G. Starrs

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.

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.

Two-point concrete resistivity measurements: interfacial phenomena at the electrode–concrete contact zone

Ac impedance spectroscopy measurements are used to critically examine the end-to-end (two-point) testing technique employed in evaluating the bulk electrical resistivity of concrete. In particular, this paper focusses on the interfacial contact region between the electrode and specimen and the influence of contacting medium and measurement frequency on the impedance response. Two-point and four-point electrode configurations were compared and modelling of the impedance response was undertaken to identify and quantify the contribution of the electrode–specimen contact region on the measured impedance. Measurements are presented in both Bode and Nyquist formats to aid interpretation. Concretes mixes conforming to BSEN206-1 and BS8500-1 were investigated which included concretes containing the supplementary cementitious materials fly ash and ground granulated blast-furnace slag. A measurement protocol is presented for the end-to-end technique in terms of test frequency and electrode–specimen contacting medium in order to minimize electrode–specimen interfacial effect and ensure correct measurement of bulk resistivity.

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.

Monitoring and characterisation of sand-mud sedimentation processes

Estuaries and tidal inlets are often characterised by the presence of both cohesive and non-cohesive sediments. Knowledge of the sedimentation behaviour of sand-mud mixtures is therefore crucial to the understanding and prediction of the time-dependent structure (i.e. mixed or segregated), composition and erodibility of sediment bed deposits developing within these environments. In the current study, a series of settling column tests are conducted to investigate the hindered settling and initial bed consolidation phases of a range of sand-clay mixtures to determine the parametric conditions under which bed segregation occurs. A new, non-invasive, electrical resistivity measurement technique is employed to capture both temporal and spatial changes in the density, porosity and composition of the evolving sand-clay bed deposits, complimented by time-lapsed images of the sedimentation process within the column. The results show that the formation of segregated (sand-clay) bed layers with bed deposits is largely controlled by the initial fractional composition (i.e. relative sand and clay concentrations). Specifically, mixtures with low clay contents are shown to form well-defined (sand-clay) layer segregation within the resulting deposits, while higher clay contents result in more transitional segregation patterns or no layer segregation (for very high clay concentrations). The physical mechanisms under which these different segregation types can be generated are illustrated through predictions from an existing polydisperse hindered settling model. This model indicates that the degree of bed segregation, and time scale over which this occurs, correlates well with the difference in predicted hindered settling characteristics and upward displacements associated with the sand and clay fractions, respectively. In this regard, the new experimental dataset provides validation for the polydisperse model (for the first time), with the combined data and model predictions providing new insight into mixed (sand-clay) sedimentation processes.

A durability performance-index for concrete: developments in a novel test method

Implementation of both design for durability and performance-basedstandards and specifications are limited by the lack of rapid, simple, sciencebasedtest methods for characterising the transport properties and deteriorationresistance of concrete. This paper presents developments in the application ofelectrical property measurements as a testing methodology to evaluate therelative performance of a range of concrete mixes. The technique lends itself toin-situ monitoring thereby allowing measurements to be obtained on theas-placed concrete. Conductivity measurements are presented for concreteswith and without supplementary cementitious materials (SCM’s) fromdemoulding up to 350 days. It is shown that electrical conductivitymeasurements display a continual decrease over the entire test period andattributed to pore structure refinement due to hydration and pozzolanicreaction. The term formation factor is introduced to rank concrete performancein terms of is resistance to chloride penetration.

The Influence of Multiple Micro-cracking on the Electrical Impedance of an Engineered Cementitious Composite

The mechanical and electrical properties of an engineered cementitious composite (ECC) while under tensile loading are presented. Direct tensile tests were conducted on dog-bone shaped ECC samples, with simultaneous mechanical and electrical measurements taken during the loading process. Regarding electrical property measurements, these were recorded at 13 spot frequencies over the frequency range 100 Hz–1 MHz. When presented in Nyquist format, the ECC displayed the typical impedance response comprising a spur at the low-frequency side of the curve and a semi-circular arc at the high-frequency side. It was evident that tensile straining of the dog-bone samples resulted in the entire impedance response being progressively displaced to the right, indicating an overall increase in impedance, and in an increase in the prominence of the bulk arc. The increase in impedance can be attributed to increased damage resulting from micro-cracking.

Assessing the performance of engineered cementitious composites under cyclic wetting and drying

The application of electrical property measurements for assessing the permeation properties of Engineered Cementitious Composite (ECC) is demonstrated. To this end, a multi-electrode, two-point sensor was embedded within the surface region of two ECC specimens, with one being reinforced with the standard polyvinyl alcohol fibers and the other with no fiber reinforcement. These specimens were then exposed to cycles of wetting and drying in a laboratory environment. Electrical measurements were undertaken at regular intervals during this cyclic exposure regime, in order to monitor moisture movement within the surface region. It is shown that the multi-electrode sensor offers a simple, yet powerful, investigative technique for studying the spatial and temporal response of the surface region over an extended period of time and, hence, a comprehensive assessment of the protective qualities of this region. It is also shown that the proposed technique could be exploited to study the influence of surface cracking. A novel data presentation technique is introduced to facilitate visualization of key features during the cyclic testing regime.