Citation: Safitri, E.; Kusworo, R.A.; Kristiawan, S.A. Shrinkage of Micro-Synthetic Fiber-Reinforced Mortar. Infrastructures 2023, 8, 7. https://doi.org/10.3390/ infrastructures8010007 Academic Editor: Pedro Arias-Sánchez Received: 19 October 2022 Revised: 18 December 2022 Accepted: 28 December 2022 Published: 31 December 2022 Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). infrastructures Article Shrinkage of Micro-Synthetic Fiber-Reinforced Mortar Endah Safitri, Ridan Adi Kusworo and Stefanus Adi Kristiawan * SMARTCrete Research Group, Civil Engineering Department, Sebelas Maret University, Kota Surakarta 57126, Indonesia * Correspondence: s.a.kristiawan@ft.uns.ac.id Abstract: Repair materials have been developed in this research by adding micro-synthetic fibers in cement-based mortar. In addition, accelerator is incorporated in the mortar to obtain high early strength of the repair materials. Their shrinkage behavior is of interest. This study aims to determine the shrinkage of the micro-synthetic fiber-reinforced mortar and propose models to reflect their shrinkage behavior. The results show that rapid developments of shrinkage are observed at an early age where the 3-day shrinkage already attains about 40–50% of the 84-day shrinkage value. Moreover, after 14 days of age the shrinkage curves tend to approach the asymptotic value. The ACI 209.2R-08 and CEB-MC 90-99 models do not reflect the shape of the shrinkage curves of the micro-synthetic fiber-reinforced mortar. Therefore, this research proposes a modified ACI 209.2R-08 and CEB-MC 90-99 that can describe the shrinkage behavior of the micro-synthetic fiber-reinforced mortar. The accuracy of the modified models has been confirmed quantitatively using the method of best fit line, residual analysis, and coefficient of error. Keywords: micro fiber; shrinkage; shrinkage prediction model; repair mortar 1. Introduction Damage of concrete to various extents may be observed in structural concrete building and it affects the strength and serviceability of the structure. Therefore, repair to the particular damage is necessary to restore the safety and functionality of the concrete structure [1–3]. Moreover, good repair to the damaged concrete can improve the overall performance of concrete structures, increase their strength and stiffness, improve the appearance of concrete surfaces, make them watertight, and prevent aggressive substances from entering the steel surface [4]. A variety of repair systems are available today, and their pertinence is governed by the type of damage. For example, the patch repair system is a method suitable for recovering the spalling of concrete [5,6]. One repair material for concrete patching is cement-based mortar. However, cement- based mortar is known to have relatively low tensile strength when compared to its compressive strength. As a result, the repair material will be at a high risk of cracking due to the emergence of tensile stress induced by the differential shrinkage between the repair material and the concrete substrate [7,8]. Therefore, randomly dispersed micro-fibers can be incorporated into the mixture of this cement-based mortar to increase its tensile strength and toughness properties [9,10]. The increase in these properties is governed by the capability of the micro-fibers to prevent or control the initiation, propagation, or coalescence of cracks [11], so that the mortar with added micro-fiber material can improve its resistance against cracking due to the differential shrinkage stress that occurred in the patching system [12,13]. Shrinkage in concrete can be related to the loss of water content in the capillary pores due to drying. Several studies showed a linear relationship between drying shrinkage and water loss [14,15]. Shrinkage must be carefully assessed as it affects the durability of repair material as indicated in the preceding paragraph. When two materials with different shrinkage properties are combined, such as in repairing a concrete structure with repair Infrastructures 2023, 8, 7. https://doi.org/10.3390/infrastructures8010007 https://www.mdpi.com/journal/infrastructures