Fiber-Reinforced Concrete (FRC) incorporates structural or non-structural fibers into the concrete matrix and is applied using a spraying technique—an approach that has evolved significantly since Carl E. Akeley patented shotcrete in the U.S. in 1911. Today, this method is widely established for applying concrete layers onto preconditioned surfaces.
In tunneling, the most common applications of shotcrete include: protective layers, structural support (either temporary or permanent), and final linings, used both in new construction and in the rehabilitation of existing structures.
To ensure proper bonding between the fibers and the concrete matrix, the concrete must achieve a minimum compressive strength of 20 N/mm² (approximately 2,900 psi). The optimal water-to-cement ratio typically ranges between 0.40 and 0.45. The fiber dosage is determined based on project requirements, the specific application, anticipated rebound, and the characteristics of the fiber itself. Manufacturers should recommend an appropriate dosage based on the required performance and their field experience, and it is advisable to conduct preliminary on-site testing.
Structural fibers can partially or fully replace conventional reinforcement, enhancing the mechanical properties and durability of hardened concrete. This can lead to cost savings and improved safety. Non-structural fibers, on the other hand, help control shrinkage cracking, provide some fire resistance, and improve application efficiency by reducing rebound. Both fiber types can be applied using either dry-mix or wet-mix shotcrete processes. However, in dry-mix applications, hose lengths over 150 meters (approximately 500 feet) are not recommended due to economic and performance considerations.
Typically, fibers used in sprayed concrete are 30 to 40 mm (1.2 to 1.6 inches) long with a diameter of around 0.50 mm (0.02 inches). It is essential that fibers are clean and free from any contaminants that could impair their adhesion to the concrete.
The structural design of tunnel and slope support using shotcrete is based on engineering tools that consider the early-age strength of the material (young concrete) and must ensure long-term performance throughout the structure’s service life.
As noted in the NIOSH (2014) publication, since Fiber-Reinforced Concrete (FRC) is often used in combination with other support systems, its load-bearing capacity cannot always be determined through purely analytical methods. Therefore, empirical approaches—often supported by rock mass quality indices—are commonly used, especially in weak ground conditions.
Toughness is the key parameter in FRC design. The TPL index (Toughness Performance Level) is used to define different levels of toughness based on the fiber dosage. To achieve optimal lining performance, the design must balance concrete strength, fiber anchorage, and the application method, all tailored to the specific project conditions.
The design should account for both short-term and long-term loading scenarios, as well as intermediate phases that may occur during the construction process.
The structural design of fiber-reinforced sprayed linings demands a thorough analysis that considers:
There are various methods and tools available for the structural analysis of fiber-reinforced linings:
It is essential to evaluate the performance of Fiber-Reinforced Shotcrete (FRS) through specific tests that verify its properties both during the trial phase and throughout project execution. These tests must be conducted using final materials, including a control mix (without additives or fibers) to establish a baseline for comparison.
Preliminary testing is used to assess how fiber addition affects compressive strength, flexural tensile strength, and toughness—always benchmarked against a plain concrete mix. In tunneling applications, energy absorption capacity and residual strength are critical parameters, as the performance of FRS is largely defined by its post-crack behavior.
There are various testing methods for evaluating the energy absorption (or toughness) of FRS, typically categorized into two main groups: direct tests, using beams or panels to measure load-deflection behavior and fracture energy; and indirect tests, involving alternative methods to infer toughness and residual strength from other mechanical responses.
These tests are conducted on properly sized core specimens, either extracted from in-situ sprayed concrete or from test panels.
These methods allow for in-situ measurements without the need for prior surface preparation. However, they require calibration curves tailored to the specific type of concrete being tested.
Current research and testing on Fiber-Reinforced Shotcrete (FRS) aimed at improving fire performance focus primarily on the incorporation of polypropylene microfibers. The goal is for the material to withstand high temperatures, such as those encountered during a fire, for at least two hours.
These microfibers are typically dosed at a minimum of 2 kg/m³ and serve a critical function: reducing both water vapor pressure and internal stresses caused by thermal deformation within the concrete when exposed to fire. Upon reaching their melting point (approximately 160 °C), the fibers melt and disappear, leaving behind tiny channels inside the concrete matrix. These channels facilitate the release of accumulated vapor, helping to minimize phenomena such as explosive spalling and crack formation.
It is important to select synthetic fibers with appropriate dimensions to ensure they do not remain suspended during the spraying process. This not only improves the quality of the application but also reduces health risks associated with potential inhalation.
Permanent fiber-reinforced shotcrete (FRS) linings are increasingly used worldwide because they reduce both construction costs and the carbon footprint compared to traditional concrete linings. As permanent structural elements, they must achieve the same service life as the structure they protect.
For this reason, their design must consider not only short- and long-term loads but also intermediate conditions that may arise during the construction phase. Unlike temporary shotcrete, these linings typically incorporate structural fibers instead of steel reinforcement, particularly as a corrosion prevention measure.
The appropriate fiber dosage should be defined based on ground conditions and the specific characteristics of the sprayed concrete mix. Furthermore, to ensure proper performance and durability, it is essential to conduct preliminary testing with representative mixtures.
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