Relative humidity in the interior of concrete exposed to natural and artificial weathering

The moisture content is a crucial parameter for most of the degradation processes suffered by concrete. Thus, a certain water content is needed to develop alkali-silica reaction, frost attack, or steel corrosion, while in contrast carbonation can only progress if the concrete is relatively dry. The importance of the concrete moisture state has been studied for many years in the concrete literature, and the internal relative humidity has been addressed mainly by those researchers interested in creep and shrinkage. However, despite the numerous works on the subject, almost no data can be found on the monitoring of the moisture content or of the internal relative humidity in structures subjected to real weathering conditions. In general the extensive studies have been made in the laboratory in well-controlled chambers to examine water isotherms. In addition, modelling has been developed assuming general isothermic conditions. However, natural weathering usually implies irregular changes of temperature and relative humidity, which induce continuous nonsteady-state conditions in the interior of the concrete. In the present paper, Values of the internal relative humidity of concretes submitted to natural and artificial weathering are presented. From these, it is possible to deduce that the temperature is the main factor influencing the concrete internal relative humidity in samples sheltered from rain, while rain periods are the main factor in unsheltered samples. In the environment tested, two kinds of temperature cycles are acting: the day-night cycle and the seasonal cycle. The paper discusses the phenomenological features of the observed evolution of the internal relative humidity and presents some interpretations on the mechanisms of water transport induced by the external environment. (C) 1999 Elsevier Science Ltd. All rights reserved.