What is the difference between ready-mix and site-mixed in quality control terms?

• A. There is no difference in quality control.
• B. Ready-mix provides centralized batching with QA/QC; site-mixed is generally less controlled with greater variability.
• C. Site-mixed is more controlled.
• D. Ready-mix has greater variability.

Answer: (B)

The key idea is how batching control affects consistency and compliance with specifications. Ready-mix concrete is batched at a centralized plant with formal QA/QC processes: calibrated scales, documented mix designs, precise dosing of cement, aggregates, water, and admixtures, moisture content adjustments, temperature control, and routine testing (such as slump and air content, plus strength testing on cylinders). This system produces consistent workability, strength, and performance, with batch records that help ensure all deliveries meet the design requirements.

Site-mixed concrete, produced on the job site, usually has less formal QA/QC. It relies more on on-site practices and manual or portable equipment, with greater potential for human error, inconsistent water addition, variable aggregate moisture, differing mixing times, and fewer or less-detailed batch records. These factors lead to greater variability in properties like slump, air content, and ultimately strength and durability.

So the main difference is centralized batching with robust QA/QC in ready-mix, versus on-site mixing with typically less control and higher variability in site-mixed concrete.