Modified pullout test for indirect characterization of natural fiber and cementitious matrix interface properties

Fiber-matrix interfacial bond characterization using conventional fiber pullout test is unsuitable for natural fiber based cementitious composites. Therefore, a modified pullout test method is proposed in this study for the indirect characterization of interfacial properties of natural fiber and cementitious matrix interface. To this end, a cementitious matrix medium is prepared by mixing Type I Portland cement, clay and water in a specific ratio such that suitable consistency is obtained and setting time is delayed with the purpose of avoiding the formation of adhesive chemical bond between fiber and matrix interface. For experimentation, wheat straw natural fibers are chosen in this study and pullout test specimens are prepared by embedding the fibers in cementitious matrix medium for lengths of 10 and 20 mm. Pullout tests are carried out at 1 mm/min displacement rate, and load-displacement curves are obtained. While the peak pullout load and pullout energy obtained from modified pullout tests are observed to increase by 93.2% and 304% with an increase in embedment length from 10 to 20 mm, the interfacial sliding shear strength remained constant for wheat straw fibers at 2.63 KPa. Slip softening is observed to be the governing pullout mechanism for wheat straw natural fiber in the modified pullout test. For indirect validation of the proposed method, the trends of peak pullout load and interfacial sliding shear strength for polypropylene and steel fibers are obtained from both modified and standard pullout test and are compared. Similar trends are observed for polypropylene and steel fibers in both the tests which to a certain extent confirms the validity of the proposed method. (C) 2019 Elsevier Ltd. All rights reserved.