(2) Stabilization With Liquid Asphalt. For an estimate of liquid asphalt
requirements, use the following expression:
where:
p
=
.02(a) + .07(b) + .15(c) + .02(d)
p
=
percent of residual asphalt by weight of dry aggregate.
a
=
percent of aggregate retained on No. 50 sieve.
b
=
percent of aggregate passing No. 50 and retained on No. 100 sieve.
c
=
percent of aggregate passing No. 100 and retained on No. 200
sieve.
d = percent of aggregate passing No. 200 sieve.
Section 9. RIGID PAVEMENT DESIGN
1. BASIC FACTORS. Design criteria for rigid pavements are outlined below.
a. Load Bearing Capacity. Rigid pavements distribute superposed wheel loads
over effective areas much larger than tire contact areas, greatly reducing
stress intensity on subgrade and eliminating the need for high quality bases.
While bases normally are not required for structural reasons, they usually are
required for other reasons.
b. Bending Stresses and Curvature. Assume subgrade reaction or support at
every point proportional to vertical deflection of the slab at the point and
elastic pavement. Assume subgrade support is continuous with no areas where
slab has deflected away from the subgrade.
(1) Analysis. Stress analysis utilizes a basic relationship between
bending moment and radius of curvature at any point:
EI
M = ))))
r
where:
r
=
radius of curvature of the pavement,
M
=
bending moment of the pavement,
E
=
modulus of elasticity of the pavement,
I
=
moment of inertia of the pavement.
(2) Maximum Stress. Maximum pavement stresses occur when wheel loads are
near joints and exterior edges of slabs.
c. Effects of Friction and Warping. Frictional forces between slab and
subgrade interfere with pavement expansion and contraction, often resulting in
pavement cracking. Design properly spaced contraction joints to control crack-
ing caused by contraction. Excessive expansion and blowups occur infrequently
because most pavements are laid in warm weather. However, expansive aggregates
or wide temperature variations can cause blowups.
(1) Vertical Temperature Gradients. Vertical temperature gradients in
slabs cause warping. If the top of a slab is cooler than the bottom, the slab
tends to curl up at the edges because of tension in the top surface. Tension
is an additive to tensile stresses caused by external load applied to slab
edges.
(2) Stress Analysis. Analysis for warping stresses usually is not consi-
dered in pavement design; use an adequate factor of safety instead.
d. Subgrade and Base Uniformity. The thickness design procedure assumes a
constant modulus of subgrade reaction. Local variations in subgrade modulus
cause increased stresses with possible pavement overstressing and decreased
life; therefore, provide a uniform subgrade.
(1) Pavements. Pavements perform better where construction traffic can
be kept off the subgrade.
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