What is Underwater Concreting?

Underwater concreting is a critical concreting process performed for offshore construction. It involves the process of carefully placing fresh concrete underwater. As construction in water results in many difficulties, the start to end of underwater construction requires special care, supervision and planning. 

For example, the construction of the foundation and piles to support the bridge crossing a river, or a dock construction.

Fig.1. Underwater Concreting; Image Courtesy: https://menafn.com/

The main objective of any underwater construction project is to create a sry and water free environment to safely place and cure concrete that helps to give a structurally stable and durable structure.

The two important know how on underwater concrete construction are:

• Underwater Concrete
• Underwater concreting methods and techniques

Underwater Concreting

The underwater concreting involves mixing the concrete on the surface in the conventional manner and then placing this underwater by various methods. A properly mixed concrete is a stable material that when dried gains twice the density of water and remains unaffected by water. During the process, the concrete is immersed and allowed to set by providing it a drying environment.

There are many risks associated with the vibration and levelling of the concrete placed underwater. The concrete can be washed out in situations of placing. So, some anti-washout admixtures and placing techniques must be employed.

Underwater Concreting Methods

The various underwater construction methods are:

1. Tremie method
2. Grouting method
3. Using bags
4. Bucket placing
5. Pre-placed aggregate method
6. Pumping

1. Tremie Method

This method involves placing a tremie pipe into the water and lowering it to the bottom. The pipe is then filled with concrete, and as the concrete flows out of the pipe, the pipe is slowly raised to maintain a continuous flow of concrete. This method is commonly used for constructing underwater foundations and other structures.

2. Grouting Method

The grouting method involves injecting a cementitious material into the soil or rock under the water to improve its load-bearing capacity. This method is often used for stabilizing underwater slopes, filling voids in rock formations, and strengthening the foundation of underwater structures.

3. Using Bags

This method involves filling bags with sand or other materials and placing them underwater to create a structure or foundation. The bags are typically made of a durable material such as geotextile or burlap and can be stacked and arranged to create a stable foundation.

4. Bucket Placing

The bucket placing method involves using a crane or other equipment to place buckets filled with concrete onto the underwater surface. The buckets are typically fitted with a release mechanism that allows the concrete to be dumped onto the surface in a controlled manner.

5. Pre-placed Aggregate Method

This method involves placing large rocks or other materials on the underwater surface and then filling the gaps between them with concrete. The rocks provide a stable foundation for the concrete and can help to prevent erosion.

6. Pumping

Pumping is a method of transporting concrete or other materials underwater using pumps. The material is pumped through a hose or pipe to the desired location, where it is then placed using a tremie pipe or other equipment. This method is commonly used for constructing underwater pipelines, foundations, and other structures.

The selection of the underwater construction method will depend on factors such as the type of structure, the water depth and conditions, the available equipment and resources, and the environmental impact of the method. Most of these methods employ caissons and cofferdams.

Requirements of Underwater Concrete

The concrete used for underwater construction must meet a few requirements, unlike conventional construction. ACI 304R-00 "Guide for Measuring, Mixing, Transporting, and Placing Concrete" and ACI 301-16 "Specifications for Structural Concrete" provide guidelines and specifications for the requirements of concrete used for underwater construction. 

Here are some of the main requirements as per ACI standards:

1. Slump: The slump of the concrete mix used for underwater construction should typically be between 150-200 mm (6-8 inches) to ensure good workability and flow. However, the specific slump requirements may vary depending on the project specifications and site conditions.
2. Compressive Strength: The compressive strength of the concrete should be at least 28 MPa (4,000 psi) at 28 days or as specified by the project specifications. The strength requirements may vary depending on the type of structure and the expected loads.
3. Water-Cement Ratio: The water-cement ratio (w/c) of the concrete should not exceed 0.45 to ensure good durability and reduce the risk of corrosion. A lower w/c ratio can also help to reduce the risk of shrinkage and cracking.
4. Air Content: The air content of the concrete should be between 3-6% to ensure good workability and resistance to freeze-thaw cycles. The air content should be monitored and adjusted during the mixing and placing process to ensure consistent results.
5. Aggregates: The aggregates used in the concrete mix should be free of any harmful substances such as clay, silt, or organic matter. The aggregates should be well-graded and meet the specifications for size, shape, and cleanliness.
6. Admixtures: Admixtures such as water reducers, set retarders, and superplasticizers may be used to improve the workability, set time, and durability of the concrete. The admixtures should be selected based on the specific requirements of the project and added in accordance with the manufacturer's recommendations.
7. Chloride Ion Content: The chloride ion content of the concrete should be limited to prevent corrosion of reinforcing steel in the underwater environment. The maximum allowable chloride ion content varies depending on the type of structure and the expected exposure conditions.

Overall, the concrete used for underwater construction must be carefully designed, mixed, placed, and cured to ensure it meets the specific requirements of the project and the unique challenges of the underwater environment.

Applications of Underwater Concreting

• To construct footing blocks in water to support structures
• To tie various elements in composite action
• To fill the pre-excavated holes in the seafloor to act as a leveling mat
• To construct solid ballast to add weight and lower the center of gravity
• To fill the base of the gravity platform to obtain uniform bearing and provide shear transfer

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