Investigating the effect of microfibre on the printability and strain hardening properties of Strain-Hardening Cement-based Composites(SHCC)

Abstract

This report describes the effect of microfibre reinforcement on the printability and strain hardening properties of Strain-Hardening Cement-based Composites (SHCC). To determine this effect, a study was conducted with three different fibre types and two different concrete matrices. In total, this has led to 6 different concrete mixtures. The three different fibre types are PVA(Poly-Vinyl-Alcohol) 6mm length, PVA 8mm length and HDPE(High Density Poly Ethylene) 6mm length. The difference between the two matrices is mainly in the water-cement ratio, which leads to a stronger and weaker matrix. The strong matrix is the C mix and the weaker one is the D mix.
The printability of the different mixtures is determined by means of a slump flow test. Printability can be divided into buildability and pumpability. Buildability describes how well the printed concrete layers hold together and do not collapse. Pumpability of the mixture says how well it is able to be pumped. If the slump flow test shows that a mixture behaves stiff, than it will have a higher buildability and lower pumpability. If the mixture behaves very fluidly in the slump flow test, it will have a lower buildability and higher pumpability.
The strain hardening properties of the SHCC are determined by doing compressive tests, four-point bending tests and tensile tests. These tests determine the compressive strength, flexural strength and tensile strength of each mixture. It was also found out to what extent each mixture is able to stretch, bend and then exhibits strain hardening. After the tests, the samples were also visually examined to see how the bending and cracking took place.
The weak D mix seems to show the most strain hardening. The fibres in this matrix can transfer the tensile stresses better, because in this matrix they are more able to stretch.
The research has shown that the mixtures with PVA 6 mm in the fresh state are very stiff and are therefore the least suitable for 3D printing. In addition, the mixtures with the PVA 6 mm microfibre reinforcement show the least strain hardening. In the four-point bending tests it was already noticeable that the samples failed at very low deflection and there was little cracking. It was therefore decided not to include these mixtures in the tensile tests in order to save time.
HDPE microfibre has shown to have good printability and significantly outperforms PVA 8 mm in strain hardening. The HDPE fibres provide a very flexible and stretchable material. Compared to PVA, HDPE shows a much finer cracking pattern, which has a positive effect on the durability of the concrete. All in all, it can be concluded that HDPE performs best and would be the best choice for application in 3D printing.