THEORY of the CRAM Pavement The theory of the CRAM Pavement is founded on the selection of materials based on their compatibility with the stress, temperature, and moisture environments. In this way the individual material properties are utilized to form the optimum structural section for the intended use. This is in contrast to the concept of conventionally designed asphalt pavements where pavement materials are ordered in the structural section in accordance with their quality, with progressively "better" quality materials placed nearer the pavement surface.

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As in conventional engineering structures, pavement failures occur when stresses or strains at critical locations are exceeded. The maximum tensile strain on the bottom of the asphalt concrete layer has been found to correlate well with the number of load repetitions to cause fatigue failure, which is manifest when cracks initiate at the bottom and propagate upward through the layer. Also, the maximum compressive strain on the subgrade has been found to correlate with the number of load repetitions to cause accumulative plastic deformations in the subgrade soil. Failure in an asphalt pavement structure is thus expressed by the fatigue-caused cracking and/or subgrade deformation-caused distortion, either or both resulting in an unacceptable riding surface. The CRAM Pavements combines materials in the structural section to efficiently distribute imposed stresses and strains. Graphical representations presented in accompanying Figures 1 & 2 display the stresses developed beneath the center of a wheel load for CRAM and conventional pavements, respectively.
The CRAM Pavement effectively utilizes the surface course for imposed radial compressive stresses, the aggregate for compressive and shear stresses, and utilizes the lowermost asphalt concrete primarily for the radial tensile stresses, and produces a reasonably uniform reduction of the vertical stresses through the full depth of the structural section. In contrast, in conventional pavements the asphalt concrete is utilized for both the compressive and tensile radial stresses and is thereby subjected to a major stress reversal. The aggregate base below experience only minor compressive stresses. Also, vertical stresses reduce rapidly through the asphalt concrete and slowly through the aggregate base. The comparative analysis presented in Figure 3 shows that generally a two-fold increased efficiency in the use of pavement materials can be obtained. This efficiency in the CRAM pavement results in corresponding minimum initial and long-term costs while providing maximum protection from adverse environmental effects.

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