Effect of the water/cement ratio on the improvement of pullout behaviors using nano-SiO2 modified steel fiber and the micro mechanism

Abstract

This study investigated the influence of the water/cement (W/C) ratio on the improvement of pullout behaviors using nano-SiO2 modified fiber and a corresponding micro-mechanism that includes the change in the three main phases: pores, calcium hydroxide (CH), and unhydrated cement (UHC) in the fiber–matrix interface. The relationship between interfacial microstructures and pullout behaviors at different W/C ratios is analyzed and discussed. The results show that the fiber–matrix interfacial microstructure is improved with a decrease in the W/C ratio. A further improvement in the matrix near the fiber was achieved using nano-SiO2 modified steel fiber with 0.40 and 0.30 W/C ratios; the improvement degrees and scopes on the phases were different for different W/C ratios. The pullout behaviors were enhanced with a decreasing W/C ratio; a significant enhancement could be obtained beyond the threshold value of 0.30. The enhancement ratio obtained via the nano modification of average bond strength was relatively stable; that of the pullout energy was considerably affected by the W/C ratio. The pullout behaviors were consistent with the interfacial microstructures when the W/C ratios were 0.40 and 0.30. The bond between the fiber and the matrix, radial confinement pressure, and surface roughness after debonding become the main factors that determine the interfacial bond properties once the W/C ratio drops below 0.30.