Traffic opening criteria can be based on age, compressive strength, flexural strength, or some combination. Compressive strength is the most common test for strength in the field, however, the measurement of cracking resistance is indirect. Flexural testing is more representative of the stresses a pavement experiences and is therefore a better indicator of cracking resistance. However, flexural strength testing is more complex when compared to the simplicity of compressive strength testing. Regardless of the strength test chosen, testing needs to be done after 1, 3, 7, and 28 days to properly establish the strength gain for conventional concrete (ASTM C39). Most states have at least an age and compressive strength requirement, although, the age requirement is often secondary so the pavement can be opened whenever the strength criteria is met even if it is before the age criterion. For conventional concrete, the age, compressive strength, and flexural strength criteria ranges between 7 to 14 days, 2600 to 3500 psi, and 400 to 750 psi, respectively. PennDOT 408 requires various compressive or flexural strengths depending on the slab thickness and length (Table 1). However, often with the desire to open a pavement to traffic early, high early strength concrete is used instead of conventional concrete. High early strength concrete has its own set of criteria differing between states as well. For high early strength concrete, the age, compressive strength, and flexural strength criteria ranges between 4 to 24 hours, 1200 to 3500 psi, and 290 to 420 psi, respectively (Zayed, 2018). PennDOT 408 requires a minimum of 1,200 psi compressive strength to open to traffic for high early strength concrete. PennDOT also allows compressive strength to be measured using the standard destructive method or using the non-destructive maturity method. Table 1: Strength criteria from PennDOT 408 (2021)
Slab Thickness, in 6.0 7.0 8.0 9.0 10.0 +
Strength for Opening to Traffic, psi Slab Length < 10 ft Slab Length ≥ 10 ft MR (3rd point MR (3rd point f’c f’c loading) loading) 3000 490 3600 540 2400 370 2700 410 2150 340 2150 340 2000 275 2000 300 2000 250 2000 300
The existing criteria, regardless of the state, do not account for time of construction, design features, early age traffic load conditions, climate conditions, edge support conditions, and other factors that affect early age pavement performance (ACPA, 1994). The current methods also ignore the effect early opening has on the long-term performance. This limits the accuracy of cost-benefit decisions for the life of the pavement when considering when to open to traffic. Another restriction of the current criteria is that there is no separation between traffic levels. The stresses applied by lightweight, or passenger vehicles will be lower than heavy load vehicles. Therefore, smaller vehicles could be allowed on the pavement before the strength
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