UFC 3-260-02

30 June 2001

by yearly climatic variations must be taken into account by dividing the traffic into increments during

which variation of the resilient modulus of the asphalt concrete is at a minimum. One procedure is to

determine the resilient modulus of the asphalt concrete for each month, then group the months when the

asphalt concrete has a similar resilient moduli. Thus, it may be possible to reduce the traffic to three or

four groups. Also, since the asphalt concrete is a major structural element, the failure of this element

due to fatigue cracking must be checked. The flow diagram for design of the asphalt concrete

pavements is given in Figure 11-7.

10. PAVEMENTS WITH A STABILIZED BASE COURSE. For a pavement having a chemically

stabilized base course and an unbound aggregate subbase course, damage must be accumulated for

subgrade strain, for horizontal tensile strain at the bottom of the bituminous concrete surfacing, and for

horizontal tensile strain at the bottom of the chemically stabilized layer. Normally in this type of

pavement, the base-course resilient modulus is sufficiently high ($ 690 MPa (100,000 psi)) to prevent

fatigue cracking of the bituminous concrete surface course (where the bituminous concrete surface

course has a thickness equal to or greater than the minimum required in Tables 8-2 through 8-5), and

thus this mode of failure is only a minor consideration. For most cases, a very conservative approach

can be taken in checking for this mode of failure; i.e., all the traffic can be grouped into the most critical

time period and the computed bituminous concrete strain compared with the allowable strain. If the

conservative approach indicates that the surface course is unsatisfactory, then the damage should be

accumulated in the same manner used for conventional flexible pavement. For the pavement having a

stabilized base or subbase, checking the subgrade strain criteria becomes more complicated than for

conventional flexible or bituminous concrete pavements. Two cases in particular should be considered.

In the first case, the stabilized layer is considered to be continuous, with cracking due only to curing and

temperature. In the second case, the stabilized layer is considered cracked because of load. The first

step in evaluating the stabilized layer is to compute the horizontal tensile strain at the bottom of the

stabilized layer and the vertical compressive strain at the top of the subgrade under assumptions that the

stabilized layer is continuous and has a modulus value as determined by the flexural resilient modulus

test. To account for the increase in stress due to loadings near shrinkage cracks, the computed strains

should be multiplied by 1.5 for comparison with the allowable strains. If the analysis shows that the

stabilized base will not crack under load, then it will be necessary to compare the adjusted value of

subgrade strain with the allowable subgrade strain. If this analysis indicates that the adjusted strain is

not less than or equal to the allowable strain, then the thickness should be increased and the process

repeated, or the section should be checked under the assumption that the base course will crack and

behave as a granular material. The cracked stabilized base course is represented by a reduced resilient

modulus value, which is determined from the relationship between resilient modulus and unconfined

compressive strength shown in Figure 11-8. When the cracked base concept is used, only the subgrade

criteria need to be satisfied. The section obtained should not differ greatly from the section obtained by

use of the equivalency factors in Table 9-1 or 9-2. A flow diagram for the design of this type of pavement

is shown in Figure 11-9.

11. PAVEMENTS WITH STABILIZED BASE AND STABILIZED SUBBASE. This type of pavement is

handled almost identically to a pavement with a stabilized base. If the base is a bituminous-stabilized

material, then the cumulative damage procedure must be employed to determine if the subbase will

crack. If the analysis indicates that the subbase will crack due to loading, an equivalent cracked-section

modulus is determined from Figure 11-8, and the pavement is treated as a bituminous concrete

pavement. If both the base and subbase courses are chemically stabilized, then both layers must be

checked for cracking. A conservative approach is taken by checking for cracking of one layer by

considering the other stabilized layer as cracked and having a reduced modulus. The vertical

compressive strain at the top of the subgrade is computed by use of the flexural modulus or the reduced

modulus, as appropriate. If either of the two layers is considered uncracked, then the computed

11-10

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