Some publications indicate this ratio is a constant value, with around 0.6 by Hausmann 8 and a stable linear relationship between hydroxide concentration and chloride with a slope of 0.83 by Gouda 9. 7, whose results shown that passivation current will be increased when the chloride concentration exceeds hydroxide. The / ratio is mentioned earlier by Venu et al. Unfortunately, such a threshold cannot get a consensus according to the previous literatures. Numerous researchers have confirmed that corrosion initiation in reinforced concrete structures is triggered when the / ratio outnumbers certain critical value 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20. Such processes may significantly shorten the service life of reinforced concrete structures, causing a widespread concern in construction engineering. However, if the alkalinity locally compromises, for example in the case of chloride permeation, carbonation or sulfation, a high corrosion risk may be initiated, which will be further enhanced by the presence of water (usually entraining harmful ions), oxygen and potential difference. Normally, the embedded steel bar is protected by the surrounding concrete thanks to the passive film formed on its surface to keep it from corroding by providing a highly alkaline environment. Corrosion in reinforced concrete is often induced by the chloride permeation and exacerbated by temperature gradients, humidity changes and potential differences 1. However, its durability is strongly associated with the steel corrosion accompanied by electrochemical reactions. Reinforced concrete is universally recognized as an outstanding artificial construction material, with excellent mechanical performance and workability. Moreover, the release rate of core materials could interact with environmental pH value, in which the release rate is found to increase remarkably with decreasing pH value, but is inhibited by high pH levels. The test results exhibited that the release process of the microcapsules is a function of time. The aim of corrosion protection of steel bar is achieved through the constantly-stabilized passive film and its stability is promoted using continuous calcium hydroxide released from the microcapsule, restoring alkaline conditions. The release process is characterized over a designated range of pH values and its release characteristics of the microcapsules, triggered by decreasing pH value, are captured by observing that the core crystals are released when exposed to a signal (stimulus). Precisely controlled release behavior enables corrosion protection in the case of depassivation. Its feasibility for hindering the corrosion of steel rebar by means of lifting the threshold value of / is discussed. A novel microcapsule-based self-immunity system for reinforced concrete is proposed.
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