A mining company in the southwestern United States needed to test fiber reinforced concrete (shotcrete) used to line the mine shafts in a new copper mine. The flexural tests in accordance with ASTM C1609-10 are critical to ensure the concrete has the strength needed to properly support the shaft. The flexural specimens are created as the concrete is being applied and must be tested within 8-12 hours of being applied making it impossible to use a test lab.
The purpose of ASTM C1609-10 is to not only determine the peak strength of the concrete but also to find the residual strength from the fibers after the first crack appears. For this reason, the test extends well past the initial peak. The standard also specifies that the test must be performed in servo control at very slow deflection rates with servo feedback from the average of two deflection sensors placed on either side of the specimen. The initial crack in the specimen and the associated jump in deflection (servo feedback) make servo control very challenging.
Instron supplied a custom 300KN frame that was designed to perform both the ASTM C1609-10 testing and also tests in accordance with ASTM C1550-10a on fiber reinforced concrete slabs. A standard third point flex fixture (W-6814) was supplied along with custom sensors and mounting jigs to measure the average deflection.