Benny Suryanto

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.

Bidirectional Multiple Cracking Tests on High-Performance Fiber-Reinforced Cementitious Composite Plates

This article describes how an experimental program was conducted to investigate the effects of simultaneous opening-sliding of multiple cracks on the behavior of high-performance fiber-reinforced cementitious composites (HPFRCCs). For this purpose, 12 HPFRCC plates were tested in bending and under two constitutive principal stress directions. To facilitate reorientation of the stress fields, the plates were precracked and then sawed with certain orientations. Finally, the plates were retested in bending to failure. The results showed that the change in principal stress direction had a substantial effect on macroscopic plate behavior, as marked by reductions in strength and initial stiffness. The effect of stress field reorientation on the cracking pattern was, however, minimal. Regardless of the orientation of the new principal stress direction to that of precracking, a somewhat orthogonal crack pattern always appeared. To characterize the mechanisms involved, the relationship within the constant moment span of each plate is presented and discussed.

A testing methodology for performance-based specification

Abstract 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 period of time. Only then can there be a move from prescriptive durability specifications (minimum grade, maximum water-binder ratio, and minimum cement content) to performance-based methods. The situation is also made more complex by the range of cements now available – BS EN 197 defines a total of 27 products in the family of common cements. Implementation of both design for durability and performance-based standards and specifications are limited by the lack of rapid, simple, science-based test methods for characterizing the transport properties and deterioration resistance of concrete. This paper presents an overview of performance-based specification and developments in the application of electrical property measurements as a candidate testing methodology in evaluating the relative performance of concrete mixes. The technique lends itself to in situ monitoring, thereby allowing measurements to be obtained on the as-placed concrete.

Space-averaged constitutive model for HPFRCCs with multi-directional cracking

This paper focuses on numerical modeling of high-performance fiber-reinforced cementitious composites (HPFRCCs), specifically polyvinyl alcohol engineered cement composites (PVA-ECCs) in the context of a space-averaged, fixed-crack approach. Compression, tension, and shear models are proposed. The compression and tension models include internal unloading and reloading paths. The shear model considers the shear stress transfer contributed by surface friction and fiber bridging in a phenomenological manner. The applicability of the models is verified against recent experiments on precracked PVA-ECC plates subjected to principal stress rotation, demonstrating that the proposed models replicate various responses of the plates. The degradation of initial stiffness and overall strength of plates with precracks at different angles is represented well. Lastly, this paper demonstrates the ability of the models to replicate the average strains spanning bidirectional multiple cracks occurring at the bottom surface of the precracked plates.

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.

Heterogeneous Fiber-Particle Composite Subjected to Principal Stress Rotation

Test results of 20 Engineered Cementitious Composite (ECC) plates under 4-point bending are reported. 13 plates were pre-cracked to allow rotation of the principal stress directions, in order to permit the study of shear and tensile stress transfer at multiple cracks. Coarse aggregates were investigated as a possible means to improve shear-transfer. When subjected to principal stress rotation, ECC exhibited a nearly orthogonal crack pattern, indicating little contribution from the shear transfer mechanism. A reduction in flexural capacity was observed, depending on the relative angles of principal tensile stress applied. When coarse aggregate was added to ECC, significant reductions in flexural capacity and flexural ductility were observed. In a situation when principal stress direction rotated, however, the test results show that coarse aggregate in the amount of 15% of the maximum packing density of the aggregate used was effective to control the orientation of secondary cracks in cracked ECC and to maintain a comparable level of flexural capacity irrespective of the reorientation angle of principal stress field and the angle of pre-existing cracks.

Effects of Shear Transfer on the Directions of Principal Strain Field in Cracked Concrete with Hooked Steel Fibers

This paper investigates the post-cracking behavior of steel fiber reinforced concrete (SFRC) panels from an analysis perspective based on a smeared, fixed crack approach. The analysis results show that the addition of hooked steel fibers improves the average tensile stress of the concrete and, when added beyond 1% by volume, limits the amount of crack-shear slip in the concrete effectively. The analysis reveals that experimentally observations of smaller angles of inclination of concrete principal strain than those of concrete principal stress at intermediate load levels is due to this limited crack slip. Finally, the analysis identifies that hooked steel fibers tends to be less effective than transverse reinforcement in confining shear cracks, thereby resulting in a lower shear transfer capacity.