Laboratory assessment of early-age durability benefits of a self-healing system to cementitious composites
Graphical abstract
Introduction
Cracks induced by shrinkage not only decrease the load-carrying capacity but also negatively impact the durability of concrete by accelerating the ingress of deleterious species (e.g., water, CO2, Cl- and SO42-) [1]. As one of the main culprits that cause early-age cracking in concrete (or other cementitious composites), shrinkage has been extensively studied over the past three decades [2]. Moisture loss due to the evaporation of pore water develops a negative pressure inside the pores of cement matrix, which results in a shrinkage stress. For plain cement matrix at early age, (localized) shrinkage stress could easily exceed its weak tensile strength and thus form microcracks.
Shrinkage of cementitious materials usually occurs in two stages (i.e., the early and late stages), which are the periods before and after the final setting of fresh mixture [3]. Commonly, autogenous shrinkage, drying shrinkage and thermal shrinkage are three main types of shrinkage that could occur at both stages [4]. While autogenous shrinkage is a major concern in early-age cracking of high performance concrete (with water-to-cement mass ratio - w/c - smaller than 0.4), the development of drying shrinkage is regarded as the most significant reason for cracking of regular concrete (w/c ratio greater than 0.45) at early stages [4]. An array of factors affect the risk of drying shrinkage, including concrete properties (e.g., mixture design, aggregate properties and gradation, geometric shape, and addition of chemical admixtures and fiber reinforcement), curing conditions (e.g. temperature, relative humidity and wind velocity), and curing methods [5]. Concrete structural elements with a higher w/c ratio, a higher surface-to-volume ratio, and a lower aggregate content are more susceptible to drying shrinkage. For mitigating drying shrinkage of cementitious materials, there are two main approaches. The first one is to preserve the water content of cement matrix, such as covering the exposed surfaces of concrete elements with plastic warps. The second one is to improve the early-age mechanical properties to allow better resistance to the initiation or propagation of cracks, by adding chemical admixtures, fibers, etc. in fresh concrete.
Although the autogenous healing ability of cementitious materials exhibits the potential of mitigating microcracks caused by shrinkage [[6], [7], [8]], there are few studies investigate the potential of autonomous healing systems for mitigating the microcracks caused by shrinkage. Additionally, successfully using an external self-healing system to heal the microcracks caused by shrinkage and improve the durability of cementitious materials would broaden the application aspects of external self-healing systems in cementitious materials [[9], [10], [11], [12], [13]]. The type of healing agent and the size of the capsules are crucial factors that influence associated performance [14]. Compared with organic healing agents, inorganic healing agents generally feature lower viscosity, which makes the releasing behavior of healing agent faster and easier [14]. Urea-formaldehyde (UF) microcapsules with calcium nitrate (Ca(NO3)2) is a mature and promising self-healing system for cementitious materials [[15], [16], [17], [18], [19], [20]]. The diameter of microcapsules can be readily controlled by tuning the agitation rate during the synthesis process [16,17]. Moreover, the healing agent, Ca(NO3)2, could provide some additional corrosion inhibiting properties to the rebar embedded in concrete [21]. Fiber reinforcements are usually employed to improve the resistance of concrete to shrinkage induced cracking [5,[22], [23], [24]]. In particular, polyvinyl alcohol (PVA) microfibers admixed at 0.25% by volume in a concrete were found to greatly reduce the width of shrinkage-induced microcracks in the concrete, by almost 90% [23]. In this study, the authors developed a new self-healing system consisting of UF microcapsules/Ca(NO3)2 and PVA microfibers and hypothesized that the PVA microfibers would improve the healing performance of the UF microcapsules/Ca(NO3)2 by controlling the crack width.
In this context, this work presents an exploratory laboratory study to evaluate the effectiveness of this innovative self-healing system on improving the transport properties of cementitious composites. The durability characteristics (i.e., gas permeability, chloride migration coefficient, and pore structure of cementitious composites) and compressive strength were tested. The temporal evolution of total shrinkage of cementitious composites with/without a self-healing system during the 35-day curing period was monitored as well. To shed light on the effectiveness of this innovative self-healing system on healing microcracks caused by shrinkage and its associated effects on cement matrix, this study characterized the microstructure and hydration products of cementitious composites through SEM, TGA and XRD.
Section snippets
Experimental
Since this work presents the first time of assessing the performance of this self-healing system in mitigating shrinkage during curing and improving the durability of cementitious composites, some experimental design parameters are explained as follows. Because the ASTM C157 procedure may be only satisfactory for measuring the free shrinkage in cement composites with high w/c ratios [2], a w/c ratio of 0.6 was selected for this study. Another reason of choosing this w/c ratio is to minimize the
Morphology and size distribution of UF microcapsules
Fig. 1 illustrates the morphology of the UF microcapsules synthesized under different agitation rates. The UF microcapsules synthesized under 350 rpm, 700 rpm and 1100 rpm were observed with the magnifications of 100 × , 100 × and 220 × , respectively. With the function of analyzing particles of ImageJ, the size distribution of the UF microcapsules synthesized under specific agitation rates could be obtained, as shown in Fig. 2. The mean diameters of the microcapsules synthesized under 350 rpm,
Overall performance evaluation
This section presents the key experimental results from this study to enable a comprehensive understanding of how the self-healing system of UF microcapsules and PVA microfibers affects the overall performance of cementitious composites. Fig. 14 depicts the summary of positive and negative effects of this self-healing system, plotted by normalizing the tested properties of cementitious composite specimens with this self-healing system to those of control specimens. All the data of self-healing
Conclusions
This exploratory work aimed to evaluate the feasibility of a self-healing system (UF microcapsules/Ca(NO3)2, with or without PVA microfibers) to improve the durability of cement mortar by mitigating the shrinkage occurred during the curing process. The effects of this self-healing system on the properties of cement mortar with three dosages were also investigated, including compressive strength, gas permeability, chloride migration and porosity. In addition, representative samples were further
Funding
This study was funded by the USDOT Center for Environmentally Sustainable Transportation in Cold Climates (CESTiCC) and Simpson Strong-Tie Company through the WSU Excellence Fund.
CRediT authorship contribution statement
Jialuo He: Investigation, Data collection, Data processing, Analysis, Writing – original draft, preparation. Xianming Shi: Conceptualization, Funding, Methodology, Supervision, Writing – review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors acknowledge the funding support by the USDOT Center for Environmentally Sustainable Transportation in Cold Climates (CESTiCC) and the WSU Excellence Fund donated by Simpson Strong-Tie Co. X. Shi acknowledges the USDOT sponsored National UTC TriDurLE for funding his time in preparing the manuscript. The authors would like to extend their appreciation to Dr. Owen Kelly Neill, Dr. Thomas Williams, Dr. Lang Huang, and Dr. Zhitao Chen for their assistance in collecting or analyzing the
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