- Research Article
- Open Access
Strain and Cracking Surveillance in Engineered Cementitious Composites by Piezoresistive Properties
© Jia Huan Yu and Tsung Chan Hou. 2010
- Received: 1 January 2010
- Accepted: 3 August 2010
- Published: 18 August 2010
Engineered Cementitious Composites (ECCs) are novel cement-based ultraductile materials which is crack resistant and undergoes strain hardening when loaded in tension. In particular, the material is piezoresistive with changes in electrical resistance correlated with mechanical strain. The unique electrical properties of ECC render them a smart material capable of measuring strain and the evolution of structural damage. In this study, the conductivity of the material prior to loading was quantified. The piezoresistive property of ECC structural specimens are exploited to directly measure levels of cracking pattern and tensile strain. Changes in ECC electrical resistance are measured using a four-probe direct-current (DC) resistance test as specimens are monotonically loaded in tension. The change in piezoresistivity correlates the cracking and strain in the ECC matrix and results in a nonlinear change in the material conductivity.
- Acoustic Emission
- Structural Health Monitoring
- Cementitious Composite
- Gage Factor
- Engineer Cementitious Composite
Cracking in cementitious composite can result from a variety of factors including externally applied loads, shrinkage, and poor construction methods. Identification of cracks can be used to evaluate the long-term sustainability of structural elements made of cementitious composite. For example, small cracks affecting only the external aesthetic of the structure should be differentiated from those that reduce its strength, stiffness, and long term durability. Priorities should be given to cracks that are deemed critical to the structure's functionality (e.g. safety, stability).
After suspicious cracks are encountered, nondestructive (e.g., ultrasonic inspection) and partially destructive (e.g., core holes) testing can be performed by trained inspectors to determine crack features (e.g., location and severity) below the structural surface. Perhaps the best approach for automated structural health monitoring of concrete structures entails the adoption of the sensors available in the nondestructive testing (NDT) field. In particular, passive and active stress wave approaches have been proposed for NDT evaluation of concrete structures. Acoustic emission (AE) sensing is foremost amongst the passive stress wave methods. AE employs piezoelectric elements to capture the stress waves generated by cracks ; while AE has played a critical role in the laboratory, its success in the field has been limited to only a handful of applications . In contrast, active stress wave methods have been proven more accurate for crack detection in the field. This approach entails the use of a piezoelectric transducer to introduce a pulsed ultrasonic stress wave into a concrete element and use the same transducer or another to measure the pulse after it has propagated through the element. A direct extension of the active stress wave approach is the electromechanical impedance spectra method. This approach measures the electromechanical impedance spectrum of a piezoelectric transducer to detect cracking in the vicinity of the surface mounted transducer . With digital photography rapidly maturing, many researchers have also adopted the use of charge-coupled device (CCD) cameras to take photographic images of concrete structural elements; subsequent application of digital image processing techniques automates the identification of crack locations and widths .
Compared to other NDT methods, utilization of the electrical properties of cement-based materials for crack detection has gained less attention from the civil engineering community. In fact, the unique electrical properties of cementitious composites render them a smart material capable of measuring strain and the evolution of structural damage . The measurement of electrical properties of cementitious composite is proved capable of detecting serious as well as minor cracks. In particular, ECC is piezoresistive with changes in electrical resistance correlated with mechanical strain. When ECC materials are mechanically strained, they experience multiple saturated cracking and change in their electrical resistance.
In this paper, the piezoresistive property of cementitious materials is proposed as a novel approach for sensing strain and cracking in PVA ECC by utilization of their electrical resistance. The exploration of ECC materials piezoresistivity sets a scientific foundation for the use of the material as a self-sensing material for structural health monitoring in the future.
Material Mixing proportion of PVA-ECC.
Fiber Volume Fraction(%)
High modulus polyvinyl alcohol fiber (12 mm Kuralon-II REC-15 fibers supplied by Kuraray Company) was used as the reinforcing fiber. Ordinary Portland type I cement, Class F normal fly ash and silica sand were used as the major ingredients of the matrix. Silica sand with 110 m average grain size was used as the fine aggregates. Melamine formaldehyde sulfate was applied as superplasticizer (SP) to control the rheological properties of fresh matrix. SP neutralizes different surface charges of cement particles and thus disperses the aggregates formed by electrostatic attraction. However, it has been reported that SP fail to preserve the initial flowability with time due to the high ionic strength in dispersing medium . Appropriate weight and adding sequence of the constituent must be determined because very little difference results in considerable change of the property of acquired PVA ECC mixture. Coarse aggregates are not used as they tend to increase fracture toughness which adversely affects the unique ductile behaviour of the composite. In addition, no coarse aggregates are present thereby rendering the material as electrically homogeneous.
In this section, ECC test specimens roughly cm in size are cast for electrical resistivity measurement of ECC. The measured resistivity of ECC test specimens is investigated using four-point probe methods with direct current (DC). As the name suggests, the four-point probe method employs four independent electrodes along the length of a specimen.
Before the piezoresistivity of ECC can be characterized, the conductivity of the material prior to loading should be quantified. Time dependency is a direct result of the measurement technique and the dielectric properties of the material itself. Under an applied steady (static) electric field, The change in electrical conductivity is often viewed as an intrinsic feature of the material and has been used to understand the materials' chemical, rheological, and mechanical properties.
The higher initial resistivity encountered as the specimens cure can be easily explained. Since more and more ions are trapped by the hardening hydration byproducts, it is harder to mobilize the ions, which is consistent with a higher resistivity. The electric properties of the cementitious material are characterized chiefly by their initial resistivity at early stage.
Gage factors of ECC based on 4-point DC probe measurement.
This study exploits the piezoresistive properties of engineered cementitious composites (ECCs) so that they can be used as their own sensors to quantify the resistivity-strain relationship. ECC plate specimens were monotonically loaded in axial tension to induce strain hardening behavior in the material. As a result of linear changes in electrical resistance due to tension strain, ECC specimens could potentially self-measure their strain in the field. The resistivity of ECC specimens at different times after casting was monitored by 4-point probe resistivity measurement. The initial resistivity changes with hydration degree and increases with DC polarization. An interesting feature of the material lies in the detectable change in resistance-strain sensitivity when strain hardening initiates. The change in piezoresistivity correlates the cracking in the ECC matrix and results in a nonlinear change in the material conductivity. Additional work is underway exploring the theoretical foundation for ECC piezoresistive behavior.
Financial supports from Laboratory of Novel Building Materials Manufacturing and Inspection in Shenyang Jianzhu Universiry are gratefully acknowledged. The authors would like to express their gratitude to Professor V. C. Li and J. P. Lynch, University of Michigan, for their helpful discussion on properties of ECC.
- Yu JH, Li VC: Research on production, performance and fibre dispersion of PVA engineering cementitious composites. Materials Science and Technology 2009, 25(5):651-656. 10.1179/174328408X327731View ArticleGoogle Scholar
- Yu JH, Dai L: Strain rate and interfacial property effects of random fibre cementitious composites. Journal of Strain Analysis for Engineering Design 2009, 44(6):417-425. 10.1243/03093247JSA513MathSciNetView ArticleGoogle Scholar
- Shah SP, Choi S: Nondestructive techniques for studying fracture processes in concrete. International Journal of Fracture 1999, 98(3-4):351-359.View ArticleGoogle Scholar
- Mindess S: Acoustic emission methods. In Handbook on Nondestructive Testing of Concrete. Edited by: Malhotra VM, Carino NJ. CRC Press, Boca Raton, Fla, USA; 2004.Google Scholar
- Park G, Cudney HH, Inman DJ: Impedance-based health monitoring of civil structural components. Journal of Infrastructure Systems 2000, 6(4):153-160. 10.1061/(ASCE)1076-0342(2000)6:4(153)View ArticleGoogle Scholar
- Lecompte D, Vantomme J, Sol H: Crack detection in a concrete beam using two different camera techniques. Structural Health Monitoring 2006, 5(1):59-68. 10.1177/1475921706057982View ArticleGoogle Scholar
- Chung DDL: Damage in cement-based materials, studied by electrical resistance measurement. Materials Science and Engineering R 2003, 42(1):1-40. 10.1016/S0927-796X(03)00037-8View ArticleGoogle Scholar
- Kong HJ, Bike SG, Li VC: Effects of a strong polyelectrolyte on the rheological properties of concentrated cementitious suspensions. Cement and Concrete Research 2006, 36(5):851-857. 10.1016/j.cemconres.2006.02.006View ArticleGoogle Scholar
- Perry CC, Lissner HR: The Strain gage Primer. McGraw-Hill, New York, NY, USA; 1962.Google Scholar
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