The temperature history of the structure is the basic information needed to evaluate the in-place maturity index (expressed as the temperature–time factor or equivalent age). Therefore, a device is needed to record temperature as a function of time. Analog strip chart recorders or digital data loggers connected to thermocouples embedded in the concrete have been used in the past. The measured thermal history was converted to a maturity index using maturity functions such as Equation 5.1 or Equation 5.6. The
calculations have been automated using a personal computer with spreadsheet software.75 This approach,
however, requires manual entry of temperature values at regular time intervals, and is not practical. A more convenient approach is to use commercial “maturity meters.” These are instruments that monitor the temperature history and automatically perform the maturity index calculations. Concrete temperatures are monitored with reusable probes or with expendable thermocouple wires. The earliest models were single channel instruments based on the Nurse–Saul function with a datum temperature
of -10rC (14rF). In 1977, Freiesleben Hansen and Pedersen28 introduced a single-channel maturity
computer based on the Arrhenius equation. This instrument computed the equivalent age according to Equation 5.6 and used the relationship for activation energy given in Equations 5.6a and 5.6b.
Multichannel maturity computers have been developed that uses thermocouple wires as sensors, and some permit the use of either the Nurse–Saul function or the Arrhenius equation. In addition, the user can specify the value of the datum temperature or the activation energy. These multichannel meters, such as shown in Figure 5.15, permit each channel to be activated independently as the corresponding
sensor is embedded in fresh concrete. Tikalsky et al.76 compare the features of several commercial maturity
meters.
FIGURE 5.15 Example of multichannel maturity meter using thermocouple wires to monitor in-place temperature.
Hansen77 developed a disposable “mini maturity meter” as an alternative to the more expensive
maturity computers. The active component of this device is a glass capillary containing a fluid, as shown in Figure 5.16A. The instrument is based on the principle that the effect of temperature on the rate constant for evaporation of the fluid from the capillary tube is governed by the Arrhenius equation. Thus, the evaporation of the fluid and strength development of concrete are influenced by temperature in a similar manner. By choosing a fluid with activation energy similar to the concrete, the amount of evaporation at a given time is indicative of strength development in the concrete. The meter contains a fluid that is reported to have activation energy of 40 kJ/mol. The capillary tube of the mini-maturity meter is attached to a card that is marked in units of equivalent age at 20rC (68rF). The device was developed primarily for use during construction, and hence the scale is limited to an equivalent age of 5 days at 20rC (68rF). The card is attached to the removable cap of the plastic container. The meter is activated by removing the cap and breaking the capillary at the “0 days” mark (Figure 5.16B). The cap is replaced on the plastic container and the container is inserted into the fresh concrete. When it is desired to read the meter, the cap is removed and the position of the fluid is noted in units of equivalent days at 20rC (68rF).
Another novel idea for a “maturity meter” is based on measuring the chemical shrinkage of cement
paste as it hydrates.78 In this approach, cement paste at the same water–cement ratio as the concrete is
placed in a vessel and covered with water. The vessel, known as a dilatometer, contains a small-diameter tube to monitor the decrease in water level as the paste hydrates and undergoes chemical shrinkage (Figure 5.17A). A layer of low-volatility fluid such as oil is used to prevent evaporation of water from the tube. The vessel would be placed in the fresh concrete so that the paste would experience the same temperature history as the concrete. The fall in water level with time would indicate the chemical
shrinkage. Geiker63 has shown that there is nearly a linear relationship between chemical shrinkage and
strength, and that this relationship is independent of curing temperature. In addition, the ultimate value of chemical shrinkage was found to be affected by initial curing temperature in a similar manner as ultimate strength of mortar specimens. Thus, a meter based on chemical shrinkage would automatically account for the effects of early-age temperature on limiting strength. This is a significant advancement over maturity instruments based solely on temperature history because it would allow estimation of absolute strength rather than relative strength. To use such a device, a strength vs. chemical shrinkage correlation curve would be developed for the particular concrete mixture. Such a relationship is shown schematically in Figure 5.17B. The dilatometer would be filled with a sample of the field concrete, and measurement of in-place chemical shrinkage would be used to estimate the in-place strength. A practi- cable field instrument and test procedure based on chemical shrinkage has not been developed.
FIGURE 5.16 A disposable maturity meter that uses the evaporation of a liquid from a capillary tube as an indicator
of maturity.
Container Cap
Capillary Card
(A) Capillary Intact
0 1 2 3 4 5 (B) Capillary Broken 0 1 2 3 4 5
In summary, a variety of commercial devices are available that can automatically compute the in-place maturity index. The user should keep in mind that maturity index calculations are based on specific values of datum temperature or activation energy. Hence, they will only correctly account for temperature effects if these values are applicable to the materials being used. For instruments based on the Nurse–Saul function it is possible to correct the displayed temperature–time factors for a different value of datum temperature using the following equation:
(5.44)
where
Mc = corrected temperature–time factor
Md = displayed temperature–time factor
T0 = the desired datum temperature
T0d = the datum temperature used by the instrument
t = the elapsed time from when instrument was turned on
The readings of maturity computers based on the Arrhenius equation cannot be corrected for a different
value of the activation energy. The user of such an instrument should refer to Figure 5.11 for an under-
standing of the effect of activation energy on the age conversion factor used to compute equivalent age.
5.4.3 Maturity Method Combined with Other Methods
There are several factors that can lead to errors in the estimated in-place strength based on the maturity method:
• Errors in batching that reduce the potential strength of the concrete
• High early-age temperatures that reduce the limiting strength of the concrete
• Improper curing procedures that cause concrete to dry below a critical level and cause hydration to cease
• Use of activation energy or datum temperature values that are not representative of the concrete mixture
Because of these limitations, it is not prudent to rely solely on measurements of in-place maturity to verify the attainment of a required level of strength before performing a critical construction operation.
Therefore, as is discussed later, the ASTM standard practice11 requires that maturity testing be supple-
mented with other tests. One approach is to use the maturity method along with other in-place tests of
the concrete. For example, the maturity method has been used along with pullout tests.79,80 As will be
discussed, by combining the maturity method with other tests, the amount of required testing may be reduced without compromising safety.
FIGURE 5.17 (A) Schematic of a maturity meter based on measuring chemical shrinkage; (B) schematic of
strength–shrinkage relationship for estimating in-place strength.
Graduated Tube Oil
Water Paste
(A) Dilatometer (B) Strength-Shrinkage Relationship
Chemical Shrinkage
Strength
One of the considerations in using in-place tests (such as pullout or probe penetration) is to know when these tests should be performed. If they are performed before the concrete reaches the required strength, they have to be repeated after additional curing. Premature testing may occur when a period of unusually cold weather occurs during the time between placement and testing. On the other hand, if the tests are performed after the concrete is well above the required strength level, unnecessary delays in construction activities may have occurred.
The proper time to perform the other in-place tests can be determined by measuring the in-place maturity. From the preestablished strength–maturity relationship, the user determines the maturity index value corresponding to the required strength. The in-place maturity index is monitored. When the necessary maturity index is attained, in-place tests are performed and the concrete strength is estimated from the correlation curve for that test. If the resulting estimated strength equals or exceeds the required strength, the construction activity can proceed. If the estimated strength is significantly less than the required value, the engineer may consider the following questions before deciding on the course of action: • Were the sites of the in-place tests close enough to the locations of the temperature sensors, so
that the maturity index values were indicative of the maturity of the concrete that was tested? • Were proper curing procedures used to ensure an adequate supply of moisture?
• Is the value of the activation energy or datum temperature used to compute the maturity index reasonably accurate for the materials being used?
If the answer to any of these questions is “no,” engineering judgment is required to decide whether the construction activity may proceed after additional curing, or whether additional curing and retesting are needed before beginning the activity.
If the answer to all of the above questions is “yes,” it is reasonable to conclude that the low estimate of the in-place strength occurred because the concrete tested does not have the same potential strength as the concrete used to develop the strength–maturity relationship. In this case, the engineer must decide whether to require additional curing before beginning the scheduled activity, or whether there is sufficient concern to question the quality of the concrete.