- Asphalt concrete pavement is subjected to several damaging actions from traffic
loads, water (from precipitation and/or groundwater sources), and temperature. The
durability of the asphalt-aggregate mixture, its ability to withstand these damaging
actions for long periods, is a very important engineering property. While the durability
of the asphalt-aggregates mixture depends on several factors such as the mixture's
properties, construction methods, traffic loads and environmental conditions, they have
to be evaluated to predict their field performance. Based on mixture evaluations, the
mixtures that fail the test would have to be modified by additives or by changing the
The first objective of this thesis was to evaluate asphalt-aggregate mixtures for
water damage using the Environmental Conditioning System (ECS), and rank the
asphalt and aggregate types based on water sensitivity. The second objective was to
relate the ECS ranking of the asphalt and aggregate types to Oregon State University
(OSU) and University of Nottingham, UK (SWK/UN) wheel tracking test results, and
to Net Adsorption Test (NAT) results. The third objective was to evaluate open-graded
mixtures and rubber modified mixtures for water sensitivity using the ECS.
The ECS test results indicate that performance ranking of mixtures by asphalt
type or aggregate type alone cannot be made for the ECS test results due to the
significant interaction between asphalt and aggregate. Water sensitivity in the ECS is
significant for combinations of asphalt and aggregate. The ECS test results have shown that ECS performance ranking after one cycle is not statistically significant and does not correlate with ranking after three cycles. The results show that the ECS test program has similar aggregate rankings to those of the NAT and SWK/UN test program, while good agreement exists between SWK/UN wheel tracking results and the NAT test program results. However, poor agreement exists between the OSU wheel tracking results and those of the other two tests. Poor or very little agreement exists among the wheel tracking test results, ECS, and NAT test results in terms of asphalt type rankings.
When considering the comparisons of materials ranking by different test procedures, one must keep in mind that the mechanisms leading to varying "performance" are not the same. The testing reported herein was aimed at measuring water sensitivity, but all the tests do not do so directly. The NAT procedure addresses only the potential for stripping (adhesion) and is not capable of evaluating cohesion loss. The other tests (ECS, OSU and SWK/UN wheel tracking) included all the mechanisms simultaneously, and these provided a gross effect without clearly separating the cause of failure in each case.
Open-graded mixtures used by Oregon Department of Transportation (ODOT) performed well in the ECS in terms of water sensitivity. In the ECS evaluation, six mixtures passed the criteria of 75 % established for Indirect Retained Strength (IRS) test by ODOT, and one mixture was marginal. However, only one mixture passed the IRS evaluation, and another mixture was marginal. This confirms that the IRS test is a very severe test and is not suitable for water sensitivity evaluation of open-graded mixtures. Finally, the IRS test evaluation would suggest that these mixtures would fail prematurely after construction, but all of these mixtures have been used in projects which have been in service for more than three years with no visible signs of distress, or failures.