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Introduction
Underwater concrete is a specialized high-performance type of concrete widely used in modern industry, typically for constructing bridges, dams, and structures whose foundations are built underwater. Unlike ordinary concrete, underwater concrete placement requires different techniques due to its distinct properties to ensure successful application.
A new material called geopolymer has recently gained widespread attention as a green technology-based binder that can replace ordinary Portland cement (OPC). Numerous studies have shown that geopolymers possess strength and chemical resistance comparable to OPC. However, to date, only limited research has investigated the use of geopolymers as underwater concrete materials. Reviews indicate that the requirements for underwater concrete materials, besides high strength, must include washout resistance, workability, and durability. This review demonstrates that geopolymers provide excellent strength, durability, and workability in accordance with EFNARC standards. Finally, future research opportunities are discussed, considering the potential of geopolymers to replace OPC as underwater concrete material.
What is Geopolymer?
Geopolymers are mineral or organic polymers with three-dimensional structures and superior mechanical and physical properties. These polymers are environmentally friendly alternatives to Portland cement. One of their applications includes reducing greenhouse gas emissions.
Industrial Growth of Geopolymers
Given industrial growth, geopolymers will play an important role in sustainable development in the 21st century. The geopolymer industry and challenges related to applying these new cements have attracted attention. Due to their advantages, geopolymers can be a good alternative to Portland cement.
Advantages of Geopolymer
Introduction
Concrete is widely used globally as a construction material due to advantages such as raw material availability, simple formulation, ease of production, and flexibility for molding into various shapes. Traditionally, construction concrete is based on ordinary Portland cement (OPC). However, it is well known that calcination of limestone and fossil fuel combustion during OPC production contribute about 5-7% of the total worldwide CO2 emissions. The main phase of OPC includes tricalcium silicate (C3S), dicalcium silicate (C2S), and tetracalcium aluminoferrite (C4AF). These compounds aid early hydration and contain considerable amounts of calcium hydroxide (Ca(OH)2), calcium silicate hydrate (C-S-H), and calcium aluminate hydrate (C-A-H), which are susceptible to attack by seawater constituents (SO4^2-, Cl-, CO3^2-). Previous studies have shown that geopolymers emit less CO2 during their life cycle and have excellent durability against seawater attack. Therefore, they act as a suitable substitute for OPC. For this reason, a new binder material called geopolymer is promoted as an OPC alternative.
Geopolymers are synthesized by combining aluminosilicate reactive materials with strong alkaline solutions such as sodium hydroxide (NaOH) and can be cured at room temperature. Geopolymers are mineral materials that are hard, weather-resistant, and can withstand high temperatures between 1000°C and 1200°C. In addition to properties like hard surfaces, thermal stability, and precise molding, geopolymer is a new material currently used in construction.
The main goal of geopolymer is to provide an alternative material alongside Portland cement for construction. Therefore, the durability and mechanical performance of geopolymer concrete are key concerns and depend on chemical composition, chemical bonding, and porosity. Many studies focus on identifying the optimal mix design for geopolymer concrete. This review concentrates on previous research regarding geopolymer properties that highlight their potential for underwater concrete applications. Hence, the requirements for underwater concrete are discussed, followed by a review of previous research on geopolymer properties such as self-compacting ability and durability against seawater.
Construction Methods
When major structures are built in rivers, harbors, or coastal areas, placing concrete underwater is often required. Underwater concrete placement is one of the most critical and dynamic operations that determines the success of underwater structures. Concrete tends to deteriorate when exposed underwater, leading to layered features, heterogeneity, and low strength. Therefore, it is crucial to ensure the fresh concrete mix does not segregate.
Washout Resistance of Geopolymer
Rheologically, geopolymers must have low yield stress but high plastic viscosity to resist washout and segregation, especially in flowing water. Water turbulence and surface velocity between concrete and water directly affect the level of washout. Research by Hwalla et al. and Roviello et al. identified the relationship between workability and washout loss of geopolymer samples. Geopolymers with higher viscosity showed better performance.
Summary and Future Work
From the conducted reviews, it can be concluded that underwater concrete materials, besides high strength, must possess washout resistance, workability, and durability. These properties ensure the material can self-compact in forms without the need for mechanical compaction. OPC is typically used as underwater concrete material. Most studies focus on the effects of additives such as pozzolanic materials.
Underwater Concrete Structure Repair
Inspection of underwater concrete structures is necessary for repair and maintenance works. Methods, types, and objectives of underwater concrete inspection are discussed. The service life of underwater concrete structures, such as bridges, docks, and other marine structures, depends on maintaining the physical condition of both the superstructure and substructure. Therefore, performing adequate inspection, maintenance, and repair of the entire underwater concrete structure is critical. Underwater concrete inspection is generally neither easy nor cost-effective, and thus is less frequent than inspections above the waterline. However, underwater inspection is a major part of assessing submerged structures. Agencies such as transportation authorities and port authorities in the US and Canada set intervals for underwater inspections as part of preventive maintenance programs. This article discusses reasons for underwater inspection, factors considered during inspection, inspection objectives, and various inspection levels.
Why is Underwater Concrete Inspection Needed?
Besides maintenance inspections, it may be required for specific conditions such as new loading requirements, modifications or expansions of structures, or new construction to ensure compliance with specifications and contract documents. When a new owner acquires a structure, underwater inspection might be mandatory. Underwater inspection is also necessary after disasters such as earthquakes, ship collisions, storms, or floods.
Underwater Repairs
When underwater structures begin to fail, sending divers to repair them is more practical than draining all surrounding water. Divers typically perform welding and repairs on dams, bridges, power plants, pipelines, and retaining walls. They sometimes also repair ships to keep them afloat.
There are two main types of underwater welding: wet and hyperbaric. Wet welding, as the name implies, involves welding entirely underwater. It is the most challenging type because the diver works under pressure and moves slowly.
Hyperbaric welding (dry welding) involves isolating the repair area from water, often in a chamber lowered to the weld site. Engineers pump water out of the chamber so the diver is not fully submerged.
Commercial divers use various tools including underwater torches, saws, and welding rods to perform their hazardous tasks. They often must maneuver in tight spaces or polluted waters.
Therefore, it is not surprising that becoming an underwater welder requires years of training, beginning with two to five years of land-based welding. Few can claim to help repair a city, but underwater welders hold this reputation.
Engineering Marvel
Building underwater structures on land is expensive but much safer than sending divers equipped with electric drills to the sea floor. Thus, engineers use dams, caissons, driven piles, and off-site buildings to complete much of their construction before adding the final underwater touches. When underwater construction sites require repairs, skilled welders perform the work.
Even knowing how underwater construction works, seeing a submerged structure and understanding the many construction stages it has undergone is truly impressive.
References:
A - Centre of Excellence for Geopolymer & Green Technology (CEGeoTech), Universiti Malaysia Perlis (UniMAP), Perlis, Malaysia
B - Faculty of Mechanical Engineering Technology, Universiti Malaysia Perlis (UniMAP), Perlis, Malaysia
C - Faculty of Chemical Engineering Technology, Universiti Malaysia Perlis (UniMAP), Perlis, Malaysia
D - Faculty of Civil Engineering Technology, Universiti Malaysia Perlis (UniMAP), Perlis, Malaysia
E - Faculty of Engineering, University of Plymouth, Plymouth PL4 8AA, UK
F - Chemistry Department, Sepuluh November Institute of Technology, Surabaya, Indonesia