The sea refuses to bend materials that are weak. By the time the object reaches the shore, its ancestors have been eaten away, washed away and snapped out of place by the constant movement of the water, salt and the shifting ground. The precast concrete marine supports are in existence because someone many years ago realized that factory-manufactured concrete will last much longer in saltwater than it will if it is poured on-site in a tide zone. This article explains the nature of these supports, how they are designed to last and where they are found in actual coastal construction.
The Actual Definition of Precast Concrete Marine Supports
Structural concrete marine supports, piles, pile caps, beams and platform sections that are produced in a controlled factory environment and delivered to a marine or coastal site for installation. When the components are delivered, they are ready for installation, without having to be poured in the sea or tidal zone, which is very difficult and hazardous.
This is for a variety of products that operate in a coordinated manner. Piles are driven into the seabed to support a structure. Piles are topped with pile caps to spread out load across several points. These caps are then connected with beams and deck panels to form the usable platform above the water level which may be a jetty deck, a pier or a berth structure.
Why Precast Beats Cast-in-Place in Marine Settings
It’s easy to say to pour concrete at a marine site, but it’s not so easy to do. In reality, it is a logistical and tides-and-waves nightmare with a curing process that despises the saltwater issues before the concrete sets.
The solution to this is to produce the product in a controlled factory environment, with precast manufacturing doing just that. This control enables the precise proportions of mixture, uniform placement of reinforcement and uniform compaction, resulting in a denser and more uniform concrete that contains less voids and capillary pathways. A few voids results in fewer ways for the aggressive chloride ions to attack the concrete and reach the steel in the concrete.
It’s a big difference in marine environments where the biggest threat to any concrete structure sitting near saltwater is reinforcement corrosion over time.
The Real Enemy: Chloride and Sulfate Attack
Before one can appreciate the need for special engineering to preserve marine supports, it is necessary to first appreciate the reasons for the destruction of ordinary concrete near the sea. The chlorides in the seawater eat away the surface of the concrete and slowly corrode the steel reinforcement. When chlorides get to that steel and are present in enough quantity, corrosion starts and corroding steel expands, causing the surrounding concrete to crack from the inside out.
A second threat is sulfates. The long-term studies on the concrete piles in the seaside have shown that continuous contact with aggressive seawater causes sulfate attack (SA), which is a chemical reaction that causes degradation of the concrete matrix itself instead of corrosion of the reinforcement. Biological attack on specific piles has even been found to be another form of degradation in some coastal areas, through decades old piles.
This does not imply that concrete is not suitable for marine construction. This implies that the mix design of marine concrete is fundamentally different from the concrete for a typical structure foundation location inland.
How Marine-Grade Concrete Mix Design Resists
Normal concrete formulations are not used by precast concrete producers creating marine supports. They produce special mix designs with the specific intention of achieving high strengths, low permeability and high resistance to chemical attack.
Formulations of High-Performance Concrete (HPC) are developed for special applications, such as for marine construction, and usually result in much more impermeable and dense concrete than normal concrete mixtures. That high density gives a much better resistance to the ingress of chloride and sulfate.
The process is critical and the use of Supplementary Cementing Materials, typically fly ash, ground granulated blast furnace slag and silica fume is vital. These materials provide a reaction with the calcium hydroxide formed during the hydration of the cement, creating a smaller pore structure in the concrete and additional barriers to the movement of aggressive ions. Some studies have also suggested using specific types of aggregates (such as granite rather than some limestone) and specific types of cement (in order to withstand sulfate attack in coastal environments).
Types of Precast Concrete Marine Supports
Precast Concrete Piles
They are the building blocks for most marine structures and are driven into the sea bed to convey structural loads to the stable foundation below the water. In marine construction, the use of prestressed precast piles is widespread due to the benefits of prestressing for the resistance they offer against bending and handling stresses during pile driving.
Pile Caps
Pile caps are located on top of a pile cluster to spread the structure load of the overlying deck over several piles instead of focusing at only one point. This helps to reduce risk and enhance general structural stability.
Install Deck Beams and Deck Panels
Precast beams and deck panels are then concreted between piles and caps to create the deck surface of a usable jetty, pier, or berth. Ideally precast components are installed quickly and have a higher quality than pouring a deck directly on open water.
Seawall/Break Wall Units
Seawalls and breakwaters, which are used to absorb and dissipate wave energy before it reaches the shoreline or a protected harbor area, are another use of precast concrete.
In the Case of Marine Supports, They Are Being Utilized Wherever They Can
Precast concrete marine supports are visible in all places where infrastructure meets the sea. They are used in ports and harbours for berth structures, jetties and cargo handling platforms. Precast seawalls and breakwater units can be used to provide protection against shoreline erosion and storm surge. Coastal water crossing structures utilize marine piles and pile caps as their foundation system, whereas platforms and access structures of offshore and nearshore industrial facilities are supported by marine piles.
In quick-growing coastal districts, fast construction of marine precast works typically has to meet demanding port extension and waterfront development schedules, another factor making precast construction preferred over cast in place options in today’s marine construction projects.
How to Make a Successful Marine Support Project.How to Make a Successful Marine Support Project
There are a few things that always make the difference between projects that will last for decades and those that begin to have difficulty in the first few years of operation.For marine precast projects, a few things always make the difference to having a project that is going to last decades rather than experience problems within the first few years of use.
- Mix design specificity. For marine supports, mix designs must be developed for the exposure conditions that are specific to that site and not a “durable concrete” specification taken from a different project.
- Reinforcement protection. In addition to the mix design, the reinforcement may require further protection in aggressive marine conditions, such as a greater concrete cover thickness or corrosion resistant reinforcement in areas with heavy corrosion.
- Quality Assurance in production. As precast marine products are made off-site, the quality of that production process, the compaction, the curing, the placement of reinforcement, is directly related to the quality of the product as it is placed in the sea water environment.
- Installation expertise. Even a properly constructed pile or pile cap must be installed correctly. Cracks can be induced during moving and driving due to wrong handling that may affect durability before the structure is even in use.
There Are Common Misunderstandings That Need to Be Addressed
There are a few misconceptions that crop up around and around marine concrete. Others believe that high strength concrete will perform well in proximity to salt water because it is strong, but this is not necessarily true: Strength does not equate low permeability and it is the permeability that determines chloride resistance. Others believe that the precast structure is just a quick and easy alternative to cast-in-place with no difference in durability, and that is not the case; it is the controlled manufacturing process that is the reason precast marine components are durable to begin with.
Bringing It Together on
Precast concrete marine supports were developed to address a truly challenging engineering challenge – the creation of a structure capable of resisting structural loading for decades of exposure to constant attack by saltwater, chlorides and sulfates. They all play their part, and are built in the controlled environment of a factory, using mix designs that have been specially designed to withstand marine degradation.
Knowing this difference—ordinary concrete versus purpose-built marine concrete—that’s important whether you’re specifying a project or simply trying to figure out why that jetty down the road has been standing solid for 30 years while the seawall across the street, which has been poorly constructed, has begun to crumble. The sea is a generous provider to precision and marine supports build in precast precision in every layer.
