8 Transportation Research Record: Journal of the Transportation Research Board, No. 2577, Transportation Research Board, Washington, D.C., 2016, pp. 8–16. DOI: 10.3141/2577-02 The use of encapsulating healing agents that allow the self-healing of concrete has emerged as a potential solution to the current decay and costly maintenance of concrete infrastructure; however, many heal- ing agents are expensive. The objectives of this study were to develop a procedure for the production of urea–formaldehyde microcapsules of calcium nitrate and to evaluate the effects of the microcapsules on self-healing efficiency in concrete. Calcium nitrate was selected for microencapsulation, given its low cost and its effect on acceleration of the setting of unhydrated cement. The results indicated that the agitation rate and the temperature had a linear correlation with the microcapsule diameter and the shell wall thickness, respectively. A higher agitation rate resulted in a smaller microcapsule diameter, whereas a higher tempera- ture resulted in a thinner shell wall. The morphology of all microcapsules synthesized was virtually the same throughout the experimental matrix, with the shell walls of all the microcapsules observed having a smooth exterior surface and a rough interior surface. All microcapsules produced were also observed to have a broad particle size distribution. This char- acteristic was attributed to the type of surfactant used in the process. Analysis of the effects of the microcapsules on the self-healing efficiency of concrete showed that the modulus of elasticity increased after healing for all concrete specimens prepared with self-healing agents. The largest increase in the modulus of elasticity was observed at a microcapsule content of 0.50%. The results also showed that concrete specimens into which microcapsules were incorporated at any concentration had greater surface resistivities than control specimens. Concrete structures exposed to natural environments are prone to cracks because of many factors, such as excessive loadings, restrained shrinkage, as well as harsh environmental conditions (1). Cracks are detrimental to the structure because they either weaken its mechanical properties or lower its durability by creating pathways for harmful agents to enter the structure and attack the steel reinforcement and concrete (1). Many countries, including the United States, Germany, and South Korea, have recently experienced extraordinary deterio- ration of their civil infrastructure, such that the annual amount spent on repair and rehabilitation has exceeded the amount spent on the construction of new infrastructure (2). In the United States alone, the annual economic impact of the maintenance, repair, and replace- ment of deteriorating structures is estimated to be $18 billion to $21 billion (2). In addition, ASCE has stated that over the next 5 years, about $2.2 trillion will be needed to repair and retrofit the existing infrastructure. It is estimated that half of those concrete repairs will fail and require rerepair, a fact that raises concerns about the quality of the repair methods (2). The concept of self-healing has recently emerged as a promising method to repair that deteriorating infrastructure. Given the fact that cracking is the major failure mechanism leading to deteriorating structures, special attention has been given to crack repair (2). One way to fix those cracks is through the insertion of microcapsules into the concrete mix. Such microcapsules store the healing agent and provide a mechanism for the self-healing process to take place when damage occurs. When cracks form, the microcapsules rupture and release the healing agent to seal the crack (3, 4). For this mecha- nism to be effective, the microcapsules must have enough strength to be intact in the concrete matrix and should rupture only when the concrete is damaged. For durability, the microcapsules must resist leakage and the healing agent must be able to diffuse into cracks over a long period of time (4). Both the microcapsule size and the microcapsule diameter affect the healing process to a great extent (5). For this reason, study of the size and diameter of microcapsules is critical for the optimization of crack healing. Many studies were conducted to determine the best size of microcapsules for an optimum self-healing process (5). Studies have reported that a suitable size for microcapsules is 100 µm. Although this size is suitable for crack healing in many applications, many studies attempted to decrease the microcapsule size (5). How- ever, to achieve an efficient self-healing mechanism, it was proven that smaller microcapsules must be included in higher weight ratios. However, it was shown that at a given weight, larger microcapsules have more healing properties, since they give more healing agent per unit area. Thus, in many applications selection of the optimal micro- capsule size is critical for enhanced self-healing performance. Further- more, when the crack size is small (≈3 µm), the self-healing process can be achieved with as little as 1.25% microcapsules by weight of cement or with microcapsules with diameters of less than 30 µm. Thus, it is of strong importance to study the properties of microcapsules if an enhanced self-healing mechanism is to be achieved. This study had four objectives: (a) develop a procedure for the production of urea–formaldehyde microcapsules of calcium nitrate; (b) characterize microcapsule properties, such as diameter, shell wall thickness, and morphology; (c) relate changes in the production parameters, such as agitation rate, heating time, and temperature, to such properties; and (d ) evaluate the effects of microcapsules Microencapsulation of Calcium Nitrate for Concrete Applications Marwa M. Hassan, Jose Milla, Tyson Rupnow, Mohamed Al-Ansari, and William H. Daly M. M. Hassan, Bert S. Turner Department of Construction Management, 3134A Patrick F. Taylor Hall; J. Milla, Department of Engineering Science, Col- lege of Engineering, 214 Old Forestry Building; and W. H. Daly, Department of Chemistry, College of Science, 712 Choppin Hall, Louisiana State University, Baton Rouge, LA 70803. T. Rupnow, Louisiana Transportation and Research Center, 4101 Gourrier Avenue, Baton Rouge, LA 70808. M. Al-Ansari, Depart- ment of Civil and Architectural Engineering, College of Engineering, Qatar Uni- versity, H204, Corridor 6, P.O. Box 2713, Doha, Qatar. Corresponding author: M. M. Hassan, marwa@lsu.edu.