William John McCarter

The A.C. response of hardened cement paste

Abstract 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.

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.

Monitoring pozzolanic activity by direct activation with calcium hydroxide

Abstract 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.

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.

NEAR-SURFACE SENSORS FOR CONDITION MONITORING OF COVER-ZONE CONCRETE

Abstract 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.

Monitoring the early hydration mechanisms of hydraulic cement

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.

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.

A parametric study of the impedance characteristics of cement-aggregate systems during early hydration

Abstract 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.