Effect of absorption kinetics of superabsorbent polymers on printability and interlayer bond of 3D printing concrete

This study examines the effect of the material characteristics of superabsorbent polymers (SAPs) on the printability and interlayer bond strength of 3D printing concrete. Three types of SAP, including two acrylamide-co-acrylic polymers with coarse and fine particle sizes (S1 and S2) and an acrylic copolymer (S3), were employed. The investigated mortar mixtures had a fixed initial mini-slump flow of 195±10 mm and yield stress of 290±25 Pa. The fluidity of SAP-incorporated mortars was maintained by either adjusting the superplasticizer dosage or increasing the water content. Test results indicate that particle flocculation is dependent on SAP absorption kinetics shortly after the end of mixing. The use of S1 and S2 SAPs exhibiting high retention ability enhanced thixotropy, whereas the S3 SAP with rapid desorption reduced thixotropy. The S2 SAP improved the bridging effect of nucleation at early age due to its finer size, leading to a significant increase in the structuration rate at 30 min. Pull-out strength test and splitting tensile strength test were used to investigate the interlayer bond strength. The S1 SAP was the most significant in maintaining a higher internal relative humidity (IRH), thus resulting in higher 28-d interlayer bond strength, while the S2 SAP exhibited an intermediate ability to maintain elevated IRH, thus resulting in higher increase in 7-d interlayer bond strength.