Articles · 2026-09-07 · 3 min read
Sixty percent stronger than specified
Field data on concrete that arrives far above the strength on the drawing, and what the extra cement costs.
The US Federal Highway Administration ran its Mobile Concrete Trailer on 17 concrete paving projects across 13 states. The 28-day strength requirements were being met in 7 days; at 28 days they were exceeded by more than 60%; at 56 to 90 days the concrete was more than 80% over the requirement. The fact sheet names the primary cause: high cement content (FHWA, 2017).
Paving is not a building frame, but the mechanism is the same one the ready-mix industry describes for itself. A November 2024 Concrete International article by NRMCA authors is titled, without hedging, "Excessive Overdesign of Concrete Mixtures for Strength: Causes and Solutions" (Snell, Obla and Carino, 2024).
Two layers of margin
The producer designs the mix above the specified class to pass conformity testing. The designer often specifies a class above what strength demands, because exposure, early striking or habit set it. Each margin is defensible. Stacked, they set the cement content, and cement is where the cost and the carbon of the mix sit.
What the scan looks at
The specified class on the general notes and schedules, against the exposure class and the strength the element needs. A C40/50 column that could be C35/45 is a flag for the engineer of record, not a finding. The cost of the class step depends on the local plant, which is why the site does not put a number on it.