One essential concrete procedure used in offshore building is underwater concreting. It entails the procedure of properly submerging new concrete. Underwater construction involves a lot of challenges, therefore careful planning, monitoring, and attention to detail are needed from beginning to conclusion.
For instance, building a wharf or the foundation and piles needed to sustain a bridge across a river.
Any underwater construction project’s primary goal is to establish a dry, water-free environment in which concrete may be placed and allowed to cure safely, resulting in the creation of a long-lasting, structurally sound structure.
The following are the two key competencies in underwater concrete construction:
- Submerged Concrete
- Underwater Concrete Techniques and Methods
Underwater concreting is the process of mixing concrete conventionally on the surface and then submerging it using a variety of techniques. When concrete is mixed correctly, it becomes a stable substance that is impervious to water and gains double the density of water when dry. The concrete is submerged and given a drying atmosphere to help it set during the procedure.
The shaking and leveling of concrete submerged in water carries several concerns. In certain conditions, the concrete may wash out. Therefore, it is necessary to use specific anti-washout admixtures and putting procedures.
Techniques For Underwater Concreting
The many techniques for building undersea are:
- Tremie technique
- Grouting technique
- With bags
- Setting up buckets
- Prepositioned aggregate technique
1. Tremie Technique
Using this technique, a tremie pipe is submerged in water and lowered to the bottom. After that, the pipe is filled with concrete, and to ensure that there is a constant flow of concrete out of it, the pipe is gradually lifted. Underwater foundations and other buildings are often built using this technique.
2. Use Of Grouting
To increase the soil or rock’s ability to support weight, a cementitious substance is injected into it using the grouting technique. Underwater slope stabilization, rock formation void filling, and underwater construction foundation strengthening are common applications for this technique.
3 Using Carriers
Using this technique, sand or other materials are filled into bags, which are then submerged to form a foundation or structure. The bags may be piled and placed to make a strong foundation since they are usually composed of a durable material like burlap or geotextile.
4. Putting in Buckets
The bucket placing technique is putting concrete-filled buckets on the undersea surface with the aid of a crane or other machinery. Usually, the buckets have a release mechanism that makes it possible to carefully pour the concrete onto the surface.
5. Aggregate Method Pre-Placed
Using this technique, big boulders or other items are placed on the underwater surface, and the spaces between them are subsequently filled up with concrete. In addition to giving the concrete a solid base, the rocks can aid in halting erosion.
Pumping is the process of employing pumps to move things beneath water, such as concrete. The material is put using a tremie pipe or other equipment after being pushed through a hose or pipe to the appropriate area. This technique is frequently applied to the building of undersea foundations, pipelines, and other infrastructure.
The kind of structure, the depth and conditions of the water, the equipment and resources available, and the method’s influence on the environment will all play a role in the decision on which underwater building technique to choose. Most of these techniques make use of cofferdams and caissons.
The Prerequisites For Submerged Concrete
Unlike normal building, the concrete used for undersea construction needs to fulfill certain specifications. Guidelines and specifications for the needs of concrete used for underwater construction are provided by ACI 304R-00, “Guide for Measuring, Mixing, Transporting, and Placing Concrete,” and ACI 301-16, “Specifications for Structural Concrete”.
As per ACI standards, the following are some of the primary requirements:
Slump: To provide adequate workability and flow, the concrete mix slump used for underwater construction should normally be between 150 and 200 mm (6 and 8 inches). However, the precise slump needs could change according on the site’s characteristics and the project’s parameters.
Compressive Strength: Depending on the project standards, the concrete’s compressive strength should be at least 28 MPa (4,000 psi) after 28 days. The kind of structure and the anticipated loads may affect the required strength.
Water-Cement Ratio: To guarantee high durability and lower the risk of corrosion, the concrete’s water-cement ratio (w/c) shouldn’t be greater than 0.45. Another way to lessen the chance of shrinkage and cracking is to have a lower w/c ratio.
Air Content: For optimal workability and resistance to freeze-thaw cycles, the concrete’s air content should be between three and six percent. To guarantee consistent outcomes, the air content should be checked and changed as needed during the mixing and placement process.
Aggregates: There should be no hazardous materials, such as silt, clay, or organic matter, in the aggregates used in the concrete mix. The aggregates must to be properly graded and adhere to the size, shape, and cleanliness requirements.
Admixtures: To increase the concrete’s workability, set time, and durability, additives including water reducers, set retarders, and superplasticizers can be used. The admixtures have to be chosen in compliance with the project’s particular requirements and added as per the manufacturer’s suggestions.
Chloride Ion Content: To stop reinforcing steel from corroding in an underwater environment, the amount of chloride ions in concrete should be kept to a minimum. The kind of construction and the anticipated exposure circumstances determine the maximum permissible chloride ion content.
All things considered, meticulous planning, mixing, placement, and curing are necessary to guarantee that the concrete used for underwater construction satisfies the project’s particular needs and the particular difficulties presented by the underwater environment.