with innovative corrosion protection techniques/technologies to enhance the durability of civil infrastructures. Corrosion resistant rebars manufactured using Cu, P, Ni and Cr are capable of bearing the humid conditions of the coastal areas. TATA TISCON 500D rebars de-signed with diminished levels of impurities like S and P showed better corrosion resistance properties. Compared to CTD rebars, TMT rebars with tempered martensite zone showed improved corrosion re-sistance properties. Cement based coated steel rebars (inhibited cement slurry coated, cement polymer composite coated rebars) found to be more compatible for reinforcement corrosion control, in compari-son to galvinized and epoxy coated rebars. Fly ash, slag or silica fume, blended cements are finer as compared to OPC. The fineness reduced the permeability of blended cement concrete and improved the durability. Ternary blended cements, due to the fineness of the material and increased alumina, silica contents showed improved mechanical properties and better protection for steel in concrete. Owing to the unique molecular structure and high ion exchange capacity, chloride scavengers reduced the corro-sion susceptibility. Epoxy-polyurethane based surface coatings enhanced the durability as barrier type of coatings. Corrosion inhibiting admixtures such as calcium nitrite, citrate and stannate due to the for-mation of an insoluble film enhanced the corrosion resistance properties of reinforcements. Migrating corrosion inhibitors (MCI) based on amino alcohol, amines and nitrites showed superior corrosion resistance properties when compared to admixed inhibitor systems. Depending upon the environmental conditions, the implementation of corrosion protection technologies definitely enhanced the durability of civil infrastructures. Methods: The durability of the civil infrastructures such as National Highway Bridges, Nuclear Reactors, Elevated water tanks, On-shore and Off-shore concrete structures will be enhanced by adopting Innovative Corrosion protection Techniques/ Technologies such as i) Corrosion resistant rebars, ii)Coatings to steel rebars, iii) Blended cements, iv) Chloride scavengers for marine structures and v) Corrosion inhibiting admixtures for fresh structures. For existing structures, the following innovative technologies such as i). Coatings to concrete surface ii). Migrating corrosion inhibitors will be implemented. Results: After adopting the appropriate innovative corrosion protection techniques/technologies, the effect of treated structures will be monitored by conducting NDT techniques. Conclusion: Each and every innovative corrosion protection techniques/technologies have its own merits, demerits and limitations. Proper selection of method of corrosion protection strategies depending upon the environmental conditions can only lead to saving of money and enhancing the durability of civil infrastructures. In order to enhance the service life of civil infrastructures, it is essential to provide additional protective measures. Keywords: Protection strategies, corrosion resistant steel rebars, coatings to steel rebars, blended cements, admixtures, migrating corrosion inhibitors, chloride scavengers, repair/renovation strategy.

Capillary-Porous Effects in Concrete during Ice Abrasion

The ice drift velocities up to 2 m/s in the Sea of Okhotsk can cause severe abrasion due to mechanical contact with offshore concrete structures. The abrasion process of concrete includes cement paste and aggregate particles removed from the concrete surface. Testing procedures in the ice abrasion laboratory of Far Eastern Federal University coupled to a numerical simulation model of ice sheet-structure contact are described. The existing models account only for mechanical ice-concrete interaction, ignoring water flow into the concrete surface due to hydraulic pressure, substructure of ice feature on ice-structures interface. Theoretical and experimental research comprises the study of physical and mechanical as well as hydraulic porous processes resulting in the gradual structure refusal which is linked with emerging significant pulsating pressures in the contact layer of interacting work material. The obtained results will contribute to the development of effective protective means against ice abrasion.

Non-linear finite element analyses applicable for the design of large reinforced concrete structures

In order to make non-linear finite element analyses applicable during assessments of the ultimate load capacity or the structural reliability of large reinforced concrete structures, there is need for an efficient solution strategy with a low modelling uncertainty. A solution strategy comprises choices regarding force equilibrium, kinematic compatibility and constitutive relations. This contribution demonstrates four important steps in the process of developing a proper solution strategy: (1) definition, (2) verification by numerical experiments, (3) validation by benchmark analyses and (4) demonstration of applicability. A complete solution strategy is presented in detail, including a fully triaxial material model for concrete, which was adapted to facilitate its implementation in a standard finite-element software. Insignificant sensitivity to finite element discretisation, load step size, iteration method and convergence tolerance were found by numerical experiments. A low modelling uncertainty, denoted by the ratio of experimental to predicted capacity, was found by comparing the results from a range of experiments to results from non-linear finite element predictions. The applicability to large reinforced concrete structures is demonstrated by an analysis of an offshore concrete shell structure.

Understanding the effect of curing age on the chloride resistance of fly ash blended concrete by rapid chloride migration test

Chloride ingress is considered as the most responsibility to steel corrosion for offshore concrete structure during service. Fly ash has been shown to effectively enhance the resistance of concrete to chloride penetration. Despite the numerous contributions of investigation regarding the chloride migration behavior of fly ash blended concrete composites by using rapid chloride migration (RCM) test, there is still a lack of critical understanding of curing age effect on the chloride migration in fly ash concrete under RCM testing system. The present study aims to investigate the effect of curing age on the resistance of fly ash blended concrete to chloride ion under RCM test, also considering the factors of water-to-binder ratio and fly ash content at the same time.Based on the RCM tested results, it is found that the effect of curing age condition on the chloride migration resistance is depended on the amount of fly ash incorporated in concrete. Short term curing age can lead to increase the chloride migration coefficient of concrete with low fly ash content. But resistance to the chloride migration can still be enhanced at this curing stage when high percentage of fly ash is supplied. Long term curing age can benefit the chloride migration resistance, regardless of how much fly ash is used in concrete. However, this positive effect becomes less significant when increasing the curing time at the late curing stage. The concrete microstructure is also explored to elucidate the macro-behavior of chloride migration.

내부식성 콘크리트 개발을 위한 기초연구로서 개질유황 혼합 콘크리트의 물성 평가

Due to the increased construction of offshore concrete structures and the use of de-icing salts for the purpose of snow removal, the needs for the development of anti-corrosive concrete are increasing. To solve these problems, an evaluation of the mechanical and durability properties for concrete were conducted by mixing modified-sulfur as 0 %, 5 %, 10 %, 15 % cement weight ratio. Both strengths and the properties affecting durability such as water absorption coefficient, chloride ion permeability, accelerated carbonation resistance, rapid freezing and thawing, and chemical resistance were evaluated. All evaluations performed were according to the test specifications associated KS. The results indicate that mixing of modified-sulfur lowed chloride ion permeability and improved chemical resistance.

Effect of Area Ratio of Coarse Aggregate on Erosion Resistance of Offshore Concrete Structure

This study investigates the effects of the compressive strength and area ratio of coarse aggregate of concrete for offshore construction on the erosion depth. To evaluate the erosion resistance of concrete, test was performed according to ASTM C 1138 using a plain mix with design strength of 30 MPa using only Portland cement and five other mixes including different quantities of fly ash and blast furnace slag. The test results showed that the erosion depth is influenced not only by the compressive strength but also by the area ratio of coarse aggregate.