July_1971 — Page 35

ed for a purpose other than ferroce- ment, such as ceiling plaster meshes, chicken wire or woven sieve meshing. This fact gave a relatively narrow range for experimentation, which was mainly confined to Eastern Europe. When meshes specifically designed for ferrocement will be developed, the trend will probably be towards types which raise the steel subdivision within the section; since the greater the subdi- vision, the closer to the homogeneous is the behaviour of the section.

The presence of finely subdivided steel greatly enhances the favourable characteristics of ferrocement. Fig. 1 shows this in a comparative graph of the stress-strain relationship in bending tension for three types of reinforce- ment, used to reinforce a concrete section.

The steel area in the tensile zone is nearly the same in the three cases A, B, and C. Type A reinforcement consist of ordinary bars, type B is made up of small diameter mesh, and type C is similar to B but composed of even finer diameter and closer wire spacing.

Fig. 1 demonstrates three types of similar reinforcing and their strain capabilities. X marks the onset of cracking visible to the naked eye, X2 marks the point when no new cracks appear and when the existing ones can only widen under increasing load.

From this figure it can be seen that the more highly dispersed the rein- forcement the greater is the possible deformation without deleterious ef- fects on the carrying capacity of the section because of the more gradual and less interrupted cooperation be- tween steel and concrete due to the very high and numerous local concrete deformations.

The consequence of this is the small width and greater number of cracks, which on average are 15-times smaller than in comparable reinforced concrete members, a factor of im- portance in corrosion resistance.

The index of steel dispersion is now generally adopted as "K", known as "coefficient of cover"; Kx U

=

Fb

where U is the sum of perimeters of steel in the direction of action of the force per unit length, i.e. this is the area of steel in contact with concrete, Fb is the gross volume of concrete contained in the space where U is being considered. The demarcation between reinforced concrete action and ferrocement has been proposed at Kx = 10 mm-1. Below this value a given element can be considered as acting as ordinary rein- forced concrete.

Fig. 2 shows the cracking widths related to strain values for a typical

600

400

ferrocement element subjected to bending tension. For ordinary R.C. the line of strain vs. crack width would be considerably steeper. The limiting width of crack when corrosion starts to take effect in ferrocement is of the order of 100 microns or 0,1 mm (see Fig. 3).

Fig. 3 shows the decrease of strength in a steel wire of diameters between 0.5 to 1.2 mm when immers- ed in salt water and its relation to crack width. (Period of immersion 28 days artificially accelerated corro-

sion).

Role of fine grained concrete

In ferrocement the concrete aggre- gates are most finely grained. It is not unusual to find the maximum aggre- gate size to be less than 1.2 mm (less than 0.05in). The situation is primarily due to practical considerations, limited thickness and congestion of reinforce- ment. This in turn dictates a minimal

Strain E x 106

Fig. 2. Relation of strain to cracking width.

1:00

10

20

30

440

50

160

70

80

190

Stress

500

AC

A

1000

1500 2000

C

2500 3000

B

Fig. 1.

Strain E x 10-6

Fig. 1. Comparative behaviour of three types of reinforcement

using the same concrete section in bending.

Far East BUILDER, July 1971

Percentage effective strength

100

200

300

400

500

700

800

100

90

80

70+

60+

50

Fig. 3. Relation of strength to cracking width.

Fig. 3.

Cracking width microns

33

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