Polymer fibres make flexible concrete to withstand earthquakes

Tom Shelley reports on a novel technology that gives normally brittle materials the ability to absorb significant amounts of deformation



ECC or Engineered Cementitious Composites are mortars reinforced with typically 2 per cent by volume chopped PVA (Poly Vinyl Alcohol) fibres that can endure 3 to 7 per cent tensile strain without breaking or loss of strength.

ECC material is the invention of Professor Victor Li at the University of Michigan, who first published the idea back in 1992, but it is now the subject of a world-wide research effort that includes institutions in Japan, China, Korea, Australia, Denmark, the Czech Republic, Germany, Poland, Switzerland, Italy, Brazil and South Africa. These studies are mainly aimed at improving earthquake resistance in large buildings and civil engineering structures, at enhancing durability in infrastructures, and for effecting repairs. The material is already being used for these purposes in new, large buildings being constructed in Japan.

It is, however, a low cost material with a plethora of other possible engineering applications. During strain hardening, multiple micro-cracks, no more than 60 microns wide, form along the length of tensile specimens. The fibres are covered with a nanometre thick coating that allows them to slip when the material overloads, so that fabrications can be made to bend substantially instead of fracturing. The coatings are crucial, because used on their own, PVA fibres bond excessively well to cement concrete and rupture when cracks develop. The amount of coating material is around 1 per cent of the base fibre material by weight. Stress strain curves of the finished composites show a similar appearance to those normally associated with hard metals rather than ceramics. Typical bend strengths are in the range 10 to 15MPa. Compressive strengths are up to 70MPa.

The base material is the usual mixture of cement, sand and water, but without large particles of aggregate. The mix ingredient types, amounts and particles dimensions are adjusted to help mixture consistency and improve fibre distribution. The ingredients may be mixed in conventional concrete ready-mix trucks.

The material is optimised to attain 500 times the strain capacity of normal types of concrete. While most of the applications are in civil engineering, it can also be extruded into pipes and hollow pillars. Concrete in mechanical engineering is normally used to add weight, such as in flywheels in washing machines, but it is also possible to make fibre reinforced concrete materials lighter in weight. Light weight mixes made by adding glass micro-bubbles with a controlled size distribution show superior mechanical performances over other approaches. For example, it is possible to achieve a tensile strength of 4MPa and a strain capability of more than 4% in a light weight PVA-ECC with 2% fibre at a density of 1450 Kg/m3. The material has a compressive strength of 41MPa. As a demonstration project, the material has been used to make a two person canoe weighing only 140 pounds (64kg).
Quite large concrete ships, but without engines, and many concrete barges were constructed by the United States during World War II. Ten are still afloat and are used as parts of a breakwater on the Powell River in British Columbia, Canada. The oldest known concrete ship was a dingy built by Joseph Louis Lambot in Southern France in 1848. The first ocean-going concrete ship, the 84 foot Namsenfjord, was launched On August 2, 1917, by N.K. Fougner of Norway. Numerous small concrete boats were built in the U.K in the 1910's. One of these, a coaster named the Violette, was built in 1917 may currently be seen at Hoo on the River Medway. It is now on the UK's National Register of Historic Vessels.

Fibre reinforced ceramics have been considered for use as car engines, although reinforced with metal fibres rather than polymer fibres. Fibre reinforced ceramics are routinely used as brake disks in high performance cars.

Engineered Composites
Concrete Ships
Pointers

* By optimising mix ingredient composition, it is possible to make materials that can endure strains of up to 7 per cent in tension.

* Typical bend strengths are up to 14MPa, tensile strengths are up to 4MPa, and compressive strengths up to 70MPa.

Author
Tom Shelley

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