Testing Fresh Concrete: Part 2

Following ASTM standards for testing wet-cast fresh concrete is critical for manufacturing a high-quality precast concrete product.

By Mitch Rector

Editor’s Note: This article is intended to serve as a reference guide for entry-level production workers. This second article of the series explains the basics of slump-flow testing, air content testing and creating concrete cylinder test specimens.

Click here for Testing Conventional Fresh Concrete: Part 1

Testing your concrete is a lot like going to the doctor for a check-up. Being proactive about your concrete’s quality even when there are no detectable issues is important just like an annual physical. But how would you feel if during your check-up the doctor was not paying attention or did not know the proper steps to take? It would probably cause you to feel uneasy and search for a new doctor.

Just like in medicine, knowledge, accuracy and consistency are critical. A quality control technician must diagnose any potential concerns with a concrete mix before they become too big. This is why understanding the proper air content, slump and strength of your concrete is key to delivering a quality precast product.

Slump-flow testing

Admixtures used in self-consolidating concrete allow for highly flowable concrete that can work its way around reinforcement and requires no vibration or consolidation. However, this highly flowable mix cannot be used with ASTM C143, “Standard Test Method for Slump of Hydraulic-Cement Concrete.” In these cases, a slump-flow test in accordance with ASTM C1611, “Standard Test Method for Slump Flow of Self-Consolidating Concrete,” can be used to monitor the consistency of fresh SCC. It is important to know that C1611 is applicable for SCC with up to 1 inch coarse aggregate. C1611 may not be used for mixes with a nominal maximum size greater than an inch. Larger aggregates can inhibit flow and encourage concrete segregation.

NPCA file photo

The equipment needed for performing a slump-flow test includes:

• A mold that conforms to that described in C143.
• A smooth base plate with a minimum diameter of 915 millimeters.
• A strike-off bar that conforms to ASTM C173, “Standard Test Method for Air Content of Freshly Mixed Concrete by the Volumetric Method.”
• A measuring device such as a ruler or rigid measuring tape. This instrument must be able to measure to an accuracy of 1/4 inch.

The slump-flow test should be performed on a level surface. An uneven surface could cause the concrete to flow in one direction instead of evenly. Once all the equipment is gathered, it is time to begin the test.

• Once obtaining a sample of freshly-mixed SCC, you have 5 minutes after obtaining the final portion of the composite sample to start the slump-flow test.
• Dampen the base-plate surface and the interior of the mold.
• Place the mold in the center of the plate. C1611 allows for the mold to be placed either upright or inverted. An inverted mold may be easier to fill, but be sure not to tip it over.
• Fill the mold in one lift with SCC until it’s filled slightly above the rim. Do not rod or tap the mold during the slump-flow test.
• Strike off the surface of the mold using the strike-off bar in a sawing motion and remove the concrete from the base of the mold.
• Raise the mold vertically a distance of 9 inches in 3 seconds. Do not twist or rock the mold as you raise it. The test must take no longer than 2 1/2 minutes from the start of filling the mold to the complete removal of the mold. If the time goes over, repeat the test.
• Once the concrete has stopped flowing, measure and record the largest diameter of the spread to an accuracy of 1/4 inch. Then measure the next diameter at a 90-degree angle. If the two diameters differ by more than 2 inches, then the test is invalid and must be repeated.
• The slump flow is calculated by averaging the two diameters.

Air content testing by pressure method

Air content is an important part of a concrete mix. Being able to determine the entrained and entrapped air in concrete will help predict how it will behave during freeze-thaw cycles. Entrained air will create microscopic voids that water can travel to during freezing, preventing concrete from cracking. Whatever the target air content is for a mix, you must be able to accurately measure it. One of the primary ways of measuring air content is through ASTM C231, “Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method.” This test is applicable for normal and heavyweight concrete with maximum size aggregates no larger than 1 1/2 inches. C231 is not applicable for lightweight concrete or aggregates with high porosity. It is important to know that while C231 features a procedure for Type-A and Type-B air meters, this article covers only the requirements for Type-B meters. C231 can be performed after performing ASTM C138, “Standard Test Method for Density (Unit Weight), Yield, and Air Content (Gravimetric) of Concrete,” and will begin assuming the technician has completed a C138 density and yield test.

Air content amount is typically specified in areas where freeze-thaw damage may occur. NPCA file photo

In addition to the equipment needed for C138, the C231 test requires:

• A Type-B air meter consisting of a 0.20 cubic-foot, cylindrical measuring bowl and a cover assembly with an air chamber, pressure gauge and petcock.
• A funnel with a spout for fitting into the spray tube of the cover assembly.

After placing and consolidating a sample for C138, the steps to perform an air-content test include:

• Clean the flanges or rim of the measuring bowl, cover assembly and assemble the apparatus. Close the main air valve and open both petcocks before using a rubber syringe to inject water through one petcock until it emerges from the opposite petcock.
• Close the air-bleeder valve on the air chamber and pump air into the chamber until the gauge hand reaches the initial pressure line. The gauge hand can be stabilized by tapping the gauge by hand.
• Close both petcocks and open the main air valve.
• Tap the sides of the measuring bowl with the mallet and stabilize the gauge if necessary by tapping with your hand.
• Record the air reading percentage on the pressure gauge before releasing the main air valve.

Air content testing by volumetric method

ASTM C173, “Standard Test Method for Air Content of Freshly Mixed Concrete by Volumetric Method,” is notorious among QC technicians for including many steps and for taking a great deal of time to complete. This makes it important to familiarize yourself with the proper procedure to prevent potential mistakes. From a QC standpoint, the main benefit of testing air content by the volumetric method compared to the pressure method is the volumetric method is valid for all types of concrete. From a personal standpoint, the volumetric method provides a great workout.

The equipment needed for a successful air-content test includes:

• An air meter consisting of a 0.075 cubic-foot measuring bowl, top section, watertight clamp and watertight cap. The top section must feature a scale graduated in increments no greater than 0.5% and measure from 0 at the top to 9% of the volume of the measuring bowl.
• A funnel with a spout long enough to be inserted into the graduated neck of the air meter.
• A tamping rod with a 5/8-inch diameter and a length of at least 4 inches greater than the depth of the measuring bowl, but less than 24 inches. The rod must have a hemispherical tip for tamping the concrete.
• A 1/8-inch-thick steel or 1/4-inch-thick polyethylene bar with a height of 3/4 inch and length of 12 inches.
• A calibrated cup equal to 1% of the volume of the measuring bowl.
• A measuring vessel for isopropyl alcohol with a minimum capacity of 1-pint and 4-ounce gradations.
• A rubber syringe able to hold at least 2 ounces of fluid.
• A 1-quart pouring vessel for water.
• A scoop for filling the bowl.
• 70% by volume Isopropyl Alcohol. It is used to prevent foam from building up in the graduated neck during the test. Rubbing alcohol is commonly used as long as it is 70% by volume isopropyl alcohol.
• A 1.25-pound rubber mallet for tapping the sides of the measuring bowl.

Testing using the volumetric method involves shaking and rolling a sample in order to displace air from the concrete specimen where measured. Because you are adding water and alcohol to the concrete during the test, the sample should not be returned to the rest of the concrete.

• Dampen the interior of the measuring bowl and fill with two equal layers of concrete. Each layer must be rodded 25 times. After rodding a layer, tap the side of the measuring bowl with the mallet 10-to-15 times.
• Strike off the excess concrete with the strike-off bar. Wipe the lip of the bowl clean to ensure the top section can fully connect to it.
• Wet the inside of the top section of the meter and attach the top section to the measuring bowl. Secure the top section with the watertight clamp.
• Add 1 pint of water by inserting the funnel into the top of the assembled air meter. Then add and record the amount of isopropyl alcohol used. After, add water until it appears in the graduated neck and remove the funnel. Use the syringe to add water until the bottom of the meniscus is level with the zero mark on the graduated neck. Next, attach the watertight cap.
• Invert the air meter and shake for 5 seconds before returning the meter to its upright position. The inversion and shaking process must be repeated for at least 45 seconds.

The air meter is then tilted 45 degrees and rolled 1/2 turn several times before turning the meter 1/3 turn and repeating the process for 1 minute. If any liquid is leaking from the meter, the test is invalid and a new test must be started.

• After rolling, set the meter upright and loosen the cap to let the pressure stabilize. The liquid is stabilized when it does not change more than 0.25% in a 2-minute period. If it takes more than 6 minutes to stabilize or there is more foam than 2% on the scale, the test must be discarded and restarted with a larger amount of alcohol.
• Record the liquid level from the bottom of the meniscus to the nearest 0.25%. If the air content is greater than 9% and cannot be read, add enough calibrated cups of water to bring the liquid level back to the graduated neck. Record the number of calibrated cups used.
• Retighten the cap and repeat the rolling process for 1 minute and record the liquid level once it stabilizes.
• If the second reading is within 0.25% of the initial reading, record it as the final meter reading. Otherwise, record the second reading as a new initial reading and repeat the rolling process for a third time. If the reading is within 0.25% of the new initial reading, then record it as the final meter reading.
• If the third reading has changed by more than 0.25%, discard and restart the test using more alcohol.
• Finally, disassemble the air meter and dump out the measuring bowl. The aggregate should fall out and be washed clean of any paste. If there are any portions of undisturbed concrete, the test is invalid.

Making and curing concrete test specimens

Strength is the foundation of most concrete mixes and is why we see buildings constructed 2,000 years ago still standing. ASTM C39, “Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens,” covers the proper testing procedures for compressive strength, but knowing how to prepare an accurate, representative sample is important. ASTM C31, “Standard Test Practice for Making and Curing Concrete Test Specimens in the Field,” provides the guidelines for molding cylinder and beam specimens for use in a compressive strength test. While beam specimens can be created, this article covers the creation of cylinders.

Understanding how to prepare an accurate, representative concrete sample is important for use in a compressive strength test. NPCA file photo

The equipment required to create a cylinder specimen include:

• Cylinder molds that adhere to ASTM C470, “Standard Specification for Molds for Forming Concrete Test Cylinders Vertically.” The cylinders can be 4 by 8 inches or 6 by 12 inches.
• A tamping rod of the appropriate diameter in accordance with C31, depending on cylinder size.
• A 1.25-pound rubber mallet for tapping the sides of the mold.
• A scoop for filling the mold.
• A trowel or other finishing tool.

The steps to create a sample include:

• Write information identifying the sample on the mold. Make sure the information is readable and will not be obscured later. Do not mark on the caps of the mold because caps can be easily removed and misplaced.
• Fill the molds in the appropriate number of layers. A 4-by-8-inch mold must be filled in two layers of equal volume. A 6-by-12-inch mold must be filled in three layers of equal volume. Rod each layer 25 times and tap the outside of the mold 10-to-15 times with the mallet.
• If the concrete falls beneath the mold at any point during consolidating the top layer, stop and fill with the excess concrete.
• Strike off the top surface using the tamping rod, trowel or handheld float.
• Immediately move the specimen to a curing place for storage.

Be vigilant

Letting a mix through that has failed a test can have disastrous results. A QC technician’s goal isn’t to pass every concrete mix, but to be vigilant about mixes that don’t pass. If something is not able to pass a test, it is important to alert your supervisor immediately. After all, wouldn’t you want to know if your doctor found something during a check-up?

Mitch Rector is a technical services engineer with NPCA.