(2) Subgrade Modulus. Make plate bearing tests in each area of
substantially different soil types or determine probable differences in
subgrade module from previously made tests for comparable subgrade conditions.
The subgrade modulus design value can vary for different areas.
Provide adequate surface drainage to prevent ponding of water
and adequate subsurface drainage to prevent loss of subgrade bearing capacity.
See NAVFAC DM 5.3 for surface drainage and Sections 5 and 7 for subsurface
and base drainage.
f. Frost. Use special design procedures in areas having freezing
temperatures and frost-susceptible subgrade soil. (See Paragraph 5, below.)
2. RIGID PAVEMENT STRESSES. Design criteria for specific stresses are given
a. Wheel Configuration, Total Load, and Tire Pressure. Stresses developed
in rigid pavement depend on total load, wheel configuration and spacing, and
(1) Maximum Stress. Maximum stress is primarily a function of
individual wheel loads when axle spacing exceeds about 4-1/2 feet.
(2) Pavement Stress. For a given wheel load, pavement stress increases
with increasing tire pressure.
b. Modulus of Subgrade Reaction. Obtain subgrade modulus (K) for a plate
loading test. Make sufficient tests to obtain results for areas of different
soil types. See Section 2.
(1) Highway Design. Normally, large numbers of tests are not required
for highway design because of limited influence of subgrade modulus on
required thickness of pavement.
(2) Estimated Value. Estimated values of subgrade modulus are acceptable
if adequate subsoil investigations have been made. For approximate values of
various soils, see Table 2. Do not use estimated values of subgrade modulus
exceeding 300, unless substantiated by field-bearing test results. Values in
excess of 500 should not be used regardless of test results.
c. Flexural Strength of Concrete. Determine concrete flexural strength at
failure when tested at 28 days by third-point loading; use ASTM C78. Check
results for probable reliability if compressive strengths are known from
average relationships of Figure 10. Do not use this figure in lieu of flexural
(1) Flexural Strength. For a typical plot of flexural strength variation
with age of concrete, see NAVFAC DM-21.
d. Thickness Design Procedure. Use design curves of Figure 11 with stress
equal to concrete flexural strength divided by a factor of safety of 2.0, and
with subgrade modulus to obtain required thickness of slabs as shown by the
dotted line. Thickness shall be rounded off to nearest 1/2 inch. Consider the
need for modifying computed slab thickness when unusual concrete behavior
occurs. Specific data for this modification are not available; obtain
experience of local highway departments. Conditions indicating modification of
computed thicknesses are:
(1) Abnormally slow rate of increase in concrete strength.
(2) Decrease of 28-day strength of concrete with time.
(3) High moisture absorption by aggregate or concrete with resulting
abnormal shrinkage. This can develop excessive curling or tensile stresses.
3. BASES. Provide bases under rigid pavements, primarily to maintain design
load bearing capacity of pavement. In some cases, structural benefits from the
use of bases result in more economical construction.
a. Uses. In addition to their structural functions, bases are used to:
(1) Prevent "pump" of subgrade.
(2) Provide uniform bearing surfaces for pavement slabs.
(3) Replace soft, expansive, or highly compressible soils.
(4) Replace frost-susceptible soils to protect subgrades from