Resistance to fatigue of self-healed concrete based on encapsulated polymer precursors

Moving cracks are often present in concrete structures and in those circumstances any self-healing technique for concrete must satisfy specific performance requirements, to guarantee its increased durability. These requirements include the capability of withstanding multiple cycles of crack movement without failing to keep healed cracks sealed. This paper shows early results from a testing protocol suggested by the authors to assess the performance of polymers as healing materials for moving cracks. Ultrasound (US) shear waves were used for continuous monitoring of small prismatic mortar specimens containing a single healed crack under a cyclic load. The maximum amplitude of US waves transmitted across healed cracks was correlated to the area effectively healed and the magnitude of crack movement. A decreasing trend of the maximum amplitude during cyclic loading was observed for strain levels on the polymer corresponding to 70% of its strain limit, but soundness at lower strain levels was confirmed after 300 cycles

Moving towards a realistic implementation of self-healing concrete based on encapsulated polymer precursors

Extensive research has already been performed on self-healing concrete based on a triggered release of liquid polymer precursors from a carrier, especially during the past decade. However, tests on large concrete specimens are still seldom performed and the self-healing techniques used are often not practical or effective if implemented at a large scale. This paper presents an analysis of the most relevant properties for carriers that are critical for moving the technology closer to a realistic implementation. The study contemplates the assessment of the dimensions of tubular glass capsules that result in a high survival rate if directly added to concrete during mixing, while still being able to rupture during realistic crack formation in the host concrete matrix. Finally, a trial implementation of randomly dispersed glass tubular carriers in concrete beams is presented. This system is composed of a typical OPC concrete with tubular glass capsules added during the mixing process. It is shown that this system works and the crack planes cross several capsules that subsequently release the polymer precursor into the crack. However, the dosage used needs to be increased if a satisfying healed area is to be achieved

Monitoring crack movement in polymer-based self-healing concrete through digital image correlation, acoustic emission analysis and SEM in-situ loading

A study was performed to assess the fitness of continuous monitoring methods to detect failure due to excessive strain on polymers bridging moving cracks in the context of self-healing concrete. Testing of several polymer precursors with distinct properties also allowed conclusions regarding the requirements for polymers in this application. Acoustic emission (AE) analysis was performed in parallel with digital image correlation (DIC) at the macro-scale. In addition, a micro-scale study was performed with tensile tests inside an SEM chamber. Detection of failure through AE analysis coupled with DIC was possible only in case of failure due to brittle fracture of a rigid foam after 9% strain, which generated high-energy acoustic events. Direct observation of interfaces with SEM insitu loading allowed determination of failure of a rigid foam due to cracking of the polymer matrix and detachment at the interface with the cementitious matrix, with an onset at 5% strain and complete detachment at 16% strain. For a flexible, continuous film of polymer, detachment occurred before 50% strain. Assuming adequate adhesion, polymers with high elongation (>100%) and modulus of elasticity much lower than 10 MPa are required if cracks subjected to a realistic amplitude of movement are targeted. (C) 2016 Elsevier Ltd. All rights reserved

Simulation-aided design of tubular polymeric capsules for self-healing concrete

Polymeric capsules can have an advantage over glass capsules used up to now as proof-of-concept carriers in self-healing concrete. They allow easier processing and afford the possibility to fine tune their mechanical properties. Out of the multiple requirements for capsules used in this context, the capability of rupturing when crossed by a crack in concrete of a typical size is one of the most relevant, as without it no healing agent is released into the crack. This study assessed the fitness of five types of polymeric capsules to fulfill this requirement by using a numerical model to screen the best performing ones and verifying their fitness with experimental methods. Capsules made of a specific type of poly(methyl methacrylate) (PMMA) were considered fit for the intended application, rupturing at average crack sizes of 69 and 128 μm, respectively for a wall thickness of ~0.3 and ~0.7 mm. Thicker walls were considered unfit, as they ruptured for crack sizes much higher than 100 μm. Other types of PMMA used and polylactic acid were equally unfit for the same reason. There was overall good fitting between model output and experimental results and an elongation at break of 1.5% is recommended regarding polymers for this application

Self-healing of dynamic concrete cracks using polymer precursors as encapsulated healing agents

Self-healing concrete aims at the autonomous healing of small cracks, with widths in the order of a few hundreds of micrometers. While the existing research on this topic, based on several different healing agents and mechanisms, focuses mainly on the self-healing of early age cracks, this study aims at assessing the strain capacity of polymers used as healing materials for dynamic cracks and the viability of using their respective polymer precursors as encapsulated healing agents. Loading tests were performed on cracked, healed mortar specimens, along with the acquisition of microscopic images and capillary water absorption tests. The series of tests allowed the identification of healing agents with good flowing properties that resulted in efficient bridging and sealing of cracks and determining the strain capacity of healed cracks, which was shown to be at best between 50 % and 100 % of its initial width