Strong storage solutions

Edo Vonk, VSL International, highlights the basic guidelines for inspection and maintenance of pre-stressed concrete tanks for LNG and LPG storage, emphasising the differences between the various systems.

he use of pre-stressed concrete containments has proven to be a cost-effective, durable, and safe solution for liquid storage facilities. Their size can range from small (several metres in diameter) to very large (up to 100 m dia.), and their height varies depending on the application.

The basic principle of pre-stressed concrete tanks relies on the tensile strength of steel tendons, which are tensioned against the circular concrete structure. This induces a uniform axial compression into the concrete cross-section under the hoop stresses of the tendon. While concrete itself is a material that exhibits relatively high compressive strength, it does not allow the transfer of significant tensile stresses. Under tensile stresses, the concrete will crack and lose its structural integrity, unless reinforced by steel reinforcement bars, as well as foregoing its leak tightness. Putting the concrete into compression by the use of pre-stressing for all prevailing service load-cases is an elegant solution that has demonstrated its efficiency and reliability for approximately a century.

For LNG and LPG storage tanks, the concrete wall acts (in most cases) as the outer secondary containment designed to react the internal tank pressure in accidental scenarios only, i.e., after failure of the primary steel alloy containment membrane. In addition, this outer secondary concrete structure provides a robust barrier against external abnormal loads (impact, explosions, etc.) and aggressive environments, in particular where exposed to industrial emissions or chloride-laden marine air. The concrete containment is hence a safety-critical element of any LNG or LPG storage facility, and tank designers, operators, and owners have to ensure that the concrete containment could fulfil its critical role throughout the life of the facility.

Similar to any other pre-stressed concrete applications, in particular bridges, pre-stressing systems require regular inspection and maintenance in order to ensure their integrity throughout the service life of the structure, and the structure can be prone to hidden defects, in particular corrosion of the high tensile steel.

Basic guidelines for inspection and maintenance of pre-stressing systems

In this context, it is important to establish some basic guidance for the inspection and maintenance of pre-stressing systems in LNG and LPG storage tank applications. To the knowledge of the author, such guidelines are currently lacking for LNG and LPG storage tank applications, and it is therefore the objective of this article to present a snapshot of the state-of-the-art in the inspection of pre-stressing systems and discuss its application and relevance in the case of LNG and LPG storage tanks.

Corrosion of high-tensile steel

Pre-stressing systems use very high-strength, cold-drawn steel wires (up to approximately 2000 MPa tensile strength) with low ductility (below 5% elongation at rupture) and small cross-sections (typical wire cross-sections below 10 mm dia.) These steel wires can be potentially prone to failure by corrosion. While corrosion failure mechanisms of conventional structural steel are mainly governed by gradual section loss, the failure mechanism of high-tensile cold-drawn steels are typically controlled

by local accelerated corrosion (pit or crevasse corrosion) and subsequent rapid failure under crack propagation or notch effects. In general, such failures are sudden and characterised by very short advance warning.

Pre-stressing systems

Two distinctively different systems can be found in LNG and LPG storage tanks. The pre-stress is either applied by spirally winding a continuous high-strength steel wire under load around the outer-concrete perimeter (as seen in older generations of pre-stressed concrete tanks) referred to as ‘spiral wrapping system’, or by embedding high-tensile steel strands within reservations (ducts) inside the concrete wall proper referred to as ‘embedded post-tensioning system’. This is in addition to the general circular pre-stressing of the tank wall. Secondary post-tensioning systems might be present in a tank, either in vertical direction, or as concentrated horizontal tendons in the upper ring beam to react to the outwards dome forces of the roof structure.

Differences between the systems

In the context of inspection and maintenance, it is very important to understand the differences in detailing and behaviour of the two systems, as well as the prevailing deterioration mechanisms.

Figure 1. LNG concrete storage tank during construction.

Figure 2. Corrosion of strands inside a pre-stressed tendon.

Spiral wrapping systems

After wrapping of the steel wires in a continuous tensioning process, their corrosion protection is achieved by applying a layer of shotcrete (spray-applied cementitious material). The corrosion protection is based on the principle of electro-chemical passivation of the steel surface in the high-alkaline cementitious environment. For long-term corrosion protection, the encapsulation needs to be: (a) continuous, hence it shall be free of voids, (b) largely impermeable, i.e., dense and without larger continuous pores or cracks, and (c) remain at the depth of the steel surface highly alkaline throughout the service life. Several environmental effects can lead to the breakdown of one or several of these characteristics which will lead in turn to local or widespread corrosion of the pre-stressing steel. The two most important mechanisms to cause breakdown of the protective function are concrete carbonation, which leads to a loss of passivity of the steel and chloride-induced corrosion in marine environments. In addition, the presence of sulfates or other pollutants in industrial environments can also lead to accelerated corrosion phenomena.

Carbonation is triggered by the diffusion of carbon dioxide from the atmosphere into the outer concrete layer. This then triggers chemical reactions, lowering the pH value of the cement matrix. Once the carbonation front has reached the embedded pre-stressing steel, the steel’s electrochemical potential becomes more active. This then triggers general surface corrosion, eventually leading to section loss and failure of the steel cross-section. At the same time, the corrosion products are highly expansive, resulting in cracking and spalling of the concrete and ingress of moisture and corrosive substances.

Chloride induced pitting corrosion is caused by the local access of chlorides to the steel surfaces. In the case of storage tanks, the most frequent source of chlorides is airborne chlorides in marine environments deposited at the concrete surface. Even higher deposition rates can be observed where tanks are located close to the seashore in direct marine spray. Chlorides migrate via pores and cracks deep into the concrete and eventually reach the steel surface. This is where they can cause a local breakdown of the passive film which then results in very local and accelerated pitting corrosion.

Failed wires will re-anchor itself by bond in the shotcrete layer to some extent after initial slip so that the first wire breaks might not become immediately visible. More widespread failure will however lead to larger concrete spalling and eventually failure and uncontrolled release of the continuously wound tendons. This bears a significant health and safety risk in normal service conditions in addition to the tank containment losing its safety relevant function.

For spiral wrapping systems, inspection shall hence focus on the depth of carbonation, the presence of chlorides (or other corrosive agents), and the presence of cracks, voids, spalling, or porosity which can all cause early and accelerated corrosion of the post-tensioning system. Wire failures are a pre-cursor of full system failure.

Embedded post-tensioning systems

These systems consist of a duct embedded into the concrete prior casting, creating a void in which the high-tensile steel tendon can be threaded afterwards. The tendon is made in most cases by a number of prestressing strands, with each strand formed by seven 5 mm dia. wires. The pre-stressing tendons are individually anchored in a steel anchorage block using conical steel wedges. After tensioning to the targeted force, the pre-stressing tendon is

injected with a cementitious grout, providing passivation of the pre-stressing steel strands and therefore corrosion protection. While older systems used corrugated, spiral wound steel ducts as void formers, more recent systems use corrugated plastic ducts combined with system couplers to obtain a leak-tight additional barrier. These systems are considered today the state-of-the-art in durability.

The detailing of the concrete storage tank wall is such that embedded post-tensioning tendons have much larger concrete covers than in the case of spiral wrapping system. Consequently, embedded post-tensioning systems are better protected against corrosion induced by external effects. Based on several decades of research and observations in the corrosion of post-tensioning systems in bridges and other concrete structures, it is recognised that there can be other internal mechanisms that can trigger localised corrosion. When the tendons are injected with cementitious grout after stressing, it is possible that air voids get trapped inside and/or that the grout mix segregates as a result of improper detailing, unstable mix design, insufficient mixing, incomplete venting, or issues of general workmanship. Where air voids are present, it has often been found that the remaining oxygen and moisture can be sufficient to cause local corrosion of the exposed pre-stressing steel. This process will be significantly accelerated in the presence of chemically-different segregation products, leading to the formation of local corrosion cells due to a difference of electrochemical potential. It is known from bridges that such failures can occur as early as only a few years after completion or delayed until activated by the access of moisture from the outside.

Any failure of strands will remain hidden as the tendon can re-anchor itself within the corrugated duct. This will, however, still lead to a local loss of pre-stress in the concrete wall and hence a safety risk for the storage tank operation.

Non-destructive testing

Recent years have seen a rapid improvement in non-destructive testing technology applicable to the inspection of a pre-stressed concrete structure to ensure the integrity of a client’s asset is confirmed where the three most suitable are:

z Ground-penetrating radar (GPR) – GPR has established itself as a rapid scanning technique for concrete structures. It can detect changes in material conductivity and hence density.

It is best used to detect voids and areas of delamination while they are still hidden within the structure. Radar signals are, however, being fully reflected by metallic objects and can hence not be used to find voids within metallic post-tensioning ducts. With recent advances of data processing algorithms supported by fundamental research into concrete conductivity, GPR has today been successfully used to also map hot-spots of chloride contamination and moisture. It can therefore be used to detect pathologies in spirally-wound, as well as embedded post-tensioning, systems. Compared to other technologies, such as electrochemical potential mapping, GPR can achieve much higher scanning rates and help to reduce the cost of full tank mapping.

z Ultrasound-pulse echo arrays (UPE) – The integration of pulsed ultrasound measurement principles with a hand-held array of transducers and advanced imaging capability has resulted in powerful equipment for the detection of voids in concrete and post-tensioning systems. Different to radar, ultrasonic signals travel well through dense materials, such as steel, which allows the detection of voids even within metallic duct systems. As it requires physical coupling, the achievable measurement productivity onsite remains low and UPE should hence remain reserved for local investigations following some initial findings from document studies, visual, or GPR surveys.

z Magnetic flux leakage measurement (MFL) – This is an electromagnetic measurement principle making use of the principle that a sudden reduction in steel cross-section can lead to a leak of an induced magnetic field which can be detected at the concrete surface. The magnetic field is either induced by strong electro- or permanent magnets, and the flux is measured by set of coils. The testing unit needs to be moved along the tendon, and gets typically mounted on a rail system for quick scanning and efficient handling. MFL is the most suitable method to find wire breaks.

The combination of advanced non-destructive testing methods with engineering review of existing tank documentation and environmental conditions, periodic visual inspection, and limited intrusive material sampling are the prerequisite to keep ageing pre-stressed LNG and LPG storage tanks remain in safe state. The interpretation of results requires the implication of necessary experts in pre-stressing systems to derive relevant conclusions for any maintenance, repair, or strengthening measures.

Figure 3. Sudden failure of a pre-stressing tendon due to corrosion.

Figure 4. Advanced ground-penetrating radar (GPR) map showing increased chloride concentrations.