Maturity Testing Procedure

The maturity method is a two-step process (Figure 1). First, a relationship is established between the maturity values and the concrete strength as meas-used by tests of beams or cylinders. The development of the maturity-strength curve is done at the beginning of construction using project materials. The application covers only one mixture. If there are changes in mate-rial sources, mix proportions, or mixing equipment, another correlation must be run.

Preliminary testing is necessary before a technician can accurately analyze concrete in the field. Using the actual job mixture concrete materials, test specimens are prepared with thermocouples or micro-processors embedded in them. The temperatures are monitored and beams or cylinders are broken to develop a relationship between the strength values and the temperature-time factor (TTF). The strength-maturity equation is developed by performing strength tests at various ages, computing the corresponding temperature-time factors at the test ages, and plotting the strength as a function of the logarithm of the temperature-time factor.

A best-fit line is then plotted through the data, as shown in Figure 2. Test data from one field project indicate that the maturity curves may be more reproducible when using compressive strengths rather than flexural strengths. The second step is instrumenting and monitoring the concrete pavement. Temperature probes or microprocessors are embedded in the concrete and the temperature is measured periodically.

Installation of thermocouples is a relatively simple matter. Thermocouples wires are attached to a small wooden dowel and inserted to the desired depth (see “Location of Test Probes”) in the fresh concrete, shortly after placement. Alternatively, a wooden stake can be driven into the base and the thermocouples wires attached to the portion of the stake that will be covered by the concrete. The lead wires are then attached to the device, which may be located at some distance away. The lead wires may be placed in a shallow groove troweled into the pavement surface to allow for pavement finishing and texturing without interference.

Installing self-contained microprocessors is also as simple as inserting them to the desired depth. They may be secured to a wooden dowel if necessary to ensure that the depth is correct. The data (lead) wires must extend out of the pavement slab for later connection to the hand held computer or data reader. Embedded microprocessors contain a memory chip, temperature sensor, microcomputer and battery. Some hand held computers can be pre-programmed with the maturity curve to output strength directly from maturity data stored in the microprocessors. The equipment is capable of logging the temperature and/or maturity as long as the battery remains effective, typically several months (Figure 3).