The Physicomechanical Behavior and Microstructure of Air-Entrained 3D Printable Concrete

Abstract

The effects of air-entraining admixture (AEA), curing process, and core direction were studied as parameters in terms of their effect on the physicomechanical properties of 3D printed concrete. Four different AEA dosages (0%, 0.1%, 0.15%, and 0.2% by dosage of binder) were used to prepare the mixtures used for 3D printing. The influence of anisotropy on the printed concrete was studied by testing the cores drilled from the horizontal and vertical directions of the 3D concretes. The study also examines the efficiency of a specific spray-deposited chemical curing technique for improving the strength characteristics of 3D concrete. The addition of AEA reduced the unit weight significantly, thereby making the concrete ideal for lightweight structural members. Ultrasonic pulse velocity (UPV) testing on 3D printed samples showed that voids and air entrainment during the deposition process reduced the wave velocities. The ultrasonic wave velocity of cores drilled parallel to the printing direction was found to be slightly higher than cores perpendicular to the printing direction. However, the observation from the nondestructive tests did not agree with the compressive strength studies. The compressive strength of the cores drilled perpendicular to the direction of printing was higher than those drilled parallel, which can be due to the layers formed in the concrete during the filament deposition process. The 3D printing process reduced the compressive strength and induced anisotropy. The compressive strengths were reduced with air entrainment in the sample that was not 3D-printed. However, in the 3D-printed samples, 0.1% of AEA increased the vertical and horizontal strength compared to the samples without AEA. AEA in small quantities was found to be capable of improving the rheology of the mix and reducing the possible defects during the printing process, which resulted in better interlayer bonding. The durability aspect of air entrainment was studied using capillary water absorption, and it was found that the capillary water absorption increased with the addition of AEA. Microstructural studies using the Brunauer–Emmett–Teller (BET), Mercury Intrusion Porosimetry (MIP) and Scanning Electron Microscopy (SEM) on the 3D concrete revealed that the volume and size of the pores in the hardened matrix increased with the addition of AEA. The mechanical and durability studies showed that the optimal percentage of AEA is 0.1%.