Maturity and strength: maturity methods can speed up concrete construction and increase safety.

Concrete in a test cylinder and concrete in a structure seldom cure at exactly the same temperature. During cold weather, the structure may cure more slowly, or it could cure more quickly as cement hydration heats up the concrete in the forms. But since contractors don't know the in-place strength,

That's where the maturity test comes in. Concrete maturity can directly indicate the in-place strength of concrete. And since maturity can be read continuously, you know the strength in real time, even at an early age. Maturity testing provides a lot of benefits:

* Post-tensioning tendons can be stressed sooner.

* Forms can be stripped sooner and with confidence that the operation is safe; rented forms can be returned sooner.

* In-place strength can be monitored at critical locations and in the youngest concrete.

* Cold weather effects on strength gain can be monitored, and heating systems can be shut down sooner.

* Some of the systems now available provide tamper-proof data to prove that the concrete gained the proper strength and wasn't subjected to unusually high or low temperatures.

* Compressing the schedule can allow contractors to get paid sooner and reduce worker hours.

* The number of test cylinders or beams that must be made and tested is greatly reduced.

* Low or high temperature (or too great a temperature gradient) can trigger an alert.

* Traffic can be allowed onto concrete surfaces as soon as the needed strength is attained.

The concept

a concrete's maturity is the extent of the cement's hydration. Maturity is measured by taking the differential of the time-temperature curve. Yes, that's calculus, but don't let it scare you off.

If you were to put a temperature sensor into some concrete and record the temperature every hour, then plot that data, you would get a curve something like the one shown in Figure 1. In the first part of the curve, the temperature rises from its initial temperature due to the heat generated by the concrete hydration. In the second part of the curve, the concrete cools down with the temperature rising and falling mostly dependent on the air temperature. In this figure, TO--the datum temperature--is the theoretical point at which hydration stops. That value is usually assumed to be 14[degrees] F. The maturity at any time is simply the area under the curve.

[FIGURE 1 OMITTED]

Two formulas can be used to calculate this area to provide a value of maturity or a maturity index. The simpler one, called the Nurse-Saul equation, provides a value called the temperature-time function (TTF). This equation works well within a temperature range of about 23[degrees]F to 86[degrees]F, and many maturity meter manufacturers use it because of its simplicity.

The second equation, the Arrhennius equation, provides a value called equivalent age. Although a little more complicated, this equation provides more accurate results when the temperature varies widely. An interesting aspect of this is the concept of equivalent age. Say you have two cylinders that cure at different constant temperatures as shown in Figure 2. The first cures for 6 days at 40[degrees]F above the datum temperature, which we will assume is 14[degrees]F (40+14=54[degrees]F). The second cures for 3 days at 80[degrees]F above the datum 80+14=94[degrees]F). Although there is a 3-day difference in actual age since the cylinders were cast, they have the same equivalent age--the same maturity.

[FIGURE 2 OMITTED]

Why does maturity matter? Testing has proven that for a given concrete mix, the same maturity equals the same strength, regardless of curing temperature. That means that for the two cylinders in Figure 2 (assuming they are made from the same batch of concrete) even though one is 3 days old and one is 6, they should have the same strength. Now all we have to do is develop a curve that shows us the compressive (or flexural) strength of the concrete at any maturity (or equivalent age), as shown in Figure 3.

[FIGURE 3 OMITTED]

This maturity relationship has been known since the middle 1950s but was not used much outside the lab since easy-to-use equipment was not available, in 1987, ASTM first approved C 1074, which standardized the procedure for developing the strength-maturity relationship. Although there are ways to get around this whole procedure (more later), the typical steps are to first develop the mix design that you plan to use in the structure (or pavement or slab). Then, cast and cure 17 cylinders in the lab using that mix--with time-temperature sensors embedded in the center of two of those cylinders. Several suppliers make sensors that will provide temperature readings over time, or directly provide TTF or equivalent-age values.

At various times (1, 3, 7, 14, and 28 days, according to C1074), a maturity value is taken from the cylinders with the sensors, and cylinders are broken (two cylinders per test, plus an extra in case of a bad break) to get compressive strength values. Now we have a relationship between the maturity index (actually equivalent age or temperature-time factor) and the strength of the concrete. If you know the maturity index of this concrete at any time, then you know its strength.

Equipment

At the most basic level, all we really need to determine the maturity index within a batch of concrete is a temperature sensor and a way to record the temperature over time. That could be as simple as a tape that plots temperature versus time. You could then use the Nurse-Saul equation or the Arrhenius equation to calculate the maturity index. New maturity meters, though, calculate for you and provide the maturity index directly. Here are a few commonly used systems.

intelliRock: Developed by Nomadics Construction Labs, the intelliRock system uses embedded, expendable loggers. Once activated, these devices continuously record temperature and convert it to either TTF or equivalent age. When you want the data, simply connect a handheld reader to two leads that are left protruding from the concrete, and download.

This system has been used on many recent projects. "intelliRock has helped us go from being behind on the project to ahead of schedule," says Pare Culbertson, project engineer for McCarthy Construction on the 4th Avenue Jail Project in Phoenix. "In 5 months, our cost [for the maturity meters] has been around $3500, and our benefit is calculated at $175,000. We have been able to reduce the number of days we wait to remove reshores from 7 to 2."

Con-Cure: This maturity system has reusable sensors that plug into a plastic sleeve embedded into the concrete. The sensor is attached to a small maturity meter that mounts on the outside of the form. When contractors buy this system, Con-Cure president John Gnaedinger works with them to determine how many sensors and meters are needed (typically, 2 to 10), where to locate them, and how to interpret the results. His recommendation for developing the maturity-strength relationship is slightly different (tom the ASTM procedure. He advises making cylinders when the concrete is poured, curing them at about the same temperature as the field concrete, and monitoring their equivalent age.

"Equivalent age is the key," Gnaedinger says. "If you have temperature data in the slab, you can compare it to the equivalent age of the cylinders. Then when you break a cylinder, you know the strength. This way you create the maturity-strength curve as the job is progressing."

Onset: This company has been making miniature, battery-powered data loggers for many years. The Nevada Department of Transportation has used Onset's data logger to monitor maturity.

International Road Dynamics: IRD's system uses a wireless temperature "tag" that is embedded in the concrete and then read by a pocket PC that receives a radio-frequency signal.

Limitations

For the maturity system to be accurate, the concrete in the structure must be the same mix as the concrete being used to monitor the maturity. If the concrete mix design changes during the job, it is more difficult to create the correlation curves. Gnaedinger, however, feels that there could be instances where "maturity testing could actually drive consistency, rather than the other way around."

To make sure that the concrete you're getting on the job is the same as the concrete used to develop the maturity relationship, verification cylinders should be made from every few batches. Make four cylinders, embed a maturity meter in one, then test the others at some known maturity. If the strength is within 10% of that predicted by the maturity-strength correlation curve, then everything is OK--otherwise, an investigation should be conducted to find out what's changed in the mix.

Cost, of course, is a factor. The intelliRock system has a starter kit for $1500 with 25 data loggers--you will eventually need more sensors (at $25 each) since they remain in the concrete, but a single reader can be used indefinitely. Con-Cure's starter package with two meters and 10 sensors also costs $1500, and though you'll seldom need to buy more sensors, if you need more meters for other locations, it will cost more.

Another cost that should be taken into account is becoming properly trained to use the meters and interpret the results. Most manufacturers will provide this service for a minimal fee.

The bottom line on cost, though, is that the maturity system has the potential to pay for itself very quickly, especially in the wintertime or if there are any time pressures on a job. "On cold-weather projects," says Frank Kozeliski of New Mexico-based Gallup Sand & Gravel, "you've got the cost of propane plus a guy to keep an eye on things. That adds up in a hurry."

"We're seeing a lot of use of maturity meters on pavement and also post-tensioned jobs," says Tommy Ruttura, Ruttura & Sons, Farmingdale, N.Y. "We love them, because we can immediately see when we can pull the tendons. Maturity meters--that's the best deal going right now."

For more information on the manufacturers mentioned in this article, contact them directly as noted below.

Con-Cure, www.con-cure.com, 866-386-4800, or circle 54.

International Road Dynamics, www.irdinc.com, 303-355-5998, or circle 55.

Nomadics, www.intellirock.com, 405-372-9535, or circle 56.

Onset, www.onsetcomp.com, 508-759-9500, or circle 57.

William D. Palmer Jr. is editor-in-chief of CONCRETE CONSTRUCTION.