We live in a society of mass information, where anyone with a computer can publish and share their thoughts with the world. The downside to this rapid finding and sharing of information is that it is easy for misinformation, or flat out wrong information, to be repeated enough to be perceived as fact. From this false sense of knowing, myths are born and I hear a lot of myths in our industry. Most are based on facts, but have either been taken out of context, or have become obsolete due to new technology or research.
In our industry, many of these myths pertain to the use of steel fiber reinforcement in concrete. To help set the record straight, I have developed a Concrete Mythbuster Series which will allow me to “bust open” some of these myths by providing the facts in the context that is relevant and applicable. I hope you’ll find it informative, entertaining and worth sharing.
To start the series, I want to bust open a statement I frequently hear regarding fiber performance.
Myth or Fact: Per ASTM A820 a Type I steel fiber will outperform a Type II steel fiber.
This is clearly a myth. The performance of steel fiber reinforced concrete is based on the performance of the composite material. ASTM A820 does not deal with the performance of the composite material but deals only with the properties of the steel fiber. ASTM A820 states:
“Five general types of steel fibers are identified in this specification based upon the product or process used as a
source of the steel fiber material.”
This specification provides for the measurement of dimensions, tolerances, and required physical properties of the steel only, as well as the testing procedures to establish conformance to these requirements. In other words, it is a specification for the manufacture of steel fibers.
Why does CFS use a Type II material? First of all it must be remembered that ASTM A820 did not exist when steel fibers were being developed in the late 1950’s and 60’s. Material for the production of steel fibers was picked for engineering reasons. The bond between the fibers and the matrix is very important in the performance of the composite material. ACI 360 states:
“Steel fiber bond to the matrix is enhanced by mechanical anchorage, surface area, alloying, surface roughness or a combination of these.”
It has long been understood that a circular fiber is compact in form and leads to the smallest possible ration of lateral surface area to cross section. In fact New Fiber Technology found that by using a rectangular shape the surface area is increased by 28% for more bonding area when compared to a circular drawn fiber with the same area. So if bond is important, you should start with a raw material that gives you the best chance to achieve the best bond possible. Per the statement in ACI 360, surface roughness also enhances the bond. In the manufacture of a cut sheet fiber rectangular fiber, the edges are rough. These serrated edges also help to enhance the bond between the fiber and matrix.
So let’s jump forward to today. ASTM A820 is in effect and serves the industry well but if you want to use the advantages I previously discussed a manufacturer has to use a Type II source material. We at CFS want to produce the best fiber for the concrete industry and the best fibers start with cut sheet material described in ASTM A820 Type II.
Some will say that you can improve the bond by using mechanical anchorage. This is true but isn’t it better to increase the bond first by using the properties of the raw material and then further improving it by incorporating mechanical anchorage in the manufacturing process.
Now, regarding performance. I recently attended a seminar at North Carolina State University and a well known structural engineer said, “Concrete is going to crack. It is our job to control the width of the crack.” This is what steel fibers do best when used properly. An important property of crack control with fibers is fiber count. The higher the fiber counts, the greater the potential for fibers to cross a crack. The more fibers that cross a crack, the lower the stress in each fiber. The lower the stress, the smaller the crack. An ASTM A820 Type II steel fiber will give you the highest fiber count per pound at the lowest price per pound. Add to this the ease of mixing, placing and finishing of a Type II steel fiber and the overall performance of a Type II steel fiber will more than exceed your expectations.