Shrinkage behaviour of 3D printing concrete mixture

Abstract

Concrete is one of the most widely used construction materials across the world. Although, the traditional construction techniques are being improved for efficiency, cost and material optimization, there are disadvantages as well, such as the use of formwork, highly labourintensive and limited architectural freedom for design and material savings. 3D printing of concrete structures is one of the emerging technologies promising the automation in construction industry. However, 3D printed concrete structures pose higher risks of shrinkage as compared to the conventional concrete structures due to their different construction techniques and material composition. Shrinkage can raise issues such as reduction in strength of the structures, poor durability and aesthetically unpleasant structures. The research aims to study the shrinkage behaviour of the 3D printing concrete mixture in different restrained and curing conditions. Based on the experimental data, a finite element model has been developed to predict the shrinkage behaviour over time. The free shrinkage test was performed on cast samples and 3D printed samples of dimensions 160x40x40 mm3. The specimens were kept in two curing conditions: covered with plastic sheet and (2) uncovered or exposed to environment from the time of casting. The shrinkage behaviour was studied for up to 60 days at relative humidity of 65% and temperature 20°C. A higher shrinkage values as compared to normal and highperformance concrete is observed in both 3D printed and cast samples. The 3D printed samples showed a higher shrinkage of 1518% as compared to cast samples due to the 3D printing processes. Restrained ring test were performed for three curing conditions: (1) exposed to environment, (2) covered with plastic sheet and (3) sealed with waterproof tape. The sample exposed to drying cracked within 3 days followed by covered with plastic sheets in 4 days and the specimen in third condition cracked within 8 days. The experiment indicated that the autogenous shrinkage is so high in the concrete mixture to induce shrinkage cracking. A finite element model has been developed to simulate the shrinkage behaviour of the concrete mixture based on the experiment results of the free shrinkage test of cast specimens. The capillary tension approach has been used as the theoretical framework. It uses the KelvinLaplace equation to calculate the capillary stresses and the Bentz deformation model to calculate the corresponding shrinkage strain. It is applicable for ambient relative humidity of more than 40%. The required input for the model are: degree of hydration over time, elastic modulus and distribution of relative humidity over time. The degree of hydration has been calculated using HYMOSTRUC software. The experimental results of the free shrinkage test for cast samples have been simulated in ANSYS. An inverse analysis has been used to obtain the material parameters required to model the moisture diffusion process. Transient heat equation has been used to model the diffusion process. Since the concrete mixture has a low watercement ratio, the effect of autogenous shrinkage has been taken into account as well. The calibrated finite element model gave an error of 810% as compared to the experimental results. The moisture profile obtained from the model has been compared to an analytical model to validate the model. It shows good agreement, especially for upto 28 days. Finally the developed finite element model has been applied on three case studies to assess the accuracy of the model. First, the restrained ring test has been simulated to validate the model based on the prediction of the cracking time. It showed that to accurately predict the shrinkage induced stresses, the creep and the relaxation phenomenon plays a crucial role as well. Second, the model has been used to simulate the shrinkage in a 3D printed flower pot restrained at varying time. Third, a parametric study has been designed using the developed model. Overall the study concludes that the shrinkage in 3D printing concrete mixture is quite high as compared to the normal concrete even if conventionally cast. The 3D printing processes can increase the shrinkage further. The developed finite element model can be used as a tool to estimate the shrinkage strain.