Inside the lab: Where concrete is put to the test
June 7, 2022
This paid piece is sponsored by Knife River Sioux Falls.
Think of a concrete plant like a bakery.
Add flour, eggs, milk or water into a mixture, and when you adjust the amounts or take something away, the result changes once it’s in the oven.
“It’s the same with concrete,” said Sam Slagle, ready-mix quality control supervisor at Knife River Sioux Falls. “You need to make sure the mixture is correct for the project you’re trying to service.”
It’s Slagle and his team’s job to ensure that the material used on Knife River job sites in the Sioux Falls area does just that.
In this case, the “test kitchen” is a quality control laboratory located near the plant in western Sioux Falls.
“There’s a lot of technology and chemistry involved,” Slagle said. “There are many variables involved, which can make problem-solving a challenge sometimes.”
“For a ready-mix company to have a lab like this is very unique, especially the way it’s integrated with our larger operation,” he said. “Most companies don’t have the emphasis put on internal testing like Knife River has put into our lab. We have invested a lot to ensure our products meet the needs of our customers.”
Here’s a closer look at the kinds of tests and services the Knife River team offers out of the Sioux Falls lab.
Compressive strength
Everything from sidewalks and driveways to large commercial buildings, including the footings, walls and raised concrete decks, must withstand compression.
“We test to make sure the material can support whatever pressure is going to be applied to it once it’s in place,” Slagle explained.
Concrete cylinders, which are slightly larger than an aluminum can, are cast and cured. They are placed into a “breaker machine,” which hydraulicly presses on the cylinder until it fails. The pressure at which it breaks is then recorded.
Flexural strength
More specialized projects, such as a bridges and airport runways, require flexural strength testing. It measures the force required to bend a beam and determines the resistance to flexing.
“You want to check the strength of a bridge — flexural strength indicates how much the concrete is going to bend and move when a load is applied to it,” Slagle said.
“So in those cases, we do a flexural test, and some projects combine both a compressive and a flexural test.”
Shrinkage
This specialized test allows the Knife River team to measure how much a specific concrete mix might change in length because of internal chemical reactions or varying water amounts in the mix.
“We’re the only ones we know of locally conducting this test,” Slagle added. “It’s done in a controlled environment, so we know the moisture, humidity and temperature, and we measure how the product is going to expand and contract based on the material we put in the mix.”
Results of all the tests “demonstrate the confidence of a mix or show what areas we can improve upon to make sure we deliver the product customers are looking for,” he said.
In-place temperature and maturity strength
Now here’s a cutting-edge approach to concrete testing: With this unique service, a Bluetooth-enabled sensor is placed into the fresh concrete as it is poured. As it cures, the technician can run a smartphone over the slab, read the temperature from the sensor inside the concrete and calculate the correlated strength.
“Customers are still discovering we offer this and that it can eliminate testing delays and allow them quicker access to a cured slab and, therefore, can accelerate their construction schedules,” Slagle said.
These sensors are a good noninvasive alternative to break tests, which can be time-consuming and limited because you can test only as many cylinders as were made during the initial pour.
“With this precise approach, we’re able to help make decisions on-site related to the concrete strength and provide real-time information,” Slagle said.
Field testing
The Knife River team also performs air tests and slump tests in the field, “which help us understand the resistance to freeze-thaw cycles and the workability of the concrete,” Slagle said. “We also measure the concrete temperature as it is placed because it can have an effect on the set time and final performance of the product.
“We test concrete to avoid errors of the past. We are always looking to improve our product, and with new technologies in testing coming, it’s an exciting time to be in this industry.”
Knife River prides itself on producing and delivering a high-quality, consistent product. The work of Slagle and his team at the lab is another way to ensure the customer’s expectations continue to be met.
