Despite our need for durability and high performance, the most common design criterion for concrete is compressive strength. Usually we specify the 28-day strength, that is, the compressive strength after 28 days’ moist cure at 73 ˚F. But unless the job site is in San Diego, the concrete will cure at temperatures well above or below that. Even moderate variations in temperature affect how quickly the concrete gains strength. That matters when we want to remove the formwork or posttension the concrete. Doing either one too soon could put people in danger. But if we wait too long, the project suffers unnecessary delays. ASTM C1074 provides guidelines for predicting early-age strengths with reasonable accuracy.
ASTM C1074, “Standard Practice for Estimating Concrete Strength by Maturity Method,” uses maturity calculations to predict strength gain. Like most chemical reactions, cement hydration is faster at higher temperatures. To an extent, you can trade off time and temperature—longer times at lower temperatures or shorter times at higher temperatures. However, high curing temperatures can adversely affect the strength at later ages.
In its simplest form, maturity is just the product of time and temperature above the datum. More sophisticated maturity models use the Arrhenius equation. For those you need to determine (or assume) the value of the activation energy. Either way, maturity models predict early-age strengths much better than later-age strengths.
That’s because the rate-controlling mechanism is not the same at later ages. At first, the cement is in direct contact with the water, and hydration begins. But as hydration products form around the cement grains, the water has to diffuse through them to get to the cement. That delays further hydration, and the rate of strength gain slows. The good news is that in practice we care most about early-age strengths.
Calibrating the maturity curve using ASTM C1074
To predict strength gain, you need to calibrate your concrete mixture. The best time to do this is while you’re qualifying the mixture to make sure it meets the specification. You can just add the maturity calibration to the other tests you’re doing. You need at least 15 cylinders for the maturity calibration. Embed thermocouples in two of them and record the time-temperature history. Use two cylinders to obtain the compressive strength at each of the test ages—usually 1, 3, 7, 14, and 28 days. If any cylinders deviate from their mean strength by more than 10%, test an extra cylinder.
We care about early-age strengths for several reasons. In prestressed concrete, the production facility is on a 24-hour cycle. In that case the concrete needs to be strong enough at 16 to 18 hours to remove the forms and let down on the prestressing. For cast-in-place concrete, you might want to remove the forms at 3 or 4 days. For slip forming, you might want to jump the forms every 2 days. Posttensioning or opening to traffic might also be the basis for early-age strength criteria. Based on the project requirements, you could modify the test ages to better define your maturity curve at the relevant time(s). Or you could just test additional cylinders at those ages.
Sometimes pavements use flexural strengths rather than compressive strengths as the acceptance criteria. In that case, you can calibrate the maturity curve using the flexural strength test (ASTM C192).
Using ASTM C1074 in the field
Once you’ve calibrated the maturity curve, you can use it to decide when the concrete is strong enough for whatever you want to do. That could be anything from removing the forms to opening to traffic.
You can also use it to predict “what ifs”. For example, let’s say the weather is about to turn cold. How will that affect your schedule? How much would it help to insulate the forms before the cold front comes in?
Because the field data you’re collecting are much the same as for thermal control—a time-temperature history—the addition of maturity testing may not entail much extra work or instrumentation. But the additional information can be extremely helpful.
On one project, the contractor made concrete columns with a low portland-cement content in January in Minnesota. I’d advised him to do maturity testing so he’d know when to remove the forms. But he opted not to spend the money for calibration. Every day he’d call me to ask when he could remove the forms, but without the maturity data I had no idea.
Another contractor decided to follow my recommendation to calibrate his concrete. He’d originally intended to remove the forms at 7 days, but the maturity data indicated that 4 days was long enough. He ended up saving a full month off his original schedule without compromising safety or quality. Because that month was November, he avoided having to tent and heat the building. So he saved money as well as time. He told me he would use maturity on future jobs.
Less testing in the field
Some projects use temperature-matched curing to determine the strength of concrete in place. That is, data from thermocouples in the concrete control the curing temperature of the cylinders. You get a very accurate indication of the in-place strength, but you have to decide when to break the cylinders. You have a limited number of cylinders, so you don’t want to break them too soon.
Some people use companion cylinders for this purpose. But even if you cure the cylinders right next to the structure, the curing temperature isn’t going to be the same. Cylinders are much smaller than the structure they represent, so the curing temperature will be lower in the cylinders. That means they won’t gain strength as quickly. They’ll give you a conservative indication of the in-place strength, so it’s safe to use them to tell you when to remove the forms. But you may end up wasting a lot of time you don’t have. Maturity gives you a much more accurate estimate of the in-place strength.
Maturity data could help you know when to break the cylinders to verify the strength. It can also reduce—but not eliminate—the number of cylinders you have to break. You still need to know—and document—the strength of the concrete as delivered. You need to know that the concrete at the site is substantially the same as what you tested in the lab. But you probably won’t need as many cylinders as without maturity testing.