Technical Insights by Sean Peterfreund, SE, and Mark Trim, CPEng
For fiber-reinforced concrete structural linings, specifically sprayed concrete (i.e., shotcrete), the bending strength is determined after the concrete has cracked (i.e., exceeded the peak flexural tensile strength). This post-crack strength parameter is called “residual flexural tensile strength.” For the past two decades the underground industry, particularly civil tunnel construction, has used various test methods to verify this residual strength value. The test used would likely be based on what was done previously (especially in countries where laboratories may only be certified and/or equipped to perform one test method) and the designer’s or contractor’s preference.One of the challenges with verifying residual flexural tensile strength is that concrete no longer behaves elastically post crack. The behavior of fiber reinforced concrete, post crack, will vary (strain hardening or softening) depending on the mix design and in particular the type of fiber used. Even defining the ultimate limit state (ULS) can depend on the performance required. Most international design standards provide guidance on ULS, which is typically assumed to occur when the fibers are near the point of failure (pull-out or snap through). To design and verify fiber reinforced concrete linings, the designer must determine the performance needed from the lining and the test method to verify performance assumptions. It is critical that the designer use a test method that matches the design method. That is, the test method must return a residual flexural strength value from the test result that can verify the design.
Over the past couple of years we have seen designs/specifications that specified one or two test types: one uses a beam as the sample, and the other a round panel. Both have their pros and cons, but only the beam test can verify the design without additional testing. This method (ASTM C1609 or EN 14157) must be used if the fiber reinforced concrete lining is designed as a structural lining (to withstand bending and axial forces). The round panel test (ASTM C1550) does not directly verify the residual tensile value. It only provides an indication of toughness, not the residual tensile strength. To verify the residual flexural strength values using round panels, correlation testing with a beam test is needed. See E.S. Bernard (2002)* for a more detailed discussion of the importance of correlation testing if round panels are used to verify performance of fiber reinforced concrete structural linings.
The design is not complete until it is verified by proper testing and inspections. For fiber reinforced concrete linings, selection of the test methods is critical and must be revisited on every project.
In a future “Part 2,” we will summarize some shotcrete research testing we are performing with partners to investigate the conversion factors provided in the various codes and standards.
* Bernard, E.S. 2002. Correlations in the behaviour of fibre reinforced shotcrete beam and panel specimens. Materials and Structures, RILEM, 35: 156–164.