How much do I need?


Old 12-27-99, 10:07 AM
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Ineed to fill an area that measures 9 feet long x 5 feet wide x 4 inches deep.
How much concrete do i need to mix?,and,what is the formula for estimating such work?
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Old 12-28-99, 01:24 AM
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For concrete and mortar mixes and quantity calculations, the standard units are Cubic Feet and Cubic Yards.

Reviewing basic math:
First, I am not doing this to be a "wise ass". Instead of providing the quantities, possibly you will be able to help yourself, and then someone else later on.

Cubic volume is determined by multiplying unit length, times unit width, times unit depth, with all measurements converted to the same unit of measure.

Decimal feet is a Whole Number plus a Decimal Fractionthat represents Whole Feet plus Whole Inches, and Fractions of an Inch.
Samples: 9.00 feet, 14.2873 feet

The Reciprocal of a whole Number is a Decimal value when multiplied by the Whole Number that produces a result of "1". One use of Reciprocals is substitution of a Whole Number Divisor with a Decimal Multiplier.

Converting a measurement given in Feet, Inches, and Fractions of, to Decimal Feet:
Sample: 13 feet 7 and 5/16 inches.

Start with the Inch Fraction: divide the numerator '5' by the denominator '16'
5 / 16 = 0.3125
Afterward add Whole Inches and divide the result by 12 (12 Inch units per Foot unit).
7.3125 / 12 = 0.609375
Afterward add Whole Feet = 13.609375 Decimal feet.

Inverse transition to Feet, Inches, and Fractions of an Inch:
Starting with 13.609375 Decimal feet, subtract Whole Feet and multiple the result by 12.
.609375 * 12 = 7.3125
Afterward subtract Whole Inches and multiply the result by the desired Fraction Base.
.3125 * 16 = 5
Reassemble the parts: 12' 7 5/16"

Since there are 27 Cubic Feet in One Cubic Yard (3 feet * 3 feet * 3 feet) we can substitute division of Total Cubic Feet by "27" with a multiplier, by dividing 1 by 27 (represented as a Fraction 1/27) and using the result.
Thus 1 / 27 = 0.037037037 (carried to eight places).
And 27 * .037037037 = 1 (an error exists at eight places, but it's close enough).
Multiplying Total Cubic Feet by .037037037 is the same as division by 27 (close enough).

Your problem:
9 feet long x 5 feet wide x 4 inches deep.
Converting and multiplying: 9 * 5 * (4 /12 Inches to Decimal Feet) = 14.33 Cu. Ft.

Reciprocal of 27: 1/27 = 0.03703703 (to eight places).
Converting to Cubic Yards: 0.03703703 * 14.33 = 0.53074063 Cu. Yd.
Using division: 14.33 / 27 = 0.53074074 Cu. Yd.

Quantity = 14.33 Cu. Ft. or .53 Cu. Yds.

Quick Calc @ 4" thickness: Total Sq.Ft. / 80 (81 really) but use 80.

If you want to use Sacked Ready Mix Concrete:

Multiply Total Cu. Ft. by 120, then divide by the Sack Weight (50, 60, 80).
Afterward add a sack or two for waste, shrinkage, unevenness of grading, ect..
OR: multiply Total Cu. Ft. by 123, then divide by the Sack Weight (50, 60, 80).

What do you do with excess sacks? Throw them away, give them to your Inlaws.
Why have waste in the first place? Because!..., the best teacher is undoing ruined work. I promise, you'll get it.

Sacked Ready Mix Water to add.
Gal. Approx. Equ.
80# .75 (3 Quarts)
60# .5635 (2 Quarts)
50# .46875 (6.75 Cups)

To mix your own using 3/4 Mix (AKA: crushed recycled concrete) plus Portland Cement:

Two common volume ratios are 1:5 and 1:4 1/2 (same as 2:9). 1 volume measure of Portland Cement to 5 or 4 1/2 volume measures of 3/4 Mix (fine and course aggregates combine). Using 3/4 mix is relatively linear:
6 sacks Portland + about 1 Cu. Yd. 3/4 mix = 1 Cu. Yd. Concrete.
Don't expect high quality finishes when using 3/4 Mix.

To mix your own from raw ingredients:

Mix Ratio by volume: 6:6:14:19 - 1 1/2

In concrete mix ratios the former means:
1st. '6' a six gallon paste - 6 gallons of water per sack of Portland Cement - Type IA.
The amount of water is adjusted according to the wetness of the aggregates used (mainly sand). Very Wet Sand = 4 1/2 gallons water.
Wet 5, Damp 5 1/2.

2nd. '6' the total number of sacks of Portland Cement per Cubic Yard of concrete.
Use Type IA (general purpose with Air entraining admixture agents added. The "A" suffix).
If mild concentrations of Sulfates are known to exist in the Soil, Water, or Aggregates, use Type IIA.

1st. number '14' the amount of fine aggregate (usually clean well graded sand) in Cubic Feet per Cubic Yard of concrete.

2nd. number '19' the amount of course aggregate (usually clean well graded rock) in Cubic Feet per Cubic Yard.

- 1 1/2 is sometimes used to denote the maximum size of the course aggregate.
In the former 6:6:14:19 1 1/2 max. aggregate size is accepted and understood in many locations. The mix is generally known as "6" "6".

The ratios are not "Written In Stone". Available Ingredients happen to vary in quality from Region to Region. Should a "Local" tell you that the ratio is "Off" for a 6 6 mix in your area, pay attention, because he or she may be right. Trial batching and testing is always a good idea. Fineness and coarseness of the aggregates used (Sand and Rock) and their sharpness play rolls in batch proportioning.

The mix proportions provided are suitable for most jobs the "handyman" encounters.
It is not the only mix ratio, nor is it the most economical mix for any given purpose. Mixing, placing, and curing concrete when Soil, Air, and Mix temperatures are below 50 degrees, requires special handling. When ordering Transit Mix be silent about mix ratios. Tell the operator the job and purpose, allow and trust them to select the proper mix.

Reducing for a trial batch
Water to suit: 1 : 2 1/4 : 3
Water: 1/2 gallon (adjusted for aggregate wetness) for every 8 pounds Portland Cement.

Let that be a lesson: never ask a simple question, because some dummy will answer it, and then tell you more than you want to know.

Old 12-28-99, 08:42 AM
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Issac: You need .55 cubic yards, or 1/2 a yard. Which is about 14 cubic feet. You will waste a little. The formula is length
x width x height all divided by 27 which will give you cubic yards I agree with the ps from dale. Good Luck Jack the Contractor
Old 12-28-99, 10:29 AM
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To Dale ,and jack;

Thank you both for your quick response.
Dale,your response has been an enlightening experience.thanks

Old 12-28-99, 01:47 PM
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Hey, Dale,
I have to say, as a former Math Teacher, that was GREAT! (the only comment I would have is to divide by 27 instead of multiplying by the reciprocal of 27, but you mentioned that possibility as well!)
And can I just tell you that if and when I go back to teaching (instead of working for I will USE this example to show the kids WHY they need to learn Math, even when there are calculators.
So thanks a lot!
PS Have you thought more about our offer?

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