Time-Dependent Stiffness Increase of Foamed Asphalt Stabilized Base Material

Abstract

Foamed asphalt stabilized base (FASB) is a cold-recycling process that
combines reclaimed asphalt pavement and/or recycled concrete with a small amount of
foamed asphalt binder. The combination of water and binder causes a gain in stiffness after
placement due to drying effects and the irreversible bonds formed between the binder and
aggregate during curing. To investigate this process, a series of experiments was conducted on
a 20 cm thick FASB base layer for a lane addition project in Maryland. In-place overall
stiffness of the section was measured over seven consecutive days using a Zorn lightweight
deflectometer (LWD) and a Humboldt GeoGauge. The stiffness increase with time was
compared to that for a conventional 20 cm thick granular aggregate base (GAB) on top of the
same subgrade. The stiffness values measured by each of these in-situ devices were different
because of differences in applied stress states and zones of influence and the influence of
subgrade stiffness on the overall response. The stiffness increases for both the FASB and
GAB layers were corrected based on elastic 2-layer assumptions which revealed that the
equivalent stiffness of FASB layer increases by a modulus ratio (FU) of 8.2 while this factor is
about 3.9 for the GAB layer. FU was predicted using partially saturated soil mechanics, which
affirmed that the stiffness gain in FASB layer is significantly higher than can be explained by
mere drying of granular materials. This higher rate of stiffening in FASB layer reveals the
effects of curing and the development of adhesive bonds between the binder and coated
aggregates. The long-term post-construction stiffness was also measured using a Dynatest
falling weight deflectometer (FWD) on the final pavement structure 4 months after
construction. Backcalculated layer moduli showed that the final stiffness of the field-cured
FASB was about 2524 MPa, 15 times higher than that for the GAB material