**Permissible Shear Stress in Concrete**

The permissible shear stress in concrete is an important factor in the design of structural elements such as beams, slabs, and columns. It determines the maximum amount of shear force that can be safely resisted by the concrete without causing failure. The permissible shear stress depends on various factors, including the grade of concrete and the percentage of steel reinforcement in compression.

**Grade of Concrete**

The grade of concrete refers to its strength and is determined by the compressive strength of the concrete cube at 28 days. Commonly used grades of concrete include M15, M20, M25, M30, etc. The permissible shear stress in concrete is influenced by its strength because higher-strength concrete can resist higher shear forces.

**Percentage of Steel in Compression**

The presence of steel reinforcement in a concrete member significantly increases its shear strength. Steel reinforcement acts as a tension member and helps to resist shear forces by transferring them to the compression zone of the concrete. The percentage of steel reinforcement in the compression zone affects the permissible shear stress in concrete.

**Interaction of Grade of Concrete and Percentage of Steel in Compression**

The permissible shear stress in concrete is determined by a combination of the grade of concrete and the percentage of steel reinforcement in compression. Higher-grade concrete can resist higher shear forces, while a higher percentage of steel reinforcement in compression increases the shear strength of the concrete member.

When designing reinforced concrete members, such as beams, the permissible shear stress is calculated using the following equation:

τ = 0.4 √f'c + (0.8 ρ fy) / 3

Where:
τ = permissible shear stress in concrete
f'c = compressive strength of concrete
ρ = percentage of steel reinforcement in compression
fy = yield strength of steel reinforcement

This equation considers both the grade of concrete (through f'c) and the percentage of steel reinforcement in compression (through ρ). It shows that the permissible shear stress in concrete is a function of both these factors.

In conclusion, the permissible shear stress in concrete depends on both the grade of concrete and the percentage of steel reinforcement in compression. Higher-grade concrete and a higher percentage of steel reinforcement in compression result in a higher permissible shear stress, allowing the concrete member to resist higher shear forces.