Design basis — allowable shear strength of concrete depends primarily on which property? In reinforced concrete design, the allowable (nominal) shear strength assigned to concrete sections is primarily related to the concrete’s:

Introduction / Context:
Codes specify nominal shear strength of concrete members as functions of concrete grade and section parameters. While shear cracking is a tension-driven phenomenon, empirical provisions link shear resistance to compressive strength for simplicity and calibration to test data.

Given Data / Assumptions:

• Reinforced concrete design per common codes.
• Uncracked web in shear with or without minimum shear reinforcement.
• Nominal shear stress limits derived from tests.

Concept / Approach:

Concrete’s tensile capacity is low and variable; codes avoid relying on it directly. Instead, nominal shear strength Vc (or tau_c) is correlated to f_ck (compressive strength) and member parameters (e.g., percentage of tension steel, depth). Hence allowable shear is primarily expressed as a function of compressive strength rather than a separate measured shear or tensile strength alone.

Step-by-Step Solution:

Verification / Alternative check:

Design tables list tau_c values by concrete grade, confirming dependence on compressive strength along with minor adjustments for reinforcement percentage.

Why Other Options Are Wrong:

• Direct shear strength tests are uncommon for design basis.
• Tensile strength influences cracking but is not the direct code parameter.
• “None” contradicts code practice.

Common Pitfalls:

• Assuming shear strength scales linearly with tensile strength; code calibration is empirical.

Final Answer:

Compressive strength (characteristic cube/cylinder strength).