Cracking comes in many forms. The easiest to fix is plastic shrinkage cracking. That is easily cured by the introduction of any fiber, with the stretchy polypropylene and other inexpensive plastic fibers representing the most common solution.
After the first 12 or so hours, the plastic fibers have done their job. They are largely ineffective against any cracking that occurs after that. Drying shrinkage cracking occurs mostly over the next month, especially if curing is inadequate, or hot and dry conditions prevail. Drying shrinkage cracking requires a stiff, structural fiber to prevent or reduce cracks from opening. PVA fibers or AR glass can be used to reduce drying shrinkage cracking. But this is where PVA shines. Millions of tiny cracks, too small to see with the naked eye, begin to open up as drying and hydration continue over several weeks. PVA fibers have created a hydrophilic bond during this period, so the low-elongation fibers restrain these microscopic cracks from opening any further. They are usually so small that water cannot penetrate to the rebar below, helping to reduce or prevent long-term rusting and spalling.
Japan's national railway system, Japan Railways, has selected PVA fibers for crack and spalling prevention after a long series of side-by-side tests with polypropylene, acrylic and nylon fibers. Their tests showed that PVA fibers are unequalled in crack prevention, particularly drying shrinkage cracking, where most long-term damage begins. The dosage they selected for PVA crack control: 1.6lbs per cubic yard (0.975kb/m3). The fiber they selected: the REC100Lx12mm.
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Download a Powerpoint presentation on tests using PVA-ECC for a road repair product.
ECC_Patch.ppt (22mb)
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Normal DOT concrete patch after 4 months
PVA-ECC patch kept hairline cracks under 0.002" after 4 months
Photos courtesy of Dr. Victor Li. |
