Recovery of Chloride Penetration Resistance of Cement-Based Composites Due to Self-Healing of Cracks.

This study proposed a method of applying coating on uncracked surfaces of test specimens in the electrical migration-diffusion test for the evaluation of the chloride penetration resistance of cracked cement-based composites. It was shown that, by applying the proposed method, the recovery of the chloride penetration resistance from self-healing of cracks can be evaluated more accurately because the application of surface coating reduces the test time and the error introduced by over-simplification. Based on observations of the self-healing-induced recovery of chloride penetration resistance, a phenomenological model for predicting the progress of crack self-healing in cement-based composites was suggested. This model is expected to evaluate the chloride penetration resistance more accurately in actual concrete structures with cracks.

Using the Steady-State Chloride Migration Test to Evaluate the Self-Healing Capacity of Cracked Mortars Containing Crystalline, Expansive, and Swelling Admixtures.

Interest in self-healing-crack technologies for cement-based materials has been growing, but research into such materials remains in the early stage of development and standardized methods for evaluating healing capacity have not yet been established. Therefore, this study proposes a test method to evaluate the self-healing capacity of cement-based materials in terms of their resistance to chloride penetration. For this purpose, the steady-state chloride migration test has been used to measure the diffusion coefficients of cracked mortar specimens containing crystalline, expansive, and swelling admixtures. The results of the present study show that the time to reach a quasi-steady-state decreased and the diffusion coefficients increased as the potential increased because of the potential drop inside the migration cell and self-healing that occurred during the test. Therefore, use of a high potential is recommended to minimize the test duration, as long as the temperature does not rise too much during the test. Using this test method, the self-healing capacity of the new self-healing technologies can be evaluated, and an index of self-healing capacity is proposed based on the rate of charged chloride ions passing through a crack.

Combined Effects of Set Retarders and Polymer Powder on the Properties of Calcium Sulfoaluminate Blended Cement Systems.

This study investigates the effects of set retarders on the properties of polymer-modified calcium sulfoaluminate (CSA) and Portland cement blend systems at early and long-term ages. The fast setting of the cement blend systems is typically adjusted by using retarders to ensure an adequate workability. However, how the addition of retarders influences the age-dependent characteristics of the cement blend systems was rarely investigated. This study particularly examines the effects of retarders on the microstructure and strength development of polymer-modified CSA and Portland cement blend pastes and mortars from 2 h to 90 days. The macro- and microstructural properties are characterized by compression testing, powder X-ray diffraction, mercury intrusion porosimetry, and scanning electron microscopy with energy dispersive spectroscopy. The test results reveal that the use of retarders delayed the strength development of the cement blend systems at the very early age by hindering the production of ettringite, which was cumulative to the delaying effect of polymer, but it increased the ultimate strength by creating denser and finer pore structures with the evolution of hydration products.