Question: I got a request for a 4000 psi mix design, but the specs call for a 0.45 maximum water/cementitious material (w/cm) ratio. I have a mix that averages 4600 psi with 495 pounds of cement and 270 pounds of water, but the ratio is 0.55. Can’t I just reduce water content to 222 pounds to meet the specification?
Answer: I’m amazed how often I’m asked this, and by how many people get it wrong.
Engineers sometimes specify lower or higher water/cementitious material (w/cm) ratios than necessary to achieve strength because they’re trying to influence other characteristics, such as durability or cracking. (For more information, see “Why a Maximum W/CM Ratio Spec?”)
While your solution is mathematically logical, concrete doesn’t care about getting the math right. It cares how its ingredients fit together. Mixing rock, sand, and cement creates voids that must be filled with water. Additional water is necessary to separate the particles or else the concrete will be unworkable.
Believe it or not, the amount of water required to produce a given slump with a given set of materials doesn't change much over the typical range of cement contents used for everyday concrete. If your normal mix requires 270 pounds of water per cubic yard and you batch the mix with 222 pounds of water, the mix won’t be workable and eventually someone will add the missing 48 pounds of water (about 5 gallons) back in. The mix will revert to its original 0.55 ratio, giving an engineer who specified 4000 psi the proper strength but not the other characteristics he wanted.
To properly design the mix, divide required water content by desired w/cm ratio. In your example: 270 pounds of water / 0.45 = 600 pounds of cement. The mix will be around 5800 psi, far greater than specified, but that’s OK. Strength isn’t the only goal.
To reduce the amount of cement, you must first reduce the amount of water. Use an admixture, optimize the combined aggregate grading, add fly ash, or obtain aggregate that has a better particle shape. You can add up to 4% entrained air, but be careful. Entrained air can create new problems, such as greater slump and strength variability or slowed bleeding, that compromise finishability.