Volume 39, Issue 4 (July 2011)
Influence of Feedback Control on Flexural Toughness of Fiber Reinforced Concrete in ASTM C1399 Tests
The influence of feedback control on the measured flexural toughness of fiber reinforced concrete (FRC) remains elusive. Some tests such as ASTM C1609/C1609M-07 require closed-loop control, while others such as ASTM C1399/C1399M-10 are considered control independent, and hence open-loop testing is allowed. Recent field experience has indicated that results from even ASTM C1399/C1399M-10 tests may be test control dependent. Towards this end, a test program was initiated to understand the influence of feedback control in ASTM C1399/C1399M-10 tests. Tests were performed on specimens of two different concrete strengths and one dosage of a polymeric fiber under both open-loop and closed-loop environments. In addition to performing the analysis using the ASTM C1399/C1399M-10 approach, Ri values as per the Canadian Highway Bridge Design Code (CHBDC-S06-16) were calculated. Ri values are derived from the Average Residual Strength (ARS) values obtained from ASTM C1399/C1399M-10 tests. The results indicate that while the influence of feedback control on the measured ARS values in the case of normal strength FRC is only marginal, its influence on high strength FRC is significant. The same applies to the Ri values calculated in CHBDC-S06-16, where the results indicate that based on the published minimum acceptance criteria, the choice of feedback control may in fact govern the acceptance or rejection of a given FRC material. In the context of these findings, it is recommended that beyond a certain compressive strength, ASTM C1399/C1399M-10 tests should only be performed in a closed-loop environment.