Concrete 3D Printing Technology: Potential & Problems
Concrete 3D printing (C3DP) is a new construction technology in which building components are "3D printed" with specially designed concrete based on a predefined virtual model. The technology has gained significant attention both in the construction industry and academia as it has the potential to automate building processes, create complex geometries without using formwork, and reduce labor cost and waste.
However, the high plastic shrinkage cracking tendency is a critical concern with concrete 3D printed elements. This article discusses the research at IIT Tirupati on studying early-age shrinkage susceptibility of 3D printed concrete using non-contact-based digital image correlation techniques. The advantages of the developed test method and how it can be used to formulate shrinkage mitigation strategies are also discussed.
Advantages of Concrete 3D printing
Concrete 3D printing has drawn much interest in the construction industry in recent years. It is a technique by which a building component is fabricated in an automated manner by layerwise deposition of specially designed concrete using a concrete 3D printer. Automation has shown to be advantageous in many industrial sectors. For instance, industries like automotive, aerospace, and consumer goods production sectors use automation to reduce the cost and duration of their manufacturing processes [1].
Another main attraction of C3DP is that it allows the fabrication of complicated geometries that are difficult to create using traditional construction methods [2]. The C3DP technique lowers the cost of labor and eliminates formwork while improving production efficiency, accuracy, and worker safety. C3DP also reduces waste generation in the building sector.
In summary, a compaction and form-free technique is made possible by 3D printing, giving designers previously unheard-of freedom to adapt irregular geometries [3]. However, the rheological properties of the materials used in 3D concrete printing and quick-setting characteristics are critical for ensuring continuous pumping during extrusion and retaining shape integrity after extrusion [4].
Real-world uses of C3DP have been increasing for more than a decade. Some of the recent landmarks in C3DP construction are shown in Figure 1. The Nijmegen Bridge, also known as De Oversteek, spans the Waal River in Nijmegen, Netherlands, and combines modern architectural style with historical relevance (Figure 1a).
Figure 1b shows a shape-optimized highway water culvert at Cornwall, United Kingdom, designed using the C3DP technology. Another notable construction project in the Netherlands is the Milestone House project, which highlights the architectural freedom possible through 3D printing and how it can be used to create innovative residential buildings (Figure 1c). Figure 1d is a 3D-printed footbridge named Striatus in Venice, Italy. It is an unreinforced structure made by assembling concrete blocks without using any mortar. The project combines the latest technology of C3DP with the principles of historic unreinforced masonry [15].
Early Research on C3DP
Most of the early research on C3DP focused on understanding the rheological requirements and the development of printable concrete mix compositions [5]. Test methods were developed to assess early-age material requirements like extrudability, buildability, pumpability, and open time [6]. Following this, there were studies on the mechanical properties of 3D printable concrete and how it differs from conventional mould cast concrete. Researchers focused on assessing the factors affecting the interlayer bond strength [7] and developing test methods to evaluate the mechanical properties [8].
The current research needs are in developing structural design and reinforcement strategies in C3DP, developing advanced 3D printer systems that involve an accelerator addition, inline mixing at the nozzle for rapid stiffing of the concrete mix after extrusion [9], and the durability of C3DP elements [10].
Concerns in C3DP
Another aspect that requires significant attention is shrinkage. Shrinkage has become a major concern in C3DP. For instance, Figure 2 shows severe cracks developed in C3DP elements due to early-age shrinkage. Factors such as the absence of formwork, high binder content, and the addition of fine materials like silica fume substantially increase the early-age shrinkage in the 3D printable mixes. Shrinkage has also been found to adversely affect the bond strength between layers of C3DP elements [11,12].
Research at IIT Tirupati
One of the main emphases of the research group at IIT Tirupati has been to examine the early-age shrinkage susceptibility of various C3DP mixes. The main challenge is the lack of a suitable test method for assessing early-age shrinkage. Conventional shrinkage measurement techniques, such as ASTM C157 [16] for free shrinkage or ASTM C1581 [17] for restrained shrinkage, require physical contact with the specimen or need specialized moulds in which the specimen has to be cast. These methods are impractical for a formwork-free construction like C3DP.
Further, providing physical contact with fresh 3D-printed concrete is challenging. Non-contact-based shrinkage measurement techniques, such as digital image correlation techniques, can offer a viable solution to this problem [18]. DIC works by illuminating the specimen surface with white light and capturing images before and after deformation [19]. More details on DIC can be found in a recent review article by the same authors [20].
Figure 3 shows the early-age shrinkage measurement based on DIC, developed at the Building Materials Laboratory, IIT Tirupati. This method uses concrete specimens of 400 mm length, 40 mm width, and 10 mm height for shrinkage measurement. The samples are kept at a constant temperature of 25 ± 2°C, a relative humidity of 60 %, and exposed to a constant wind flow of 3 m/s. A digital camera with a resolution of 24 MP was positioned above the specimen, focusing on the two marker points inserted in the fresh concrete and kept 300 mm apart.
The core of the measurement process involves continuously capturing images of the specimen every minute for 6 hours and then using a sub-pixel-based MATLAB program to precisely track the change in the marker point's position. The marker pins, colored with high-contrast black and white regions, enabled accurate edge detection. The program locates the edge between these regions using an acquisition model derived from the partial area effect [21]. Figure 4 shows three trials of early-age shrinkage measurements on an OPC-based 3D printable mixture composition. No significant difference can be observed between the three curves. For the three trails, the maximum shrinkage attained was 7395, 7752, and 7802 µm/m, respectively (a maximum difference of only around 5 %). Therefore, reproducible shrinkage readings can be obtained using the developed shrinkage measurement test setup and image processing algorithm.
The research team at IIT Tirupati is also exploring methods to mitigate early-age shrinkage cracking in C3DP. Based on research on traditional concrete, several approaches can be used to alleviate shrinkage, such as shrinkage-compensating binder systems to counteract the increased shrinkage [22], admixtures containing shrinkage-reducing and shrinkage-compensating agents [23], and adding internal curing [24] and fibers [25]. The suitability of these methods for C3DP should be investigated. Although less explored, another approach can be applying curing compound immediately after printing the concrete. Figure 5 shows the result of a preliminary study done at IIT Tirupati using wax and polymer-based curing compounds. We can observe that applying curing compounds substantially reduced the shrinkage strain, about 68 % in the case of wax-based curing compound and 71 % in the case of the polymer-based compound. The curing compounds work based on the membrane curing technique [26,27]. They form a membrane on the surface of the concrete that serves as a barrier for reducing the rate of evaporation from the concrete, thereby reducing early-age shrinkage. The curing compound could be applied on the fresh concrete as early as 5 minutes from the onset of casting. However, more studies are required (such as restrained shrinkage studies to assess the reduction in cracking potential) to examine the possibility of using curing compounds as a shrinkage mitigation strategy.
Conclusions
Concrete 3D printing has the potential to transform the construction industry. The technology could be crucial for sustainable, cost-effective, and safer construction and could play a pivotal role in shaping the future of the construction sector. Many landmark projects have already been executed around the world by concrete 3D printing, such as the shape-optimized Nijmegen Bridge recently built in the Netherlands, which showcases the capabilities of this technology.
However, shrinkage is a significant problem in 3D-printed concrete elements. Research at IIT Tirupati on early-age shrinkage measurement and its mitigation in 3D-printed concrete, using a digital image correlation technique, is based on capturing photos of a fresh concrete prism element and precisely tracking the movements of two embedded marker pins in the sample using a sub-pixel-based edge detection algorithm. The method is robust, and reproducible shrinkage measurements could be obtained using this approach.
A possible method to mitigate the early-age shrinkage cracking tendency is to apply a suitable curing compound on the concrete surface immediately after an element is printed. Preliminary results at IIT Tirupati indicate that curing compounds can potentially reduce early-age shrinkage by up to 70%. However, more studies are required to further assess its effectiveness and develop other possible shrinkage mitigation strategies that are cost-effective and efficient.
Acknowledgment
The authors would like to acknowledge MYK Arment Private Limited for providing the curing compounds used in this study and supporting our research.
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