Technical Insights by Mark Trim, CPEng, and Sean Peterfreund, SE
Part 1 of this series examined various existing test methods to verify post-crack values of shotcrete (fiber reinforced sprayed concrete). Part 2 presents shotcrete research McMillen Jacobs Associates has undertaken with local industry partners in Sydney, Australia: Professor Stephen Foster and Research Fellow Ahsan Parvez from the School of Civil and Environmental Engineering of the University of New South Wales (UNSW), and a local area contractor.
Our main research goals were to perform direct tensile testing of shotcrete in accordance with Australian Standard (AS) 3600:2018; and investigate and improve the underground industry’s understanding of residual tensile strength (post-crack) performance of shotcrete by indirect tests (flexural beam test BS EN 14565*) and direct tests (direct tensile test AS3600). One potential benefit to contractors is improved accuracy in developing approved shotcrete mix designs, allowing optimization of the amount of steel fibers required to produce a compliant and durable shotcrete mix design.
Direct tensile testing was performed at UNSW’s Heavy Structures Laboratory. Flexural beam testing was provided by the local contractor. The research test program included 18 direct tensile tests, 18 flexural beam tests, and 6 unconfined compressive strength (UCS) tests. Specimens were tested after curing for at least 28 days. This is the first time that the direct tensile test has been performed on shotcrete and that direct tensile and flexural beam testing methods have been compared.
The plot in the figure (right) shows a typical result of stress versus crack opening displacement (COD) in pure tension. Results should not be assumed to be similar to those of flexural beam tests, which typically report stress versus crack mouth opening displacement (CMOD). If the designer is using AS3600 or AS5100, the direct residual test results (COD) can be used directly with the Australian Standard design model. However, for use in common design models,† these results need to be matched by inverse analysis with flexural beam test results. Inverse analysis converts the stress-CMOD relationship into stress-COD.‡ Once flexural beam results are expressed in the stress-COD relationship, they can be matched (calibrated) with the direct test stress-COD results. Once calibrated, the resulting correlation factor can be used on all future flexural beam tests.
Based on initial results, it was concluded that the inverse analysis methods developed for steel fiber reinforced concrete can be used for steel fiber reinforced shotcrete. Furthermore, the residual tensile results were within the expected range for fiber type and dosage used. Based on the success of the testing and because this was the first time direct tensile testing was used on shotcrete, McMillen Jacobs and our partners will publish the results and findings once final data post-processing is complete.
Because of the intricacy of the test setup and amount of testing time needed (compared to flexural beam), direct tensile testing during production of a major civil project is likely not feasible. However, preproduction
testing for developing the mix design and possibly limited production verification at certain milestones are considered practical and worthwhile. Additionally, this direct tensile test method could be used assist contractors in resolving nonconformances related to verification of residual tensile strength.
* Standard industry test for shotcrete performance.
† Such as RILEM/NZS 3101-2, fib Model Code 2010, and DAfStb.
‡ For a discussion on inverse analysis, see Foster et al., 2017 (Structural Concrete 19).
For more information contact Mark Trim at firstname.lastname@example.org.