Introduction to Fiber Reinforced Concrete
Fiber-reinforced concrete (FRC) is extensively used as primary or secondary reinforcement in infrastructure projects. Recent advancements in serviceability-based design methods have allowed the adoption of FRC since we can now design for a specific deflection, crack width, stiffness, or load capacity level. Fibers have several advantages in comparison to traditional reinforcement which includes increased ductility, durability while reducing the construction time, and labor costs. FRC can be used to enhance handling and placement of precast concrete segments with the added benefit of reducing job-site labor requirements. FRC also improves the post-cracking behavior considerably, and increases energy absorption, defined as toughness several folds. With better crack control characteristics than conventional steel-bar-reinforced concrete, the use of FRC generally results in improved durability over the life of the structure.
The structural design of FRC sections is based on post-cracking residual strength provided by randomly oriented fibers inside the cementitious matrix. A minimum mechanical performance of FRC must be guaranteed for its structural use for which, ACI 544.4R recommends conducting the ASTM C1609 beam test on a closed-loop system. Our facilities at ASU are equipped to conduct a host of characterization techniques for FRC using strength and fracture tests on conventional and non-conventional materials.
Full-Scale Testing Facility at ASU
Precast concrete tunnel segments are installed to support the tunnel bore behind the tunnel-boring machine (TBM) in soft ground and weak rock applications. The TBM advances by reacting against the completed rings of precast concrete segments that typically provide both the initial and final ground support as part of a one-pass liner system. These segments are typically designed to resist the permanent loads from the ground and groundwater, as well as the temporary loads from production, transportation, and construction (ACI 544.7R). FRC can result in improved durability and toughness at SLS (service limit state) since they can reduce crack spacing and crack width. FRC can also be used to improve the behavior at ULS (ultimate limit state) where they can partially or totally substitute conventional reinforcement.
TBM advancing by reacting against precast tunnel segments
Herrenknecht tunneling video,
demonstrating tunnel boring machine operation
The performance of tunnel segments compared to conventional designs can be tested with full-scale testing of the precast segments. The design methodology developed at ASU utilizes parameterized stress-strain models generated using closed-form solutions that were verified with full-scale testing of tunnel lining segments in the large-scale structures lab at ASU. The precast tunnel segments must be tested for flexure loading and compression point loading whose setup and instrumentation are shown in figures below.
Schematic drawing with location of instrumentations
Total Instrumentation in Flexure Testing to measure:
- Deflection (LVDT x4)
- Crack-Width (String Sensor x4)
- Strain in Tension and Compression (Strain gage x4)
- Digital Image Correlation (DIC)
Digital Image Correlation (DIC)
Load Frame for full scale testing
Total Instrumentation in Compression Testing to measure:
- Displacement (String Sensor x4)
- Strain (Strain gage x5)
Load Frame for compression testing