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The influence of ongoing cement hydration and multiple microcrack formation on the electrical impedance of an engineered cementitious composite (ECC) is presented. Impedance measurements were obtained over the frequency range 20 Hz–1 MHz and displayed in the Nyquist format. In addition, the permittivity and conductivity were de-embedded from the measured impedance and presented in both time and frequency domains to elucidate the nature of conduction and polarisation processes. It was found that, over a curing period of 90 days, the ECC displayed a classic impedance response. Both conductivity and relative permittivity were found to be frequency dependent due to bulk relaxation processes operating within the composite. Tensile straining was shown to result in a detectable change in the impedance response, but retained a similar overall profile. When presented in the frequency domain, a downward displacement in both conductivity and relative permittivity profiles was evident with increasing tensile strain. It is shown that the relative permittivity at the high-frequency end could be exploited as a potentially useful indicator for strain/damage detection. The influence of microcracking on the piezo-resistive response of the composite is discussed based on crack patterns obtained from both visual observations and digital image correlation.

The ambient environment has a considerable influence on the permeation properties of the near-surface zone of exposed concrete (i.e. the cover zone). Monitoring the mass transport and flow processes and properties within this region is crucial in evaluating the long-term performance of concrete for a particular exposure condition. This paper presents an experimental study on both the spatial and temporal moisture movement within the surface region of concrete with and without supplementary cementitious materials. Prior to exposure, the samples were conditioned under two regimes representing poor and good curing; the samples were then exposed to a simulated hot environment with a diurnal temperature fluctuation of ∼20–40°C and ∼60% ambient relative humidity. Moisture movement within the surface region was monitored using discretised electrical conductivity measurements which, together with gravimetric measurements, allowed evaluation of the volumetric uptake and sorptivity of the concrete and the rate and depth of water penetration into the concrete cover zone; it is shown that when these are combined, the degree of saturation, effective porosity and total porosity of the surface region could be estimated. By evaluating the conductivity prior to and after water absorption, the zone of influence of wetting/drying action (i.e. the convective zone) could be evaluated.

A number of sophisticated techniques, such as NMR imaging and γ-ray attenuation, have now been used to study moisture transport in porous construction materials. This paper presents initial developments in the application of discretized, electrical conductivity measurements as a relatively simple technique to monitor water movement within cover-zone concrete. Using a 3-phase model and the Archie relationship for porous rock formations, conductivity measurements are used to evaluate moisture-content profiles during water ingress and, through the use of Boltzmann's transformation, it is shown that these profiles collapse onto a master-curve. Having obtained the master-curve during absorption, the capillary transport coefficient (Dθ) can then be obtained; however, the current work focuses on presenting the methodology as an alternative testing technique.

Satisfactory guidelines for ensuring adequate reinforced concrete durability can only be developed by monitoring concrete performance under a range of field exposure conditions over an extended per...

Environmental Scanning Electron Microscopy (ESEM) is used to study the origin of micro-crack healing in an Engineered Cementitious Composite (ECC). ESEM images were acquired from ECC specimens cut from pre-cracked, dog-bone samples which then subjected to submerged curing followed by exposure to the natural environment. The mineralogical and chemical compositions of the healing products were determined using the EDX facility in the ESEM. It is shown that the precipitation of calcium carbonate is the main contributor to micro-crack healing at the crack mouth. The healing products initially appeared in an angular rhombohedral morphology which then underwent a discernable transformation in size, shape and surface texture, from relatively flat and smooth to irregular and rough, resembling the texture of the original surface areas surrounding the micro-cracks. It is also shown that exposure to the natural environment, involving intermittent wetting/drying cycles, promotes additional crystal growth, which indicates enhanced self-healing capability in this environment.

Alternating current (a.c.) electrical property measurements are presented on an Engineered Cementitious Composite (ECC) reinforced with a hybrid mix of polyvinyl alcohol fibers (fixed dosage) and straight steel fibers of varying dosages (0.15–1.0% by volume), with the aim at elucidating the influence of conductive inclusions on the nature of conduction and polarization processes within the composite. Measurements were undertaken over the frequency range 1 Hz–10 MHz at 7, 14 and 28 days after casting and the data presented in a range of formalisms to aid interpretation. When plotted in the frequency domain, the work shows that steel fibers enhance the polarizability of the material, particularly within the frequency range ~10 Hz–10 kHz. When presented in Nyquist format, this feature manifests itself as an intermediate arc forming between a high frequency arc (>10 kHz) and a low frequency arc (

The self-healing performance of an engineered cementitious composite (ECC) exposed to the natural environment is presented. Fifteen dog-bone shaped ECC samples were preloaded after 14 d of curing and then placed outside in an open area at Heriot-Watt University (Edinburgh campus). Ultrasonic pulse velocity measurements were used to determine the rate and extent of the self-healing capabilities of the ECC in the natural environment. The results showed that, while the more highly damaged samples displayed the greatest decrease in ultrasonic velocity, they also displayed initial accelerated healing, which implies an increased quantity of individual cracks in the more damaged samples rather than an increase in individual crack widths. It was also found that the self-healing of microcracks in the ECC was robust. Narrow hairline cracks (

Flexural bond strength is an important performance characteristic of masonry structures yet there is no guidance for lime-mortared stonework in design codes of practice. This study investigates the bond strength of natural hydraulic lime (NHL) and natural cement mortared sandstone masonry. To this end, the flexural bond strength of masonry couplets, built with mortars of three hydraulic strengths and one natural cement and having a water-content adjusted to achieve a similar consistency, was measured with the bond wrench test. Practical mortar compositions and natural curing conditions were used within the experimental programme. Bond strength was found to be directly related to binder hydraulicity and sandstone pre-wetting time – a positive effect in the case of the former and a negative influence in the case of the latter. Pre-wetting time, however, had a greater influence on the feebly hydraulic lime binder (NHL 2) than on the moderately (NHL 3.5) and eminently hydraulic (NHL 5) lime binders. The results presented will assist in improving our knowledge of lime mortared sandstone masonry and in the development of design guidance.

The electrical conductivity of a range of concrete mixes, with and without supplementary cementitious materials (SCM), is studied through multiple cycles of heating and cooling over the extended temperature range −30/+70 °C. When presented in an Arrhenius format, the experimental results display hysteresis effects at the low-temperature end of the thermal cycle and, in those concretes containing supplementary cementitious materials at higher water/binder ratios, hysteresis effects were evident over the entire temperature range becoming more discernible with increasing number of thermal cycles. The depression in both the freezing and thawing point could be clearly identified and was used to estimate pore-neck and pore-cavity radii. A simplified approach is presented to evaluate the volumetric ratio of frozen pore water in terms of conductivity measurements. The results also show that the conductivity and activation energy of the concrete specimens were related to the water/binder ratio, type of SCM, physical state of the pore water and the thermal cycling regime.

The remit of the UK–China Science Bridge is to accelerate deployment of research knowledge in sustainable solutions for the built environment developed at Queen’s University Belfast, to high-ranking partner universities and research institutions in China. To achieve this, proof of concept studies are being undertaken at key Chinese universities and research centres to assess the relevance and transferability of Queen’s University-developed technologies. Particular attention is given to non-destructive testing and sensors for monitoring the durability of structures as well as a novel form of construction called ‘flexible arch’. Chinese counterparts aim to set up monitoring systems in concrete bridges using sensor systems from Queen’s. Data from these monitoring stations will be used to predict the service life and structural health performance of concrete bridges in marine environments. The aim of this paper is to introduce this international collaborative programme and research, allowing other organisations to benefit from the outcomes.

The current study monitors both the short- and long-term hydration characteristics of concrete using discretized conductivity measurements from initial gauging, through setting and hardening, the latter comprising both the curing and post-curing periods. In particular, attention is directed to the near-surface concrete as it is this zone which protects the steel from the external environment and has a major influence on durability, performance and service-life. A wide range of concrete mixes is studied comprising both plain Portland cement concretes and concretes containing fly-ash and ground granulated blast furnace slag. The parameter normalised conductivity was used to identify four distinct stages in the hydration process and highlight the influence of supplementary cementitious materials (SCM) on hydration and hydration kinetics. A relationship has been presented to account for the temporal decrease in conductivity, post 10-days hydration. The testing procedure and methodology presented lend itself to in-situ monitoring of reinforced concrete structures.

In establishing the reliability of performance-related design methods for concrete – which are relevant for resistance against chloride-induced corrosion - long-term experience of local materials and practices and detailed knowledge of the ambient and local micro-climate are critical. Furthermore, in the development of analytical models for performance-based design, calibration against test data representative of actual conditions in practice is required. To this end, the current study presents results from full-scale, concrete pier-stems under long-term exposure to a marine environment with work focussing on XS2 (below mid-tide level) in which the concrete is regarded as fully saturated and XS3 (tidal, splash and spray) in which the concrete is in an unsaturated condition. These exposures represent zones where concrete structures are most susceptible to ionic ingress and deterioration. Chloride profiles and chloride transport behaviour are studied using both an empirical model (erfc function) and a physical model (ClinConc). The time dependency of surface chloride concentration (Cs) and apparent diffusivity (Da) were established for the empirical model whereas, in the ClinConc model (originally based on saturated concrete), two new environmental factors were introduced for the XS3 environmental exposure zone. Although the XS3 is considered as one environmental exposure zone according to BS EN 206-1:2013, the work has highlighted that even within this zone, significant changes in chloride ingress are evident. This study aims to update the parameters of both models for predicting the long term transport behaviour of concrete subjected to environmental exposure classes XS2 and XS3.

The conduction and dielectric behaviour of two different grades of natural hydraulic lime is presented over the frequency range 1 Hz–1 MHz, with measurements taken over the initial six months after gauging with water. Samples containing embedded electrodes were exposed to both a natural atmosphere (20°C and 65% relative humidity) and a natural atmosphere with a carbon dioxide concentration maintained at 400 ppm which was used to accelerate the carbonation process. A decrease in relative dielectric permittivity and rise in conductivity, with increasing frequency, was observed at all stages over the time-scale presented. When plotted in the complex plane, the impedance featured a bulk response comprising two depressed semicircles and a low frequency spur, the latter being associated with the electrode/sample interface. The complex impedance plot, together with the application of an equivalent circuit model, indicated a dual arc feature with carbonation and hydration contributing to bulk impedance response. This study demonstrates the applicability of electrical property measurements to monitor the combined processes of hydration and carbonation in this group of materials.

An extensive chloride-profiling programme was undertaken over an eight-year period on nine concrete pier-stems placed at a marine location to represent XS1, XS2 and XS3 environmental exposure conditions as defined in European Standard EN 206-1. The pier-stems were 2·0 m high and octagonal in plan with each vertical face 0·66 m wide and were constructed in groups of three (one each placed at the locations defined above): one group, which was used as a benchmark, represented plain Portland cement concrete; the second group had a waterproofing agent (Caltite) added at the time of mixing; and the third was treated with silane. Chloride concentration profiles were taken at a number of locations on each pier-stem, which were subsequently used to evaluate the temporal change in effective diffusion coefficient and surface chloride concentration for the different exposure conditions. In addition, it allowed assessment of the relative performance of the plain concrete, Caltite concrete and silaned concrete pier-stems with regard to chloride ingress.

This paper reports an approach by which laboratory based testing and numerical modelling can be combined to predict the long term performance of a range of concretes exposed to marine environments. Firstly, a critical review of the test methods for assessing the chloride penetration resistance of concrete is given. The repeatability of the different test results is also included. In addition to the test methods, a numerical simulation model is used to explore the test data further to obtain long-term chloride ingress trends. The combined use of testing and modelling is validated with the help of long-term chloride ingress data from a North Sea exposure site. In summary, the paper outlines a methodology for determining the long term performance of concrete in marine environments.

It is recognised that the addition of carbon fibres to a brittle cement matrix results in a less dense composite with enhanced ductility, improved impact resistance and increased toughness. In addition, the reinforcing effect of fibres in the cement often produces superior flexural strength and marked improvements in post-cracking behaviour. Further, carbon fibres influence the electrical properties of the composite which could, potentially, make it a smart material, with a range of applications. Despite attention directed towards the mechanical and electrical properties of carbon fibre reinforced cement (CFRC), there is a dearth of information of the influence of fibre additions on the rheological properties of the resulting composite. To this end, this paper describes an investigation using the Viskomat NT into the influence of carbon fibre additions (fibre length in the range 3–12 mm and volume in the range 0–0.5%) on the rheological properties of CFRC. Within the limitations of the instrument and testing procedure it is shown that CFRC’s conform to the Bingham model: increasing fibre volume and fibre length increase both the yield stress and plastic viscosity.

The temporal change in electrical properties is used to study the hydration kinetics of chemically activated fly-ash systems at room temperature. Four low-lime ashes and one high-lime ash were used in the experimental programme and the electrical response was measured in terms of the in-phase (i.e. conductance) and quadrature (i.e. capacitance) components of the sample admittance. The low-lime ash and high-lime ash exhibited markedly different electrical response. The work highlights the applicability of the testing methodology as a non-invasive, non-destructive technique in the assessment of the efficacy of a particular activator or combination of activators.

It has been reported in a previous study [Cem. Concr. Res. 29 (1999) 377; Cem. Concr. Res. 33 (2003) 197] that when a low-lime fly ash is introduced as a cementitious component, there are significant changes in the electrical response of the system, most notably, an enhancement in dielectric constant and the emergence of a plateau region in the complex plane. The changes in electrical behaviour were attributed to the spherical nature of the fly-ash particle, resulting in an enhancement in the double-layer polarization effects on the particle surface. In this paper, data are presented for three low-lime ashes, each displaying differences in their electrical behaviour. Through a systematic series of tests, a new interpretation for the origin of the characteristic electrical response is presented.

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.

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.

With more demands being made on reinforced concrete, 100-year guarantees of durability will become a necessity. Lifetime calculations, and prediction of the residual service-life of structures, require quantitative information on cover-zone properties and threshold values for corrosion initiation. It is clear that there exists a need to determine quantitatively those near-surface characteristics of concrete which promote the ingress of gases and/or liquids containing dissolved contaminants. In addition, in-situ monitoring of the temporal change in such properties could assist in making realistic predictions as to the in-service performance of the structure; likely deterioration rates for a particular exposure condition or compliance with the specified design life. This paper details covercrete sensor arrangements; format of data presentation and information that can be obtained from embedded sensors. Such sensors could, ultimately, form part of a high-level monitoring strategy and should be considered at the design stage.

A combination of gravimetric, internal relative humidity, and electrical measurements were used to study moisture changes and spatial distribution of moisture within the surface 100 mm of concrete. Specimens were subjected to a repeated drying and wetting regime. The testing program highlighted the influence of binder type, water/binder ratio, initial curing regime, and cyclic drying/wetting on surface-zone absorption and desorption characteristics. It is shown that electrical measurements taken at discrete points within the surface 100 mm can be used to estimate the zone of concrete most influenced by evaporative and wetting processes. A relationship was established between the integrated area between conductivity profiles and cumulative water uptake by the surface region. Over the 12-month test period, electrical conductivity measurements also indicated ongoing hydration and pozzolanic reactions.

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.

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.

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.

This paper utilizes electrical methods to follow the early hydration characteristics of pozzolanic materials activated with powdered calcium hydroxide at room temperature (20°C). The electrical response is measured in terms of conductance and capacitance. Four materials are studied: ground granulated blastIurnace slag, microsilica, metakaolin and pulverized fuel ash (the last from three different sources). The work shows that monitoring the temporal change in electrical parameters can exploited as a potentially usefit! test method for identifying the stages of hydration and studying the rate of reaction over these stages. To this end, different formats of data presentation are exploited. It was shown that the capacitance of the material displays a marked frequency dependence over the range of the current study (10a Hz to 10kHz); the influence of frequency of applied field on conductance was less pronounced.

The current work focuses on the application of a.c. impedance spectroscopy techniques to cemenfltlOus systems while still in the liquid state (i.e. during the dormant period). A variety of formats of data presentation are presented and exploited to understand the origins of polarization processes within these systems. The theoretical treatment also allows simulation of the experimental data to provide an interpretation of the bulk impedance response. Data are obtained over the frequency range 1 Hz to 20 MHz, but interpretation and analysis is focused, in the main, on the range 1 kHz to 1 MHz.

In this paper the change in conductivity of pozzolan/CH mixtures, at ambient temperatures, is monitored for periods extending over two days. Such a method allows the chemical activity within the mixture to be followed and it is shown that a number of features/parameters can be identified which are related to the pozzolanic reactivity of the material. In particular, a pozzolanicity index is developed and is based on data obtained from conductivity measurements. The index reflects both the rate of pozzolanic reaction and time to setting. Work is presented on a range of cement replacement materials.

The work presented in this paper forms part of a wider research programme being undertaken on water and ionic movement within the cover region of structural concrete and the evaluation of the properties that, ultimately, determine durability. The developments in monitoring techniques employed in the testing programme are detailed, with particular emphasis on the application of electrical methods. Data are presented that highlight the testing methods and the interpretation of the results. Examples are given of other uses of the data to provide additional information on the electrical and physical properties of concrete, and on its moisture state. In particular, it is shown that it is possible to determine such factors as rates and depths of water penetration, degree of saturation and porosity of the concrete cover, and the resistivity of the concrete at the level of the reinforcement.

This Note presents the theory and application of two methods for measuring the electrical resistivity of repair mortar for use in cathodic protection systems. One method employs the use of steel pins embedded within the sample and the other uses external steel plates placed on the sides of the sample. It is shown that, if the correct geometry of pin arrangement is chosen, the results obtained using both methods compare favourably.

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

The theory of unsaturated flow in porous materials has been applied to the study of water penetration into concrete. This theory predicts the classical relationship between the cumulative volume of water absorbed and the square root of the elapsed time. In this Paper data are presented on both the volume of water absorbed and the depth of penetration of water, for a range of concrete mixes, as a function of time.

This paper describes an electrical model for cement and concrete applicable over a wide frequency range and at various stages of hydration. The model contains a non-Debye dispersive element to explain the experimental data which are presented.

A sensor system, developed in a previous laboratory study [McCarter WJ, Emerson M, Ezirim H. Properties of concrete in the cover-zone: developments in monitoring techniques. Mag Conc Res 1995;47(172):243–51; Chrisp TM, McCarter WJ, Starrs G, Basheer PAM, Blewett J. Depth related variation in conductivity to study wetting and drying of cover-zone concrete. Cem Conc Comp 2002;24(5):415–27], is used to obtain the spatial distribution of electrical conductivity within the cover-zone of concrete specimens subjected to a range of natural exposure conditions. The testing methodology enables in-situ monitoring of the electrical conductivity and technical issues relating to site measurements are discussed. The work also highlights the need for continual monitoring of the covercrete as the electrical conductivity changes with time, depth and environment.

Mass transport processes in hardened Portland cement-based binders are of considerable importance, not only in connection with the durability of reinforced concrete but also in the immobilization and isolation of radioactive waste.

Synopsis The objective of the work presented in this paper is to investigate the changes in the electrical properties of cement paste during the first 24 hours after gauging with water. It is demonstrated that the electrical response characteristics of setting cement paste can be used as an effective means of studying the progress of cement hydration and for monitoring structuralchanges occurring within the paste

Cement is one of the most widely used construction materials with one billion tonnes used annually. From an engineering point of view, it is essential that cement sets and hardens in a correct manner, indeed, modification of the setting and hardening characteristics of cement by the use of admixtures is becoming widespread in the construction industry. The reaction between cement clinker and water is a complicated chemical process which results in a rigid matrix capable of sustaining load. The increase in strength of the cement matrix is the consequence of hydration and crystal formation within the paste. Understanding the mechanisms of hydration and how they can be modified could result in new cement blends and admixtures tailor-made to suit any particular set of design criteria. In this paper it is shown that the temporal change in electrical response can be used to monitor the progress of hydration, and give an insight into mechanisms of hydration. Data are presented for several cement paste consistencies over the frequency range 20 Hz to 300 kHz.

Etude de l'influence de deux additifs (sucre et chlorure de calcium) sur l'etape initiale de l'hydratation du ciment, par mesure de parametres electriques

On montre que la reponse electrique de ciment alumineux et de melanges de ciment Portland/ciment alumineux peut etre utilisee pour enregistrer les caracteristiques de l'hydratation initiale et que la vitesse de variation de ces parametres electriques peut donner une indication de la vitesse de progression des reactions

This paper presents the influence of cyclic wetting and drying on the absorption characteristics of cover-zone concrete. Portland cement concrete and concrete with partial replacement with fly ash and ground-granulated blast-furnace slag were used in the experimental program, with water and chloride solution applied at the suction surface. Electrical techniques were utilized in conjunction with a surface-applied, volumetric-gain method to study water and ionic movement. Monitoring the variation of electrical response across electrodes embedded within the cover region provided information on the depth and rate of water penetration, ionic ingress, and evidence of continuing hydration and pozzolanic activity. By combining depth-of-penetration data with the cumulative volumetric uptake of the samples, the effective porosity of the surface region could be estimated. Ponding studies were also undertaken on replicate samples to study ionic ingress.

The work described in this paper forms part of a wider research programme being undertaken on water and ionic movement within the cover region of structural concrete. In-situ monitoring techniques described previously were used to measure the depth of penetration of water, cumulative volume gain and sorptivity of concrete samples containing different types of aggregate. The effects of dissolved ions (chlorides) in the invading water were also monitored. As a direct result of the monitoring techniques employed in the testing programme, it is possible to present many of the results in considerably more detail than has previously been the case. Possibilities for evaluating the electrical properties of concrete in the cover zone, and at rebar level, are also discussed. In addition, a method of estimating chloride concentration within the pore water is suggested.

A.C. impedance spectroscopy is now gaining favour as an investigative technique for monitoring the development of cement microstructure. This paper extents the range of application of the method by showing the compositional dependence of the complex impedance of cement-aggregate-water systems (ie. concrete and mortar), with attention being directed towards this material while still in the plastic state. In the work presented, the composition of such systems was varied over a wide range and a number of features of the complex response were identified which can be linked to cement-paste content, cement type and aggregate content. Measurements were, in the main, taken over the frequency range 1Hz–15MHz.

The complex impedance response of fresh concrete was investigated over the frequency range 100 Hz–10 MHz, together with resistivity measurements on the interstitial water phase. The work corroborates and extends previous studies both in the range of concretes tested and modelling of the data. It is shown that the contained water-phase within the concrete exerts a considerable influence of the impedance response and, as such, is related to the type of cemetitious binder. When the bulk resistivity of the concrete is normalized by that of the contained water, a direct relationship exists between this value and the fractional water content of the concrete. Possibilities exist for the application of impedance spectroscopy techniques in the quality control of structural concrete.

In situ electrical measurements are used in conjunction with surface-applied, volumetric gain methods to study cover zone concrete under a wetting and drying regime. Ordinary Portland cement (OPC) concretes and OPC with partial replacement with ground granulated blast-furnace slag 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 after drying, provides information on water and ionic penetration into the cover zone; drying response, and evidence of continuing hydrationlchloride-binding effects.