The amount of cement in a concrete mix is driven by how the mix must perform in service. The design specifies the target compressive strength, the exposure conditions it will face (like freeze–thaw, deicing chemicals, sulfates), durability requirements, and how workable the mix needs to be to place and finish it properly. Cement content is chosen to meet all those performance goals, not just to hit a single ratio. The water–cement ratio is a tool used to achieve the desired strength and durability, but it doesn’t by itself set the minimum cement content. You can adjust water and cement to meet a ratio, but doing so without considering strength, durability, and workability can compromise performance. Other factors like color or texture aren’t determinants of cement content, and ambient humidity affects curing, not the basic amount of cement required to reach design performance. So, the minimum cement content is determined by the design requirements for strength, exposure, durability, and workability, rather than by the water–cement ratio alone.

Ready-mix concrete comes from a plant where materials are batched precisely and mixed under controlled conditions, with QA/QC checks to verify each batch meets the specified performance. This centralized batching and testing ensure consistent strength, workability, and air content from one delivery to the next, and it helps manage temperature and curing considerations so the concrete behaves predictably on site. Site-mixed concrete, by contrast, is mixed on the job site with looser control over proportions, mixing time, and conditions. Small errors in measuring materials, variations in mixing efficiency, and on-site adjustments (like adding extra water) introduce greater variability in the final strength and performance. That’s why the statement highlighting centralized batching with QA/QC for ready-mix, and less control and more variability for site-mixed, is the accurate one. The other choices contradict common practice: QA/QC is essential for ready-mix; site-mixed does not reliably guarantee higher strength; and there is a measurable difference in quality control between the two.

When you design a concrete mix, the cement portion is the most expensive part, while the aggregate (rock) is relatively inexpensive. Increasing the proportion of rock means more of the concrete’s volume is filled with aggregate and less with cement paste. Since you’re replacing costly cement with cheaper rock, the overall cost per cubic yard goes down. This is the main practical advantage of a mix with a higher rock content. Of course, changing the ratio can affect placement, strength, and durability, but the cost reduction is the primary reason this approach is advantageous.

Slab flatness and levelness are quantified using FF and FL ratings, which are measured with a profiling device or a long straightedge as defined by project documents. This method provides objective, repeatable numbers that reflect the actual surface geometry across the slab, and the project specs specify the target FF/FL values and how to measure them. Visual inspection alone can’t quantify how far a surface deviates from perfect flatness or levelness, and a laser level by itself typically doesn’t capture the full, continuous surface profile needed for FF/FL assessment. Counting cracks addresses cracking damage, not the surface geometry being measured.