APPROXIMATE RULES OF THUMB
By Ray Zaremba, Manager
For Adjusting Concrete Mix Proportions: There are a number of handy, practical rules which are used in making adjustments in concrete mix proportions. While the rules are useful for making quick changes in concrete mixes, the adjustments should only be used for emergencies and considered a temporary measure until the mix can be exactly redesigned.
Cement content decreased 25 pounds per cubic yard/increase sand
21 pounds (Specific Gravity 2.65)
F.M. of sand increases 0.10/increase sand 25 pounds per cubic yard/decrease coarse aggregate 25 pounds.
^/A Ratio increases 1%/increase water 2.5 pounds and cement 4 pounds.
D.R.W. increased 1 pound/increase coarse aggregate 20 pounds and decrease sand 20 pounds.
Approx. 22 Ibs of sand and 1 gallon of water are of equal volume. If you are consistently using 25 gallons of mix water per yard for example, but after a rain storm you are using only
22 gallons per yard, add 70 pounds sand per yard to return to proper yield. (22 x 3 gallons = 66 pounds plus 5% for water in sand generates 70 pound.)
Water increased 1 gallon/slump increased 1 inch/cement increased 15 pounds.
1% reduction cement content equals 10 psi compressive strength loss (500 Ib P.C. per cubic yard)
1" increase in slump equals 150-200 psi compressive strength loss.
1% increase in sand/aggregate ratio equals 40 psi compressive strength loss.
10°F increase in concrete temperature equals 1% decrease in air content.
10°F increase in concrete temperature equals 150-200 psi compressive strength loss.
10°F increase in concrete temperature equals 7 pounds increase in water content to maintain equal slumps.
1% increase in air content over design equals 3-5% compressive strength loss (medium and rich mixes).
Perhaps You've Forgotten... That if You Add Only One Gallon of Water to a Yard of Properly Designed 3000 psi Concrete Mix:
• you increase the slump about one inch
• you reduce the compressive strength by as much as 200 psi
• you waste the effect of 1/4 bag of cement
• you increase the shrinkage potential about 10%
• you increase the possibility of seepage through the concrete by up to 50%
• you decrease the freeze-thaw resistance by 20%
• you decrease the resistance to attack by de-icing salts
• you lower the quality of the concrete in many other ways.
If more workability is needed, ask the laboratory that designed the mix to adjust it. It may not need more water.
That a Drop of Only 1% In the Entrained Air (say from 5% to 4%) Will Almost Certainly...
...reduce the yield by over 1/4 cubic foot per yard, a loss of one cubic yard in a hundred.
...have the same effect on workability as leaving out about 50 pounds of sand per yard.
...reduce the slump by about ½ inch.
...increase the water demand by up to 4% or about one gallon per yard for the average 3000 psi mix.
...increase the chances for segregation and bleeding.
...decrease durability by about 10%.
...decrease resistance to the action of de-icing salts.
Since many factors such as temperature, mixing time, aggregate size and shape, sand gradation and other things affect the amount of air entrained by a given quantity of air-entraining agent, it pays to check the air content frequently and keep it at the designed level.
Approximate Concrete Strength Relationships:
Compressive strength can be used as an index of flexural strength, once the empirical relationship between them has been established for the materials involved. The flexural strength or modulus of rupture of normal-wight concrete is often given as 8 to 10 times the square root of the compressive strength (PCA). Waddel's Concrete Construction Handbook cites a modulus of rupture range between 12 and 20 percent of the 28 day compressive strength and averages about 15% for concrete with a compressive strength of 3,500 psi.
The origin of some of these "rules of thumb" is unknown and no attempt has been made to identify the originating source. Suffice to say they are merely approximate values which have gained acceptance in the industry and should be continued to be used with discretion.