Table 6: Example set of strength data collected from Golden Triangle Construction Company

Step 3: Obtaining Strength Data

Strength data must be collected in a laboratory before construction begins. In the example, three cylinder and three beam specimens were tested for compressive strength and flexural strength, respectively, for 1, 3, 5, 7, and 14 days. The obtained average compressive and flexible strength for each age along with the corresponding computed maturity are shown in Table 6.

In this study, the earliest strength measurement was taken according to ASTM C1074 after 24 hours from the beginning of concrete curing. However, the long-life concrete mix exhibited a very high 24-hour compressive and flexural strength. This suggests that the pavement might be opened to traffic at an even earlier age. If it is desirable to open to traffic earlier than the current lab strength testing ages, the first strength testing should be conducted at earlier time. Based on the results of this study as well as the results of the study sponsored by the MnDOT, testing after 6, 12, and 18 hours is recommended in addition to testing after 1, 3, 5, and 7 days.

Table 6: Example set of strength data collected from Golden Triangle Construction Company.

Age Cylinder Maturity (oC-hrs) Compressive Strength (psi)

Beam Maturity (oC-hrs)

Beam Shear Wave Velocity (m/s) Modulus of Rupture (psi)

1 day 881 3311 699 2621.67 597 3 days 1998 4329 1810 2719.72 741 5 days 3190 4426 3010 2720.56 719 7 days 4401 5040 4226 2766.39 819 14 days 8604 5237 8411 2797.78 824

Step 4: Creating the Maturity – Strength Curves

To develop relationships between the flexural strength and maturity, the following steps may be taken:

1. Estimate ultimate (long-term) flexural strength of concrete, . 2. Transform Equation (11) as follows:

(1 3)

3. Compute and

Perform a linear regression with as an independent variable and as a dependent variable to determine coefficient and . 4. Determine the coefficients of the maturity-flexural strength relationship using the following equations:

Applying this approach to the beam maturity and flexural strength data summarized in Table 6 and estimating ultimate flexural strength to be 850 psi leads to the following values for these coefficients:

273.44

The coefficients of the relationship found in Step (3) between the compressive strength and maturity can be obtained in a similar manner. Applying this approach to the data summarized in Table 6 and estimating ultimate compressive strength to be 5,500 psi leads to the following values for these coefficients:

508.09

Figure 22 shows the resulting maturity – flexural strength and maturity – compressive strength curves along with the corresponding lab-measured strengths. A good correspondence between the lab-measured strength is observed. The modified models also present more reasonable strength estimations right after concrete placement than the conventional models as seen in Figure 13.