As the above table indicates, carbon fibers are the closest to asbestos in a number of properties. The focus of this section is on Japanese developments in the use of chopped and short carbon fibers in concrete in a form known as carbon fiber cement concrete (CFCC) or carbon fiber reinforced concrete (CFRC).
As developed in Japan, CFCC has little resemblance to conventional concrete. It contains no coarse aggregate and typically contains between 3 to 15 percent by volume chopped and short carbon fiber elements. Three types of carbon fiber are used in CFRC in Japan: pitch-based carbon fiber, polyacrlonitrile-based carbon fiber, and Mitsui Mining form.
The first two materials are well known to the composites industry. The last was developed by the Mitsui Mining Co. as a cheaper material form with affinity for concrete slurry. A major concern in the addition of fibers to concrete is the bonding between the two. The production procedure is shown in Figure 2.1. The resulting fiber has a "fuzzy" form with a strong affinity for concrete. The outcome is due to a combination of factors including the surface fuzziness and surface chemistry obtained by skipping the stabilization stage during pyrolysis. Table 2.3 compares the properties of three commercially available carbon fiber varieties.
In its use in polymer concrete, as with fiber reinforced concrete, the optimum form of the fiber may well be different from that used in aerospace applications. Further, the different requirements for civil engineering applications could result in the viability of lower cost fuzzy forms that could not be used previously in composites. Not all production is used in concrete and often special varieties are produced for use in CFCC for chemical stability, bonding issues and economics.
Based on the specific needs for a commercially viable form usable in concrete, Mitsubishi Kasei introduced the DIALEAD chopped fiber form made of pitch some years ago. Due to improved surface characteristics, it can be mixed in a normal top loading mixer without the need for special additive or a special mixer. The form is a variation on the K223 reinforcement widely used in thermoplastic composites. Table 2.4 provides a comparison of two forms of DIALEAD.
Fig. 2.1. Process used for production of the Mitsui Mining fiber.
0.5 - 0.8
30 - 35
1.5 - 1.8
0.65 - 0.80
35 - 40
Reinforcement in Thermoplastic Composites
3 - 6
Reinforcement in Concrete
10 - 18
Although the performance levels of the fiber used in concrete are lower, they are at levels sufficient to show significant improvement in the performance of concrete. In addition to the direct improvements in performance in tensile and flexural strengths, the use of chopped carbon fibers in concrete results in other generic advantages, especially in building construction, as shown in Table 2.5.
Carbon Fiber Reinforced Concrete
Difficult to transport
Easier to transport with 50% less weight
Quality control problems, corner damage and cracking during handling
Better quality control, tougher and hence easier to handle
Bar arrangement is difficult, formation of ribs is difficult
Casting and molding of complex architectural forms is possible, increased used in decorative outcrops
The following section highlights CFRC application areas drawing on examples of its use in existing structures.