Testing Conventional Fresh Concrete: Part 1

Following ASTM standards for testing conventional 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 first part of a two-part series explains the basics of sampling mixed concrete and performing a temperature test, slump test and density test.

Why do we test fresh concrete? One important reason is to gain an awareness of performance indicators while casting a precast concrete product. But we are also looking for trends or variations in test results that may indicate an issue.

Section 1.1.3 of the National Precast Concrete Association Quality Control Manual for Precast Concrete Plants requires that both the plant quality control manager and the assigned backup personnel hold American Concrete Institute Concrete Field Testing Technician – Grade 1 certification. This requires understanding the following four ASTM standards related to concrete sampling and testing. Additional testing standards will be reviewed in the January-February issue of Precast Inc. These standards are critical in ensuring every manufactured precast concrete product is of the highest quality.

Sampling freshly mixed concrete

It is every technician’s duty to ensure all concrete test samples represent the whole mix, not simply the first concrete to come down the chute or out of the bucket. If a technician only collects a sample from one part of a mix, it can easily lead to test results that do not represent the properties of the rest of the mix. Therefore, following ASTM C172, “Standard Practice for Sampling Freshly Mixed Concrete,” is key to obtaining a representative sample of a mix.

There are four ways to obtain a sample of fresh concrete, depending on the type of mixer used. Sampling from stationary mixers – not paving mixers – involves passing a receptacle such as a wheelbarrow completely through the discharge stream of the mixer. Be careful not to use portions from the very beginning or very end of the discharge.

Samples from a revolving drum truck mixer are collected by taking two or more portions at regular intervals from the middle of the concrete discharge. Like with stationary mixers, portions from revolving drum truck mixers should not be taken from the beginning or end of the discharge and should be collected by passing a receptacle through the entire discharge stream. If the rate of discharge needs to be adjusted to more easily collect the sample, adjust the rate of revolution and do not change the size of the gate opening. Samples taken from continuous mixers should be collected after a discharge of at least 5 cubic feet and at least two portions from the discharge need to be collected at regular intervals.

No matter the type of mixer, there are some common steps to follow:

• No more than 15 minutes may pass between taking the first and final portions of a sample.
• Tests for slump, temperature and air content must begin within 5 minutes of obtaining the final portion of the sample.
• Strength test specimens must be molded within 15 minutes of completing the composite sample.

Temperature testing

The reaction between cement and water can generate a large amount of heat. Monitoring this heat is an important way to predict the properties of hardened concrete. Concrete that has a higher initial temperature will likely develop early strengths more quickly, but at the cost of a suboptimal later strength. Alternatively, concrete that has a low starting temperature will likely develop a lower early strength, but will usually end up with a higher ultimate strength. Temperature can affect more than just strength, though. It can change the properties of admixtures in the concrete. Measuring the concrete’s temperature accurately is one of the first things to do to ensure a consistent high-quality product.

When done correctly, temperature testing is one of the easiest tests to perform. The equipment required for performing proper temperature testing listed in ASTM C1064, “Standard Test Method for Temperature of Freshly Mixed Hydraulic-Cement Concrete,” includes:

• A container large enough to provide at least 3 inches of concrete clearance around the measuring device in all directions.
• A temperature-measuring device that is capable of being inserted into the concrete at least 3 inches and able to measure temperature to an accuracy of 1 degree Fahrenheit. Lasers or temperature guns may not be used for ASTM C1064.

Measuring the concrete’s temperature is a vital step in order to ensure a high-quality precast concrete product. NPCA file photo.

Ensuring the accuracy of the temperature-measuring device is important for performing the test correctly. This can be done by suspending the measuring device, along with a reference temperature-measuring device, in oil or bath liquid that is kept at a constant temperature within 0.5 degrees Fahrenheit throughout the verification process. The measuring devices cannot touch the sides or bottom of the fluid’s container and must be left in the fluid for at least 5 minutes. The measuring device should be rejected if there is a difference of more than 1 degree between the two devices.

According to ASTM C1064, it’s important to confirm the accuracy of the temperature-measuring device prior to taking the temperature of the freshly mixed concrete sample by following these steps:

• Begin the temperature testing within 5 minutes of collecting the sample.
• Press the temperature-sensing portion of the measuring device at least 3 inches into the freshly mixed concrete, ensuring 3 inches of cover in all directions.
• Close the void caused by placement by lightly pressing the concrete around the surface of the device. This keeps ambient air from affecting the measurement.
• Leave the device in the freshly mixed concrete for 2-to-5 minutes and record the temperature to the nearest whole degree Fahrenheit. The most common mistake is removing the device to take the temperature. This is a problem because once is it taken out of the concrete, it will no longer be measuring the concrete’s temperature. Keep the probe in the concrete while reading the measurement to guarantee a proper reading.

Slump testing conventional wet-cast concrete

Slump is a measure of concrete’s fluidity and consistency. Even though changes in water content do affect slump, it is vital to understand that slump is not a measure of the water content. Factors like air content, temperature or the presence of admixtures impact slump. ASTM C143, “Standard Test Method for Slump of Hydraulic-Cement Concrete,” is the method specified in NPCA’s QC Manual. With only 2 1/2 minutes from the start of the test to removal of the slump cone, it is important to understand all the concepts and steps beforehand. There is no time to pause and double-check.

An orderly workspace is the first step in completing a successful slump test. The following items are needed:

• A 12-inch-tall device in the shape of the frustum of a cone with a top diameter of 4 inches and a base diameter of 8 inches. The cone must be free of any dents, damage or debris. Cones that can clamp on to a nonabsorbent base plate are commonly used, as long as the clamps do not deform or move the cone and the base contains the slumped concrete.
• A tamping rod with a 5/8-inch diameter and a length of at least 4 inches greater than the depth of the cone, but less than 24 inches. The rod must have a hemispherical tip that is used to tamp the concrete in the cone.
• A rigid or semi-rigid measuring device such as a ruler or metal measuring tape. This instrument must be at least 12 inches and marked in 1/4-inch increments.
• A scoop for filling the cone.

A slump test must be completed from start to finish in 2 1/2 minutes, so the quality control technician must know the concepts and steps beforehand. NPCA file photo.

Once all the instruments have been gathered, it is time to begin the test.

• Dampen the interior face of the cone and place it on a nonabsorbent surface large enough to hold the slumped concrete.
• Hold the cone firmly in place by standing on the foot plates or clamp it to a base plate.
• Fill the cone in three layers, each equal to 1/3 the volume of the cone. A good practice is to mark lines on the cone that correspond to these levels to ensure consistency.
• After placing each layer of concrete in the slump cone, rod the concrete with the tamping rod exactly 25 times. For the second and third layers, the tamping rod should penetrate the layer below by 1 inch.
• Place the final layer of concrete, ensuring it exceeds the top of the cone before rodding. If concrete drops below the top of the cone during tamping, more can be added.
• Strike off excess concrete protruding above the slump cone opening by rolling and screeding the tamping rod once rodding of the final layer is complete.
• Remove any excess concrete from the base of the cone, then raise the slump cone vertically 12 inches in 5 seconds.
• Place the slump cone next to the slumped concrete, then measure the slump of the displaced center of the top surface of the sample relative to the top of the cone. If any shearing or falling away happened during the test, it should be disregarded and restarted with another portion of the sample.

Density and yield testing

Concrete made using normal-weight aggregate commonly has a density of approximately 150 pounds per cubic foot. This can vary depending on the mix design, air content and admixtures. High-density concrete will often exhibit high strength, abrasion resistance and cracking resistance. Conversely, lower-density concrete featuring high air content can exhibit higher resistance to freeze and thaw cycles by allowing water to navigate to the voids during freezing. Whether high, low or normal density, the proper way to test density is one of the key tasks a technician will need to perform. NPCA’s QC Manual states that density tests must be performed at least once per week or for every 150 cubic yards of fresh concrete, whichever occurs first. If lightweight concrete is used, density tests must be performed at least once every month or for every 100 cubic yards of lightweight concrete. Density tests must comply with ASTM C138, “Standard Test Method for Density (Unit Weight), Yield and Air Content (Gravimetric) of Concrete.”

The equipment required to test density is:

• A balance or scale accurate to 0.1 pounds or within .3% of the test load, whichever is greater. The scale should be zeroed before the test.
• A tamping rod with a diameter of 5/8 inch and a length of at least 4 inches greater than the depth of the cone, but less than 24 inches. The rod must have a hemispherical tip that is used to tamp the concrete in the cone.
• A cylindrical container that conforms to ASTM C29 and C231.
• A flat plate that is at least 1/4-inch-thick metal or 1/3-inch-thick glass or acrylic. The plate must be more than 2 inches wider than the inner diameter of the cylindrical container.
• A rubber or rawhide mallet with a 1 1/4-pound head if the cylindrical container’s volume is 1/2 cubic foot or smaller, or a 2 1/4-pound head if the container is larger than 1/2 cubic foot.
•  A scoop for filling the cone.

Unlike the slump test, a density test is not timed. However, the following procedures should still be completed in one act:

• The interior of the cylindrical container should be dampened but clear of any standing water. Record the weight of the empty container.
• Fill the cylinder to 1/3 of its volume before rodding 25 times. Tap the sides of the cylinder 10-to-15 times with the mallet to close any voids. Repeat the process two more times, rodding 1 inch into the previous layer each time.
• There should then be roughly 1/8 inch of excess concrete, which is then struck off in accordance with ASTM C138 using the strike-off plate.
• Weigh the filled container to find the mass of the concrete.
• Calculate the density and yield.

While the steps here provide an overview of each test, it is important to consult the proper ASTM standard for each test. The standards contain more detailed information regarding what tolerances are allowed.

QC technicians are responsible for maintaining the reputation of their company’s products. They not only need to know the proper equipment to use and steps to follow for fresh concrete testing, but also must perform these tests consistently to minimize variation. It is easy to become distracted in a plant environment and lose count of how many times a sample has been rodded, forget to calibrate a scale or let a slump test run too long. Therefore, it’s important to carefully follow each test as it helps maintain consistency and results in high-quality precast products.

Mitch Rector is a technical services engineer with NPCA.