From the previous blog Part-I of this series, the concept of underwater concreting and how it became the preferred concreting method for construct structures whose foundation is laid on water. The main advantage of underwater concreting is that it prevents the issues of washout by the addition of addictive components.
With the emergence of better technologies, techniques for underwater concreting has become innovative and has led to the discovery of better methods which will be explained in the following section.
Different techniques are used for underwater concreting. Although there are many methods available, some of the most widely used techniques are described here.
- Tremie method
- Pump method
- Toggle bags method
- Bagged Concrete method
Let us see how underwater concreting is performed through each of the following methods
This is one of the common methods used for Underwater concreting. Advantage of this method it allows pouring a large amount of high flowable concrete. It consists of a Tremie pipe in which the upper end is connected to a hopper and the lower end is continuously submerged in fresh concrete. The reason for immersing the tremie pipe lower end in concrete is to prevent intermixing of both concrete and water.
The tremie pipe can be generally configured in three different ways such as constant length which is raised during concreting, pipe with different sections which can be dismantled during concreting and telescope pipe. Aluminium alloy pipes should be never used in the tremie method as it can negatively affect the concrete and leads to chemical reactions between them.
In order to prevent blockage due to aggregate size, The pipe should have an adequate diameter. Usually, it is in the ranges between 200- 300 mm, but sometimes 150 mm and 450 mm can be used but the size of the aggregates should be also considered.
This technique is a much more convenient technique for underwater concreting. It is basically a modified version of the tremie pipe and is effective in concreting areas that are typically difficult to access. The main advantage of pumping technique is that it allows the pouring of concrete from the mixer to formworks directly, which the tremie method lacks.
Toggle Bags Method
This technique is preferred when only a small amount of concrete is required. This technique involves the use of a reusable canvas bag that is sealed at the top with a chain and is secured with toggles. The toggles are filled with concrete and dropped carefully into the determined location, then through an opening at the bottom of the bag, the concrete is discharged.
Bagged Concrete Method
The method is used for renewing ballast or temporarily seal holes. These bags are produced from strong fabrics and have an estimated capacity of 10 -20 litres. This is then carried by divers to the selected position. The concrete slump is typically between the ranges 19- 50 mm and 40 mm is the maximum aggregate size that can be used.
As mentioned earlier, although many different methods are available, every technique has its own set of merits and demerits, therefore, it is up to the engineer to accurately determine which technique suits best for the construction.
The engineer must also make sure that the concrete mix is prepared as per recommended specifications, because, a weak preparation of concrete mix results in eventual collapsing.