Hardened properties of layered 3D printed concrete with recycled sand

Highlights

• The incorporation of recycled sand decreases the mechanical properties of 3D printed concrete.

• The strength development of 3D printed concrete with recycled sand in the later curing age is obvious.

• Recycled sand have limited effect on the anisotropy of compressive and flexural strengths.

Abstract

3D concrete printing has received worldwide attention while the development on new cementitious material compositions for 3D printing is inadequate. This study employed the recycled sand instead of natural sand to achieve 3D concrete printing and investigated the hardened properties of this extrusion-based material. The effect of replacement ratio of recycled sand, curing age, nozzle height and anisotropic behavior were evaluated based on the compressive tests, tensile splitting tests and flexural tests. Moreover, the digital image correlation (DIC) technique was adopted to capture the strain behavior and failure pattern of this layered and printed concrete. Owing to the high un-hydrated cement paste attached to the recycled sand and the internal curing mechanism, the compressive and flexural strengths of the 3D printed concrete with recycled sand were a little lower than those specimens without recycled sand. The compressive, tensile splitting and flexural strength of 3D printed concrete with recycled sand had obvious anisotropy. The replacement of recycled sand had limited effect on the anisotropy of compressive and flexural strength, but had certain effect on the tensile splitting strength. Since recycled sand is one of those major products derived from the construction and demolition waste, it is believed that the employment of recycled sand to the mix of 3D printed concrete will significantly improve the sustainability of 3D printed concrete structures.

Introduction

Concrete, which is the most consumed building material, has the greatest impact on the total emissions of the construction industry [1,2]. Therefore, there is a great demand to develop new technologies, or use recycled materials to reduce emissions and save natural resources in the construction industry [3,4]. On one hand, preparation of environmentally-friendly concrete with recycled materials from waste concrete, is of great significance due to the shortage of natural resources; On the other hand, it is noticed that, currently, a new innovative construction process called concrete 3D printing has received worldwide attention, and recent developments show that this is a promising technique. The advantages of 3D printed concrete buildings are characterized by less labor requirements, allowing complex and diverse architectural designs, and absence of formwork [5,6]. Therefore, turning waste concrete into recycled sand can not only be used in the conventional cast method, but also may be employed in the preparation of 3D printed concrete, which will consume waste concrete and significantly reduce the demand for natural sand.

At present, a large number of studies have been reported on the concrete preparation with recycled coarse aggregate [7,8], while the research on mortar made of recycled sand is relatively limited. This is mainly because recycled sand is often considered not suitable as fine aggregate for cement-based materials. Compared with natural sand, the recycled sand usually has low density and high water absorption since its porous characteristics and the attached old cement paste [[9], [10], [11]]. Negative effect will be induced on the fresh and hardened properties of mortar and concrete with the addition of recycled sand, such as inferior working performance, low mechanical properties, and more shrinkage [12]. Evangelista and Brito [13] suggested that the replacement ratio of recycled fine aggregates should not up to 30%. However, the situation may be different if recycled sand is adopted instead of natural sand for 3D printed concrete. With the favorable mix proportion, the application of recycled sand in 3D printing may be a reasonable way to consume recycled sand [14,15].

Since Khoshnevis first introduced the concept of Contour Crafting (CC) into concrete 3D printing [16,17], this technology has been constantly investigated around the world due to a series of problems related to equipment, materials, structures, etc [[18], [19], [20]]. Although the influence of workability was limited, the hardened performance of concrete or mortar may still be reduced due to the incorporation of recycled sand. Moreover, in order to achieve 3D concrete printing as a feasible construction technology, it is increasingly important for printed products to conform to the conventional building standards, as this is the key part of the breakthrough for 3D printed concrete technology in practical engineering. As mentioned above, unlike traditional mould cast concrete construction processes, 3D printed concrete based on contour crafting is a layered manufacturing technique, of which the biggest feature is that the printing process introduces quantities of interfaces to the final product. The filament deposition and stacking process may mechanically lead anisotropy for the printed products [21,22]. The location and orientation of these interfaces surely have an impact on the mechanical behavior of the structure. On one hand, if the impact of the induced interface, material composition and loading direction in the 3D printing process cannot be understood clearly in the hardened state, the later structural design will be unsafe; On the other hand, studying and quantifying the mechanical anisotropy of 3D printed structures, as well as the differences from traditional mould cast products, is of great significance for future optimal design.

In fact, not many studies focus on the mechanical behavior of strength development of 3D printed concrete are available at present. Le et al. [23] first reported the influence of the layer-by-layer manufacturing process on density, compressive strength, flexural strength, tensile bond strength and dry shrinkage, and found that the induced anisotropy inevitably affected the hardening properties. Similar results were also found by the tests conducted by Paul et al. [24] and Ma et al. [25], which showed that the printing directions had a large effect on the mechanical properties of the 3D printed specimen. The influence of print time interval was studied by Sanjayan et al. [26] with compressive and flexural strength test in different loading directions. It is found that the results of both compressive and flexural strengths were depended on the loading directions, though the orthotropic phenomenon was more obvious in compressive than flexural strength. Other manufacturing parameters including nozzle height and surface dehydration were considered in the study taken by Wolfs et al. [21]. They stated that if the interval time between two printed filaments prints was short enough, the direction of the interface had a very limited effect on the mechanical properties of the test specimens. However, they also indicated that with the interlayer time interval increased, the interlayer bonding strength decreased.

From the literature review, it can be found that the investigations on hardened properties of extrusion-based 3D printed concrete are paid much attention recently and it is really crucial for future practical application. The development on new cementitious material compositions for 3D printing can be found from some investigations and reports. The possibility of using recycled glass, geopolymer, fly ash, alkali activated slag and other materials have already been discussed [[27], [28], [29], [30]]. The application of these new materials in 3D concrete technology shows great potential and has attracted increasing attention. However, those available literatures referring to the possibility of new option of sand/aggregate cannot be found, which is an area that has not been explored. Therefore, this study has been working on the possibility of using recycled sand instead of natural sand to achieve 3D concrete printing. The corresponding anisotropic mechanical properties will be evaluated and compared with mould cast concrete, based on compressive test, tensile splitting test and flexural strength test.