At present, the application of sea sand is still limited by its own characteristics. Sea sand contains chloride ions that can corrode steel. It cannot be directly used to replace river sand to configure concrete in traditional reinforced concrete structures. Therefore, from the perspective of material purification and protection, the engineering community first proposed anti-rust measures such as sea sand desalination, adding rust inhibitors, and steel reinforcement protection. In my country, the sea sand desalination method is the main method, and the sea sand desalination measures need to increase investment. Due to "interest-driven", many desalinated sea sand of unqualified quality have flowed into the building materials market, and a large number of quality sea sand has appeared in coastal areas due to abuse or misuse. Unqualified housing construction, known as "sea sand house", has caused huge economic losses to the country and the people.

In order to solve the problem of rust, the material replacement method is considered to be the most effective technical way. Replacing steel and ordinary concrete with Fiber Reinforced Polymer (FRP) combined with sea sand concrete can not only avoid the occurrence of rust, but also directly use sea sand and sea water without desalination to save fresh water resources; FRP is lightweight. It has high quality and good designability, and can be combined with sea sand concrete to form a series of structural forms; and FRP has strong corrosion resistance. Existing research has shown that the chloride environment has no significant effect on its performance, which can ensure long-term durability of the structure. . When the combination of sea sand concrete and FRP is applied to the construction of oceans and islands and reefs, sea sand and sea water can be directly obtained from the sea, making use of vast marine resources, greatly reducing the transportation volume of raw materials, taking them from the sea, using them in the sea, and having other materials incomparable advantages. Therefore, a new type of marine-oriented FRP sea sand concrete composite structure is proposed, which provides a new technical solution for my country's marine engineering construction, and also provides a new idea for the application of sea sand concrete in the engineering field.

1 Application of reinforced sea sand concrete

The sea sand resource reserves are large, and its application in the field of engineering construction has been widely concerned. For this reason, scholars at home and abroad have first carried out research on the properties of sea sand. The results obtained show that the firmness and gradation of sea sand are both The properties of sea sand concrete have also been extensively studied, including compressive strength, axial compressive strength, flexural strength, split tensile strength, elastic modulus and compressive stress-strain curve, etc. Compared with river sand concrete under the same conditions, it is found that the mechanical properties of the two are basically the same, and the mechanical properties of the two do not need special consideration in the structural design. The standard value and design value of each performance index, "Concrete Strength Inspection and Evaluation Standard" GB/T 50107 The provisions on the mechanical properties of concrete are also applicable to sea sand concrete.

The problem that restricts the application of sea sand concrete in the engineering field is that sea sand contains a lot of chloride ions, which will lead to rapid corrosion of steel. When sea sand is directly used in reinforced concrete structures, the presence of chloride ions will reduce the pH value of concrete, thereby destroying the passivation film on the surface of steel bars, forming galvanic cells to corrode steel bars, causing concrete cracks, and finally leading to structural damage due to durability issues. To this end, engineers and researchers have put forward many solutions, mainly to deal with sea sand, concrete or steel bars, which can be divided into three categories according to the objects to be dealt with.

(1) Sea sand desalination method

The sea sand desalination method is to reduce the chloride ion content in sea sand and screen out harmful substances through purification measures. There are three main treatment processes: natural placement method, fresh water flushing method and mechanical method. The natural placement method is to accumulate sea sand for more than a few months, reduce its chloride ion content by rainwater erosion, and use it after passing the sampling test. The fresh water flushing method uses fresh water to flush the sea sand. This method has a better chlorine reduction effect and requires less investment in equipment. The mechanical method is to use machinery to sort and wash the sea sand. This method has high efficiency and requires a short time, but requires a complete set of equipment for classification, centrifugation, water supply and drainage, and requires a large investment and a large amount of fresh water.

(2) Add rust inhibitor method

The method of adding rust inhibitor is to treat concrete. When mixing concrete, a certain amount of rust inhibitor and admixture is added to inhibit the corrosion effect of chloride ions on steel bars and improve the reinforcing properties of sea sand concrete. The method is simple and reliable, and can be directly mixed with seawater and sea sand to make concrete. It has been widely used in Japan, and the practical application effect is good, but the quality requirements of the rust inhibitor are relatively high.

(3) Reinforcement treatment and protection method

The steel bar treatment and protection method is to add a coating on the surface of the steel bar to avoid the corrosion of the steel bar by chloride ions, including metal coatings such as galvanized and chrome plating and non-metallic coatings such as epoxy resin. The thickness and quality of the coating should be guaranteed when using. At the same time, it is also required that the coating does not affect the bonding performance between the steel bar and the concrete, the process requirements are high, and the actual rust prevention effect is not significant. Another idea is to improve the density and corrosion resistance of surface steel through steel metallurgy technology; however, this method is costly and difficult, and has not yet been well developed.

At present, reinforced sea sand concrete structures have been applied to a certain extent in my country, mainly using the sea sand desalination method. However, due to the increased cost of sea sand desalination, my country's sea sand purification process and market supervision mechanism are not complete. The quality of desalinated sea sand is unstable, and many sea sand concrete buildings still have serious steel corrosion problems. The phenomenon of "sea sand houses" in coastal areas is more common, so the use of sea sand as construction sand has been greatly questioned, hindering the Wide range of applications for desalination of sea sand.

In response to this situation, my country has issued many relevant regulations in recent years to strictly control the chloride ion content in sea sand and concrete mixtures. "Sand should be purified" is a mandatory provision and must be strictly implemented; in addition, the specification also increases the requirements for chloride ion content, which not only strictly limits the content of chloride ions in various raw materials, but also imposes restrictions on the total amount of chloride ions in concrete mixtures. The water-soluble chloride ion content of the proposed clear limit. These strict control methods combined with the perfect sea sand market supervision mechanism can ensure the quality of reinforced sea sand concrete buildings. However, sea sand desalination needs to consume a lot of fresh water resources, and in areas where river sand is scarce and sea sand is urgently needed, fresh water resources are generally relatively tight, and the application of sea sand desalination is still limited.

2 Combined application of sea sand concrete and FRP

As a new type of high-performance structural material, FRP has strong designability. It has been combined with ordinary concrete to have many rich structural forms. On this basis, ordinary concrete is replaced by sea sand concrete, and the The corrosion resistance of chlorine salt has formed a series of new application forms: FRP reinforced sea sand concrete structure, FRP profile sea sand concrete composite structure, FRP pipe sea sand concrete composite member, steel pipe-FRP-sea sand concrete composite column, etc.

2.1 FRP reinforced sea sand concrete structure

FRP bars are used in sea sand concrete instead of steel bars, which mainly use the good corrosion resistance of FRP materials to avoid the problem of corrosion of steel bars. Moreover, FRP bars are light in weight and high in strength, and have the advantages of corrosion resistance, non-magnetic properties, and fatigue resistance. There are various forms of FRP bars, which can be FRP longitudinal bars and FRP stirrups similar to steel bars, both of which form a FRP bar cage and are used in sea sand concrete; or FRP grids can be used as bars for sea sand concrete. The ribs are interlocked together, which is convenient for use in plate members, avoiding the binding of longitudinal and transverse ribs, fast and convenient construction, and good two-way force performance.

2. 2 FRP profile-sea sand concrete composite structure

With the progress of the FRP production process, especially the maturity of the pultrusion process, the section form of the pultruded profile is flexible and highly designable. The application of FRP profiles in engineering has increased rapidly. Using it in combination with sea sand concrete can give full play to the material properties, so that the upper concrete is under compression, the lower FRP profiles are under tension, and the shear connection between the two ensures the synergy between FRP and concrete. work; and FRP profiles can be used as permanent formwork to facilitate construction. This type of composite member is especially suitable for bridge structures in offshore and cold regions, and can resist the harsh environment of chloride salt corrosion.

2.3 FRP pipe sea sand concrete composite member

Using the prefabricated FRP pipe as the formwork and pouring concrete into the pipe to form the FRP concrete pipe is the most reasonable FRP-concrete composite structural member. The FRP pipe not only has a good constraining effect on the concrete, but also greatly improves the strength and deformation capacity of the concrete; it can also act as a formwork to improve the construction speed. Since FRP is resistant to chloride salt corrosion, sea sand concrete can be poured into the pipe to form a FRP pipe sea sand concrete component. The combination form is reasonable, the mechanical properties are excellent, and it is environmentally friendly and has broad application prospects. Scholars from all over the world have conducted many studies on FRP pipe concrete, including the axial compression performance, flexural performance, seismic performance, mechanical performance and long-term mechanical performance of FRP pipe during construction, fiber types and The winding angle and the analysis and calculation model have been deeply studied. The sea sand concrete is used instead of the ordinary concrete, and the stress mechanism of the FRP concrete tube is unchanged. It is known that the mechanical properties of the sea sand concrete and ordinary concrete are basically the same under the same conditions. Therefore, for FRP The research and application of the pipe ordinary concrete member can be quickly extended to the FRP pipe sea sand concrete composite member.

2.4 Steel pipe-FRP-sea sand concrete composite column

The CFST column is a high-performance component that has been widely used in the engineering field. Cha Xiaoxiong et al. discussed a reasonable and safe application of sea sand concrete in the CFST column. The specific method is to place the FRP pipe in the steel pipe. Between the inner wall and the sea sand concrete, let it isolate the chloride ions in the sea sand to contact the inner wall of the steel pipe, and at the same time, it can be used to increase the restraint effect of the outer tube on the concrete, and improve the bearing capacity and deformation capacity of the component. Combining FRP and sea sand concrete through a reasonable structure can form a series of high-performance components. When used, unpurified sea sand and seawater are directly used to prepare concrete, saving resources and protecting the environment, which can be used in the development of environment-friendly buildings. play an important role. When used in coastal and marine applications, it can obtain local materials, greatly reduce the transportation volume of raw materials, and reduce the difficulty of construction, which is of great significance to marine construction and development.

3 New FRP sea sand concrete composite structure facing the ocean

3.1 Presenting the background

The ocean occupies about 71% of the earth's surface and is the direction for the expansion of human activities in the future. my country is a big ocean country, with an ocean area of ​​about 3 million square kilometers, more than 5,000 islands of various sizes, and a continental coastline of 18,000 kilometers. Entering the new century, my country's marine development has been accelerating. It is estimated that by 2050, the marine industry will become one of the pillar industries of my country's national economy. The related technologies and basic theories of the marine industry are the strategic focus of my country's medium and long-term scientific and technological development plan. What is more urgent is that my country's maritime sovereignty is facing increasingly severe challenges. There are a large number of islands in the territorial waters that urgently need to build permanent buildings to demonstrate sovereignty. It has become an urgent task to protect maritime sovereignty through engineering construction. Therefore, there is an urgent need to carry out research on marine-oriented engineering construction technology.

Offshore engineering construction is faced with extremely harsh natural corrosive environments. According to the environment, it can be divided into offshore, coastal, island and offshore, etc., all of which are subjected to different degrees of environmental effects such as chloride ion erosion, alternation of dry and wet, humidity and heat, freeze-thaw and so on. . Steel is highly susceptible to corrosion in humid and high chloride ion environments, which has become a common and unavoidable problem for steel structures and reinforced concrete structures in long-term service in marine environments. The survey shows that most marine reinforced concrete structures have serious corrosion problems. For example, 18 wharfs along the southern coast of my country were investigated, and 16 wharfs were found to be significantly corroded, of which 9 were severely corroded; According to the survey, 55.6% of the piers have beams and slabs corroded; the investigation of 66 berths of 14 piers along the northern coast of my country found that all berths were damaged by corrosion. Using corrosion-resistant materials is one of the main ways to solve this problem. FRP is a non-metallic material composed of carbon fiber, glass fiber and other reinforcing materials and a resin matrix. It has inherent corrosion resistance, and the chloride environment has no significant effect on its performance. It is a structural material suitable for marine environments.

For offshore and island construction, there are also construction problems such as long-distance transportation of raw materials, lack of construction equipment, lack of fresh water and auxiliary materials, and long maintenance periods. The farthest offshore distance of islands in the South my country Sea is more than 1500km, and the farthest offshore distance of islands in the East China Sea is about 530km. If cement, steel and other building materials are transported from land, and seawater desalination, sea sand and rough bone are carried out at the construction site The non-chlorination treatment of the materials will consume a lot of time and cost; the available mechanical equipment is also limited, and the construction is difficult; the maintenance and repair materials also need to be transported from land after completion. If the combined structure of sea sand concrete and FRP is used for construction, these problems can be avoided. The sea sand and sea water used can be directly obtained from the sea without purification treatment, thus reducing the transportation volume of raw materials. Moreover, the proportion of FRP is small, only 20% to 25% of that of steel, and has the advantages of high strength and convenient molding, which is convenient for transportation and construction, and can reduce the difficulty of construction when used in marine conditions.

Based on the principle of "take from the sea and use it for the sea", we propose a new type of FRP sea sand concrete composite structure, which meets the major needs of my country's island reef construction and provides a new direction for the application of sea sand concrete in the engineering field. and development opportunities.