Control of Yield and Unit Weight Impact on Pavement Performance Courtesy of PCA

Courtesy of PCA

Jan Olek, Purdue University ACPA Scholar [email protected] February 13, 2014

PCCP Quality Control Workshop, Session 3 – Yield and unit Weight

Slide 1 of 27

• The unit weight (density) of freshly mixed concrete is the weight of a material that will fill one unit of volume (e.g.1 cu yd or 1 cu ft). • The unit weight is calculated as follows: Weight of Concrete Unit Weight = Volume of Concrete • It is typically expressed in pounds per cubic foot (lb/ft3) or in pounds per cubic yard (lb/yd3) • Unit weight is determined in accordance with ASTM C 138 (AASHTO T121) February 13, 2014

PCCP Quality Control Workshop, Session 3 – Yield and Unit Weight

Slide 2 of 27

• The yield is the actual volume of fresh concrete produced in a batch. It is usually expressed in cubic yards (yd3) • The yield is calculated by dividing the total weight of the materials batched by the unit weight (density) of the freshly mixed concrete Weight of Concrete Volume of Concrete = Unit Weight • Yield is determined in accordance with ASTM C 138 (AASHTO T121) February 13, 2014

PCCP Quality Control Workshop, Session 3 – Yield and Unit Weight

Slide 3 of 27

• Producing concrete with correct yield (and being able to verify that yield) is extremely important to every ready mix producer and to every contractor • As an example, contractor may claim that he has not received all of the concrete that was ordered • In such cases concrete supplier has to be able to confirm that the cubic yard batch weights he is using really do yield 27 cubic feet of concrete.

February 13, 2014

PCCP Quality Control Workshop, Session 3 – Yield and Unit Weight

Slide 4 of 27

• Procedures used for determining unit weight are described in ASTM C 138 • The common method for measuring yield is based on a simple unit weight test, usually using a 1⁄2cubic- foot container. • A sample is collected from each of three different truckloads in accordance with ASTM C 172 and the weight test is run on each sample.

J. G. Rose Yield of Concrete, PUBLICATION #C860313, The Aberdeen Group,, 1986 February 13, 2014

PCCP Quality Control Workshop, Session 3 – Yield and Unit Weight

Slide 5 of 27

• An average unit weight is calculated. • Total weight of the batch is then divided by the average unit weight to determine the volume (or yield) of freshly mixed concrete in the batch. • This procedure is illustrated in example that follows (for this example an adjusted unit weights were batched). J. G. Rose Yield of Concrete, PUBLICATION #C860313, The Aberdeen Group,, 1986 February 13, 2014

PCCP Quality Control Workshop, Session 3 – Yield and Unit Weight

Slide 6 of 27

SSD* Design (lbs/yd3)

Adjustment for Free Moisture in Aggregate

Adjusted** Batch Weights

water

253

-49 (38 +11)

204

cement

564

sand

1250

+38 (0.03 x 1250)

1288

Coarse Aggr.

1877

+11 (0.006 x 1877)

1888

6% air

------

Total

3944

564

3944

J. G. Rose Yield of Concrete, PUBLICATION #C860313, The Aberdeen Group,, 1986 February 13, 2014

PCCP Quality Control Workshop, Session 3 – Yield and Unit Weight

Slide 7 of 27

Total weight of the ½-cubic-foot container filled with concrete (average of three)

92.4 pounds

Empty weight of the container

20.1 pounds

Weight of ½ cubic foot of concrete (92.4 - 20.1)

72.3 pounds

Unit weight of the concrete in pounds per cubic foot (72.3 x 2)

144.6 pounds

The yield is then calculated as follows: weight per cubic yard yield in cubic feet = ——————————weight per cubic foot

3944 yield = ——— = 27.28 cubic feet 144.6 J. G. Rose Yield of Concrete, PUBLICATION #C860313, The Aberdeen Group,, 1986 February 13, 2014

PCCP Quality Control Workshop, Session 3 – Yield and Unit Weight

Slide 8 of 27

• Since the cubic yard yield is greater than 27.0 cubic feet, the overyield is 0.28 cubic foot or about 1 percent. • The unit weight test must be run very carefully because of the effect that discrepancies in the unit weight will have on the result. • That is the reason that an average of at least three tests is needed. J. G. Rose Yield of Concrete, PUBLICATION #C860313, The Aberdeen Group,, 1986 February 13, 2014

PCCP Quality Control Workshop, Session 3 – Yield and Unit Weight

Slide 9 of 27

• In a batch of concrete, the sum of the absolute volumes of cement, aggregate and water plus the volume of air is equal to the volume of concrete produced per batch. • Weights of the various ingredients must be chosen to yield a design volume (usually one cubic yard) of concrete with the desired cement content, water-cement ratio and air content J. G. Rose Yield of Concrete, PUBLICATION #C860313, The Aberdeen Group,, 1986 February 13, 2014

PCCP Quality Control Workshop, Session 3 – Yield and Unit Weight

Slide 10 of 27

• In a batch of concrete, the sum of the absolute volumes of cement, aggregate and water plus the volume of air is equal to the volume of concrete produced per batch. • Weights of the various ingredients must be chosen to yield a design volume (usually one cubic yard) of concrete with the desired cement content, water-cement ratio and air content J. G. Rose Yield of Concrete, PUBLICATION #C860313, The Aberdeen Group,, 1986 February 13, 2014

PCCP Quality Control Workshop, Session 3 – Yield and Unit Weight

Slide 11 of 27

• To calculate the absolute volume of a material for a given weight, the specific gravity of the material must be known. • Specific gravity is the weight of a material divided by the weight of an equal volume of water. • For example, the specific gravity of cement is 3.15. Thus, one cubic foot absolute volume of cement weighs 3.15 times 62.4 (the weight of 1 cubic foot of water) or 196.6 pounds. J. G. Rose Yield of Concrete, PUBLICATION #C860313, The Aberdeen Group,, 1986 February 13, 2014

PCCP Quality Control Workshop, Session 3 – Yield and Unit Weight

Slide 12 of 27

• The volume that any weight of cement will occupy in the concrete can be calculated by dividing the cement weight by 196.6. • A similar calculation is made for the other concrete ingredients as shown in the following example • The weights used in the example are based on SSD condition (specific gravities given are SSD specific gravities from which of the cubic foot of absolute volume is calculated) J. G. Rose Yield of Concrete, PUBLICATION #C860313, The Aberdeen Group,, 1986 February 13, 2014

PCCP Quality Control Workshop, Session 3 – Yield and Unit Weight

Slide 13 of 27

SSD Batch weight

SSD Batch Weight

Specific Pounds per cubic foot Gravity Absolute volume

Water

240

1.00

1.00 X 62.4 = 64.4

Cement

564

3.15

3.15 X 62.4 = 196.6

Sand

1229

2.63

2.63 X 62.4 = 164.1

Coarse aggr.

1850

2.70

2.70 X 62.4 = 168.5

• Add enough air entraining admixture to produce 6% air content. J. G. Rose Yield of Concrete, PUBLICATION #C860313, The Aberdeen Group,, 1986 February 13, 2014

PCCP Quality Control Workshop, Session 3 – Yield and Unit Weight

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•

The absolute volume calculations would then be:

February 13, 2014

PCCP Quality Control Workshop, Session 3 – Yield and Unit Weight

Slide 15 of 27

• The total yield is 26.81 cubic feet and the mix underyields by 0.19 of a cubic foot. • The logical way to make up the additional volume needed would be by adding 0.19 of a cubic foot of sand or coarse aggregate or a combination of the two. • For example, if sand were added, the amount would be 0.19 x 164.1 = 31 pounds. The mix would • then yield exactly 27.00 cubic feet. J. G. Rose Yield of Concrete, PUBLICATION #C860313, The Aberdeen Group,, 1986 February 13, 2014

PCCP Quality Control Workshop, Session 3 – Yield and Unit Weight

Slide 16 of 27

• If it did not appear that the workability of the mix would benefit from more sand, the addition could be made with 0.19 cubic foot of coarse aggregate which would weigh 0.19 x 168.5 or 32 pounds. • NOTE o If either the weight or specific gravity of an ingredient is changed, the yield will be affected. o Also, a 1 percent change in air volume will result in a yield change of 0.27 cubic foot (1 percent of 27 = 0.27).

J. G. Rose Yield of Concrete, PUBLICATION #C860313, The Aberdeen Group,, 1986 February 13, 2014

PCCP Quality Control Workshop, Session 3 – Yield and Unit Weight

Slide 17 of 27

Hypothetical mix for 1 cubic yard of concrete with saturated surface-dry (SSD) batch weights and specific gravities as shown. Air content is 6%.

W. R Malisch and B.A. Suprenant, No Minus Tolerance on Yield, PUBLICATION #J980328, The Aberdeen Group,, 1998 February 13, 2014

PCCP Quality Control Workshop, Session 3 – Yield and Unit Weight

Slide 18 of 27

* Due either to changes in aggregate moisture content or to holding back water at the plant to allow slump adjustments in the field.

This example shows that air content is one of the most critical factors affecting yield. An air content reduction from 7% to 5% reduces yield by more than 1⁄2 cubic foot. Holding back 2 or 3 gallons of water, if it’s not added at the jobsite, can also cause a significant underyield.

As an example, lets assume that specifications require a 4% to 7% air content and a 3- to 5-inch slump. Mix proportions are based on a trial-mix concrete with 6.5% air and a 4¼-inch slump. If one field-produced load hits the lower limits for both air and slump, assuming that reducing water content by 1 gallon reduces slump by 1 inch, the resulting drop in yield is 0.83 cubic foot. That’s enough to reduce the average yield of three loads below 27, even if the other two come in at 27.2 cubic feet.

he unit weight measure is incorrectly struck off with a tamping rod , concrete may mound up (b), making measured unit weight higher an the true value. Carefully striking off the surface with a flat plate (c) aves the measure just level full.

asured unit weight only 1 pound higher than the true unit weight for alweight concrete can result in a nearly 0.20-cubic-foot error in calculated yield.

When a mix underyields, the purchaser is not receiving a sufficient volume of concrete. This results in overpayment to the producer and ncreased construction costs. f the underyield is due to low air content or low cement content the mix quality may be adversely affected The importance of maintaining the required air content, cement content, and water-cement ratio cannot be overemphasized.

When a mix overyields, the purchaser receives a greater volume of concrete than was specified. This represents a loss for the producer. However, if the cement content is not increased accordingly, the cement is used with a larger volume of concrete than intended, decreasing the overall cement content and possibly resulting in decreased strength and quality of the concrete.

If the overyield is due to increased volume of water the concrete quality is reduced. A higher than designed air volume will increase yield, but may reduce strength A consistent yield, as reflected by a consistent unit weight, is one of the best indicators of mix quality. A varying yield is indicative of varying proportions of mix ingredients which will result in varying quality of the finished product

Mixes that show a consistent yield are more likely to meet quality requirements. Yield checks should be routinely conducted as part of the quality control plan.

Unit Weight Monitoring unit weight provides low-cost yet effective means to measure variability. Unit weight will flag excess water or large variations of air in the mixture. Used in combination with other test data, it can be at the heart of a QC plan.