Limit state design — design bond stress for plain bars in tension (concrete grade M30)

Introduction:
Bond stress quantifies the interface action between reinforcing steel and surrounding concrete. In limit state design, a design bond stress is specified for different concrete grades and bar types; this ensures adequate development length and anchorage in tension zones.

Given Data / Assumptions:

• Concrete grade: M30.
• Bars: plain (not deformed) in tension.
• We are concerned with the code-specified design value, not test values.

Concept / Approach:
Design bond stress (tbd) increases with concrete strength and is higher for deformed bars than for plain bars. For plain bars, codes tabulate tbd by grade. Development length Ld is later calculated using Ld = (phi * sigma_s) / (4 * tbd), where phi is bar diameter and sigma_s is design stress.

Step-by-Step Solution:
1) Identify table entry for M30 concrete and plain bars in tension.2) Read off the design bond stress value corresponding to M30.3) The typical tabulated value for M30 (plain bars, tension) is 1.5 N/mm2.4) Therefore, the correct choice is 1.5 N/mm2.

Verification / Alternative check:
Compare adjacent grades: M25 often uses about 1.4 N/mm2; moving to M30 increases to about 1.5 N/mm2, consistent with strength progression.

Why Other Options Are Wrong:
1.0 and 1.2 N/mm2 are typical of lower grades; 1.4 N/mm2 aligns more with M25 plain bars. They understate the bond for M30 and would produce unconservative (excessive) development lengths.

Common Pitfalls:
Confusing plain and deformed bars; mixing characteristic and design bond values; forgetting to adjust for bar type when computing Ld.

Final Answer:
1.5 N/mm2