Micromechanical Properties of a New Polymeric Microcapsule for Self-Healing Cementitious Materials

Abstract

Self-healing cementitious materials containing a microencapsulated healing agent are appealing due to their great application potential in improving the serviceability and durability of concrete structures. In this study, poly(phenol–formaldehyde) (PF) microcapsules that aim to provide a self-healing function for cementitious materials were prepared by an in situ polymerization reaction. Size gradation of the synthesized microcapsules was achieved through a series of sieving processes. The shell thickness and the diameter of single microcapsules was accurately measured under environmental scanning electron microscopy (ESEM). The relationship between the physical properties of the synthesized microcapsules and their micromechanical properties were investigated using nanoindentation. The results of the mechanical tests show that, with the increase of the mean size of microcapsules and the decrease of shell thickness, the mechanical force required to trigger the self-healing function of microcapsules increased correspondingly from 68.5 ± 41.6 mN to 198.5 ± 31.6 mN, featuring a multi-sensitive trigger function. Finally, the rupture behavior and crack surface of cement paste with embedded microcapsules were observed and analyzed using X-ray computed tomography (XCT). The synthesized PF microcapsules may find potential application in self-healing cementitious materials.

Validation of Self‐Healing Properties of Construction Materials through Nondestructive and Minimal Invasive Testing

“…A uniform dispersion is important to make sure that randomly existing cracks cross enough capsules to allow healing. A uniform distribution, without agglomeration, of polymeric microcapsules in a cementitious matrix was found by Lv et al However, in order to have a good contrast and a clear visualization of the microcapsules, not only the capsule dimension is important to visualize them using X‐ray CT, but also their X‐ray attenuation has to be different from the matrix they are in. Using the XCOM web database of NIST the theoretical total attenuation coefficient of each component, such as microcapsules versus matrix, can be plotted for different X‐ray energies.…”

“…While some researchers tend to obtain a uniform capsule size distribution, others are in favor of a grade of capsule diameters. As there would be a correlation between the rupture force and the particle size of microcapsules, Lv et al applied microcapsules with different grade of diameters (obtained by sieving) allowing their self‐healing system to respond to varying trigger forces and thus to heal cracks of different crack widths (see also Section ).…”

“…In the study of Liu et al X‐ray CT was used to show that the creation of cracks (with a width varying from 20 to 50 µm) in a cementitious matrix triggered rupture of ethyl cellulose capsules containing a bacterial healing agent. Also Lv et al used CT combined with data reconstruction and image segmentation software to investigate the crack zone in cementitious materials with self‐healing properties and find a relationship between the size of microcapsules and their trigger behavior. Therefore, a small region around the crack was selected as region of interest and different materials were color labeled.…”

“…The scale bar represents 1 mm in both images. Reproduced with permission . Copyright 2016, Materials (Basel).…”

Design and testing of tubular polymeric capsules for self-healing of concrete

“…These challenging requirements may be achieved by using polymeric capsules. Research on encapsulation of healing agents for self-healing concrete mainly focuses on the development of organic spherical microcapsules [1][2][3][4]. However, recent studies have also attempted to develop tubular polymeric capsules to carry the healing agent.…”