CHAPTER 2

SHORT CARBON FIBER REINFORCED CONCRETE

BACKGROUND

At the time of the 1994 JTEC study*, short carbon fiber reinforced concrete was an area of intense activity in Japan. Although carbon fibers are still used in cement slurry and concrete, this area is no longer being pursued as aggressively as it once was, primarily due to economics and codes that do not take into account the higher levels of performance. Although concrete is good in compression, it lacks toughness, tensile capacity and flexural strength. In fact, Portland Cement Paste does not compare very favorably even to aluminum in terms of standard properties (Table 2.1).

Table 2.1
Comparison of Some Basic Properties

Material

Specific Gravity

Flexural Strength (psi)

Compressive Strength (psi)

Fracture Energy (J/m2)

Portland Cement Paste

1.6 - 2

725 - 14,500

5,000 - 15,000

20

Advanced Polymer Concrete

2.3 - 2.5

14,500 - 21,725

22,000 - 36,000

300 - 1,000

Aluminum

2.7

21,750 - 38,000

42,000

10,000

Although steel reinforcement (rebar) is conventionally used in reinforced concrete to provide tensile reinforcement, there are a number of applications such as curtain walls, fascia panels, paneling for access ducts, barriers, and other cases in which cement mortar by itself could be used if tensile strength, flexural capacity and toughness could be improved. Asbestos fibers traditionally have been used as reinforcement in the chopped fiber form for applications such as thin sheet-like materials or boards (where reinforcing bars cannot be used due to thickness constraints), structural and architectural panels that must withstand high loads and/or deformations, and structural components where the fibers are added to obtain toughness and prevent cracking. The overall use of asbestos prior to the determination of it as a health hazard has been estimated to be as high as 2.5 to 3 million tons. Potential replacements for asbestos have ranged from steel fibers, polypropylene, nylon and polyethylene to glass, carbon and aramid fibers. A potential replacement for asbestos must be able to match most of the attributes that made asbestos a useful additive to cement mortar. These attributes are:

A basic comparison of some of the physical and mechanical properties of candidate fibers is given in Table 2.2.

Table 2.2
A Comparison of Some Short Fibers

Type

Diameter (microns)

Specific Gravity

Tensile Strength (GPa)

Tensile Modulus (Gpa)

Strain to Failure (%)

Acrylic

10 - 18

1.18

1

17 - 20

8 - 11

Asbestos

0.02 - 0.5

2.6 - 3.5

3 - 3.5

160 - 190

2 - 3

Carbon (Mitsui)

16 - 20

1.6

0.8 - 1

75

4

Carbon (PAN)

7 - 10

1.75 - 2

3 - 3.6

230 - 400

0.5 - 1.5

Carbon (Pitch)

18

1.65

0.5 - 1.75

30 - 32

2 - 2.5

AR Glass

2 - 6

2.78

2.5

70

3.6

Polyethylene

 

0.95

0.2

5

8 - 10

Polyvinyl Alcohol

90

1.1 - 1.4

0.1 - 0.15

20 - 25

8 - 11

Fibrillated Polypropylene

20 - 200

0.91

0.4 - 0.7

5 - 10

8

Steel

100 - 600

7.86

2

200

3.5


* Wilkins, Dick J. (ed.), JTEC Panel Report on Advanced Manufacturing Technology for Polymer Composite Structures in Japan, Japanese Technology Evaluation Center, Baltimore, MD, NTIS PB94-161403, April 1994. (http://itri.loyola.edu/polymers/).


Published: October 1998; WTEC Hyper-Librarian