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 1 day 3 days 5 days 7 days 14 days
Cylinder Maturity (oC-hrs) 881 1998 3190 4401 8604
Compressive Strength (psi) 3311 4329 4426 5040 5237
Beam Maturity (oC-hrs) 699 1810 3010 4226 8411
Beam Shear Wave Velocity (m/s) 2621.67 2719.72 2720.56 2766.39 2797.78
Modulus of Rupture (psi) 597 741 719 819 824
Step 4: Creating the Maturity – Strength Curves To develop relationships between the flexural strength and maturity, the following steps may be taken: 1. 2.
Estimate ultimate (long-term) flexural strength of concrete, Transform Equation (11) as follows:
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(1 3)
36
