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

Updated on January 17, 2019
Lissa Clason profile image

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.

To get the unit weight of a sample of concrete, you must get the weight of a full container of concrete, subtract the weight of the empty container, and divide that by the volume of the container.
To get the unit weight of a sample of concrete, you must get the weight of a full container of concrete, subtract the weight of the empty container, and divide that by the volume of the container.

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.

Writing the volume and weight of your empty container on the side can make it quicker and easier to do your calculations.
Writing the volume and weight of your empty container on the side can make it quicker and easier to do your calculations.

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:

How to Find the Volume of Your Container

r=radius in inches, which is half the diameter (measure from the inside edge of the container to the opposite inside edge)

h = height in inches (measure from inside the container)

Volume, V = π*r²*h

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:

How to Calculate Unit Weight

Mf = Mass of full container

Me = Mass of empty container

V = Volume of container

Unit Weight, D = (Mf-Me)/V

How to Calculate Theoretical Density

Mb = total mass of all materials batched

Vb = total volume of all components in the batch

Theoretical density, T = Mb/Vb

How to Calculate Yield

Mb = total mass of all materials batched

D = unit weight of the concrete

Yield, Y = Mb/(D x 27)

How to Calculate Relative Yield

Y = yield (amount of concrete that the batch produced)

Yd = yield of concrete which the batch was designed to produce

Relative Yield, Ry = Y/Yd

How to Calculate Cement Content

Cb = mass of cement in the batch

Y = yield (amount of concrete that the batch produced)

Cement Content, C = Cb/Y

How to Calculate Gravimetric Air Content

T = theoretical density

D = unit weight

Y = yield

Vb = volume of the ingredients of the batch

Air content, A = ((T-D)/T) x 100


A = ((Y-Vb)/Y) x 100

A Video of the ASTM C138 Procedure

© 2019 Melissa Clason


    0 of 8192 characters used
    Post Comment

    No comments yet.


    This website uses cookies

    As a user in the EEA, your approval is needed on a few things. To provide a better website experience, uses cookies (and other similar technologies) and may collect, process, and share personal data. Please choose which areas of our service you consent to our doing so.

    For more information on managing or withdrawing consents and how we handle data, visit our Privacy Policy at:

    Show Details
    HubPages Device IDThis is used to identify particular browsers or devices when the access the service, and is used for security reasons.
    LoginThis is necessary to sign in to the HubPages Service.
    Google RecaptchaThis is used to prevent bots and spam. (Privacy Policy)
    AkismetThis is used to detect comment spam. (Privacy Policy)
    HubPages Google AnalyticsThis is used to provide data on traffic to our website, all personally identifyable data is anonymized. (Privacy Policy)
    HubPages Traffic PixelThis is used to collect data on traffic to articles and other pages on our site. Unless you are signed in to a HubPages account, all personally identifiable information is anonymized.
    Amazon Web ServicesThis is a cloud services platform that we used to host our service. (Privacy Policy)
    CloudflareThis is a cloud CDN service that we use to efficiently deliver files required for our service to operate such as javascript, cascading style sheets, images, and videos. (Privacy Policy)
    Google Hosted LibrariesJavascript software libraries such as jQuery are loaded at endpoints on the or domains, for performance and efficiency reasons. (Privacy Policy)
    Google Custom SearchThis is feature allows you to search the site. (Privacy Policy)
    Google MapsSome articles have Google Maps embedded in them. (Privacy Policy)
    Google ChartsThis is used to display charts and graphs on articles and the author center. (Privacy Policy)
    Google AdSense Host APIThis service allows you to sign up for or associate a Google AdSense account with HubPages, so that you can earn money from ads on your articles. No data is shared unless you engage with this feature. (Privacy Policy)
    Google YouTubeSome articles have YouTube videos embedded in them. (Privacy Policy)
    VimeoSome articles have Vimeo videos embedded in them. (Privacy Policy)
    PaypalThis is used for a registered author who enrolls in the HubPages Earnings program and requests to be paid via PayPal. No data is shared with Paypal unless you engage with this feature. (Privacy Policy)
    Facebook LoginYou can use this to streamline signing up for, or signing in to your Hubpages account. No data is shared with Facebook unless you engage with this feature. (Privacy Policy)
    MavenThis supports the Maven widget and search functionality. (Privacy Policy)
    Google AdSenseThis is an ad network. (Privacy Policy)
    Google DoubleClickGoogle provides ad serving technology and runs an ad network. (Privacy Policy)
    Index ExchangeThis is an ad network. (Privacy Policy)
    SovrnThis is an ad network. (Privacy Policy)
    Facebook AdsThis is an ad network. (Privacy Policy)
    Amazon Unified Ad MarketplaceThis is an ad network. (Privacy Policy)
    AppNexusThis is an ad network. (Privacy Policy)
    OpenxThis is an ad network. (Privacy Policy)
    Rubicon ProjectThis is an ad network. (Privacy Policy)
    TripleLiftThis is an ad network. (Privacy Policy)
    Say MediaWe partner with Say Media to deliver ad campaigns on our sites. (Privacy Policy)
    Remarketing PixelsWe may use remarketing pixels from advertising networks such as Google AdWords, Bing Ads, and Facebook in order to advertise the HubPages Service to people that have visited our sites.
    Conversion Tracking PixelsWe may use conversion tracking pixels from advertising networks such as Google AdWords, Bing Ads, and Facebook in order to identify when an advertisement has successfully resulted in the desired action, such as signing up for the HubPages Service or publishing an article on the HubPages Service.
    Author Google AnalyticsThis is used to provide traffic data and reports to the authors of articles on the HubPages Service. (Privacy Policy)
    ComscoreComScore is a media measurement and analytics company providing marketing data and analytics to enterprises, media and advertising agencies, and publishers. Non-consent will result in ComScore only processing obfuscated personal data. (Privacy Policy)
    Amazon Tracking PixelSome articles display amazon products as part of the Amazon Affiliate program, this pixel provides traffic statistics for those products (Privacy Policy)
    ClickscoThis is a data management platform studying reader behavior (Privacy Policy)