Quality Control is the systematic inspection and testing of work as it gets installed (and after installation) to verify compliance with specifications. QC activities include: superintendent walkthroughs, third-party special inspection (concrete, structural welding, fireproofing, soils), trade-specific tests (hydrostatic on plumbing, megger on electrical, balancing on HVAC), and the formal punch-list process at substantial completion. On commercial work, QC deliverables include daily inspection reports, test results, and a QC tracking log.
A real QC program does not replace QA. The pattern is: train and qualify upstream (QA), then inspect and verify at the work (QC), then formally close out (punch list and final inspection). QC is the safety net that catches what QA missed, but a project that depends entirely on QC to find defects is expensive: rework cost on caught defects exceeds the cost of preventing them in the first place. The contractors with the highest gross margins almost universally invest more in QA than in QC.