Underwater concreting – materials, technology

Investigating the depths of underwater concreting reveals an intriguing world where traditional building materials are combined with aquatic environment challenges. Underwater concreting necessitates specific methods and supplies, in contrast to conventional land-based construction, in order to guarantee longevity and structural integrity beneath the waves.

The materials used in underwater concreting are carefully chosen to endure the challenging submerged conditions. These consist of aggregates that are selected based on how well they bond with cement in aquatic environments, such as crushed stones, sand, and gravel. In order to improve the concrete’s workability, decrease water permeability, and strengthen its resistance to chemical erosion from seawater, specific additives and admixtures are also used.

For underwater concreting projects to be successful, technology is essential. Preserving the integrity of the concrete mix during placement and curing is one of the main challenges. Commonly employed methods include tremie concrete pouring and pre-placed aggregate concrete (PAC). With PAC, the concrete is laid out in layers, with each layer being carefully compacted to reduce voids and maximize strength. Tremie pouring is the process of pouring concrete underwater through a funnel-shaped pipe so that it displaces water and settles in place without separating.

Moreover, developments in the design of concrete mixes are tailored to underwater applications. High flowability and little bleeding are the two key characteristics of the mixes that engineers customise to ensure even distribution and consolidation of the concrete around submerged structures. These developments contribute to the longevity and sustainability of underwater infrastructure in addition to increasing construction efficiency.

Comprehending the complexities of submerged concreting necessitates an understanding of material science in addition to specialized construction techniques. To adapt conventional building methods to aquatic environments, engineers and contractors must work closely together to ensure that every project satisfies strict performance and safety requirements. Our capacity to build sturdy and long-lasting structures beneath the water’s surface will advance together with technology.

General information

Special grades of concrete are needed for this kind of work, and they must fulfill strict requirements:

• by water resistance;
• frost resistance;
• mechanical strength;
• sulphate resistance.

In addition to the aforementioned requirements, concrete mixtures intended for this kind of work also need to have minimal heat generation during the hardening period and enhanced mobility and workability.

All of these requirements are satisfied by hydraulic concrete. The components’ arrangement and the brand that is needed depend on the structure’s intended use, the operational specifications placed on it in each specific instance (prefabricated, thin-walled, etc.), the local temperature where construction is taking place, and other factors.

GOST 4795-68 states that hardened concrete must have a grade of no less than:

• for underwater concrete – M400;
• for sealing joints and grillage structures not lower than M500;
• frost resistance – F300;
• for water resistance – W

The primary benefit of hydraulic concrete lies in its elevated water resistance, setting it apart from conventional brands. However, costly ingredients must be added to the mixtures’ composition in order to produce high-quality indicators. The cost of waterproof concrete is consequently much more than typical.

The next point is that because the solution needs to be mixed in tiny portions, the mixtures’ quick setting can occasionally be detrimental when making concrete by hand. Additionally, if the developer has budgeted for a sizable structure’s material volume, this will have a big impact on the amount of labor required and the turnaround time.

Materials

Underwater concreting technology places strict requirements on the concrete used, taking into account the working conditions and the final object’s operating standards. Thus, there are now stricter standards for the quality of fillers and cement, which must completely fill in any gaps in the concrete structure.

This method leads to a significant reduction in the amount of binder (cement) used while also improving the properties:

• heat generation decreases;
• viscosity of mixtures increases;
• durability and density of the product increase.

The components of concrete for underwater concreting are as follows:

• binder — sulfate-resistant Portland cement or pozzolanic cement;
• coarse aggregate — crushed granite;
• fine aggregate — sand;
• surface-active additives (SAA).

Cement

Pozzolanic or sulfate-resistant cement grades M400–M500 (GOST 10178–85) are used in underwater concreting as the primary binder for the preparation of concrete mixtures. These grades must meet the following requirements:

• density of cement paste — no more than 26%;
• setting start — 2 hours after mixing, not earlier;
• the alkali content in clinker should not be higher than 0.6%, lime – 0.5%, poorly soluble sediment no more than 0.5%.

The final concrete’s design grade dictates the amount of cement. The volume of the structure shouldn’t be more than 350–400 kg/m3, depending on the kind. It is preferable to utilize low-emission cement if the project calls for installing large structures.

Large and small aggregates (GOST 10268-80)

The following factors are considered when selecting small aggregates:

• grain composition;
• Module of size;
• the content of clay and dusty particles;
• The compression strength limit in water impregnated with water.

Use of pure quartz sand with a module size of 2.0–2.5 or field pavement is required as small aggregates.

The following categories are used to choose the large aggregate composition for hydraulic concrete:

• density;
• according to grain composition;
• strength;
• by the number of grains of needle and lamellar in shape;
• the content of particles of weak breeds;
• frost resistance;
• water absorption;
• by content of harmful inclusions.

Crushed stone from dense rocks with fractions of 5–40, 5–20, and 5–10 mm is known as coarse aggregate. Compressive strength greater than 1000 kgf/cm2 is allowed.

Crushed stone needs to be cleaned with water to get rid of any dust, silt, or clay particles before being fed into the mixer. Use any potable water that satisfies GOST 4797-69 requirements to wash aggregates.

Take note! Minerals such as pyrite, opal, siliceous compounds, and others that can react chemically with Portland cement’s alkaline constituents should not be present in the fillers.

Surface active additives

Complicated surfactant additives are the primary source of high-quality hydraulic concrete with the necessary characteristics. Additionally, these composites enable the creation of highly cohesive and mobile mixtures when combined with microfillers.

Fly ash, metallurgical slags, and other finely ground materials typically serve as microfillers.

Suggestions for surfactant additives:

• sulfite yeast mash (SYM);
• neutralized air-entraining resin (NAER);
• complex additives: GKZh–94+SDB.

The sulfite-yeast mash needs to dissolve in water that has been heated to between 80 and 90 degrees Celsius before using. A metal sieve with holes roughly one millimeter in size is used to filter the concentrated suspension (10–20% by volume) that is produced.

Tanks containing water meant for mixing the mixture are filled with the completed SDB solution before the other ingredients are added.

After finely grinding and diluting in warm water, the SNV additive is ready to use. 900 g of water and 100 g of neutralized resin are needed to create the composition. After being prepared, the solution is fed into a concrete mixer after being filtered through a sieve or rare fabric.

Underwater concreting is a specialized area of construction where materials and technology come together to solve particular problems. The important components of underwater concreting are examined in this article, with an emphasis on the unique supplies and cutting-edge methods that are critical to the process’s success. To achieve long-lasting and sustainable underwater structures, it is essential to comprehend these components, which range from the creation of concrete mixes that impede water penetration to the application of specialized placement techniques like tremie pipes. By exploring these methods, we are able to see how the ability to build sturdy foundations and structures below the surface of the water has been transformed by advances in materials science and engineering.

Concreting methods

The primary applications of concrete in water include the building and restoration of coastal concrete structures, as well as:

• river or sea berths;
• bridge structures;
• when equipping a waterproof (grouting) cushion;
• in special cases, during the construction of underwater parts of buildings.

Diving is an essential component of concrete work underwater, and divers’ duties encompass the following tasks:

1. Preparatory work.
2. Arrangement of concreting sections.
3. Installation of formwork.
4. Sealing (sealing cracks, etc.).).
5. Installation of concrete pipeline.
6. Process control with subsequent drawing up of the act of hidden works.

Among the preparatory tasks are:

• cleaning the work area from silt and debris;
• removing oil stains;
• removing a weak soil layer;
• installation of crushed stone preparation.

When laying concrete, the following kinds of formwork are employed:

• wooden panel;
• wood-metal;
• reinforced concrete non-reversible;
• metal removable.

Prerequisites for the formwork:

1. Impermeability.
2. The ability to rigidly withstand the pressure of concrete while maintaining the original geometric shapes.
3. Convenience during installation under water.
4. For joints, it is necessary to provide for sealing.

When installing formwork, it is reinforced from the outside using two rows of bags that are two thirds filled with sand or stone. Sections in contact with concrete when using replaceable formwork are covered with synthetic sheets or burlap that has been bitumen-impregnated.

Take note! Preventing the solution from coming into contact with water is crucial when laying mixtures, and the concreting process needs to be done continuously until the design marks are reached.

The following technological approaches can be used to classify concreting techniques in water based on the operational requirements of the structure being built:

• vertically moving pipe method (VMP);
• ascending mortar method (AM);
• pouring the mixture with buckets;
• island method;
• laying concrete in bags;
• laying with concrete pumps;
• hydrovibration;
• injectable.

VPT method

It is utilized when strong and substantial underwater structures are needed, and at depths of up to 50 meters.

In order to perform work on the future pile structure, a crossbar is installed on a work platform. A loading funnel is suspended from a 200 mm diameter pipe. The pipe is a 1-meter-long, readily detachable pipeline link.

In order to prevent the discharge of concrete mixture into the water, the working winch and suspension system must ensure easy vertical lifting of the pipe with an error of no more than 30–50 mm, as well as its rapid lowering by 30–40 cm.

Procedure for work, guidelines:

1. Work begins with lowering the pipe into the water with a minimum gap between the base, for the free exit of the solution.
2. A burlap bag is inserted into the cavity of the pipeline and the mixture is fed through the funnel.
3. Under the pressure of the concrete, the gag smoothly sinks to the bottom and, in the process of movement, squeezes water out of the pipe, as well as the air located in the upper part.
4. Concreting without repositioning the pipe continues until the material fills the entire space of the first block to a height of 0.8–1.5 m above the lower end of the concrete pipeline.
5. Then, without stopping the supply, the pipeline is raised so that its lower end is constantly located 0.8 m below the top of the already poured concrete.
6. Upon reaching the top of the first (lower) section of the pipe, the supply of the solution is stopped, the upper link is removed, the funnel is repositioned and pouring continues.
7. The concreting process continues until the volume of the mixture exceeds the design mark of the structure by at least 100 mm.

Plastic mobile mixtures with an acceptable cone settlement of 14–20 cm are used to produce work using this method.

A single pipe can cover up to a radius of 6 meters. Larger-than-expected structures are concreted using multiple pipes simultaneously, forcing the closure of nearby action zones.

The block’s width and the distance between the filling pipe axes are assumed to be no greater than 6 meters. There are multiple pipes that make up the receiving bin. Trunks are used to feed the mixture. There are valves on the trunk and loading funnel.

Method VR

At a depth of 1 to 50 meters, the ascending solution (VR) method is applied. Crushed stone or natural stone is poured into the area that has to be concreted. The mixture is then pumped through the pipe that has been installed, which spreads outward and upward to remove water from the structure’s voids.

When using the VR method for concreting, the following materials are needed:

• crushed stone of fraction 40-150 mm;
• natural stone of dense rocks of size 150-400 mm;
• sand with a grain size of 2.5 mm.

There should be no more than 45% of void volume in the stone preparation. Concrete should have a recommended strength of at least 15 MPa.

In order to use the VR method, the following devices are required:

• concrete pipes Ø 35–100 mm;
• metal funnels with a recommended bevel of 30° or more;
• for installing plugs at the beginning of concreting – pipes Ø 75 mm.

One pipe’s design rational coverage area is typically 18 m^2. A mixture feed rate of 0.3 m3/hour is advised. Depending on the technology being utilized, the ascending solution method can be classified as either pressure or non-pressure.

When there is no pressure:

1. An auxiliary shaft made in the form of a lattice structure is installed in the center of the concreted section. The cell size should be 2/3 of the smallest fraction of filler used.
2. A metal pipe Ø 90–100 mm is lowered into it to its full depth.
3. A stone mixture is poured into the space enclosed by formwork, the voids of which are subsequently filled with cement mortar (proportions 1:1 or 1:2). The cement paste spreads freely over the surface and envelops the previously laid aggregate.
4. After reaching the design mark, the mixture is raised by 10–20 cm and the pouring is stopped.

The filling pipes are installed in tiers (at different heights) when working with pressure. The lower tier is shut off and the higher tier’s pipes are activated as soon as the mixture reaches the necessary level.

The pipeline’s upper section, which is situated above the concreted structure, is severed when the necessary strength is reached, but the pipes are left in the solution after concreting is finished. When utilizing the first method is not feasible or cost-effective, the VR method is employed as a backup plan instead of the VPT.

Cement consumption is greatly decreased during the VR process, but the amount of equally expensive metal pipes is increased.

Supply of mortar with buckets

This technique is applied in structures whose design specifications permit a maximum concrete strength of 200 kg/cm2 at depths of up to 20 m:

• drop wells;
• shells of large-diameter structures;
• shell columns;
• leveling screeds for foundations, etc.

A bucket is a unique, sealed delivery container for concrete (see picture).

Work process:

• The mixture is laid in layers. A bin filled with the mixture is lowered into the water and fed to the working section. The first layer is opened and concreted.

Attention: Use a mixture with a cement content 15-20% higher than the calculated value for excellent concrete work on the bottom layer.

• The next bucket is pressed 5–10 cm into the laid mortar, and is gradually unloaded into the working section.
• Each subsequent layer of the mixture must be laid before the previous one begins to harden.

The mortar is laid in layers using this method, which has some drawbacks. Water may wash away the concrete that has already been laid as a result of this operation.

Tamping the mortar

When building structures at a depth of no more than 1.5–2.0 meters, the mixture is tamped down or the island method is employed. Some examples of such structures include:

• structures that do not require reinforcement;
• foundations on sloping banks;
• coastal protection fortifications;
• low-lying pile grillages.

In a concrete plan, the smallest block size ought to be 1.5 times larger than the design. The final value in the structure is 20% lower than the grade of concrete that was used. Cones settle to a depth of 5–10 cm.

The following order is followed when performing the work:

• sealed formwork is installed;
• using a pipe or bucket, an island is poured in the corner of the concreted section 30 cm above the existing water level.
• then, along the contour of the elevation (20–40 cm above the watershed boundary), portions of the mixture are poured, followed by its ramming with vibrating platforms or deep vibrators.

Benefit: There’s no need to divide the concrete structure into smaller pieces.

High labor intensity as a result of the process’s continuance and rapid solution delivery is a drawback.

Delivery in bags

The following situations call for the use of bagged mortar:

• during repair and construction work;
• sealing caverns;
• for leveling the bottom;
• strengthening removable formwork, etc.

Strong, thinned fabric that is able to pass cement milk, air, and water is the material used to make bags. The concrete filler, which has a grain size range of 10–200 mm, fills two third of the bags.

They are transported to the bottom in containers by a winch or crane, where divers unload them and manually reinforce them with steel rods measuring between 10 and 20 mm in diameter.

Hydrovibration and injection methods

The primary purpose of these two techniques is repair work. A feed rate of 1-3 m3/hour is used with concrete pumps to carry out injection concreting.

Work process:

1. Install a sealed formwork with an opening in the upper part.
2. In the lower part, it is equipped with a connecting nozzle through which the mixture is supplied from below.
3. The section is filled with crushed stone.
4. Concrete is continuously fed into the cavity being concreted through a nozzle.
5. The supply of the solution is stopped after the liquid mixture has exited through a special hole in the upper part of the formwork.

There are two options for the hydrovibration method:

1. The block is filled with inert components of concrete. Cement milk is poured with synchronous vibration of the filler;
2. Cement mortar is loaded into the section. Then large filler is loaded with simultaneous vibration of the mixture.

Concrete pumps are sometimes the only tools necessary for concreting in water. The sole distinction between this technology and the VR and VPT approaches is that filling pipes are not utilized in this instance (see the video in this article).

Underwater concreting poses distinct difficulties and necessitates specific materials and methods to guarantee structural soundness and longevity. The choice of materials that can survive the challenging underwater environment is one of the most important factors. In order to speed up setting times and lessen cement particle washout by water currents, accelerators are frequently added to concrete used for underwater projects.

For underwater concreting to be successful, technology is essential. Techniques like tremie pouring, in which water contamination is reduced by pouring concrete through a funnel system, are frequently used. This method assists in gradually releasing the concrete mix underwater while preserving its cohesiveness and quality. Furthermore, meticulous observation and modification of the consistency and flow of concrete are necessary to guarantee consistent placement and compliance with structural requirements.

To prevent aggregate segregation and ensure proper bonding between freshly placed and previously placed concrete sections, extra care is taken during the preparation and pouring phases. Together, engineers and divers supervise the process to make sure every underwater structure component is safely integrated and fits together as a whole.

Underwater concreting has made it possible to build important infrastructure, including bridges, dams, and offshore platforms, despite its difficulties. Underwater structures are becoming more and more reliable and long-lasting, and as a result, they are becoming an indispensable part of contemporary civil engineering projects worldwide, thanks to advancements in material science and construction techniques.

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