For newly constructed pavements made with ordinary Portland cement (OPC) concrete, after how many days of curing is the typical working (design) strength considered to be achieved for opening to traffic?

Introduction / Context:
Concrete pavements gain strength over time due to cement hydration. Agencies commonly reference a standard age for compressive strength acceptance and opening-to-traffic decisions.

Given Data / Assumptions:

• OPC concrete with conventional curing practices.
• Working strength refers to the specified characteristic strength for design.

Concept / Approach:
The reference age for OPC strength is 28 days, by which time a large fraction of ultimate strength is realized under standard curing conditions. Early openings may be possible with rapid-hardening cements, but not for ordinary Portland cement unless special provisions exist.

Step-by-Step Solution:

Verification / Alternative check:
Lab tests and maturity methods can estimate in-place strength; typical acceptance is keyed to 28-day cylinder/cube tests.

Why Other Options Are Wrong:
7–21 days are too early for OPC without accelerators; 42 days exceeds the usual reference period.

Common Pitfalls:
Confusing rapid-hardening cement schedules with OPC; inadequate curing causing understrength concrete at 28 days.

Final Answer:
28 days