KURALON™ PVA Fiber: The Fiber Helping Concrete Last Longer

Most people who walk into an office building or a school never think about what the exterior panels are made of. If they are fiber cement, there may be thousands of short synthetic fibers spread through each panel.

Many fiber cement façade panels and roofing sheets are reinforced with polyvinyl alcohol (PVA) fiber. KURALON™, which is Kuraray's trademark for PVA fiber, was Japan's first domestically produced synthetic fiber, industrialized in 1950.

Inside the cement panel, the fiber's role is to maintain high flexural strength, providing long-term reinforcement for decades in one of the most demanding chemical environments found in buildings.

National Chemistry Week runs from October 18 to 24 under the American Chemical Society Society theme “Chemistry Under Construction”, and the theme names concrete first among the materials it covers. Concrete carries enormous compressive loads and relies on reinforcement to resist tension. Steel rebar does much of that work in a building's structure. Fibers work on a smaller scale, on the cracks that start inside the concrete itself.

The chemistry of KURALON™ PVA Fiber

Part of this year's theme is getting people to notice the elements inside everyday building materials, and this bond comes down to a few of them. Every other carbon atom along a PVA chain carries a hydroxyl (OH) group, and those groups attract calcium from the cement as it hardens.

Calcium compounds form on the fiber and give the cement something to grip, so when a crack opens, the fibers stay anchored instead of pulling out. They stretch across the gap, keep carrying the load, and hold the crack faces close together. Highly crystalline chains, packed in orderly rows, give the fiber the strength and stiffness to do that work.

Kuraray has made that polymer since it brought KURALON™ to market in 1950; Kuraray makes both the PVA resin and the fiber spun from it, so it can tune the polymer, the fiber's size and its surface in-house and is intended to keep quality consistent and traceable. That control also makes it easier to tailor the fiber for new uses.

Built to Last

Outdoors, concrete also goes through wet and dry cycles, freeze and thaws, traffic loads, and chlorides. Keeping cracks narrow keeps water and chlorides out, and a fiber that does not rust gives designers more options near coastlines and on salted roads.

The crew on the pour sees the first benefit, since fresh concrete shrinks as water evaporates, and the fiber helps limit the fine cracks that often show up before it hardens. That means a more even finish and less patching afterward.

A typical concrete slab on grade mix needs only 6 to 10 kilograms of KURALON™ RF4000 grade fiber per cubic meter of concrete, well under 1% by volume. Each fiber is about 0.66 millimeters thick, slightly thinner than a credit card. At a higher dose of 2% by volume, railway roadbed concrete reinforced with KURALON™ RF4000 grade fiber withstood more than 100,000 load cycles at 13 times its designated load. Kuraray's technical literature cites a result from a 2011 report by Japan's Railway Technical Research Institute.

Where you'll find it

Fiber cement dates to 1894, and KURALON™ helped carry it into its modern form. When the industry moved away from asbestos, the new fiber had to:

• bond to cement,
• withstand alkali,
• disperse evenly on a sheet-forming line, and
• remain reinforcing for decades.

KURALON™ meets all four requirements and is now used worldwide in fiber cement boards and roofing.

Swisspearl, whose Eternit® fiber cement traces back to that 1894 invention in Austria, has reinforced its panels with KURALON™ since the 1980s. Together, the two companies developed KURALON™ A-8-S grade fiber so white panels keep their bright shade for years on a building. Swisspearl can take panels back after more than 30 years of service and recycle them into cement production, where the fiber dissolves completely. Swisspearl also counts on Kuraray to keep fiber flowing when storms and hail cause sudden spikes in demand.

Holcim reinforces its white Ductal® ultra-high-performance concrete cladding with KURALON™, which lets architects design panels thin enough to reduce the concrete a project uses.

The buildings around the Lusail Plaza Towers in Qatar, then described as the major Ductal® Envelope project to date, required 16,000 tonnes of Ductal®.

Much of the University of Michigan's research on engineered cementitious composites, often called bendable concrete, used KURALON™. These composites can stretch 3% or more under tension while cracks stay around 60 microns wide. Kuraray lists these composites among the uses for KURALON™. A 2005 University of Michigan life cycle study found that bridge link slabs made with them would use:

• 40% less life cycle energy consumption,
• 50% less solid waste generation than conventional steel expansion joints.

The application and use cases of KURALON™ extends beyond construction. It reinforces the rubber in automotive brake hoses and belts and strengthens paper and nonwovens. In each case, it bonds to the surrounding material and carries force, the same work it does in concrete.