69 Transportation Research Record: Journal of the Transportation Research Board, No. 2577, Transportation Research Board, Washington, D.C., 2016, pp. 69–77. DOI: 10.3141/2577-09 Self-healing concrete with microencapsulated calcium nitrate was investigated. The compressive strength of concrete admixed with micro- capsules (as a percentage of the weight of the cement) was tested and compared with that of control specimens of the same mix design without microcapsules. Surface resistivity tests were conducted to quantify the surface permeability of the concrete specimens with and without micro- capsules. The self-healing potential was measured by the modulus of elasticity test (ASTM C469), with measurements being taken before and after damage after 14 days. After the concrete was damaged by application of 80% of its ultimate load, all specimens were incubated by immersion in water. The results showed that the concentration of micro- capsules added and the size of the microcapsules had a direct impact on the compressive strength of the concrete. Furthermore, the concrete specimens into which microcapsules were incorporated had greater sur- face resistivity than the control specimens. The recovery of the modulus of elasticity was analyzed according to the increase from the modulus of elasticity recorded after application of 80% of the sample’s ultimate load and the increase relative to the initial modulus of elasticity of the concrete in the virgin state. Overall, the results of this study indicated that although microcapsules caused a decrease in the compressive strength of the concrete, they enhanced the self-healing capability of the concrete that was produced. To take advantage of the benefits of microcapsules, the authors recommend that future work evaluate the use of a dispersing agent to reduce the amount of microcapsules needed in the mix. In recent years, it has been widely recognized that a significant portion of the concrete infrastructure is severely deteriorated. In the United States, the associated repair and maintenance costs are estimated to be between $18 billion and $21 billion (1). Approximately half of Europe’s annual construction budget is spent on the rehabilitation and repair of existing structures. These facts highlight the global demand for an infrastructure upgrade (2, 3). Concrete cracking is inevitable, and it affects the structure’s durability if it is not significantly con- trolled or reduced. Furthermore, cracking is a major problem, par- ticularly for steel-reinforced concrete structures, as the cracks expose the reinforcement to the elements, which cause corrosion. To address this problem, continuous inspections and maintenance repairs are needed, but the implementation of these activities is challenging because of the significant amount of money required. In addition, crack repair can be difficult to achieve when the cracks are internal or inaccessible. A proposed alternative is to engineer concrete materials with superior quality and durability and, in turn, to extend the service lives of concrete structures. White et al. introduced the concept of self-healing materials through the use of microencapsulation, in which the healing agent is kept from any reactions within the struc- tural material’s matrix until it is needed (4). During a cracking event, the microcapsules rupture and release the healing agent to seal the newly formed cracks. Although such a healing mechanism was demonstrated mainly for polymer materials, recent investigations have adapted the self-healing concept to concrete and cementitious materials. Concrete that self-heals through microencapsulated heal- ing agents is a promising technology that can address the durabil- ity of concrete infrastructures. A suitable self-healing agent should (a) be easily encapsulated, (b) remain stable and reactive over the service life of the concrete structure under various environmental conditions, and (c) respond quickly to repair damage (5). Through this healing mechanism, it is possible to enable a concrete structure to repair its internal cracks and its external microcracks without human intervention. Several healing agents—such as sodium silicate, polyurethane, epoxy, and cyanoacrylates—have been explored and described in the literature (6–8). Other healing agents, such as methylmethacrylate monomer and a triethylborane system, require a catalyst embedded in concrete to seal the cracks (9). Many of these healing agents are expensive or require a catalyst to trigger the self-healing mechanism. To this end, this study evaluated calcium nitrate as an alternative heal- ing agent and measured its healing potential to take advantage of the following benefits: (a) its low cost, (b) its ability to react with the cementitious matrix and subsequently contribute to the formation of calcium silica hydrate, and (c) its setting acceleration of the hydration of unhydrated cement particles (10, 11). OBJECTIVES The main objective of this study was to evaluate the use of calcium nitrate as a healing agent and to assess its effects on self-healing mechanisms for concrete materials. To achieve this objective, a labo- ratory experiment was conducted to measure the self-healing capacity of concrete with calcium nitrate microcapsules and to compare the results with those obtained with control specimens. The study also assessed whether calcium nitrate is a suitable healing agent that may be used to enhance the durability of concrete. Effect of Self-Healing Calcium Nitrate Microcapsules on Concrete Properties Jose Milla, Marwa M. Hassan, Tyson Rupnow, Mohamed Al-Ansari, and Gabriel Arce J. Milla, Department of Engineering Science, College of Engineering, 214 Old Forestry Building, and M. M. Hassan and G. Arce, Bert S. Turner Department of Construction Management, College of Engineering, 3134A Patrick F. Taylor Hall, Louisiana State University, Baton Rouge, LA 70803. T. Rupnow, Louisiana Trans- portation and Research Center, 4101 Gourrier Avenue, Baton Rouge, LA 70808. M. Al-Ansari, Department of Civil and Architectural Engineering, College of Engineering, Qatar University, H 204, Corridor 6, P.O. Box 2713, Doha, Qatar. Corresponding author: M. M. Hassan, marwa@lsu.edu.