A new method is proposed for updating the nonlinear finite element (FE) model of a structural system. It has been recognized that in some classes of structures, the degradation of the capacity of the structure occurs with a change in the zero-crossing stiffness. A relationship is obtained between the damage parameters used in a numerical simulation and the FE model stiffness at the zero-load crossings. This relationship is used to update the state of the FE model to reflect the damage that is associated with dynamic parameters. The modal characteristics are identified using ambient vibration data. The approach has been applied to a numerical model of a RC beam-column building subassemblage under quasi-static loading to demonstrate the proposed method. For simulation purposes, a one-dimensional hysteretic load-deformation material model is used in the FE model to represent the nonlinear moment-rotation behavior of RC beam-column joints. A modal flexibility-based model updating procedure is performed to update the damage parameters based on the change in the dynamic characteristics at each zero-load crossing. Good agreement between the updating and simulated stiffness demonstrates the efficacy of the proposed method.

In this research, a series of innovative optically-based sensors, which were designed, fabricated and characterized were created for potential evaluation for applications in determining moisture ingress in a range of concrete materials subjected to various environmental conditions. The approach taken to the creation of these novel humidity sensors is using long period grating (LPG) technology in an optical fibre. Several sensor configurations are fabricated by coating LPGs and then characterizing and cross-comparing and evaluating the resulting sensor performance. The thin layer of polyvinyl alcohol (PVA), whose refractive index varies as a function of humidity level when coated onto a LPG written into an optical fibre, provides a means to change the optical propagation in the fibre and thus to induce the a wavelength shift in the attenuation bands of its transmission spectrum, which then is calibrated against the measurand, humidity. When compared to the more familiar optical fibre-based humidity sensors, using Fibre Bragg gratings (FBGs), the LPG-based devices show a much higher measurement sensitivity, with more relaxed requirements for coating thickness and uniformity.

Electrical property based sensing of the properties of cement based materials is becoming increasingly popular. This paper reports the results of a systematic investigation carried out to understand the influence of material parameters on the dielectric response of cement pastes and concretes. A simple mathematical model that links the effective conductivity and time has been proposed, which can provide the setting time of the pastes. The variation in conductivity between plain and fly ash modified pastes is brought out. A fractional factorial experimental design coupled with range analysis is used to extract the significant parameters that influence the electrical conductivity of concretes. It is found that the aggregate-to-cementing materials ratio (a/cm) influences the very early age conductivity of concretes while the combinations of a/cm and water-to-cementing materials ratio (w/cm), and fly ash content and curing condition influences the final conductivity significantly. The changes in dielectric constant and conductivity spectra are attributed to the polarization phenomena. The dispersion in electrical conductivity starts at lower frequencies for fly ash modified pastes. There is an observed dielectric enhancement also for fly ash modified pastes. The dielectric response of concrete is very similar to that of pastes, and the effect of dilution by the aggregates in evident. The relaxation times of the cement pastes were determined from a geometric analysis of the bulk arc in the Nyquist plot. This method facilitates the determination of relaxation times and depression angle of the bulk arc using limited frequency sweep. The relaxation times decrease with increase in hydration time and conform to a power law. Though only a single relaxation time is considered in this study, the relaxation time distributions as represented by the depression angle, indicate increasing material heterogeneity with increasing hydration. Representing the dielectric constant at particular frequencies as a function of the capillary porosity in the cement pastes, it is possible to detect the percolation threshold of cement pastes. The observed percolation thresholds agree well with those reported for cement based materials.

Traditional methods of condition assessment frequently rely on manual inspections of a structure to locate signs of aging and deterioration. In comparison, the sensor-based health monitoring shows several advantages including increased accuracy and frequency of the measurements, possibility of remote sensing, and ability to determine the rate of damage and remaining life of a structure. Although the embedded electrical sensors provide valuable measurements enabling prediction of concrete durability, proper calibration of the measurements is essential. As an example, the electrical conductivity measurements of concrete are shown to be simultaneously influenced by four parameters: (a) concrete microstructure, (b) pore fluid composition, (c) moisture content, and (d) temperature. Experimental evidence is provided suggesting that the sensor measurements must be calibrated to account for these parameters; otherwise, the results can be misleading. A combination of several sensors can provide the information needed for proper calibration of the measurements. Several sensors have been developed to monitor the durability of concrete. Among these, a series of electrical/electro-chemical sensors have been recently introduced that enable measuring the transport properties of concrete as well as monitoring the corrosion process of the reinforcing steel. In his paper, several types of such electrical sensors are introduced and the theoretical background behind each measurement is discussed. For example, it is shown that measurement of the electrical conductivity of concrete can be used to estimate the permeability, ion diffusivity, or moisture content of concrete.

Environmental factors are major considerations of health monitoring of structures. A new system has been developed for measuring the internal relative humidity and temperature in concrete laboratory specimens and field structures. Internal relative humidity measurements in concrete are useful for evaluating shrinkage stress development and assessing structural susceptibility to durability problems. This paper describes the system, presents the essential elements of analysis that relates RH to internal stress in porous materials, and presents field applications that shows how environment data can facilitate analysis of stress and deformation of pavement. The new system has been used in laboratory and field experiments. Advantages of the new system are summarized. Some problems encountered in measuring relative humidity, especially in the field, are discussed.