Given data:
- Width of the reinforced concrete beam (b) = 300 mm
- Effective depth of the reinforced concrete beam (d) = 500 mm
- Span of the beam (L) = 6 m

Maximum spacing of shear reinforcement:
The maximum spacing of shear reinforcement is determined based on the shear strength requirements of the beam. According to the design codes, the maximum spacing of shear reinforcement should not exceed a certain limit to ensure the beam's structural integrity.

Shear strength of a reinforced concrete beam:
The shear strength of a reinforced concrete beam is provided by the concrete and the stirrups (vertical reinforcement) placed in the shear zone. The design codes specify the minimum amount of shear reinforcement (Asv) required to achieve the desired shear strength.

Formula for maximum spacing of shear reinforcement:
The maximum spacing of shear reinforcement can be calculated using the formula:

Maximum spacing (smax) = (0.75 * d) / (tan(theta) * sqrt(fc) * Asv)

Where:
- d is the effective depth of the beam
- theta is the angle of inclination of the inclined compression strut (typically taken as 45 degrees)
- fc is the compressive strength of concrete
- Asv is the area of shear reinforcement per unit length of the beam

Calculating the maximum spacing of shear reinforcement:
Given that the width of the beam (b) is 300 mm, the effective depth (d) is 500 mm, and the span (L) is 6 m, we can calculate the maximum spacing of shear reinforcement as follows:

- First, we need to calculate the area of shear reinforcement (Asv) required. This value is determined based on the design shear force (V) and the design shear strength of the stirrups (fyv).

- Assuming a design shear force of V = 0.5 * sqrt(fc) * b * d, and a design shear strength of the stirrups (fyv) = 0.4 * sqrt(fc), we can calculate the required area of shear reinforcement:

Asv = V / (fyv * d)

- Substituting the given values, we get:

Asv = (0.5 * sqrt(fc) * b * d) / (0.4 * sqrt(fc) * d)

- Simplifying the expression, we find:

Asv = (0.5 * b) / 0.4

- Now, we can substitute the value of Asv in the formula for maximum spacing:

smax = (0.75 * d) / (tan(theta) * sqrt(fc) * Asv)

- Substituting the given values, we get:

smax = (0.75 * 500) / (tan(45) * sqrt(fc) * ((0.5 * 300) / 0.4))

- Simplifying the expression, we find:

smax = 300 mm

Therefore, the maximum spacing of shear reinforcement along the axis of the reinforced concrete beam is 300 mm.