HYDRAULIC STRUCTURES

Hydraulic structures are constructed as part of the development of water resources such as hydro-electric power schemes, dams, canals, drains, and tunnels for water supply and sewage

The concrete used in these structures is often subject to a unique set of operating conditions depending on the application. For example, water filled tunnels, drains, and spillways may convey water at a high velocity in which case the surfaces of the structures may experience cavitation damage. In other structures, such as drains, reservoirs, and tanks, the concrete surface of the structure may be subject to wicking of moisture in the splash zone which can elevate the degree of exposure and deterioration suffered. Drains may also be subject to erosion damage from tumbling rocks and suspended debris. These operating conditions require special consideration that is seldom mentioned in standard exposure classification references such as AS3600 or AS5100. It therefore behoves the designer to become familiar with the special operating conditions prevailing for hydraulic structures.

Durability Conditions

Concrete in the splash zone of a water body, and especially a salt-rich marine environment, is susceptible to more rapid deterioration due to a combination of factors, including chloride-induced corrosion, sulphate attacks, and the effects of wet-dry cycles. These aggressive conditions can significantly reduce the lifespan of concrete structures in this zone compared to zones that are either predominantly dry or wet. Exposure conditions for the concrete ten to be worst immediately above the water line because of wicking of moisture from below the waterline to the evaporation zone above the waterline. This tends to transport of dissolved minerals through the concrete which are then precipitated in the drying zone leading to expansive salt crystallisation damage. This is clearly worse in salty water than fresh water.

Hydraulic structures are often located in mountainous areas which may be subject to freezing weather. This can lead to freeze-thaw damage of the surface of concrete which is aggravated if the concrete is wet. Improving the frost resistance of concrete is usually achieved by entraining air bubbles in the paste. The American Concrete Institute document 201.2R-16 Guide to Durable Concrete provides an excellent overview of this phenomenon and guidance on how to avoid freeze-thaw damage. Freeze-thaw can be particularly problematic for shotcrete because of the difficulty of control the air content in sprayed concrete. Resistance to frost damage typically requires an entrained air content of about 6 percent. Spraying normally expels air from the concrete leaving the in-place concrete with a very low entrained air content. Specially methods of spraying the concrete must therefore be used to guarantee a high air content (with bubbles of the required size) in the placed concrete.

Water in mountainous areas can also exhibit unique characteristics that are aggressive to concrete. Peaty water is generally acidic and concrete exposed to this type of water must be developed to resist acidic conditions. Water in mountainous areas can also be very pure, which can promote calcium leaching from concrete surfaces. Pure water, especially when in contact with concrete for extended periods, can dissolve calcium hydroxide, a key component of the cement paste in concrete. This process weakens the concrete matrix and reduces its strength. Resistance to acidic water and very pure water is improved by use of a low water/binder ratio and pozzolanic additives in the mix design.

Concrete exposed to sewage must resist the unique conditions that can prevail in anaerobic environments. An absence of oxygen in sewers can promote the formation of hydrogen sulphide which leads to acid sulphate attack of concrete. Sewage networks are normally designed to prevent anaerobic conditions occurring, but this is not always successful. Concrete used in pipes and other structures exposed to sewage must therefore be designed to resist acid sulphate attack, which is one of the most aggressive exposure conditions for concrete. The usual approach is to use a low water/binder ratio, high pozzolanic content, and aggregates that are resistant to acid (i.e., not limestone or dolomite). Aluminium sulphate set accelerators should not be used in shotcrete that may potentially be exposed to acid sulphate conditions.

Erosion

Erosion of the surface of a concrete structure is possible when exposed to flat-flowing water that includes suspended particles and tumbling debris. The American Concrete Institute Guideline ACI 210R-93 Erosion of Concrete in Hydraulic Structures is a useful starting point for assessment of the resistance of concrete surfaces to erosion in a water stream. This type of damage is usually concentrated in the base of a channel and may not be relevant to the entirety of a hydraulic structure.

A related phenomenon is scour around the base of bridge piers, canal walls, and other elements of hydraulic structures that may be exposed to floods, mud-flows, and other severe events in which debris suspended in rapidly flowing water impacts a concrete surface. These intermittent events are rare but may represent the most aggressive exposure condition for some types of hydraulic structure.

Improved resistance to erosion is usually achieved by increasing the strength of the concrete surface, ensuring it is smooth, and including fibres in the mix design. This is most readily achieved by spraying without set accelerator and screeding/trowelling the surface while it is still fresh. The surface must also be well cured to ensure high strength is achieved and drying shrinkage cracks are avoided.

Cavitation

Cavitation is a destructive hydraulic phenomenon that occurs in high-velocity water flows. Local pressures drops below the vapor pressure of water in recesses and trailing edges in the water stream, leading to the formation of vapor bubbles. When these bubbles collapse, they generate intense localized forces that can damage surrounding materials.

In hydro tunnels, which carry high-speed water under pressure, cavitation is a significant concern. One of the common materials used to protect tunnel linings from such forces is shotcrete—a pneumatically applied concrete mixture. When reinforced with fibres (typically steel or synthetic), shotcrete becomes more ductile and impact resistant. However, even fibre-reinforced shotcrete (FRS) is susceptible to the damaging effects of cavitation.

How does cavitation damage occur?

In hydro tunnels, cavitation typically occurs at locations such as bends, contractions, joints and intersections, also areas where the water becomes turbulent or the surface has irregularities.

Vapor bubbles caused by pressure fluctuations in flowing water implode near the tunnel surface, the released energy creates micro-jets and shock waves. These shock waves are in a way very similar to “Pipe Hammer” that can occur in household plumbing, both are caused by rapid changes in the water flow in-turn causing pressure surges in the system.

These shock waves and micro-jets have the ability to rapidly erode the surface, particularly if the lining material lacks sufficient resistance to repeated impact forces.

In fibre-reinforced shotcrete, although the fibres improve toughness and crack resistance and its ability to absorb energy, the concrete matrix at the micro level is still susceptible to erosion. This damage is usually observed as pitting and roughening of the lining surface, this then exacerbates the turbulence, and the likelihood of further cavitation is greatly increased. Over time, this can lead to progressive material loss, compromising both the hydraulic efficiency and structural integrity of the lining.

The main effects cavitation has on Fibre-Reinforced Shotcrete

Binder Matrix Erosion and Surface Degradation.
The primary effect of cavitation is the progressive removal of the cementitious materials in the binding matrix. Fibers rely on the integrity of the matrix for anchorage and while the fibres in FRS help bridge cracks and enhance ductility, any loss of cementitious materials and the fibres are susceptible to becoming exposed and even dislodged, especially if the bond between the fibre and matrix is weak. If left untreated the cavitation only worsens and surface degradation increases.

Decreased Structural Performance
Over time, as the surface deteriorates and fibres are exposed and lost, the energy-absorbing capacity and therefore load-bearing capacity of the shotcrete decreases. In Hydro Tunnels this is particularly problematic, where water pressures and dynamic loading is high. Through the loss of surface material, the effective thickness of the lining is reduced diminishing its resistance to internal water pressures and structural resilience.

Increased Maintenance and Downtime
Cavitation-induced damage requires regular inspection and repair; this is generally in quite challenging conditions. In operational hydro tunnels repairs on shotcrete linings may require partial or complete operational shutdowns, leading to substantial operational losses. Also repair materials and applications must match or exceed the performance requirements of the original FRS lining; this is of utmost importance in areas prone to recurring cavitation.

Accelerated Corrosion of Steel Fibres
In steel fibre-reinforced shotcrete, cavitation can also lead to localized exposure of steel fibres to oxygen and moisture, this exposure will then lead to corrosion. Corrosion and rust formation will not only weaken the fibres but also cause expansive stresses in the binding matrix, leading to further cracking, spalling and surface erosion.

Strategies to minimize the effects of cavitation.

Several approaches must be employed to minimize the effects of cavitation on FRS in hydro tunnels:

Improved Surface Finishing: Ensuring the shotcreted surfaces are smooth reduces turbulence and cavitation risk. Application technique, Quality Control and monitoring are paramount during this stage.

Use of High-Performance Concrete: Incorporating supplementary cementitious materials (SCMs) e.g., silica fume, Fly ash, Metakaolin enhances density within the binder matrix and increases resistance to cavitation erosion.

Optimization of Fibre Content and Type: Blends of both steel and synthetic fibres can balance toughness with corrosion resistance.

Surface Linings and Coatings: Application of epoxy or polyurethane coatings over shotcrete can also provide a sacrificial layer that also resists cavitation forces.

Cavitation remains a serious threat to the performance and longevity of fibre-reinforced shotcrete linings in hydro tunnels. While fibres significantly enhance the material’s toughness and post-crack behaviour, they cannot alone completely prevent damage caused by cavitation. Effective design, material selection, application technique and maintenance practices are crucial in mitigating the impact cavitation has on FRS whilst also ensuring the durability of hydro tunnel infrastructure.

LINK: Concrete Institute of Australia - Shotcreting in Australia