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An Engineer's Dream

An Engineer's Dream

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    Conventional concrete breaks under flexural loading (above), while ECC merely bends under even higher pressure.

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    A mix design for ECC and regular concrete. In ECC, there are no coarse aggregates. Instead, very fine sand, fly ash, and microfibers are included.

Over the last few decades, material scientists have improved concrete mix designs using technology that has increased strength, durability, placing, and improved environmental aspects. Perhaps the brittle nature of concrete is the last technological barrier to attack.

Ever since concrete was made by the Romans 2000 years ago, it has been known for this brittleness. Concrete's brittleness has at times been responsible for catastrophic failures of structures, but more often results in a gradual deterioration that requires repeated and costly repairs. Many attempts have been made to modify concrete so it can take tensile load. Today, the most effective modification has been the introduction of fibers, typically made of steel, glass, or polymer, resulting in fiber-reinforced concrete.

It has been a dream of concrete engineers to produce a concrete that retains the beneficial properties of conventional concrete, such as high compressive strength and non-rusting. Yet at the same time, the final product should possess the tensile ductility of steel so yielding, instead of fracturing, occurs when the concrete is overloaded.

It's true that this design feature can be achieved with fiber reinforcement. However, the past strategy has been to use a lot of fibers (more than 5% in volume), often in aligned or fabric form. While such composites perform well, they are typically too costly to adopt and too difficult to mix and place in the field to become widely used. Very often, these materials require processing with sophisticated equipment available only in a research laboratory.

ECC's development

Recently, a ductile concrete material—engineered cementitious composite (ECC)—has been designed and developed to the point where it is emerging in full-scale applications, including on bridge decks and tall buildings. ECC, developed at the University of Michigan, attains metallic behavior under loading and utilizes only 2% by volume of short fibers. It can be mixed in typical ready-mix equipment and cast as self-consolidating concrete (SCC). The material can also be sprayed as shotcrete.

ECC's tensile ductility is demonstrated by its ability to undergo stretching to about 300 times that of normal concrete before it breaks. It is equally ductile when loaded in shear. In bending, ECC deforms into a curve beam just like a metal plate deforming into the plastic yielding stage. In compression, some versions of ECC reach the same compressive strength as high-strength concrete. However, the material does not explode on failure.

ECC is able to display this unique behavior because of several discoveries: How load can be gradually transferred from the mortar matrix into the reinforcing fibers when the matrix material experiences excessive loading, and how the load can be gradually transferred from the fiber back into the adjacent mortar matrix when the fiber experiences excessive loading.

In this manner, no catastrophic failure occurs either in the matrix or in the bridging fibers. Instead, local bands of material relax and shed load by passing the load to neighboring zones of material. Loads essentially are transferred away from highly loaded regions that undergo a stepping down of elastic stiffness. All of these are carried out automatically in the properly designed composite.