What is a Concrete Mix Design?
A concrete mix design is the set of ingredient proportions that a laboratory develops so that the concrete meets a strength and workability target.
What a mix design contains
The mix design states the maximum nominal coarse aggregate size. It also gives the weight and volume of each ingredient for one cubic yard (or cubic metre) of concrete. The ingredients are cement, other cementitious materials, fine and coarse aggregate, water and admixtures. The aggregate weights are for the saturated surface dry condition. FAA Item P-501 tells the laboratory to develop the mix with the procedures in the Portland Cement Association publication “Design and Control of Concrete Mixtures”.
What Item P-501 requires
Item P-501 is the FAA specification for the cement concrete surface course of airfield pavement. It applies to aircraft over 30,000 pounds. In AC 150/5370-10H, it sets these rules for the mix:
- The contractor may not place concrete until the resident project representative (RPR) reviews the submitted mix. The laboratory must hold ASTM C1077 accreditation.
- The mix must reach a 28-day flexural strength that meets the acceptance criteria. The test method is ASTM C78 on beams.
- The water-cementitious ratio must be between 0.38 and 0.45 by weight.
- The slump may not exceed 2 inches for slip-form paving, 3 inches for fixed-form paving or 4 inches for hand placement.
- Fly ash may replace 20 to 30 percent of the cementitious material by weight.
- Slag cement may make up 25 to 55 percent of the cementitious material.
- Air content comes from trial mixes. The specification gives recommended values by exposure level (mild, moderate, severe) and maximum aggregate size.
- A change of aggregate source or admixtures needs a new mix submittal.
Flexural strength is the controlling value because a concrete pavement fails mainly in bending.
How the mix shows on an inspection
A mix design cannot be seen from the air. A material defect can show on the surface. The FAA PAVER manual lists inadequate air entrainment for the climate as a cause of scaling. It generally affects several slabs that received the same batches. Scaling also follows improper curing or finishing with freeze-thaw cycles, and alkali-silica reactivity causes map cracking. A survey records where these distresses are.
Frequently Asked Questions
- Which strength does an airfield concrete mix have to reach?
- A flexural strength, measured on beams at 28 days. FAA Item P-501 requires the mix to reach a 28-day flexural strength that meets the acceptance criteria, tested to ASTM C78. The pavement design itself uses flexural strength, and AC 150/5320-6G recommends a design value between 600 and 750 psi for most airfield applications. Item P-501 lets the engineer accept on compressive strength only for pavements for aircraft of 60,000 pounds or less.
- What water-cementitious ratio does Item P-501 allow?
- The ratio must be between 0.38 and 0.45 by weight. The guidance note tells the engineer to specify a maximum of 0.45 unless severe freeze-thaw, deicer or sulfate exposure needs a lower value. A ratio below 0.38 needs approval from the engineer and the FAA.
- Can a drone inspection check the mix design of a concrete pavement?
- No. A drone survey sees the surface, not the proportions of the concrete. Material defects such as inadequate air entrainment can show as surface distress, for example scaling over several slabs. A drone survey records where the distress is.
References
- faa-ac-150-5370-10h
- AC 150/5370-10H, Standard Specifications for Construction of Airports FAA
- faa-ac-150-5320-6g
- AC 150/5320-6G, Airport Pavement Design and Evaluation FAA
- faa-paver-concrete
- PAVER Distress Identification Manual for Concrete Surfaced Airfields FAA
- faa-ac-150-5380-6c
- AC 150/5380-6C, Guidelines and Procedures for Maintenance of Airport Pavements FAA
This is a reference entry. TarmacView provides drone inspection services and doesn't sell the equipment or materials described in this glossary.
