Page 98 - MODELING OF ASPHALT CONCRETE

76 Cha pte r T h ree

up of a sum of the nonpolar and the combined effect of the polar components. The
relation is shown in Eq. (3-6) (Good and van Oss 1991):

+
Γ = Γ LW + 2 Γ Γ − (3-6)
where Γ = total surface energy
LW
Γ = nonpolar Lifshitz-van der Waals surface energy
+
Γ = acid component of the polar surface energy
−
Γ = basic component of the polar surface energy
In Table 3-1, the wetting and dewetting components and the total surface energies of
new and aged asphalts are given along with the same components of water.
Table 3-1 shows that as an asphalt ages, the surface energies change so as to reduce
healing and to make fracture easier. Thus, the nonpolar portion of the wetting surface
energies grows larger and the polar portions grow smaller as the asphalt ages. At the
same time, both of the portions of the dewetting surface energies decrease so as to
decrease the work of fracture with age. More detailed tables and figures illustrating
these effects on the asphalt binder are presented in Chap. 12. Once a microcrack, or a
crack, forms in the asphalt binder, the surface energies on each face of the crack interact
to provide the cohesive bond strength against fracture and to provide the surface energy
to promote healing. The computation of the cohesive bond strength on this interface,
both dry and in the presence of water, is presented in detail in Chap. 12.
The aggregate particles also have surface energy components which interact with
the surface energies of the asphalt binder to produce the adhesive bonding strength at
the interface between the two. The method used to measure the surface energies of the
aggregates is the universal sorption device (USD) which is illustrated in Figs. 12-5 and
12-6. The USD is used to deposit vapor molecules on the surface of the aggregate particles
in a vacuum. The accumulation of the mass of the vapor molecules on the particle
surfaces at different levels of vapor pressure is used to calculate the specific surface area
of the aggregate particles and to determine the wetting and dewetting components of
the aggregate surface energies. The method of calculating the adhesive bond strength of
an asphalt binder with an aggregate particle, both when the two surfaces are dry and
when there is water present on the interface, is presented in Chap. 12. It is demonstrated
there, and noted here for emphasis, that when water is on the interface between asphalt
and an aggregate surface, the water acts to destroy the adhesive bond. The intensity of
the action of the water varies greatly between various combinations of asphalt and
aggregates as is demonstrated in Table 12-16. This is the scientific basis for determining
which combinations of asphalt and aggregate will strip and which will not. It explains
why some aggregates will strip with some asphalt but not with others.
There are two components of moisture damage to asphalt concrete stiffness: one
due to soaking and the other due to repeated loading progressively opening adhesive
debonding interface zones along the surface of the aggregates in the asphalt concrete
mixture. The soaking damage depends upon the rate of moisture diffusion and
the amount of water that the asphalt film can hold. The rate of moisture diffusion
depends upon the relative vapor pressure in the immediate vicinity of each aggregate
particle and the thickness of the mastic film surrounding the aggregate. Each asphalt
has a unique water versus relative vapor pressure characteristic curve. Some asphalts
hold more water at the same level of relative vapor pressure than others. Based upon
measurements made to the present, this vapor pressure characteristic curve is the crucial