Princeton researchers develop new laser-cutting technique to improve fracture resistance of 3D-printed concrete

Engineers at Princeton University are utilizing lasers to evaluate a significant drawback of 3D-printed cement: its susceptibility to fractures. The researchers hope that advancements in this area could lead to broader applications of additive manufacturing in cement-based structures. The ultimate objective is to develop superior materials using additive techniques that enable innovative designs and functions.

Cement is the primary ingredient in concrete, which constitutes a significant portion of modern construction, including buildings, roads, runways, bridges, and dams. As 3D printing has demonstrated advantages in efficiency and versatility in recent years, there has been a growing interest in applying this technology to construction.

However, compared to conventionally cast concrete, 3D-printed alternatives can be prone to cracking, especially in the regions between different layers of concrete. Researchers attribute this issue to non-uniform microstructures introduced by the layering process employed in 3D printing. Princeton researchers have introduced a new test to gain a microscopic-level understanding of this cracking phenomenon. Their findings indicate that by accurately characterizing the fracture properties, 3D-printed concrete could possess the same or even greater strength than cast concrete.

In a study published in the journal Cement and Concrete Composites, Princeton researchers present a novel testing method that utilizes lasers to precisely cut grooves in 3D-printed cements. By controlling the power and speed of the laser, the researchers can manage critical features such as groove depth and shape. This control allows for significantly more accurate testing compared to conventional methods.

“Through this technology, we can develop a comprehensive understanding of the fracture properties of 3D-printed cement-based materials in various failure modes, which is crucial for eventually scaling up this technology,” explained Reza Moini, an assistant professor in the Department of Civil and Environmental Engineering at Princeton and the senior author of the study. “Additive technologies provide new opportunities to create stronger and more resilient materials by leveraging the design of materials architecture and fabrication freedom.”

The other authors of the study from Princeton, all members of Moini’s lab, include Shashank Gupta and Arjun Prihar, both Ph.D. students, and Hadi Esmaeeli, a former associate research scholar.

In contrast to cast concrete, which is poured into a mold and solidified, 3D-printed concrete involves the extrusion of cement paste through a nozzle, one strand at a time. The nozzle moves back and forth, gradually building up the concrete layer by layer.

One challenge arises during extrusion, where a thin, water-rich film tends to form around each printed strand to facilitate flow. These water-rich films can introduce significant internal flaws and heterogeneities between strands of the 3D-printed material, contributing to structural weaknesses.

The Princeton researchers examined these interfaces in greater detail to understand their relationship with fracture properties. Initially, the research team fabricated and cured samples for testing using a custom-built 3D printer that extruded cement paste. Traditional testing often involves creating notches in the material using tools like circular saws. However, such saws can act as blunt instruments, leading to imprecise and non-sharp notches, which hampers accurate testing.

Instead of employing physical tools like saws to create notches, Moini and his colleagues chose to utilize a laboratory laser. Their approach allows for the creation of testing notches precisely where they are needed, such as at the interface between the printed layers.

“The advantage of this test for brittle 3D-printed materials is that using the same sample geometry, one can capture the resistance to cracking under tension, shear, or any combination of the two,” stated Moini.

Shashank Gupta, the first author of the paper, emphasized that “this approach can help provide information about material properties as researchers collaborate with the industry to scale up concrete additive manufacturing processes for both structural and nonstructural applications.”

Graduate students in Moini’s lab are furthering these efforts by investigating architected materials and fracture characteristics. Recently, at the spring convention of the American Concrete Institute in San Francisco in April, paper co-author Arjun Prihar won a third-place award for his poster, while graduate student Krystal Delnoce secured first place. Prihar’s research focuses on understanding the fracture mechanics of sinusoidally architected designs of concrete materials using experimentation and simulation. Delnoce’s work revolves around adapting a new fracture test method for 3D-printed materials by developing a notch within the toolpath.

“Our research is contributing to solving fundamental questions about the fracture behavior of 3D-printed concrete,” stated Moini.

The deployment of lasers by engineers at Princeton University is revolutionizing the evaluation of 3D-printed cement. By precisely cutting grooves using lasers, researchers can gain a deeper understanding of the material’s resistance to fractures. This development has the potential to address a major drawback of 3D-printed cement—its susceptibility to cracking, particularly in the areas between different layers. By characterizing the fracturing properties, it may be possible to enhance the strength of 3D-printed concrete and encourage its wider application in construction.

Cement is a vital component of concrete, which forms a significant part of modern construction projects such as buildings, roads, bridges, and dams. While 3D printing offers advantages in efficiency and versatility, it faces challenges in producing concrete that is as robust as conventionally cast alternatives. The layering process involved in 3D printing introduces non-uniform microstructures, leading to potential cracking. Princeton researchers have introduced a groundbreaking testing method that employs lasers to create precise grooves in 3D-printed cement. By controlling the laser’s power and speed, researchers can accurately control the depth and shape of the grooves, enabling more precise testing than traditional methods.

Reza Moini, an assistant professor in the Department of Civil and Environmental Engineering at Princeton, believes that this technology can lead to a comprehensive understanding of the fracture properties of 3D-printed cement-based materials, thereby enabling the scaling up of additive manufacturing in construction. The ability to design and fabricate materials with greater strength and resilience through additive techniques holds immense potential.

The research team at Princeton, including Shashank Gupta, Arjun Prihar, and Hadi Esmaeeli, has examined the interfaces between printed layers to investigate their relationship with fracture properties. Traditional testing methods involve using tools like circular saws to create notches, but these can result in imprecise and non-sharp notches, making testing challenging. In contrast, the use of lasers allows for precise notching at the interfaces, enabling more accurate testing.

The researchers highlight the advantages of this laser-based testing method for brittle 3D-printed materials. It can capture the resistance to cracking under tension, shear, or a combination of the two, providing valuable insights into material properties. This information can inform the scaling up of concrete additive manufacturing processes for structural and nonstructural applications, as researchers collaborate with the industry.

In addition to the laser-based testing, graduate students in Moini’s lab are conducting further investigations into architected materials and fracture characteristics. Their research aims to deepen the understanding of fracture mechanics in concrete materials and explore innovative designs using experimentation and simulation.

The work conducted at Princeton University contributes to addressing fundamental questions surrounding the fracture behavior of 3D-printed concrete. It represents a significant step toward advancing the field and harnessing the full potential of additive manufacturing in the construction industry.

Source: Princeton University

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