International Concrete Abstracts Portal

Structural health monitoring and sensing are rapidly developing fields of study that have been successfully applied to engineered structures, such as aircraft frames. The implementation in concrete structures, however, is neither common nor currently accepted. ACI Committees 236, Materials Science of Concrete, and 444, Analysis for Concrete Structures, undertook an effort to organize a technical session and special publication that recognize new achievements in this promising field of research. The objective of this effort is to increase awareness of leading research that applies this technology to concrete structures, and thus to promote interest in the field. Many quality papers were submitted in response to the original call for papers for this session; regrettably, several worthy papers could not be included. The subject matter of the selected papers represents a broad range of topics, from the development of specific types of embedded sensors for concrete to the implementation of wireless sensor networks to managing infrastructure systems. This volume will be of interest to engineers, researchers, and students who wish to learn more about this important, dynamic, and developing topic.

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.

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.

There is a need in industry for new devices for the monitoring of chloride ion ingress in structural concrete. This work reports on the development of a reflective, gold-coated long period grating-based senor for the measurement of chloride ions in solution, with potential for evaluating the corrosion condition of concrete structures. The sensor scheme is based around a long period fibre grating (LPG)-based Michelson interferometer where the sensor was calibrated and evaluated in the laboratory using sodium chloride solutions, over a wide range of concentrations, from 0.01 M to 4.00 M. The grating response yields shifts in the spectral characteristic of the interferometer, due to the change of refracting index of the solution surrounding it. It was found that the sensitivity of the device could be enhanced over that obtained from a bare fibre by coating the LPG-based interferometer with gold nanoparticles.