Step-by-Step Process of Underwater Concrete Placement

73

Underwater concrete placement is one of the most complex operations in marine and foundation engineering. It demands precision, control, and an understanding of both materials and environment. When concrete is placed underwater, the primary challenge is preventing it from mixing with surrounding water. If the mix disperses or segregates, the resulting concrete loses strength, creates voids, and can compromise structural integrity. This is why engineers rely on specialized underwater concreting techniques such as the tremie method, the pump method, and preplaced aggregate concrete. Each of these approaches is designed to place concrete in its final position without washout or dilution, ensuring a continuous, dense, and durable structure even in submerged conditions.

The process requires not just the right equipment, but also a deep understanding of concrete behavior. Factors like water velocity, pressure, temperature, and visibility can all affect the outcome. The following sections explain how each underwater placement method works, the steps involved, and how engineers maintain quality from start to finish.

Understanding the Principles of Underwater Concrete Placement

The first principle of successful underwater concreting is minimizing contact between fresh concrete and water. When the cement paste in concrete comes into direct contact with moving water, fine particles wash out and create a weak, porous surface known as laitance. To avoid this, the concrete must be delivered directly to its intended location through a controlled and sealed path—either a pipe, hose, or chute. This ensures the concrete displaces the water rather than mixing with it.

Another critical principle is maintaining a continuous and unbroken flow. Interruptions in placement cause water to enter the pipe or tremie, breaking the seal and allowing contamination. The discharge end must remain embedded in the freshly placed concrete, usually by a minimum of 3 to 5 feet, to preserve the seal and maintain hydrostatic pressure. These core ideas are at the heart of every underwater concreting technique used today.

The Role of Mix Design

Concrete used underwater is formulated differently from regular mixes. It must be highly cohesive and resistant to washout, while still maintaining sufficient flowability to travel through long pipes or tremies. Achieving this balance is a challenge. Engineers typically lower the water-cement ratio to around 0.40–0.50 to ensure density, while incorporating additives such as anti-washout admixtures (AWA) and viscosity-modifying admixtures (VMA). These agents increase the mix’s internal cohesion, preventing segregation even when concrete is subjected to currents or turbulence.

Supplementary cementitious materials such as silica fume, fly ash, or ground granulated blast furnace slag are also commonly used. They refine the particle size distribution and make the concrete less prone to bleeding. The resulting mixture should achieve a slump between 175 and 225 millimeters for tremie or pump applications, or a slump flow of around 550 to 700 millimeters if self-compacting concrete is used. The Visual Stability Index (VSI), which indicates resistance to segregation, should ideally remain below 1.0. Laboratory tests like the CRD-C 61 washout test confirm the mix’s stability before it reaches the site. The combination of these measures ensures that underwater concrete retains its strength and integrity once placed.

The Tremie Method

Among all underwater concreting methods, the tremie method remains the most widely used, especially in large foundation slabs, cofferdams, and pier bases. A tremie is a long, watertight steel pipe, typically 150 to 300 millimeters in diameter, fitted with a funnel or hopper at the top. Its lower end is kept submerged in the fresh concrete at all times during placement.

The process begins with the preparation of the site. The foundation surface must be cleaned of debris, silt, or loose material to ensure the concrete bonds properly. Once the tremie is positioned vertically with its base resting on the placement area, a plug—often made of foam, rubber, or even a small ball of concrete—is inserted to separate the first batch from the water inside the pipe. Concrete is then poured into the hopper, forcing the plug downward. As the plug exits at the bottom, it pushes out water and allows fresh concrete to flow smoothly into place without contamination.

The critical phase starts once the flow begins. The tremie tip must stay embedded several feet within the newly placed concrete, creating a positive seal. As placement continues, the operator gradually raises the tremie in sections but never allows the discharge end to break free. The flow of concrete displaces water upward and outward, filling the form from the bottom up. If the pipe is lifted too high, the seal is lost and water can rush in, diluting the mix. Large placements often require multiple tremies working simultaneously to maintain a uniform rise of concrete across the area.

When the pour is complete, engineers intentionally overfill the form by several inches to allow the weaker top layer, often contaminated with laitance, to be removed once the concrete hardens. The tremie method’s success relies on maintaining a steady rhythm—no interruptions, no horizontal movement of the pipe, and constant monitoring of embedment depth. When executed properly, it produces dense, homogeneous concrete with minimal washout.

The Pump Method

In modern underwater construction, the pump method has become an equally reliable alternative to the tremie system. It offers better control over the flow rate and is especially effective for deep foundations, drilled shafts, and situations with limited access. Instead of relying on gravity, the pump method uses mechanical pressure to push concrete through watertight pipelines or hoses directly to the placement point.

The process starts with the assembly and priming of the pumping system. All joints and couplings must be leak-proof to prevent water from entering the line. Before pumping begins, the system is primed using a cement-sand grout or slurry to coat the interior and minimize friction. Concrete is then pumped at a controlled rate to ensure a continuous stream. As with the tremie method, the discharge nozzle must remain embedded in the fresh concrete at all times. This prevents the sudden entry of water and maintains the integrity of the seal.

The pump operator must coordinate closely with the batching plant to keep concrete arriving at regular intervals. Any pause in pumping can cause blockages or cold joints. Flow rate, pressure, and line length are monitored constantly to avoid segregation or excessive turbulence. Once the placement is complete, the pipeline is flushed clean using compressed air or water.

The pump method offers several advantages. It allows faster placement, minimizes manual handling, and performs well even in confined spaces or great depths. However, it also demands skilled operators and precise control over the concrete’s rheology. Any failure in maintaining pressure or watertightness can quickly lead to defects.

Preplaced Aggregate Concrete (PAC)

For certain underwater projects, especially structural repairs, congested reinforcement zones, or areas exposed to strong currents, engineers use preplaced aggregate concrete, also known as two-stage concrete. Unlike traditional placement, the coarse aggregate is first placed into the formwork in a dry or submerged condition. Clean, graded stones are deposited until the form is completely filled and densely packed.

Once the aggregate skeleton is ready, a highly fluid cement-sand grout is pumped from the bottom upward through embedded grout pipes. The grout fills all voids between the stones, displacing any trapped water as it rises. The process continues until grout appears at the top vents, ensuring complete saturation. Because the aggregates are already in position, the risk of washout is minimal, and the resulting concrete exhibits excellent durability and low shrinkage.

Preplaced aggregate concrete is slower to execute and requires more monitoring, but it is invaluable for repair work, underwater bridge piers, and areas where mechanical vibration is impossible. Its ability to form a dense, low-permeability matrix makes it a favored choice for underwater rehabilitation projects.

Managing Environmental Factors

The environment surrounding the pour plays a decisive role in the quality of underwater concrete. Water movement is one of the most critical variables. High currents can erode fresh concrete surfaces and wash away cement paste before it sets. Ideally, water velocity should be kept below half a meter per second. In tidal or river environments, contractors often install silt curtains or temporary cofferdams to calm the flow around the work area.

Temperature also affects performance. In cold water, hydration slows down, which can delay strength gain, while warm water accelerates setting and may cause premature stiffening. Adjusting the mix with accelerators or retarders helps maintain the desired setting time. Pressure at greater depths influences the concrete’s flow characteristics, so the pumping system and mix viscosity must be tailored to suit the placement depth. In many marine projects, visibility is limited, so divers or remotely operated vehicles (ROVs) monitor the operation to confirm placement level, pipe position, and embedment.

The formwork must be watertight and sturdy enough to resist hydrostatic pressure. Any leakage at joints can allow fine materials to escape, leading to honeycombing. Engineers generally design forms with gaskets or rubber seals and specify overpouring to allow the top layer to be trimmed later. By carefully controlling these environmental factors, underwater concreting can proceed smoothly even in challenging marine conditions.

Quality Control and Inspection

Underwater concreting demands rigorous quality control at every stage. Before the pour, laboratory testing verifies slump, slump flow, and washout resistance. During placement, field engineers record the time, volume, and elevation of the concrete surface at regular intervals. For tremie and pump methods, inspectors ensure that the discharge end remains continuously submerged and that the rate of placement keeps pace with delivery.

After hardening, cores or non-destructive tests such as ultrasonic pulse velocity and crosshole sonic logging (CSL) confirm the integrity of deep foundation elements. Inspectors also check the removed top layer to verify that laitance and weak material have been eliminated. Every parameter—from mix temperature to concrete volume—is logged to create a full placement record. This documentation helps evaluate performance and provides accountability for the project’s long-term durability.

Common Issues and Solutions

Even with careful planning, problems can occur. The most frequent issue is loss of the tremie seal, which happens when the pipe is lifted too high and allows water to enter. The solution is immediate: stop placement, clean the pipe, replug, and restart. Segregation can occur if the concrete flows horizontally over long distances, which can be avoided by using multiple delivery points or adjusting flow velocity. The formation of laitance layers is inevitable to some degree, but their effect can be minimized by overpouring and removing the upper 150 to 200 millimeters after hardening. Pump lines must be carefully primed to prevent blockages, and backup pumps should always be available in case of mechanical failure. These precautions keep the process efficient and ensure consistent quality throughout the placement.

Modern advances in admixtures, pumping equipment, and underwater monitoring have made these techniques more reliable than ever. Yet, the fundamentals remain unchanged: prepare the site thoroughly, control the flow, and never lose the seal. When these principles are followed, underwater concrete can achieve the same—or even greater—durability as concrete placed in the dry, forming the backbone of piers, jetties, foundations, and offshore structures around the world.