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

Concrete Construction, Jan, 2004 by William Palmer, D., Jr.

Concrete in test cylinder and concrete in a structure seldom 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 we don't know the in-place strength, to be safe, we have to wait to be sure the concrete is up to strength before we strip forms, stress post-tensioning cables, or allow traffic onto a road surface. We need a reliable way to determine the actual strength of the concrete in the structure.

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:

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

* 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.

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 (p. 40). 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, [T.sub.O]--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 C 1074), 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.