Uncategorized Archives - Consult Marine

Underwater Concreting

The tremie method is a simple, yet effective way of placing concrete underwater. A gravity-fed tremie consists of a rigid pipe and hopper (see photos). The key principle is to keep the end of the tremie pipe submerged within the mound of fresh concrete being placed underwater. The new concrete is always being placed inside the concrete mound. The outside of the mound is therefore the only surface exposed to the washout effects of the surrounding water. The extent of any quality loss is thusly minimised. The end product is typically a series of mounds with a coarse finish (see underwater photo – courtesy of Noel Saenger).

It is necessary for a plug to seal between the concrete and water interface during the initial free-fall of concrete. A soccer ball may be used to this effect, as seen in the photos.

It is critical that the supply rate of concrete is kept up to the hopper such that the hopper remains filled at all times. Otherwise, pockets of air could be introduced within the concrete pour.

The amount of concrete “lost” during the placement process is a function of the energy of the surrounding waters, and stickiness of the concrete. The concrete’s stickiness may be readily increased via addition of manufactured fine aggregates to the mix design.

Underwater Inspection of Piles

Sometimes defects in either steel or timber piles are not evident above water (refer to photos – courtesy of Polaris Marine). Underwater inspection of piles is therefore an important part of routine maintenance of marine structures.

Marine piles should be periodically inspected and measured by an experienced team of commercial divers. Timber piles are typically measured using calipers (refer to photo). Steel piles are typically measured using NDT (non-destructive testing) techniques, for example ultra-sonic thickness testing, or pulsed eddy current testing.

Underwater Vacuuming

During underwater foundation preparation, it is sometimes necessary to provide a perfectly prepared and clean surface. This may require bulk removal of sand, gravel, cobbles, or the like. This can be efficiently achieved using an underwater vacuum cleaner, for example an air lift, or water lift as shown (underwater photo courtesy of Chris Streater).

The water lift is generally more efficient in shallower water and permits use of lighter equipment, as there is less chance of the water lift becoming buoyant. The water lift operates upon the venturi effect. In the same way that air-flow through a carburettor facilitates the transfer of fuel, water-flow through the lift facilitates transfer of sand, gravel and even cobble-sized stones.

Underwater Power Tools

Underwater construction requires the use of specialised tools. Hydraulic-powered tools are often used due to their relative longevity and safety. In this photo a commercial diver is trimming rock using a hydraulic-powered ring saw. Using a ring-blade minimises losses due to friction, drag and inertia.
(Underwater photo courtesy of John Galpin).

Plastic Bar Chairs

Use of either steel or plastic bar chairs is generally avoided in reinforced concrete marine structures. Plastic is generally difficult to bond to, due to its low surface energy. Plastic bar chairs could therefore introduce a path for salt water to enter the concrete, along the interface between the concrete and plastic.

Marine concrete mixes often exhibit poor workability, as a by-product of specifications that call for a low water/cement ratio and high compressive strength. This may result in incomplete filling of the voids within plastic bar chairs (refer to photo).

Custom-made concrete spacer blocks are preferred (refer to photo). The concrete spacers can be custom-made to the correct height with a matching grade of concrete, producing an ideal outcome.

Pile Splice Connections

Splicing is a common method used to repair defective timber piles in the marine environment. A pile splice involves replacement of the deteriorated timber pile section with a new section made from steel, or timber. The new section of pile is connected to the existing pile stump at the seabed, and then at the top of structure. Risks associated with this repair method (at the lower connection point) include:

· Deterioration of the original stump, which can occur if seabed levels change due to dredging or slope instability (refer to photo 1).

· Deterioration of the new pile splice section (refer to photo 2)

· Deterioration of the connection. Photo 2 shows a thin steel casing (most likely a 44 gallon drum body) has been used as sacrificial form work for the grouted connection. As the casing deteriorates, confinement of the grout is lost, ultimately leading to failure of the connection.

Stainless Steel Reinforcement

Stainless steel reinforcement is sometimes used in marine structures. Often when a long service life is desired, or where the environmental conditions are extreme. Two different types of stainless steel reinforcement are available: Austenitic, or Duplex. Duplex is a mixture of austenite and ferrite. Duplex stainless steels are generally both stronger and cheaper owing to their lower Nickel content.

Within each type, there are a number of common grades:

Austenitic: 304 & 316.
Duplex (austenitic-ferritic): 2101, 2304 & 2205.

The microstructure of stainless steel can be identified on site with use of a magnet. If the stainless steel has a duplex microstructure, then a magnet will stick. If it has an austenitic microstructure, then a magnet will not stick.

In the photo attached, the diameter of the rebar is stamped on (i.e. 40mm diameter). This helps with quality control, as it is easy for the builder & checker to identify that the correct size bars have been used.