T.M. Chrisp

Electrical conductivity, diffusion, and permeability of Portland cement-based mortars

Abstract The electrical conductivity of a range of Portland cement-based mortars was studied over a period of 450 days hydration. The influence of thermal cycling on conductivity was investigated and the activation energy, E a , established for conduction processes. E a was found to be system specific and was in the range 16–30 kJ/mol (0.16–0.31 eV/ion); pozzolanic additions had the effect of increasing E a in comparison with the plain ordinary Portland cement (OPC) mortar. Results also indicate that on the initial heating cycle, microstructural changes occurred. At the end of the test period, permeability and diffusion tests were carried out and data are presented in this respect.

Characterization and monitoring of cement-based systems using intrinsic electrical property measurements

This paper highlights the application of electrical property measurements as a characterization and investigative technique in the study of cementitious systems at the micro- and macroscale. Both fixed-frequency and spectral measurements are exploited to study cement pastes, mortars and concretes and results are presented from research programs relating to the early hydration of cement-based systems, characterization of fly ash, and concrete durability. The work comprises both laboratory-based investigations and field-monitoring studies. The methodology could complement other techniques that are used in the study of cement-based materials.

Temperature-conductivity relationships for concrete: An activation energy approach

The application of electrical techniques for in situ durability monitoring of reinforced concrete structures is now receiving considerable attention.

Developments in Performance Monitoring of Concrete Exposed to Extreme Environments

AbstractThe performance of the surface zone of concrete is acknowledged as a major factor governing the rate of deterioration of reinforced concrete structures because it provides the only barrier to the ingress of water containing dissolved ionic species such as chlorides, which ultimately initiate corrosion of the reinforcement. In situ monitoring of cover-zone concrete is therefore critical in attempting to make realistic predictions as to the in-service performance of the structure. To this end, this paper presents developments in a remote interrogation system to allow for continuous, real-time monitoring of the cover-zone concrete from an office setting. Use is made of a multi electrode array embedded within cover-zone concrete to acquire discretized electrical resistivity and temperature measurements, with both parameters monitored spatially and temporally. On-site instrumentation, which allows for the remote interrogation of concrete samples placed at a marine exposure site, is detailed together wi...

Depth-related variation in conductivity to study cover-zone concrete during wetting and drying

In situ electrical property measurements are used to study cover-zone concrete under a cyclic wetting and drying regime. OPC concrete and OPC with partial replacement with GGBS and PFA were used in the experimental programme with samples exposed to water and chloride solution. Monitoring the variation of electrical properties at discrete points within the surface 50 mm, during absorption and drying, provides information on water and ionic penetration into the cover zone, drying response, and evidence of continuing hydration and pozzolanic reaction.

Immittance spectra for Portland cement/fly ash-based binders during early hydration

A range of immittance formalisms is exploited to understand the nature of conduction and polarization within Portland cement-based binders over the frequency range 1 Hz to 1 MHz. Data are presented for binders with and without aggregate additions; of particular interest was the electrical response of a binary combination of ordinary Portland cement and fly ash. Regarding the latter, when presented on a Nyquist diagram, a characteristic plateau region emerged between the electrode spur and the bulk arc. When presented in the form of dielectric constant and conductivity as a function of frequency, a region of dispersion was evident for all systems. Furthermore, it was shown that by undertaking a dielectric frequency-domain analysis of the data, the experimental results could be synthesized across the frequency range 1 kHz to 1 MHz. Rather than ascribing separate processes to each impedance zone, the observed response could be attributed to a single bulk polarization process. When transformed into the complex impedance plane, there was also good agreement between synthesized and measured response. It is postulated that polarization is as a result of double-layer effects on the grain with a relaxation frequency in the low kilohertz region.

The early hydration of alkali-activated slag: developments in monitoring techniques

Abstract Developments in the application of electrical methods in the study of the early hydration reaction kinetics of alkali-activated slag are presented. Work focuses on monitoring the change in both conductance and capacitance over the initial 24–48 h hydration with electrical measurements taken over the frequency range 1–100 kHz. In the current study, sodium hydroxide and sodium silicate are used as activators. The work highlights the frequency dependence of capacitance and, to a lesser extent conductance; in addition, over the duration of the test, both these electrical parameters undergo significant changes which can be interpreted in terms of mechanisms of hydration. It is shown that electrical methods could be exploited as a convenient technique in characterising the reactivity of these materials with alkaline activators.

An experimental study on ionic migration through saturated kaolin

This paper details technical aspects associated with electrical conductivity measurements within saturated clays and presents experimental work to show the direct application of electrical techniques to track ionic movement through clay under chemically induced flow. The study confined itself to kaolin using both naturally sedimented samples and consolidated samples. Regarding the latter, a Rowe cell was adapted to allow electrical measurements to be taken both normal and parallel to the direction of loading. Bulk conductivity measurements and measurements on the conductivity of the interstitial pore fluid were used to quantify the pore tortuosity and effective diffusion coefficient of saturated clay. In addition, predicted concentration time curves agreed well with those estimated from conductivity measurements.

Developments in monitoring techniques for durability assessment of cover-zone concrete

This paper outlines developments in the use of an embedded multi-electrode sensor to study the response of the cover-zone (surface 50mm) to the changing ambient environment. The sensor enables the measurement of the spatial and temporal distribution of the electrical properties of concrete and temperature within the cover-zone thereby allowing an integrated assessment of cover-zone concrete performance. Both laboratory and field results are presented to highlight the information that can be obtained from embedded sensors. When exposed to the natural environment, the temperature dependence of the electrical response is highlighted and standardization protocols are developed to account for this effect. The monitoring system detailed also allows remote interrogation thereby providing (if required) a continuous output of real-time data and developments in this area are presented.

The development of roundwood timber pole structures for use on rural community technology projects

Abstract Roundwood timber from forest thinnings is a plentiful and inexpensive resource in many parts of the world, generally destined for low grade applications such as pulping, board production or fuel. In recent years, there has been interest in developing higher grade uses for such material. One such application has been the construction of the KIND centre, a 350-m 2 single storey residential centre completed in 2001, in which roundwood timber framing is used for the structural frame. The paper describes the structure, its design and construction. The paper outlines the societal, environmental and economic benefits of using locally grown roundwood timber thinnings, as well as the technical merits of the material and difficulties to be faced in its use. It describes how the design philosophy evolved to maximise the use of skills available within rural communities. The paper sets out a process of design validation followed when in using a novel engineering material, including the use of physical testing and re-evaluation of conventional performance criteria. The paper is amply illustrated to show the high quality internal useable space achieved, which has maintained the essence of the rustic and rural setting in its outer appearance.