In this episode, we talk to Matthew Karalus, P.E., a field technician from the ECS Group of Companies, about low concrete strength and some of the methods used to test for concrete strength.
Engineering Quotes:
Here Are Some of the Questions We Ask Matthew:
Why is it so important to conduct a concrete strength test?
Are the concrete strength tests conducted while the concrete has been poured, or after it has already cured, or both?
What are some of the methods you use for on-site testing of concrete strength?
Are there any additional curing steps to ensure the same is happening to the poured concrete?
What are some of the causes of low concrete strength results?
When low concrete strength test results are obtained from test cylinders, what are the steps you take?
What is non-destructive testing, and when would you use it?
What are some of the additional testing methods that are less common but are still usable?
Would you like to share some career insights or wisdom?
Here Are Some of the Key Points Discussed About Conducting Concrete Tests:
Some of the concrete strength tests are:
Batch replacement time concrete test: There is a 90-minute window between when the concrete is mixed with water until it needs to be out of the truck. The longer the concrete is turning in the truck, the more it loses its strength.
Concrete temperature test: If concrete gets too hot, it will dry out and not hydrate properly. If the concrete freezes, it can become brittle. Field samples are placed under a concrete blanket by the poured concrete. If the field samples have frozen, then it is highly likely the poured concrete also froze.
The viscosity of the concrete: If you have a plasticizer in the concrete, it raises the viscosity, or slump. You will not need concrete with high viscosity in places where you need the concrete to remain in a specific shape.
Entrained air content: Interior concrete usually has no entrained air content, while most exterior concrete does. Exterior concrete needs entrained air to help with the long-term durability of the concrete by giving it room to flex.
Concrete strength tests are mainly performed before or during the pouring of the concrete. In some cases, the structural engineer might request tests to be done sometime after the concrete has hardened because the slab needs to be lifted from horizontal to vertical for concrete walls.
Casting cylinders is the main way to perform a concrete strength test. There are two ways to cure these cylinders: field-cured cylinders and laboratory-cured cylinders. Field-cured cylinders are used for testing concrete strength in places with low temperatures. Laboratory-cured cylinders are used to find the optimal strength of the concrete after 28 days.
One of the main causes of a low concrete test result is an incorrect concrete/water mix. The structural engineer is then notified about this and it is up to him to decide if he wants to use the concrete or not, as it could fail later.
If concrete strength test results are low, there are a few things that need to be considered:
There are statistical acceptances in place for concrete when the test results are slightly low, which is within 500psi of the required strength.
The structural engineer can also run tests to see if the concrete strength is acceptable for the location.
If the curing cylinders are still not meeting the standards at the 28-day stage, spear cylinders are then left for an additional 28 days to see if the concrete still does not match the requirements. This additional time can show a 20% increase in strength and could then be close enough to the requirements.
The traditional destructive testing method means to take a co...
