This article surveys the research and development of Engineered Cementitious Composites (ECC) over the last decade since its invention in the early 1990's. The importance of micromechanics in the materials design strategy is emphasized. Observations of unique characteristics of ECC based on a broad range of theoretical and experimental research are examined. The advantageous use of ECC in certain categories of structural, and repair and retrofit applications is reviewed. While reflecting on past advances, future challenges for continued development and deployment of ECC are noted. This article is based on a keynote address given at the International Workshop on Ductile Fiber Reinforced Cementitious Composites (DFRCC) - Applications and Evaluations, sponsored by the Japan Concrete Institute, and held in October 2002 at Takayama, Japan.
Textile reinforcement is standard meanwhile since there is large experience with continuous and chopped fibers. However, the prestressing of continuous fibers opens more advantages since the initial strain is anticipated and larger stiffness is obtained. The paper shows that this theoretical prediction has been validated.
Although steel fibers have been used in cement and concrete composites for more than four decades, most of the steel fibers on the market today have been introduced prior to 1980. This is in sharp contract to the continuous progress and development in the cement matrix itself. Following a brief summary of the main properties and limitations of steel fibers used in cement based composites, this paper describes the rationale and technical background behind the development and design of a new generation of steel fibers for use in cement, ceramic and polymeric matrices. These fibers are engineered to achieve optimal properties in terms of shape, size, and mechanical properties, as well as compatibility with a given matrix. They are identified as Torex fibers. Typical tests results are provided and illustrate without any doubt the superior performance (2 to 3 times) of Torex fibers in comparison to other steel fibers on the market. The new fibers will advance the broader use of high performance fiber reinforced cement composites in structural applications such as in blast and seismic resistant structures, as well as in stand-alone applications such as in thin cement sheet products.
Finite element method in conjunction with an appropriate material model may serve as a suitable tool to analyze the structural performance of Engineered Cementitious Composites (ECCs) - short fiber reinforced composites, which exhibit pseudo strain-hardening behavior and multiple cracking under tension. Several such models are reviewed and some new formulations are proposed. The new model represents a composite in multiple cracking state as an equivalent continuum with identical macromechanical properties. The constitutive law of the equivalent continuum is obtained as a relationship between overall stress and strain of a representative volume element (RVE). The RVE is modeled as a solid element intersected by fiber-bridged matrix cracks. In order to relate the relative displacements of crack faces to the bridging tractions, a generalized model of crack bridging is derived. A relationship between stress and crack density is also discussed. The resulting constitutive law is suitable for implementation in FEM, yet maintains a transparent link to a composite microstructure.
The distribution of fibers in Engineered Cementitious Composite (ECC) is one of the most important factors in terms of the mechanical performance of the composite. However, estimation of fiber distribution in ECC has been a difficult problem because of the lower contrast of organic fiber in the cement material. To overcome this problem, we demonstrated a new evaluation method for the distribution of discontinuous Polyvinylalcohol (PVA) fibers in ECC. By using a fluorescence technique on the ECC, we were able to observe PVA fibers as green to yellow dots in the cross section of the composite. After capturing the fluorescence image with a CCD camera through a microscope, the image was divided into small units of the appropriate pixel size. Then, the degree of distribution was calculated with the deviation from the average number of fibers in one unit. By adjusting the preferable unit size, we found a relationship between the degree of distribution and the ultimate tensile strain of the composite.
The use of Engineered Cementitious Composite (ECC), which has metal-like deformation and crack opening restriction ability, as a retrofit material for structures, has been the subject of high expectations. For this application, the direct spray method has been commonly accepted. This study focused on experimentally clarifying the fundamental properties of direct sprayed ECC containing high-performance polyvinyl alcohol fibers. The experiments that were performed demonstrated that direct sprayed ECC was successfully processed and showed pseudo-strain hardening performance comparable to that of traditionally cast ECC reported in the literature. Furthermore, test results simulating concrete cover cracking due to re-bar corrosion demonstrated that direct sprayed ECC has significant potential to prolong service life of R/C members in a heavy chloride environment.
Three-point bending tests and uniaxial tension tests on Hybrid Fiber Reinforced cement-based Composites (HFRCC) were carried out. HFRCC contains both specially-processed steel fiber (steel cord) and synthetic fiber. As the result of the bending tests, it was confirmed that coarse and wide cracks were observed near the notch of the specimens reinforced with only steel cord. On the other hand, HFRCC showed high strength and ductility. Furthermore HFRCC developed in this study showed multiple cracks and pseudostrain hardening under uniaxial tension. Therefore it could be confirmed that HFRCC has a sufficiently high performance to qualify as High Performance Fiber Reinforced Cement-based Composites (HPFRCC) as defined in previous studies.
Development of high-performance construction materials is one of the key issues for the sustainability of structures, and this is one of the reasons for the development of ductile fiber reinforced cementitious composites (FRCC). The purpose of this study is to observe microcracking in high-performance FRCC at the micro and meso levels to clarify the detailed mechanisms causing ductile behavior. In the experiments, a specially designed system was used to observe the surface of specimens under tensile loading by means of an electron microscope. X-ray technique with a contrast medium was also applied to observe internal cracking around a deformed bar. A formation of a number of microcracks on the surface of the FRCC was observed even before loading. These microcracks subsequently grew and/or other cracks occurred to generate multiple cracking as the load increased. The accumulation of multiple cracks produced extended nonlinearity of FRCC. In the vicinity of the deformed bar in the FRCC, multiple cracks were formed from the lug of the bar and the crack width was much thinner than in plain mortar. Thus FRCC surely contributes to dispersal of bond cracks and resistance to the expansion of cracks.
This paper presents an experimental study on the flexural fatigue characteristics of PVA-ECC and PE-ECC. The ECCs showed a unique S-N relationship and exhibited the development of multiple cracks even under fatigue loading. The development of multiple cracks was found to be dependent on fatigue stress levels, and the mechanism is discussed in reference to the static multiple cracking mechanism. The difference between the two ECCs appeared especially in the deformation capacity under fatigue loading. The deformation is shown to be affected by the number of cracks as well as the crack width, where the fracture mechanism of a bridged crack is related to either fiber rupture or fiber pullout.
This paper reports the results of an experimental program on the effectiveness of a Ductile Fiber Reinforced Cementitious Composite (DFRCC) material, which exhibit strain-hardening and multiple-cracking bahavior under flexural loadings, in retarding the corrosion of steel in Reinforced Concrete (RC) beams. Based on the collective findings from theoretically-estimated steel losses, rapid chloride permeability tests, pH value tests, as well as structural tests, it was concluded that Functionally-Graded Concrete (FGC) beams, where a layer of DFRCC material was used around the main longitudinal reinforcement, had a noticeably higher resistance against reinforcement corrosion compared to a conventional RC beam. The better performance of the FGC beams was also evident from the absence of any corrosion-induced cracking and the very low tendency of the concrete cover to delaminate as measured by a concrete-embeddable fiber optic strain sensor.
Structural performance of a cementitious damper made of HPFRCC and steel bar was experimentally observed. These dampers will be applied for reducing seismic damage as well as seismic response of RC structures under performance-based engineering. Since the stiffness of RC structures is higher than that of steel structures, dampers that are stiffer than the conventional ones mainly applied for steel structures are required for drastically reducing the seismic response of RC structures. The advantage of HPFRCC dampers is selective structural performance, strength, stiffness, and ductility, obtained by varying the configuration, bar arrangements and types of materials used. Compressive resistance, which is never a feature of conventional response control dampers, is also a unique advantage of HPFRCC dampers. Thus optimum dampers that meet high performance requirements such as damage reduction for buildings in the event of large-scale earthquakes, can be easily obtained in order to minimize the reconstruction burden. The experimental results indicate that when HPFRCC is used, an extremely large capacity of more than 6N/mm2 in average shear stress and large deflection capacity of more than 10% in deflection angle can be achieved since HPFRCC contribute to the prevention of shear failure and compression failure. HPFRCC can remarkably reduce structural damage by initiating multiple fine cracks. Thus HPFRCC can be used to create small, stiffer dampers capable of high energy absorption due to large capacities in lateral resistance and lateral deflection, as well as high compressive resistance. However, the bending capacity of dampers is drastically increased by increases in axial compression force due to restriction of their elongation. Standard methods of calculating bending and shear capacities of RC columns are inadequate for predicting the capacity and failure modes of these HPFRCC dampers. Therefore, it is recommended that further investigation on evaluation methods for lateral resistance capacity, ductility, fluctuation of axial force, and hysteresis of HPFRCC dampers is essential to develop a performance-based-design method.
This paper describes the results of an experimental investigation of an infill system that utilizes precast ductile fiber reinforced cementitious composite (DFRCC) panels in lieu of traditional materials. The system is intended for use as a retrofit strategy in steel framed hospital structures. Bolted connections are utilized both between individual panels and to the frames, allowing the system to be moved or altered in the event of changes in facility use. Results from laboratory tests of panel connections and panels are presented. The connection tests studied the effect of bolt orientation with respect to applied load, the effect of the DFRCC surface condition within the connection, and time-dependent bolt tension loss. The connection tests demonstrated the viability and the capacity of the proposed connections. DFRCC panels were tested to evaluate the cyclic load-displacement response and energy dissipation in the panels with various panel geometries, panel reinforcements and DFRCC materials. The panel tests also demonstrated the viability of the connection system used. Selected panel test results are presented here, indicating the variation in stiffness and energy dissipation possible with different panel reinforcement.
This paper is a summary report based on the discussions of the JCI-DFRCC committee. The paper attempts to summarize the terminology related to DFRCCs and the structural advantages and application concepts of DFRCCs. This attempt was made for the purpose of further discussion at the JCI International Workshop on Ductile Fiber Reinforced Cementitious Composites -Application and Evaluation- held in 2002 at Takayama, Japan.
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