247 / 2022-06-15 19:57:08

Adding hydrated lime for improving microstructure and mechanical properties of mortar for ultra-high performance concrete

Ultra-high performance concrete; Strain-hardening; Polyethylene fiber; Surface modification; Mechanism

Generally ultra-high performance concrete (UHPC) possesses excellent mechanical properties and durability, in which fiber constitutes an indispensable raw material for toughening UHPC. Polyethylene (PE) fiber has been used for preparation of UHPC with obvious strain-hardening behavior. Although silane coupling agent (SCA) has an effective modification effect on the PE Fiber and hence on mechanical properties of UHPC, the mechanism for interaction between SCA-modified fiber and the matrix remains to be a controversial issue. In this research, SCA solutions at various concentrations from 1 % to 5 % were used to modify the surface of PE fiber to verify the mechanism for the interaction between SCA-modified PE fiber and the matrix of UHPC with strain-hardening behavior.



The stress-strain curves of UHPC containing PE fiber modified by SCA (Fig. 1) manifest that the modification of fiber surface significantly affected the strain-hardening behavior of UHPC. Compared with the ultimate tensile stress (6.11 MPa) of reference group (SCA18-0), the ultimate tensile stress of UHPC incorporating PE fiber modified by 1%, 3% and 5% concentrations of SCA solution reached to 7.39 MPa, 8.01 MPa and 7.04 MPa. The direct-tensile specimens containing original pristine PE fiber failed at a typical main-crack failure mode, while the specimens incorporating SCA-modified PE fiber exhibited a multi-cracking failure mode (Fig. 2), among which the UHPC incorporating PE fiber modified by 3% concentration of SCA solution obtained the optimum multi-cracking behavior.



The contact angle between SCA-modified PE film and water was 43 °, which was approximately half of that between the unmodified PE film and water (Fig. 3), which proved the improvement of hydrophilic feature of PE fiber.  In the XRD diagrams of inorganic C–S–H (I–C–S–H), SCA solution modified C–S–H (M–C–S–H) and organic C–S–H (O–C–S–H) (Fig. 4), within the range of 0 ^o-10 ^o, the peak of M–C–S–H was broadened and interlamellar distance d001 (1.34 nm) of M–C–S–H  was greater than that of I–C–S–H (1.31 nm), reveals that a chemical reaction might happen between the hydrolysate of SCA and I–C–S–H. The FTIR spectra of inorganic C–S–H (I–C–S–H), SCA solution modified C–S–H (M–C–S–H) and dried SCA solution (D–SCA) (Fig. 5) reveals that M–C–S–H contained Si-CH₃ groups at 802 cm^-1 and 1261 cm^-1, while no Si-CH₃ group was detected in the unmodified I–C–S–H, and –OH groups in hydrolysate of SCA might participate in chemical reaction. The FTIR spectrum of M–C–S–H also suggests that hydroxyl groups of I–C–S–H condensed with hydroxyl groups of SCA’s hydrolysate.



Based on the above results, the mechanism for the effect of SCA modification of PE fiber on the strain-hardening behavior of UHPC should be a combination of both physical and chemical interactions between fiber and matrix, and a modification layer formed on the surface of PE fiber (Fig. 6). Nevertheless, too thick a modification layer is undesirable for improving strain hardening behavior of UHPC.