Engineered Cementitious Composites (ECC)

This chapter provides a broad introduction to Engineered Cementitious Composites (ECC). It describes the historical development of concrete material and the motivation behind the development of ECC. Specifically, the need for further concrete material development for enhancing infrastructure resilience, durability, sustainability, and smartness is discussed. These desirable infrastructure characteristics serve as the backdrop for much of the research and development behind ECC over the last decades.

The chapter offers a brief overview of the unique features of ECC in comparison to normal concrete and other high performance concretes. It emphasizes the distinguishing and valued high tensile ductility of ECC, even though ECC with high compressive strength has also been achieved.

This introduction chapter also describes the concept of Integrated Structures and Materials Design (ISMD) for infrastructure and environmental performance. The need for such integration and its feasibility offered by ECC is reviewed. ISMD serves as a natural framework for scale linkage from nano-scale to infrastructure and environmental scale.

One of the unique features behind the high ductility of Engineered Cementitious Composites (ECC) is a design basis that is distinctly different from that of high strength concrete. For high strength concrete or members of this family of concrete materials, high compressive strength is reached by particle tight packing. The design basis of ECC, however, is based on synergizing the mechanical interactions between fiber, matrix, and interfaces of the composite so that multiple cracking in tension is attained. This design basis is embodied in a body of knowledge known as the micromechanics of ECC.

Micromechanics of ECC serves as a powerful foundation for design of ECC for various performance needs for different target applications. In this sense, micromechanics is an effective tool for efficient design of ECC with optimized mechanical, physical, and functional properties, avoiding costly trial and error approach that seems to pervade the study of fiber reinforced concrete.

This chapter describes the details of micromechanics of ECC, relating properties from the macro- to meso- to microscales. In so doing, the relevant phenomena, material features, and mechanisms at specific length scales are incorporated into the micromechanical model. Most of the parameters in the resulting micromechanical model can be physically measured and therefore support the selection and if needed the tailoring of the ingredients that make up the ECC.

As a physics-based rational model of material behavior, micromechanics often suggests insights into material design that may appear contradictory to conventional wisdoms. These include, for example, the deliberate weakening of fiber/matrix bond and the introduction of artificial flaws into the matrix. Extensive amount of experiments at different length scales have verified the appropriateness of these concepts. Such knowledge is included in this book chapter.

Engineered Cementitious Composites have unique tensile ductility and autogenous crack width control, characteristics attractive to a variety of construction applications. For different construction approaches, such as on-site casting, off-site precasting, shotcreting, or structural member extrusion, the fresh property requirements can be distinctively different. For example, while self-consolidation behavior is desirable for casting, this behavior does not satisfy the requirements for shotcreting. Hence, it is necessary that the fresh properties of ECC be designed to suit the specific application methodology. Control of fresh properties, however, must not interfere with the hardened properties of ECC. In particular, the high tensile ductility must be properly maintained.

One of the essential challenges of processing ECC material is the uniform dispersion of fibers in the matrix. Even at a moderate fiber volume content of two percent, balling and nonuniform dispersion of fibers can result in poor hardened properties and high variability of tensile strain capacity, if the material composition and mixing procedure are not properly designed and controlled.

This chapter describes the accumulated knowledge of fresh property control of ECC that leads to enhanced uniformity of fiber dispersion. As well, techniques for achieving processing requirements for self-consolidating casting, shotcreting, and extrusion are presented. The presented information should be helpful for successful processing of ECC in the laboratory as well as the production of ECC in the field.

The mechanical properties of Engineered Cementitious Composites (ECC) serve multiple purposes. The compressive strength and tensile stress-strain relation are fundamental characteristics of the material. The tensile strain capacity defines the tensile ductility of a given mix composition and processing/curing method. The crack pattern (crack spacing and crack width distribution) is a critically important indicator of durability of the material. Appropriate test methods must be used for proper material characterization. This is particularly important in light of the fact that tensile test is not commonly used for concrete material. The tensile and compressive properties can be used as representative material qualities of ECC.

The flexural properties of ECC serve as a first indicator of structural performance, especially since beams are common structural elements. The simple test setup for flexural test (compared with that of tensile test) also makes it ideal for quality control of ECC when used in large quantities in field applications.

This chapter covers the fundamental mechanical properties of ECC in tension, compression, and flexure. Other properties covered include shear, fatigue, and creep. Together, they provide the needed database for design of structures under complex loading, including high frequency fatigue, high strain rate, and sustained loading.

As research in ECC advances from material development to structural applications, the need for accurate constitutive models that capture ECC’s response under load becomes increasingly apparent. When combined with finite element method, constitutive models of ECC can be utilized to simulate structural response. Such simulations are useful to develop a better understanding of how the unique properties of ECC, such as tensile ductility and crack width control, can be translated into advantageous structural performances. Ultimately, high fidelity numerical simulation of ECC structural behavior can lead to a reduction in the amount of experimentation needed to gain confidence in full-scale structural deployment of ECC. Further, constitutive models can be helpful in the deployment of integrated structural and materials design approach, where targeted structural performance can be downlinked to composite properties and material composition and microstructures. Such scale-linkage provides an efficient basis for ECC material design for optimal structural performance.

While major advances have been made over the last decade on constitutive modeling of ECC, the goals identified above have yet to be realized. However, as this chapter demonstrates, a variety of constitutive models have successfully captured essential experimental trends. Specifically, this chapter presents two classes of constitutive models: phenomenological models and multiscale physics-based models. The phenomenological models account for 1D, 2D, and 3D stress states as well as monotonic, cyclic, and dynamic loading. These models have been verified with experimental data with various levels of successes. The multiscale model links microscale phenomena and material features to mesoscale and macroscale material and structural responses. The advantages of explicitly modeling the opening and sliding of multiple cracks of ECC are demonstrated. The models described lay the ground work for further much needed development in this field.

An original driving force behind the development of Engineered Cementitious Composites (ECC) was the potential enhancement of structural safety given the collapse of some reinforced concrete structures under earthquake loads. Since then, extensive amounts of testing at the structural element level have demonstrated significant improvements in structural resilience characterized by delayed failure, limited degradation in structural function during the load event, and rapid recovery of structural functions postevents. These experiments have been conducted for beams, columns, beam-column connections, frames, and wall systems.

Apart from improving structural resilience, ECC has the potential to enhancing constructability by eliminating steel congestion when a large amount of steel is used to overcome severe member forces. This is accomplished by the intrinsic shear capacity of ECC so that the transverse reinforcing steel often adopted in seismic detailing is rendered unnecessary.

The compatible deformation between ECC and axial steel, even when both are loaded to beyond the elastic stage, allows large energy absorption in R/ECC members. The ability to strain-hardening to several percent tensile strain in ECC assures this compatible deformation and eliminates the commonly observed bond failure and bond splitting in concrete cover. Instead, distributed microcracking represents a commonly observed damage pattern of overloaded structural members.

This chapter describes the unique behavior of R/ECC structural members under fully reversed cyclic load and under impact load. The fundamental mechanisms behind the enhanced structural resilience are emphasized. The knowledge gained should be helpful in further structural designs by optimal utilization of the tensile ductility of ECC.

The durability of Engineered Cementitious Composites (ECC) stems from the intrinsically tight crack width even when strained to hundreds of times the strain capacity of normal concrete. The less than 100 μm crack width is autogenously controlled by the composite and does not depend on steel reinforcement. It is also independent of specimen size and thickness. As a result, transport properties such as permeation and chloride diffusion of ECC can be significantly lower than similarly strained concrete, even when steel reinforcement is used.

One of the major mechanisms of degradation of R/C structures is corrosion of steel reinforcement. The tight crack width and low transport properties of ECC render the corrosion rate of steel in R/ECC extremely low, even when multiple cracking is accounted for.

This chapter presents the knowledge accumulated on the durability of R/ECC structural elements, as well as ECC subjected to a variety of aggressive environments, including freeze-thaw with or without de-icing salt, accelerated weathering exposure, elevated temperature exposure, and high alkalinity exposure. The experimental evidence is overwhelming that R/ECC structural elements possess extreme durability. It points to the feasibility of designing R/ECC structures with significantly enhanced service life.

Concrete, representing about 80% of all engineering materials used, plays an important role in the construction of human habitat. The large material flow of concrete coupled with high energy and carbon intensity of cement, however, has drawn increasing scrutiny due to the threat of climate change on urban communities. As a new construction material, ECC must be developed taking into account environmental sustainability. This means that the embodied energy and carbon in ECC must be minimized with judicious choice of material ingredients, preferably incorporating as much materials from industrial waste streams as possible, but without compromising its performance. A particularly important performance is structural durability as this directly impacts on the resource use and emissions during the use phase of ECC structures.

This chapter introduces a comparative life-cycle analysis framework of ECC structures and its use in studying the sustainability of bridge decks and pavement overlays. The development of green ECCs, with examples in the adoption of alternative binder/filler, sand, and fiber, is surveyed. Alternative ingredients may have lower energy/carbon intensity, source from industrial waste streams, or be renewable. These studies confirm that ECC can be made green and, when combined with its contribution to structural durability through its intrinsically tight crack width, can lead to sustainable infrastructure. The fundamental life-cycle analysis tool and greening methodology provide support for continuing development of significantly sustainable ECC structures.

The development of Engineered Cementitious Composites (ECC) would not be complete without real world applications of the material. Field applications serve many purposes. First, it provides the opportunity to translate basic material properties, high tensile strain ductility of ECC in particular, to enhanced structural performance. Second, field applications provide validation of the usefulness of the unique properties of ECC that cannot be achieved with conventional concrete. Third, field experience of ECC in structures provide a feedback path to further development/refinement of the material. Thus, the applications of ECC bring life to ECC.

In this chapter, applications of ECC in the building, transportation, and water infrastructure domains are described. Emphasis is placed on connecting the infrastructure requirements to specific material characteristics. Thus, in some applications, structural resilience is the performance target. In others, enhanced service life is the most desired application goal. Still others emphasize operational efficiency such as reduction in water leakage. In some applications, improvements in construction efficiency are the driving force behind the adoption of this relatively new material.

The variety of applications can be grouped into repair, retrofit, and new constructions. The application examples also provide illustrations of the different processing methods available to ECC, including self-consolidating casting (on-site casting and off-site precasting), shotcreting (spraying), injection pumping, and hand-troweling. The information contained in this chapter should provide technical and economic insights for further and wider applications of ECC.

To support smart infrastructure development in smart cities, it is natural to expect that modern concrete can do more than just carrying load. While offering enhancements in infrastructure resilience, durability, and sustainability, Engineered Cementitious Composites (ECC) can also offer multifunctionalities. Multifunctional ECCs have the ability to adapt and respond to the changing external environment.

In this chapter, thermal adaptive ECC, self-healing ECC, photo-catalytic ECC, and self-sensing ECC are described. Self-sensing and self-healing ECC can support infrastructure service-life extension with minimal inspection and maintenance. The material detects damage so that repair is applied only when and where it is needed. Even better, self-healing restores its mechanical and transport resistance without any external intervention, thus maintaining durability on a continuous basis. After a major load event, such smart ECC can assist in rapid recovery of infrastructure functions, leading to improved community resilience. Thermal adaptive ECC offers the possibility of reducing building energy use by adapting its thermal capacity in response to external temperature change. Photo-catalytic ECC offers the possibility of maintaining aesthetics and even purifying the surrounding air of the infrastructure in an autogenous fashion. These multifunctions enable operational values of the infrastructure while retaining the basic load carrying functions. They contribute directly to the sustainability of civil infrastructure.

This last chapter of the book offers a glimpse of building smart functions into a two-century-old concrete material. It serves as a preview of future infrastructures of smart cities.