W. John McCarter

Electrode Configurations for Resistivity Measurements on Concrete

In this study, two- and four-electrode arrangements were used to evaluate the electrical resistance (hence resistivity) of saturated concrete specimens using both embedded rod-electrodes, external plate-electrodes, and combinations thereof Measurements were obtained mainly at a fixed frequency of 1 kHz. It is shown that the two-electrode external plate measurement system gave higher resistivity values than the four-electrode system; however, studying the electrical response in terms of complex impedance, over the frequency range 1 to 10 MHz, revealed that the sponge-contacting system used in the two-electrode method introduced a spurious resistance attributed to the sponge-sample interface.

Full-Scale Marine Exposure Tests on Treated and Untreated Concretes Initial 7-Year Results

An extensive chloride profiling program was undertaken on concrete pier stems erected in the vicinity of the Dornoch Bridge located at the Dornoch Firth in Northeast Scotland. The pier stems were 2 m (6.562ft) high and octagonal in plan with 0.66 m (2.165ft) wide faces. The piers were constructed in sets of three with the lowest of each set in the tidal zone and the highest in the atmospheric zone. The pier stems were placed in such a way that they would represent the exposure conditions of the actual bridge piers of the Dornoch Bridge. In all, six of the pier stems were made using plain ordinary portland cement (OPC) concrete (with three of these having the surface treated with silane); the remaining three pier stems had a concrete containing caltite as an additive. Three exposure zones were studied: the tidal zone, the splash zone, and the atmospheric zone. The tidal zone was further subdivided into two levels defined as low-level and high-level. Chloride profiles were obtained from the different regimes over a period of 7 years for all nine pier stems. This paper describes the nature of chloride ingress and the usefulness of diffusion parameters in classifying each exposure regimes. Furthermore, the effectiveness of silane and caltite in protecting concrete from chloride ingress in different exposure zones was studied.

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.

Rheology of Carbon Fibre Reinforced Cement-Based Mortar

Carbon fibre reinforced cement based materials (CFRCs) offer the possibility of fabricating “smart” electrically conductive materials. Rheology of the fresh mix is crucial to satisfactory moulding and fresh CFRC conforms to the Bingham model with slight structural breakdown. Both yield stress and plastic viscosity increase with increasing fibre length and volume concentration. Using a modified Viskomat NT, the concentration dependence of CFRC rheology up to 1.5% fibre volume is reported.