Pull-out behaviour of steel and carbon fibers in 3D-printable cement matrices of various compositions

The aim of this study was to investigate some of the factors which affect the fiber–matrix pull-out behaviour and bond strength. 3D-printable cement mixtures with various compositions were used as matrices. The effect of parameters such as type of fiber (steel or carbon), kind of aggregates (sand or limestone powder), and kind of modifier additives (metakaolin or complex nano-sized additive based on SiO2 (CNA)) on fiber–matrix pull-out behaviour were determined. The results demonstrated that the bond strength of a fiber-matrix mainly depends on the fiber type. The bond strength of a "steel wire matrix" was 2–4 times higher than the bond strength of a "carbon fiber matrix". The fiber–matrix bond strength increased as the conditions for the wetting of a fiber's surface improved, especially for carbon fiber. Microstructural investigation revealed that the congestion of hydration products in a fiber–matrix interface significantly improved the pull-out behaviour. The use of sand as an aggregate for a cement matrix enhanced the bond strength by 10–15% in "steel wire matrix" systems. The introduction of metakaolin and CNA into a cement matrix increased the bond strength by 15–20% for sand-based and powder-based cement matrices.