# ASTM C138: Unit Weight, Yield, and Gravimetric Air Content of Concrete

*Melissa has a bachelor's degree in geology, and is an ACI-certified field and lab technician at Froehling and Robertson.*

## Significance and Use of ASTM C138

Knowing the unit weight of a sample of concrete can help you figure out if your lightweight concrete is really lightweight, and is a good way to confirm you’re receiving the right mix of concrete for that particular setting.

Unit weight can be very important in multiple story buildings: if you use full weight concrete on the second floor of a building and lightweight concrete on the first floor, your building is likely to collapse under the weight.

The procedure for unit weight can also help you calculate other important parameters for a batch of concrete, such as yield, gravimetric air content, and theoretical density for the entire batch.

## Equipment Needed for ASTM C138

Scale – must be accurate to 0.1 lbs at any point within its range. The scale must be able to display a reading for the measure even when it is full to the brim with concrete.

Tamping Rod (if slump is greater than 1 inch, you can rod the concrete) – use the same tamping rod you would use for 6x12 cylinders. It needs to be 5/8 ± 1/16 inches in diameter and the length of the rod must be at least 4 inches longer than the depth of your unit weight container. The rod must also have a hemispherical tip.

Internal Vibrator (if slump is less than 3 inches, you can vibrate the concrete) – frequency should be at least 9000 vibrations per minute (150 HZ). The outside diameter of the vibrating part should be between 0.75 and 1.5 inches. The length of the vibrator should be at least 3 inches longer than the depth of the unit weight container.

Unit Weight Container – a cylindrical container that must be made of steel or another metal that does not get easily corroded by cement paste. The container must be watertight and rigid enough to not get dents in it or deform during the tamping process. The height of the container needs to be between 80%-150% of the diameter. The top rim also needs to be smooth and plane within 0.01 in and needs to be parallel to the container’s bottom within 0.5°. The interior wall of the container needs to be a smooth and continuous surface. The container from a pressure meter can be used for this purpose.

Strike-off Plate – can be made of metal or glass. If metal, must be at least ¼ inches thick. If glass or acrylic, must be at least ½ inches thick. Whatever material the plate is made of, it must be at least 2 inches greater lengthwise and widthwise than the diameter of the container, and must have a smooth surface that is plane to 1/16 inches.

Mallet – must have a mass of 1.25 ± 0.5 lbs for containers smaller than 0.5 ft³, or a mass of 2.25 ± 0.5 lbs for containers larger than 0.5 ft³.

Scoop – must be big enough to collect a representative sample from the mixture but small enough so you don’t spill any concrete when you are filling the container.

## ASTM C138 Procedure

1. The method of consolidation that you will use is based on the slump. If the slump is less than 1 inch, you will need to vibrate the concrete. If the slump is between 1 and 3 inches, you can either rod or vibrate the concrete. If the slump is greater than 3 inches, you will need to rod the concrete.

2. To perform this test, you will need to know the volume of your unit weight container. Either bring one in with a known volume (preferred to have been tested in the lab using the water method), or you can calculate the volume of your container using the formula for the volume of a cylinder:

3. Dampen the inside of the container and pour out the excess water.

4. Find the mass of the empty container on the scale and round it to 0.1 lb.

5. Place the container on a flat, level and sturdy surface.

6. a. If rodding: Fill the container with 3 layers of concrete of an equal volume, and rod each layer with 25 blows (if the container is less than 0.5 ft³ in volume), 50 blows (if the container is between 0.5 and 1 ft in volume), or 1 blow for every 3 in² (if the container is larger than 1 ft³). Rod the bottom layer all the way through. When rodding the next two layers, penetrate the layer below them by about 1 inch. After each layer is rodded, tap the sides of the container 10-15 times with the mallet. The final layer must not be overfilled.

6. b. If vibrating: Fill the container with 2 layers of concrete of an equal volume, making sure you have all the concrete in for each. Each layer will be consolidated by inserting the vibrator at 3 different points until the surface of the concrete becomes relatively smooth. For the bottom layer, do not touch the bottom or sides of the container when vibrating. For the top layer, penetrate the underlying layer by one inch.

7. When you are done consolidating the concrete, make sure that the container does not have an excess or deficiency of concrete. An excess of concrete sticking out about 1/8 of an inch above the top of the mold is optimum. If there is a deficiency of concrete in the container, a small amount of concrete may be added. If there is a lot of excess concrete, you can remove it with a scoop or trowel.

8. Strike off the top surface of the plate like in the picture below: First, place your strike-off plate 2/3 of the way over the top of your container. Move it side to side while pulling it backwards off of the container. Then, put the strike-off plate on top of the remaining 1/3 of the top of the container, and move back and forth while pushing it the other way. Lastly, angle the edge of the strike-off plate and make a couple of sweeps forwards and backwards across the top of the container.

9. Clean the excess concrete off of the sides of the container with a rag, sponge, or scrub brush, and then weigh the container full of concrete.

10. Now, you can calculate the unit weight, theoretical density, yield, relative yield, cement content, and gravimetric air content using the information you have on hand from the test you just performed and the information on your batch ticket:

## A Video of the ASTM C138 Procedure

## Quiz

For each question, choose the best answer. The answer key is below.

**What is the unit weight of a sample of concrete if Mf = 35.20 lbs, Me = 7.10 lbs, and V = 0.249 ft3?**- 112.85
- 169.88
- 7.00

**True or false: You can use an air meter bucket for unit weight.**- True
- False

**After consolidation of the final layer, how much concrete above the top of the measure is considered optimum?**- 1/2 inch
- 1/4 inch
- 1/8 inch

**If the mass of materials batched was 27300 lbs, and the unit weight of the concrete is 150.4 lb/ft3, what is the yield?**- 6.62 ft3
- 6.72 ft3
- 6.82 ft3

**What is the minimum thickness of a metal strike-off plate?**- 1/4"
- 1/2"
- 1"

### Answer Key

- 112.85
- True
- 1/8 inch
- 6.72 ft3
- 1/4"

**© 2019 Melissa Clason**