PSI - Issue 28

Dimos Triantis et al. / Procedia Structural Integrity 28 (2020) 502–510 2 D. Triantis, I. Stavrakas, A. Kyriazopoulos, E. D. Pasiou, S. K. Kourkoulis / Structural Integrity Procedia 00 (2019) 000–000 1. Introduction It is well known that the mechanical response and strength of concrete and cement-based materials at their very early ages are completely different from the respective ones of the mature materials. In general, the strength and stiffness of these materials increase with time. The rate of increase depends on the type of cement and admixtures used, and, also, on moisture and temperature conditions during curing (Topkaya et al. 2004). Several studies have dealt with the specific issue, long ago. For example, the rate of increase of the compressive strength, and that of the respective tensile one (as obtained from the standardized Brazilian disc test) as well as of the elastic modulus were studied by Lew and Reichard (1978), for ages ranging from one day to six weeks. They concluded that the compressive and tensile strength increase according to almost the same rate, while the rate of increase of the elastic modulus was slightly higher. A few years later, Oluokun et al. (1991) considered the properties of concrete specimens of age varying from a few hours to four weeks. They concluded that the familiar empirical formula of the American Concrete Institute (ACI), correlating the elastic modulus and the compressive strength of concrete, is satisfactory for specimens the age of which exceeded twelve hours. Similar conclusions were drawn, also, by Khan et al. (1995), who tested concrete specimens of ages between eight hours and three months and concluded that during the very first few hours, the respective empirical formula of ACI for the elastic modulus provides results overestimating the stiffness of the material. In general, the ascending branch of the axial stress - axial strain curve of early age concrete under uniaxial compression is quite flatter and with smaller linear portion compared to that of mature concrete. Although the rate of increase of both the strength and the modulus of elasticity against time strongly depends on the physico-mechanical properties of the constituent materials and their proportion in the mixture, it is experimentally verified that at least during the first two weeks a roughly linear relation between time and strength exists, assuming that time is plotted in logarithmic scale. A similar empirical relation is assumed for the dependence of Young’s modulus on time. Another time dependent property of concrete, influencing its mechanical response at its early age is shrinkage, a result of volume reduction due to hydration of cement and water and, also, due to drying. Shrinkage is considered responsible for cement cracking, thus undermining the durability of any structure. It is accepted that shrinkage occurs in two stages, an early one (commonly defined as the first day after placement) when the concrete starts hardening and a later (long-term) one, characterizing concrete at ages beyond 24 hours (Holt and Leivo, 2004). Nowadays, the response of concrete and cement-based structures during their very early age is still under study both in a laboratory scale and, also, in situ. For the latter it is accepted that monitoring the response of such structures becomes a pressing demand, especially during form removal. In this direction, the explosive development of innovative sensing techniques during the last decades changed drastically the way structural health monitoring of civil engineering structures is achieved. Novel nondestructive testing techniques have been introduced, based on acoustical, electrical, mechanical, optical and thermal properties of the tested materials (Voigt et al. 1993). These techniques, assisted by the rapid development of data acquisition and storage systems of huge capacity, offer to the engineering community flexible tools for Structural Health Monitoring (SHM) of concrete structures starting from their very early ages. Taking into account the above arguments and the fact that during their curing period the cementitious materials are quite sensitive to damage mechanisms and especially to early cracking, the present study aims to assess the applicability and sensitivity of a recently introduced sensing technique, when it is used to monitor the mechanical response of early aged cement-based structures. The technique assessed is based on the detection of very weak electrical emissions, and is known as the Pressure Stimulated Currents (PSC) technique. Analysis of the data provided by the experimental protocol indicates that the PSC technique can be indeed used for SHM of cement-based materials, offering valuable data regarding their structural integrity and proximity to criticality (fracture) even during their very early age. 2. Electric emissions during mechanical loading and the PSC technique The idea that the electrical emissions, produced when specific classes of materials are loaded mechanically, can be used to monitor the mechanical response of the material is closely related to the efforts of the scientific community to predict earthquakes. Stepanov (1933) was the first to detect electric phenomena during plastic deformation of ionic materials already almost one century ago. However, it was Whitworth (1975) who demonstrated and quantified such an effect in alkali halides. Although some researchers (Finkelstein et al. 1973; Nitsan 1977) attributed these phenomena to piezoelectricity, similar signals were observed in non-piezoelectric materials, attributed to additional electrification 503