In addition to concrete strength and elastic modulus, a number of other concrete properties influence CRCP performance. These include the heat of hydration, coefficient of thermal expansion, and shrinkage potential of the concrete mix. All concrete properties should be optimally selected according to site-specific conditions so that sufficient structural capacity is provided, capable of withstanding the anticipated traffic loads. The concrete should also possess the required characteristics to endure the expected environmental conditions.
Durability factors such as alkali-silica reactivity (ASR) potential, freeze-thaw damage, sulfate attack, and others that can be minimized or even avoided with proper design of the paving mixture. If possible, this should be considered during the design of the pavement through the development of project specifications and/or special provisions. Some of the more relevant concrete properties that should be considered in CRCP design include:
- Strength – Both the tensile strength andtheflexural strength are the concrete properties of interest for reinforcement and thickness design, respectively.
- The transverse crack pattern in CRCP is related to the tensile strength of the concrete. Higher tensile strength typically results in larger crack spacings. In addition, greater variability in tensile strength can result in shorter crack spacings and vice versa. The 28-day tensile strength used for reinforcement design is determined through the American Standards for Testing and Materials (ASTM) C 496 or AASHTO T 198 splitting tensile tests.
- In addition to the effect of tensile strength on cracking behavior, CRCP also requires sufficient strength to resist traffic loads. Fatigue cracking in concrete is found to correlate with the flexural stress-to-strength ratio. For CRCP, maintaining stresses at a level that is much lower than the concrete flexural strength can minimize punchout development. The 28-day flexural strength is determined using the ASTM C 78 or AASHTO T 97 third-point loading (modulus of rupture) test, and is used in most thickness design procedures.
- The concrete strength used in CRCP design does not have to deviate from that currently used for jointed concrete pavement design.
- Concrete Coefficient of Thermal Expansion – Volumetric changes in the concrete, and thus the level of stresses generated,are governed in large part by theconcretecoef-ficient of thermal expansion (CTE).
- CTE has been found to be one of the most influential factors on the behavior of CRCP.(1)Evaluation of the Performance of Texas Pavements Made with Different Coarse Aggregates (TX-01/7-3925-1)
- All else being equal, selection of aggregate types with low CTE is recommended to achieve adequate cracking patterns minimizing the potential for punchout formation. For economic reasons, however, locally available materials should be used to the greatest degree possible. Improved construction practices including an optimized concrete mixture can often compensate for higher CTE.
- Drying Shrinkage – Thisisa function of a number of factors including the water-cement ratio, cement type, cement content, admixtures used, type and amount of aggregates, and climatic conditions.
- Should be kept as low as possible to minimize volumetric changes in the CRCP that can lead to wide cracks, adversely impacting performance.
- Heat of Hydration – affects the set time, strength development, and modulus of elasticity development. In addition, the heat of hydration contributes tothetempera-ture increase in the concrete during the first hours after placement.
- If possible, take measures to reduce the adverse affects of excessive heat of hydration, as it could affect CRCP performance.
More information on the influence of these and other concrete properties can be found in guidance such as the Integrated Materials and Construction Practices (IMCP) for Concrete Manual, developed by the National Concrete Pavement Technology Center (CP Tech Center).
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