# Layer Bond Strength in 3D-Printed Concrete: The Role of Interlayer Surface Area and Printing Delay Time

## TL;DR

This study shows how increasing the surface area between layers in 3D-printed concrete can significantly improve bond strength, but only up to a certain limit.

## Contribution

The study introduces a method using retractable teeth to modify interlayer surface area and improve mechanical uniformity in 3D-printed concrete.

## Key findings

- Interlayer bond strength increases linearly with surface area up to a threshold of 120%.
- A 20% increase in surface area can lead to a 70% increase in bond strength.
- Bond strength shows a logarithmic dependence on delay time between layers.

## Abstract

What are the main findings?

Interlayer bond strength increases linearly with surface area up to a threshold.A 20% increase in surface area yielded up to 70% higher bond strength.The relative strength gain remained consistent across delay times.Bond strength shows a logarithmic dependence on delay time.

Interlayer bond strength increases linearly with surface area up to a threshold.

A 20% increase in surface area yielded up to 70% higher bond strength.

The relative strength gain remained consistent across delay times.

Bond strength shows a logarithmic dependence on delay time.

What are the implications of the main findings?

Surface modification is effective only within a defined range.The threshold depends on material properties and teeth geometry.Retractable teeth may reduce interlayer-induced heterogeneity.The method can improve mechanical uniformity in printed elements.

Surface modification is effective only within a defined range.

The threshold depends on material properties and teeth geometry.

Retractable teeth may reduce interlayer-induced heterogeneity.

The method can improve mechanical uniformity in printed elements.

Three-dimensional (3D) printing, also known as additive manufacturing of cementitious materials, appears to be a promising way to build in a way that is more time-efficient, cost-effective and, under certain conditions, environmentally friendly. This technology continues to exhibit significant inhomogeneity, which is frequently caused by the interlayer area. The presented research aims to clarify the influence of the interlayer surface area and delay time on the bond strength. This study involved reference cast and printed samples with different delay times and cast samples with different interlayer surface areas. Different interlayer surface areas were accomplished through the utilisation of a teeth shaper before casting the second layer. Research has shown that the interlayer surface area has a significant impact on layer bond strength; up to a 70% increase in bond strength can be achieved while increasing the area by 20%. The results show that the increase in strength due to a larger surface area remained constant in terms of percentage, across delay times, with a linear dependency on a specific range of conditions. After the threshold of the surface area increased, the bond strength could be compromised and lowered. This threshold is above a 120% increase in surface area for the used teeth geometry and material. The proposed technology of ejecting teeth to alter the interlayer surface area has the potential to reduce the heterogeneity of mechanical properties in 3D-printed objects, caused by the different delay time between layers, because of the print strategy or material shortage.