SUPERHYDROPHOBIC CONCRETE

Fiber Reinforced Concrete and
Engineered Cementitious Composites (ECC)

The concept of using fibers to reinforce brittle materials has been used for thousands of years. The Egyptians used horse hairs or straw to reinforce mud bricks, thus reducing the material’s brittleness. These reinforcing fibers are typically 3-dimensionally, randomly oriented and are primarily used as means to carry tensile loads, allowing for greater deformations, and even creating strain hardening behavior in composites. 

The most current application of fibers in concrete has been for architectural applications such as concrete art or curvilinear concrete wall panels. This is possible because the addition of fibers allows the concrete to be cast into almost any shape whereas when steel reinforcement is used, shapes are generally limited. There have been many other applications of fiber reinforcement in structural applications. It has been found that the use of fiber reinforced concrete instead of steel reinforced concrete can reduce labor costs (Li, 2002a). Other fiber applications include non-structural applications to prevent shrinkage and to improve durability (Li et al., 2004). 

The use of PVA fiber reinforced cementitious composites include structural members that are exposed to combined bending, shear and torsion such as concrete utility poles, bridge decks, concrete approach slabs, or members subjected to earthquake loading. For earthquake regions, the use of ECC can be used as a concrete cover to prevent spalling and to provide more confinement to concrete columns. It would also result in high compressive strain capacity to avoid loss of integrity by crushing, low tensile first cracking to initiate damage with the plastic hinge, higher shear and spall resistance to avoid integrity loss by diagonal fractures, and enhanced mechanisms that increase inelastic energy dissipation (Li, 2002b).


Fig 1. Compatibility deformation between steel reinforced concrete (left) and
ECC with steel reinforcement (right) after cyclic loading (li V., 2000)

In conventional design of structures exposed to earthquake loading, a large amount of steel reinforcement used to increase shear capacity is required to allow for plastic hinges to form. This drastically increases labor costs associated with placement of rebar. When ECC is used, it has been found that plastic hinges can still form without the increase in shear reinforcement (Mishra, 1995). Flexural members that incorporate ECC also provide improved ductility and energy dissipation capacity, allowing for less transverse reinforcement to be used in these regions (Li & Fisher, 2002). Moreover, when ECC is used, the crack widths remain small enough to maintain an adequate bond with the steel reinforcement as seen in the adjacent figure. The break in bond between the cementitious material and steel reinforcement represents failure and would have to be repaired, whereas when ECC was used, the structure could still remain functional and even pose a chance to self-repair due to the small crack sizes.

Fig 2. Stress-Deflection Behavior of PVA-ECC, Conventional FRC, and Plain Mortar (Kuraray Co) (left)
and Flexural Behavior of ECC tested at UW-Milwaukee (Sobolev k. et al., 2012) (right)

Engineered cementitious composites (ECC) have been proven to be a much more durable alternative to conventional concrete. Furthermore, ECC materials exhibit very ductile performance under tension similar to steel (Li and Herbert, 2012). The strain capacity of ECC may be increased by a factor of 200 or more when high strength reinforcing fibers are 3-dimensionally dispersed in the mortar.


Fig 3. Strein Hardening behavior of multi-cracking
PVA-ECC. (Li V., 2002)

A variety of fibers, including polymeric, steel and carbon have been examined in ECC. Most recently, research on ECC has been performed with polyvinyl alcohol (PVA) fibers due to their near perfect bond with a cementitious matrix. Unlike conventional concrete, ECC exhibits extremely large strain capacity. The material consists of only fine aggregates, therefore adding to the material’s flexibility since course aggregates typically tend to create a more brittle material due to their interlock. The use of only fine aggregates also allows for multi-cracking and thus strain hardening behavior. The ability to withstand large deformations without creating large cracks or loss in load carrying ability makes ECC a near perfect material to create an ultra-durable concrete. 

The use of polyvinyl alcohol fibers in ECC also drastically improves the durability. Once cementitious materials inevitably crack, the cracks tend to widen under continuous loading or sometimes no loading at all. The only thing stopping these cracks from widening even further is the reinforcement holding them together. In conventional steel reinforced concrete the steel is spaced in such a way that cracks are allowed to grow to such a size where water can easily penetrate. Moreover, the bond between concrete and steel is not perfect; therefore all the tensile stress within the crack region is transferred to the steel resulting in a single crack. The 3-dimensional random spacing of PVA fibers, conversely, enables multi-cracking. Multi-cracking of cementitious composites allows the tensile stresses that would normally be held together by reinforcement over a single crack to be distributed over several cracks. This phenomenon known as strain hardening behavior allows the material to hold more loads after the formation of the initial crack, unlike other materials, which lose their load carrying ability after initial cracking. The cracks that form are also small enough so that minimal amounts of water can penetrate. PVA fibers are capable of doing this due to their near perfect bond with cementitious paste. In some cases this bond may be so strong that an undesired rupture of fibers is seen. To alleviate this and to create a controlled pullout of fibers, the bond must be engineered by use of supplementary cementitious materials (Wang & Li, 2007) and precisely tailored air voids to initiate cracking in the high strength cementitious matrix.

References:

  • V. C. Li, and E.N. Herbert., 2012
    • "Robust Self-Healing Concrete for Sustainable Infrastructure", J. Advanced Concrete Technology, 10, 207-218.

  • Li V. C., Horikoshi T.,Ogawa A., Torigoe S. and Saito T., 2004
    • "Micromechanics-Based Durability Study of Polyvinyl Alchohol-Engineered Cementitious Composite",ACI Materials Journal, vol. 101, no. 3, pp. 242-248.

  • Li V. C., 2002a
    • "Large Volume, High-Performance Applications of Fibers in Civil Engineering",Journal of Applied Polymer Science, vol. 83, pp. 660-686.

  • Li V., 2002b
    • "Advances in ECC Research", ACI Special Publication on Concrete: Material Science to Applications, Vols. SP 206-23, pp. 373-400.

  • Li V. C. and Fisher G., 2002
    • "Reinforced ECC - An Evolution from Materials to Structures", in Procceding of the 1st fib Congress

  • Li V. C., 2000
    • "Reflections on the Research and Development of Engineered Cementitious Composites (ECC)", pp. 1-20.

  • Mishra D., 1995
    • Ph.D. Thesis, Ann Arbor, MI: University of Michigan.

  • Kuraray Co. LTD
    • "PVA-fiber for Ductile Fiber Reinforced Cementitious Composite".

  • Wang, S., & Li, V. C., 2007
    • "Engineered cementitious composites with high-volume fly ash"ACI Materials Journal, 104(3), 233.
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