The construction industry is becoming aware of the existence of reinforcing bars made from fiber-reinforced plastic. Fiberglass rebar has been on the market for some time, making inroads where steel rebar doesn’t work well. The first common applications have been used in corrosive environments and places where the induced fields resulting from steel reinforced concrete structures exposed to high levels of radio frequency radiations are a problem. There is now a new entry into this field, rebar made from basalt continuous filaments.

Basalt is a common volcanic rock, found throughout the world where volcanoes have erupted and sent lava to the surface. It is actually present everywhere at a depth below the surface—there is a worldwide layer of basalt rock below the sedimentary or metamorphic rock that is exposed on the surface. Where it is present on the surface due to volcanic activity, it is available in large quantities. The shield volcano pictured in image 1 is in north-east New Mexico, and is about 20 miles long and 3,000 feet deep in basalt deposits, laid down from eruptions over millions of years.

This one structure could sustain a vast basalt filament industry for many decades.

Basalt rock is quarried now for many uses, including use as road base. Throughout the areas where basalt is common, it is used instead of limestone as the common base for construction. Image 2 shows a working quarry in the volcano structure.

The basalt usually exists as thick slabs corresponding to the depth of the original lava flows, and fractured vertically through the flow. In some cases the slow cooling causes octagonal structures to develop in the basalt layers.

Before a basalt reinforced rebar can be made, one must first produce basalt continuous filaments. This process begins with crushing the basalt rock as shown above into small pieces, usually in the ½ inch range. This rock is melted in large furnaces, and the melted rock then drawn into thin fibers through special fixtures made from platinum and rhodium. These fixtures are called bushings in the industry. The drawing process is powered by special high-speed winders that can maintain constant fiber speed even as the diameter of the winder and its fiber load increases in diameter as the fiber accumulates. As the fiber is drawn from the bushing it is also stretched massively, reducing in diameter by 90% or more. Also during the 15 foot or so space between the bushing and the winder, the fiber is cooled from a liquid state to a solid glassy state described chemically as an amorphous solid. This cooling is done with mists and finally completed by passage over a brush with liquids on it. This liquid can be water in some cases; other times it is a specialized chemical formula called a sizing which enhances the adhesion of the fibers to various resins.

This process can produce filaments of various diameters with the most common sizes being between 9 and 22 microns. (A human hair is typically 100 microns as a comparison.)

Several things need to be noted here. First is that there are no chemicals or other products added to the basalt rock before it is melted. The natural composition of certain basalts is perfect for making good fibers. As a contrast, fiberglass is made from a mixture of many ingredients, some of which are not environmentally friendly. Basalt continuous filament is a green product. And we can never deplete the supply of basalt rock.

Second, the physical properties of basalt filaments are quite attractive. Compared to e-glass, the most common form of fiberglass, basalt filaments have higher tensile strength and modulus of elasticity, much better temperature tolerance, better resistance to acid and alkali damage, and do not absorb water through the core of the fiber like glass fibers do.

Compared to carbon, basalt fibers offer a much lower cost and a complete absence of conductivity and the inductance of fields when exposed to RF energy.

Third, compared to steel, basalt filaments are much stronger for the same diameter, a fraction of the weight for the same strength, and impervious to acids, alkali, and corrosion.

With this background, here is how basalt filaments are turned into basalt rebar. The basic process is called pultrusion.

It works in the simplest description by pulling filaments from as many spools of basalt roving as is necessary to make the finished product. As an example, to make a 3/8 inch basalt rebar, one would place on a rack called a creel enough spools of roving so that when they are all pulled together into a tight cylinder, the diameter of the cylinder would be 3/8 inch. During the pulling process, the rovings are drawn through a bath of liquid resin and thoroughly wetted with the resin. After wetting, the rovings are drawn through progressively smaller dies, and finally it passes through a heated die that is the final diameter that is desired. The heat in this die sets off the catalyzing process that turns the liquid resin into solid plastic.

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Image 3 is a schematic description of the process. This image shows the creation of a flat plate, but the process is essentially the same for round bars.

A shows the creel of fiber spools being pulled into the resin bath, B. From there the fibers are pulled through the heated die, C. This entire pulling process is powered by pullers or tractors, usually working in tandem so while one is pulling, the other is lining up to take up the pulling process when the first tractor reaches the end of its travel. These tractors are shown at D. Finally, after the product reaches the end of the machine, it is sawn into lengths by the in-line saw, E. The saw moves with the line to ensure uniform cutting.

Early basalt rebars were true cylinders in shape. With experience it became clear that more texture was needed to ensure good mechanical bonding between the rebar and the concrete. The system that is most commonly used is to spirally wrap a band of filament around the rebar while it is still somewhat soft, and deform it with a spiral indentation. Other factories bond a spiral of basalt filament bonded around the cylindrical bar to create mechanical bonding surface. Both systems seem to work well, and the ultimate victor in this competition is still to be determined.

Images 4 and 5 show both types of rebar.

Another important consideration is that basalt rebar can be bent, but has a strong memory like a spring. If you bend a straight rebar, it requires a lot of force, and when you release it, it returns to its original straight form. This allows basalt rebar up to a certain size to be shipped in coils of 100-500 meters. It can then be uncoiled on the job site and used in the straight form. Image 6 shows coils of rebar ready for use on a job site.

After removing the cover, image 7 shows what the coils look like.

There is over 4,000 feet of rebar shown in the stack shown at the left. Yet one man can easily move this stack by hand. These coils weigh less than 40 lbs. each. This much steel rebar would weigh tons, and would require a forklift to move.

Basalt rebar can also be provided in more conventional straight shapes (see image 8), usually bundled for ease of handling.

Using enough heat, basalt rebar can be permanently bent. However, it is probably more practical to use corners and other shapes that are pre-made. Image 9 shows various special shapes created for various construction projects.

The reality is that basalt rebar can be used much like conventional steel rebar. Some techniques need to be changed, but the basic processes are the same. Images 10, 11 and 12 show basalt rebar being used in construction jobs.

Basalt rebar clearly is ready to be used as a substitute for both steel and fiberglass rebar. It is still somewhat more expensive than steel, so it is first being used where steel has disadvantages. It can quickly replace stainless steel and epoxy-coated steel on a cost basis when regulatory hurdles are cleared. As its cost comes down with production volume, it has a chance to replace steel rebar more generally.The fact that it is not corrosive gives it a great advantage over steel. It is clear that steel in a concrete construction is a rust-spalling failure waiting to happen. Eventually moisture will get to steel wherever it is and no matter how well it is protected. It will then rust, swell, and cause the concrete to fail. With basalt rebar this issue is avoided forever.

The lack of spalling leads to one more advantage for basalt rebar. Construction codes call for spacing steel reinforcement at least 3 inches from the surface of the concrete. This delays the time when moisture will penetrate to the steel. This makes the minimum panel thickness for steel-reinforced concrete at least 7 inches. This is not necessary when using basalt rebar. A slab or panel can be made as thick or thin as is needed for structural integrity. If one inch of concrete is sufficient, a panel can be one inch thick with no risk of failure from spalling.

In summation, basalt rebar is now available as a real alternative to other concrete reinforcement systems.

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