Seawater Sea-sand Concrete Research Articles

Surface modification of fibres with silane for enhancing the durability of FRP composites as reinforcement for seawater sea sand concrete (SWSSC): A review

This review discusses the necessity of surface modification to effectively use fibre-reinforced polymers (FRPs) as reinforcements for seawater and sea sand concrete (SWSSC). FRPs can be a viable reinforcement material for SWSSC as an alternative to the traditional carbon steel reinforcement that will suffer unacceptably high rates of corrosion due to the very high chloride content of the SWSSC environment. FRPs exhibit mechanical properties similar to steels but suffer insignificant degradation/corrosion due to chloride. However, the fibres in certain FRPs (such as basalt FRPs and glass FRPs) suffer degradation due to attack by the alkaline environment of concrete, causing degradation in fibre-matrix interfacial strength and loss of FRPs’s overall mechanical strength and durability. Therefore, fibres used in such FRPs require suitable surface modification to ameliorate their degradation, enabling their practical use as reinforcements of SWSSC. While carbon fibre-reinforced polymers (CFRPs) demonstrate superior resistance to alkali-induced degradation, their widespread use in large-scale civil engineering is hindered by high costs. In this review, more economic alternatives, i.e., basalt fibre-reinforced polymers (BFRPs) and glass fibre-reinforced polymers (GFRPs) are critically reviewed. To enhance the durability of FRPs and mitigate alkali-induced degradation of the basalt/glass fibres, suitable surface modifications are necessary to safeguard the mechanical properties of FRPs. Underpinned by a comprehensive discussion on the necessity of surface modification of fibres for the use of FRPs in SWSSC environment, this review focuses on the potential of advanced approaches for surface modifications of the fibres, i.e., coatings of silanes, and silanes impregnated with graphene nanoplatelets (GNPs), graphene oxide (GO) and its functionalised derivatives or metal oxides (such as SiO2, ZrO2, Al2O3, TiO2 and CeO2). This review critically discusses the opportunities for suitable surface modification of basalt and glass fibres for enabling the durable use of FRPs as reinforcements for sweater sea sand concrete.

Enhancing compressive behaviour of seawater sea sand concrete filled hybrid carbon-glass FRP tubes exposed to seawater: Effect of thickness

This study presents a novel investigation into the impact of tube thickness on the residual compressive strength of sea water sea sand concrete (SWSSC) filled filament wound hybrid fibre-reinforced polymer (HFRP) tubes, positioning them as innovative alternatives to traditional concrete-filled steel tubes in corrosive marine environments. The research explores tube thicknesses of 2 mm, 3 mm, and 4 mm, employing a unique hybrid configuration of 50 % carbon fibre reinforced polymer (CFRP) internal layers and 50 % glass fibre reinforced polymer (GFRP) external layers. This study examines the effect of these configurations on compressive mechanical properties, microstructural characteristics, and damage progression under alkaline and seawater conditions. Additionally, the research evaluates cross-ply and hoop orientations as variables in filament winding fibre orientation. A comprehensive set of 108 hybrid tubes were filled with an alkaline solution to simulate the SWSSC environment and exposed to seawater at varying temperatures and durations. Accelerated aging experiments were conducted in the laboratory to assess the long-term compressive performance of SWSSC-filled HFRP tubes exposed to seawater. Long-term compressive strength predictions were made using the Arrhenius theory, based on short-term accelerated aging data. The accelerated test data revealed maximum compressive strength reductions of 33 %, 15 %, and 17 % for 2 mm, 3 mm, and 4 mm cross-ply HFRP tubes, respectively, after 150 days of exposure at 60 °C. For hoop HFRP tubes, the corresponding reductions were 19 %, 18 %, and 17 %. Thicker tubes (3 mm and 4 mm) with sufficient internal CFRP layers protect external GFRP layers from alkaline damage, akin to pure CFRP tubes. Thinner tubes (2 mm) exhibit similar performance to GFRP tubes as alkaline damage spreads from their less substantial internal CFRP layers to the outer GFRP layers. For cross-ply tubes, the recommended strength reduction factors are 0.6 for 2 mm, 0.8 for 3 mm, and 0.8 for 4 mm thicknesses. For hoop tubes, the suggested factors are 0.7 for 2 mm, 0.7 for 3 mm, and 0.8 for 4 mm thicknesses. This study shows the innovative potential of tailored hybrid HFRP tubes in enhancing the durability and performance of marine infrastructure.

Early Corrosion Behavior of Cr10Mo1 Alloy Corrosion-Resistant Steel Bars in Seawater–Sea-Sand Concrete

Early Corrosion Behavior of Cr10Mo1 Alloy Corrosion-Resistant Steel Bars in Seawater–Sea-Sand Concrete

Strength and ductility of concrete columns reinforced with hybrid steel-fiber reinforced polymer composite bars

Using hybrid reinforcement scheme, comprising steel-FRP composite bar (SFCB) and FRP tie, holds great promise in developing seawater sea-sand concrete (SSC) columns with high strength and ductility for marine infrastructures. However, the impacts of FRP tie configurations on hybrid-reinforced SSC columns have not been thoroughly investigated, which hinders their application. To fill this research gap, this study conducted axial compression tests on hybrid-reinforced SSC columns to assess the impacts of tie type, tie size, tie spacing, and tie size/spacing configuration. The failure mechanism of columns, compressive contribution of SFCBs, and confinement mechanism of FRP ties were revealed. The results show that within the tie spacing limit, the failure mode, elastic modulus, yield strength, and ultimate strength of SFCBs are similar. Longitudinal SFCBs effectively resist compression until concrete crushing, providing considerable compressive contributions. However, the confinement efficiency of pultruded FRP ties is limited by the slip of lap splice and the inferior bent portions. In contrast, the closed-type FRP ties exhibit significantly higher confinement efficiency, which increases the load capacity and ductility of columns by 13.6% and 82.8%, respectively. The tie spacing limit was determined and a prediction model of load capacity was proposed based on theoretical analysis and experimental validation.

Pullout behavior of recycled macro fibers embedded in ultra-high performance seawater sea-sand concrete

Recently, an eco-friendly ultra-high performance concrete (UHPC) has been developed using seawater, sea-sand (UHPSSC) and the recycled macro fiber (REF) processed from waste wind turbines, which could be easily available in remote islands and address the corrosion issue of concrete structures with steel reinforcements. The interface bond of the UHPSSC/macro fiber are lack of investigation and a significant factor influencing the tensile characteristics of macro fiber reinforced UHPSSC. In the present study, the pullout test of the single macro fiber was conducted to investigate the average bond behavior of such interface. The test variables included the macro fiber width (4, 6, and 8 mm), the embedded length (5, 10, and 15 mm), and the type of matrix. Then, the local bond-slip relationship at the fiber/UHPSSC interface was obtained by finite element analysis (FEA) to provide a more precise display of interface bond mechanisms. The results revealed that the bond of UHPSSC/macro fiber was stronger with increased fiber embedded length and width due to the greater frictional interactions and better overall integrity. Additionally, the FEA method proposed here to obtain the local bond-slip relationship was reliable and effective. This study can serve as a basis for further research on the tensile properties of UHPSSC reinforced with macro fibers.

Interface mechanical properties of CFRP-seawater sea sand concrete under simulated seawater immersion

In this study, the interfacial mechanical properties of carbon fiber-reinforced polymer (CFRP) sheet reinforced seawater sea sand concrete (SSSC) in simulated seawater immersion environment were studied. The results present that the interface bearing capacity of CFRP-SSSC decreases significantly after seawater immersion. The decreasing rate increases greatly with the extending of immersion periods. Seawater immersion also affects the failure mode of interface between CFRP and SSSC, which leads to the damaged surface expanding from the base layer of concrete to the junction section of resin and aggregate. The distribution of strain and stress along the bonding direction presents the load transfer from the loading end to the free end. The shear stress peak locates near the loading end and decreased about 15–20 % due to seawater immersion as a result of the deterioration of the interfacial property. The analysis on the microstructure of CFRP-SSSC interface proves that the chemical ions in seawater causes the hydrolyzation of epoxy resin adhesive layer. The CFRP-SSSC interface presents a lower interfacial bonding properties than CFRP reinforced normal concrete (NC). The improvement of strength grade of concrete from C30 to C50 increases the bonding properties of the CFRP about 9–12 % and mitigates the influence of seawater immersion. The obtained bond-slip curve indicates that the seawater immersion decreases the interface ductility. A modified Popovics model is applied to predict the bond-slip curve and presents a high fitting degree.

Modifying effects and mechanisms of superfine stainless wires on microstructures and mechanical properties of ultra-high performance seawater sea-sand concrete

Modifying effects and mechanisms of superfine stainless wires on microstructures and mechanical properties of ultra-high performance seawater sea-sand concrete

Novel insights on the setting process, hardened properties, and durability of sustainable ultra-high-performance seawater sea sand concrete

Using seawater and sea sand to prepare concrete could effectively alleviate the shortage of freshwater and river sand resources and promote the sustainable development of concrete. However, the current research on the setting process, hardened properties, and sustainability of ultra-high-performance seawater sea sand concrete (UHPSSC) remain considerably limited. In this paper, the UHPSSC was designed to mitigate the autogenous shrinkage and enhance the sustainability, and the effect of seawater, untreated and desalinated sea sand on the setting process, mechanical properties, durability, and sustainability was explored by various tests. Results show that the abundant ions (e.g., Cl- and SO42-) in seawater and untreated sea sand promote the dissolution and nucleation of cement, accelerating the hydration process, and seawater provides a high pH hydration environment, which enhances the pozzolanic reaction activity of silica fume and fly ash. This leads to the enhanced flexural and compressive strengths at early age, reduced flowability, and increased autogenous shrinkage. Although the seawater and untreated sea sand considerably alleviate the natural raw materials consumption, the mechanical performance at 28 d and durability are reduced, thereby decreasing the sustainability index by 7.8 %. The desalination treatment of sea sand decelerates the hydration process of UHPSSC with untreated sea sand due to the lower ion content in desalinated sea sand. The application of desalinated sea sand mitigates the adverse impact of untreated sea sand on the workability, mechanical properties, and durability. However, this desalination process requires more energy and freshwater consumption, thereby decreasing sustainability index. The fly ash incorporation could improve the workability of UHPSSC, and decrease the autogenous shrinkage value by 17.9 %. Moreover, it mitigates the environmental effect without compromising the mechanical properties and durability, leading to an impressive 42.8 % improvement in sustainability. Durability assessment reveals that the prepared UHPSSC specimen maintains outstanding mechanical properties even after 360 d of exposure to severe marine environment, indicating its excellent durability. This paper provides novel insights for achieving a cleaner and more sustainable concrete production.

Practicability and fundamental performance of alkali treated raw bamboo fiber reinforced high performance seawater sea sand concrete

In pursuit of sustainable development, there has been an increasing amount of research on plant fiber reinforced concrete and seawater sea sand concrete in recent years. In this study, a new type of alkali treated raw bamboo fiber reinforced high performance seawater sea sand concrete (ABFRHPSSC) was suggested, with its practicability evaluated and its fundamental performance investigated. The practicability was evaluated through fiber tensile test, water absorption test, single fiber pull-out test, and degraded fiber characterization test. The fundamental performance, including flowability, mechanical performance, pore volume distribution and three-dimensional distribution of pores and raw bamboo fibers, were then investigated when fiber content and length were used as two parameters. The results showed that the alkali treatment of raw bamboo fibers could increase their tensile strength and elastic modulus, reduce their water absorption, improve their bond performance, and ensure their degradation stability. The fiber content and length had significant effects on the fundamental performance. A fiber content of 0.5 % and a fiber length of 12 mm resulted in relatively optimal mechanical performance while ensuring satisfactory flowability and uniform fiber distribution. At this point, compared with plain high performance seawater sea sand concrete (HPSSC), the compressive, splitting tensile, and flexural strengths increased by 20.1 %, 33.2 %, and 33.6 %, respectively, and the pores were refined. Therefore, ABFRHPSSC has a promising future as a new environmentally friendly material.

Bond performance between sand coated hybrid glass-carbon FRP tubes and seawater sea sand concrete after exposure to elevated temperatures

Employing hybrid glass-carbon FRP tubes in concrete-filled FRP tubes for marine applications offers a balance between durability and cost-effectiveness, making them ideal for harsh, corrosion-prone environments. The investigation aimed to explore the bond-slip characteristics of sand-coated hybrid carbon-glass FRP tubes filled with seawater sea sand concrete (SWSSC) subjected to varying high temperatures. Elevated temperatures can affect the bond by causing thermal expansion and altering material properties, while sand coating enhances the bond by increasing surface roughness and improving mechanical interlocking. Utilising a push-out test, the study addresses this crucial but insufficiently studied aspect of the interface between SWSSC and FRP tubes. Thirty-three sand coated hybrid tubes were tested experimentally, with temperature being the sole variable. The samples underwent testing at eleven different temperature levels (ranging from 25°C to 250°C) to investigate the relationship between temperature fluctuations and bond-slip behaviour. The results highlighted three key factors influencing bond performance. Two factors had a positive influence: firstly, the expansion of the tube due to increased temperatures, promoting better interlocking among the concrete, sand coat, and tube; secondly, resin’s mechanical properties as a result of post-curing. Conversely, a negative impact was observed due to sand coating detachment and tube degradation resulting from elevated temperatures. Analysis of the findings revealed a pattern in bond strength retention: a decrease to 77 % renetion up to 80°C, followed by an increase to 108 % up to 220°C, and finally a decrease again to 83 % at 250°C. The findings from this study enhance the progress and utilisation of hybrid FRP tubes together with SWSSC in producing environmentally sustainable construction materials, all the while ensuring they meet the necessary structural standards.

Experimental investigations on the flexural behavior of compression-cast seawater sea-sand concrete beams reinforced with CFRP bars

To address the corrosion problems, environmental concerns, and achieve green and sustainable civil engineering practices, this study designed several compression-cast seawater sea-sand concrete (SSC) beams reinforced with corrosion-resistant carbon fiber-reinforced polymer (CFRP) bars (i.e., FRP-SSC beams) and their flexural behavior was experimentally investigated through four-point bending tests. Twelve FRP-SSC beams were designed, including three normal-cast and nine compression-cast ones. The critical parameters included the concrete casting forms, water-to-cement (w/c) ratios, and reinforcement ratios of CFRP bars (ρf). Effects of the concrete casting methods, w/c ratios, and ρf on the flexural responses of FRP-SSC beams were comprehensively studied. In addition, the flexural theorems were used to derive the prediction models of the equivalent stress block parameters (i.e., εcm, α1, and β1) and flexural strength (Mu) of compression-cast FRP-SSC beams. Moreover, based on the experimental results, effectiveness and accuracy of the proposed flexural strength design models were verified by using the existing design specifications. Finally, the results indicate that, (1) compression-cast FRP-SSC beams with the larger ρf (1.78 %) exhibited bending failure featured by compression concrete crushing, whereas that with the smaller ρf (0.75 %) shown splitting failure; (2) The cracking moments of compression-cast FRP-SSC beams increased significantly owing to the higher compressive strength and elastic modulus of compression-cast concrete (CCC); (3) Because of the pronounced brittleness of CCC, the improvement of the ultimate flexural capacity of compression-cast FRP-SSC beams was limited; (4) Under the same moment level, the maximum crack widths of compression-cast FRP-SSC beams were smaller than their normal-cast counterparts; and (5) the proposed flexural strength design models could yield more accurate, safer, and conservative predictions of flexural strength for both normal- and compression-cast FRP-SSC beams.

Multi-technique analysis of seawater impact on the performance of calcium sulphoaluminate cement mortar

The construction of offshore islands and reefs requires the development of new seawater sea-sand concrete materials. This study investigated the impact of seawater on the performance of calcium sulphoaluminate (CSA) cement mortar, with mechanism analyses by the acoustic emission (AE) and digital image correlation (DIC). For all uniaxial compression tests, cumulated AE, average AE frequency, rise angle, b-value, and βt coefficient were considered to perform the failure analysis. In addition, other techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) were also employed to highlight the effects of seawater on the hydration products and microstructure of the CSA cement. The results show that seawater not only promotes the formation and growth of ettringite (AFt) but also interacts with cement hydration products to produce Friedel’s salt and various gel formations. Compared to freshwater mixing (0 %), using seawater mixing at normal concentrations (3.3 %) enhances the compactness of the CSA cement, thereby improving the strength and ductility of the CSA mortar. Specifically, the peak load increases from 77 kN to about 86 kN when mixed with seawater at normal concentration, whereas higher seawater concentrations (6.6 %, 9.9 %) resulted in a slight decrease in strength of the CSA mortar, with the peak load reaching approximately 80 kN, yet the ductility is enhanced, as evidenced by the softening post-peak branch of the loading curve. Additionally, AE bursts have a strong correlation with the fracture of the CSA mortar, which can also be forecast by the AE time-scaling coefficient βt. Therefore, when constructing oceanic island and reef foundations, this type of seawater sea-sand concrete is preferred due to its improved ductility and strength, which enhances the structural integrity of marine constructions and provides early warning for effective marine disaster prediction.

Durability of SFCB reinforced low-alkalinity seawater sea sand concrete beams in marine environment

Seawater sea-sand concrete (SWSSC) provides an alternative to normal concrete, while Steel-FRP Composite Bars (SFCB) offer corrosion resistance in offshore and marine structures. Despite their benefits, the long-term performance of SFCB-reinforced SWSSC structures remains under-explored. This study evaluated the durability of such beams, particularly focusing on the effects of using Portland cement versus low-alkalinity calcium sulphoaluminate (CSA) cement. The investigation involved tensile and pull-out tests conducted on SFCB, and flexural tests conducted on the beam components after accelerated marine environmental exposure over periods ranging from one to six months. Results highlighted that SFCB embedded in low-alkalinity CSA cement exhibited improved retention of tensile strength, elastic modulus, and bonding strength by 32.0 %, 39.1 %, and 56.7 % respectively, compared to that embedded in normal Portland cement concrete. Microstructural analyses showed much less hydrolysis of the resin matrix and shallower corrosion of the basalt FRP (BFRP) cover on the SFCB. Furthermore, flexural tests demonstrated superior cooperative performance between low-alkalinity CSA concrete with SFCB in beam components, showing only marginal degradation in crack density, crack strength, and yielding points. The ultimate failure load of beams made with CSA concrete was 1.89 times higher than those made with normal SWSSC after 6 months of exposure. These results underscore the potential of low-alkalinity concrete in improving the long-term durability and structural performance of SFCB-reinforced marine concrete infrastructures.

Flexural performance of seawater sea sand concrete composite beams enhanced by dual-functional C-FRCM jackets

The flexural performance of seawater sea sand concrete (SSC) composite beams enhanced by the dual-functional carbon-fabric reinforced cementitious matrix (C-FRCM) jackets was experimentally investigated. Five SSC composite beams were constructed and tested in flexure under four-point loading, including one control specimen, one C-FRCM enhanced beam without the presence of the impressed current cathodic protection (ICCP) system, and three C-FRCM jacketed beams implementing the ICCP system (three different current densities: 25, 75 and 125 mA/m2). The failure modes observed and the performance of the strengthened beams with the dual-functional C-FRCM jackets were discussed in details. The results indicated that the C-FRCM jacketing effectively increased the cracking load up to 46.43 % and delayed yielding. The C-FRCM jacket had a relatively small impact on the initial stiffness of the specimens but significantly increased the load-carrying capacity (up to 18 %) and improved ductility. With the increase in current density, the load-carrying capacity gradually decreased, the crack inhibitory effect gradually weakened, but the ductility coefficient increased notably (up to 98 %). Regarding the failure modes of the composite beams, both fiber fracture and fiber slippage coexisted, and with the increase of current density, the latter became more predominant. Based on the design guidelines provided by ACI 549.4R-13, an analytical formula for bending moment was proposed and compared with the experimental results, providing a reference for the design of such components.

Study on the Effect of Fly Ash on Mechanical Properties and Seawater Freeze–Thaw Resistance of Seawater Sea Sand Concrete

When using seawater and sea sand as mixes, the mechanical properties and durability of concrete are adversely affected because the raw materials themselves contain harmful ions. Fly ash is the tailings formed in the process of industrial production, the use of which does not require the burning of clinker, reducing CO2 emissions. Moreover, it belongs to a new type of cementitious materials with low emissions and high environmental protection. Fly ash enhances the properties of concrete and reduces the effect of harmful ions on concrete. Based on the above considerations, the corresponding specimens were prepared and subjected to cubic compressive strength, flexural strength, and seawater freezing and thawing resistance tests by using fly ash admixture as the main variable. A combination of macro-analysis and micro-analysis was used to investigate the effect of fly ash on the performance of seawater sea sand concrete. The results showed that fly ash significantly enhanced the mechanical properties and resistance to seawater freezing and thawing of seawater sea sand concrete. The best improvement in compressive strength and resistance to seawater freezing and thawing was achieved at a substitution rate of 20%. The maximum increase in compressive strength was 13.22%. The maximum reduction in mass loss rate was 57.26% and the strength loss rate was 43.14% after the specimens were subjected to seawater freezing and thawing 75 times. The maximum enhancement in flexural strength was 17.06% for a substitution rate of 10%. Through microanalysis, it can be seen that the incorporation of coal ash can enhance the compactness of concrete through the microaggregate effect as well as the volcanic ash reaction to promote the secondary hydration reaction, so as to strengthen the seawater freeze–thaw resistance of seawater sea sand concrete. Finally, the damage prediction model established using the mean GM (1, 1) model of gray system theory meets the requirements of the first level of prediction accuracy and can accurately predict the damage of seawater sea sand concrete under seawater freezing and thawing.

Shear behavior of seawater sea-sand concrete beams reinforced with BFRP bars after seawater dry-wet cycling

The shear behavior of 27 basalt fiber-reinforced polymer reinforced seawater sea-sand concrete (BFRP-SSC) beams after seawater dry-wet cycling was comprehensively investigated. Parameters considered included stirrup diameter and spacing, shear span-to-depth ratio, stirrup surface treatment, cycling duration, concrete cover thickness, and seawater temperature. Results indicate that seawater dry-wet cycling increases cracking load and decreases bending stiffness in BFRP-SSC beams. Increasing shear span-to-depth ratio from 1.55 to 2.35 led to load reductions of 31.3 % and 49.7 % at 6.5 mm mid-span deflection. Seawater dry-wet cycling, particularly temperature and duration, significantly impacts the mechanical properties of stirrups. This results in diminished shear contribution and reduced efficacy in inhibiting diagonal crack propagation, leading to a shift in beam failure mode from diagonal compression failure to shear compression failure and diagonal tension failure. Increasing cycling duration from 60 to 120 days decreased tensile strength retention by 19.1 % and 33.3 % respectively. Based on experimental data, Arrhenius theory, and the time shift factor (TSF) method, a predictive model was developed for the shear capacity retention of beams subjected to seawater dry-wet cycling. Results indicate that after 100 years in a tidal environment (70 % and 90 % relative humidity), shear capacity damage factors of 0.77 and 0.52, respectively, can be applied to account for the impact of seawater dry-wet cycling on the shear capacity of BFRP-SSC beams.

Experimental Study of the Flexural Performance of GFRP-Reinforced Seawater Sea Sand Concrete Beams with Built-In GFRP Tubes.

The use of seawater sea sand concrete (SSSC) and fiber-reinforced polymer (FRP) has broad application prospect in island and coastal areas. However, the elastic modulus of FRP reinforcement is obviously lower than that of ordinary steel reinforcement, and the properties of SSSC are different from that of ordinary concrete, which results in a limit in the bearing capacity and stiffness of structures. In order to improve the flexural performance of FRP-reinforced SSSC beams, a novel SSSC beam with built-in glass FRP (GFRP) tubes was proposed in this study. Referring to many large-scale beam experiments, one specimen was used for one situation to illustrate the study considering costs and feasibility. Firstly, flexural performance tests of SSSC beams with GFRP tubes were conducted. Then, the effects of the GFRP tubes' height, the strength grades of concrete inside and outside the GFRP tubes, and the GFRP reinforcement ratio on the flexural behaviors of the beams were investigated. In addition, the concept of capacity reserve was proposed to assess the ductility of the beams, and the interaction between the concrete outside the GFRP tube, the GFRP tube and concrete inside the tube was discussed. Finally, the formulas for the normal section bearing capacity of beams with built-in GFRP tubes were derived and verified. Compared to the beam without GFRP tubes, under the same conditions, the ultimate bearing capacities of the SSSC beam with 80 mm, 100, and 200 mm height GFRP tubes were increased by 17.67 kN, 24.52 kN, and 144.42 kN, respectively.

Corrosion Resistance of Cr10Mo1 Alloy Corrosion-Resistant Steel in Seawater–Sea Sand Concrete Pore Solution

Corrosion Resistance of Cr10Mo1 Alloy Corrosion-Resistant Steel in Seawater–Sea Sand Concrete Pore Solution

Axial compressive behavior of low-alkalinity seawater sea-sand concrete column reinforced with hydrophobic longitudinal SFCBs and FRP hoops

To address corrosion problem of steel bars and shortage of freshwater and river sand in marine and offshore structures, a novel hydrophobic-modified longitudinal steel-FRP composite bars (SFCB) and FRP hoops reinforced low-alkalinity seawater sea-sand concrete (SWSSC) is developed. However, axial compressive behavior of hybrid SFCB and FRP reinforced columns is not clear and the applicability of existing design methods is low. Here, nine hybrid reinforced columns have been designed and tested. Typical failure modes of columns and axial compressive behavior of constituent materials (i.e., hydrophobic reinforcements and low-alkalinity SWSSC) are investigated. It is found that the synergistic confining effect formed by hybrid reinforcement are significant. The mesh confinement formed by densely distributed FRP hoops and longitudinal SFCBs improves the local buckling behavior, and effectively constrains the lateral expansion of concrete core, resulting in improved compressive strength and ductility of column. The compressive contribution of single longitudinal SFCB to ultimate load capacity of column can be enhanced effectively, up to 36.3 %, by increasing steel content of SFCB. Based on this, a prediction model of hybrid reinforced columns is proposed, which can accurately evaluate the ultimate load capacity. This work provides valuable experimental observations and load capacity model for hybrid reinforced column, which promote its design and application in marine engineering.

Influence of the multiple element ions contained in seawater and sea sand on the freeze-thaw resistance of cementitious materials

Preparation of concrete with seawater and sea sand can increase the pore solution salinity due to the introduced multiple ions, which can affect the freeze-thaw resistance of concrete by changing the crystallization pressure and freezing temperature. This study aims to investigate the influence of these multiple element ions in seawater and sea sand on the freeze-thaw damage characteristics of cementitious materials. The seawater sea-sand mortar (SSM) samples and reference specimens (Freshwater and river sand-FRM) were first prepared with same mix design. The accelerated freeze-thaw tests were then conducted in an environment chamber under the temperature range of [-20 °C, 7 °C]. The surface spalling, mass loss, and mechanical performance (Compressive strength, stiffness and fracture resistance) evolution during freeze-thaw cycles were characterized to examine the freeze-thaw resistance. The influence mechanism is analyzed through characterization of pore solution and structure, and Discrete Element Method (DEM) simulation. It is indicated that the introduced multi-element ions from seawater and sea sand accelerate the freeze-thaw deterioration of cementitious materials. After 150 freeze-thaw cycles, the mass loss rates of SSM and FRM samples reach above 10 % and 8 %, respectively. And the corresponding reduction rates for dynamic modulus are 51 % and 36 % for the SSM and FRM specimens. The deterioration in compressive strength is slightly higher than the loss of fracture resistance. The mechanism analysis indicated that deteriorated freeze-thaw resistance due to the introduced multi-element ions from seawater and sea sand mainly originated from the degradation of pore structure and the increase in freezing pressure. This study can serve as a solid base for the durability design and application of seawater sea-sand concrete in cold regions.