10.Environmental Effects on Pavements
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- Introduction
Pavements are exposed to the environment, which has a significant effect on their performance. The two main environmental factors of concern are the presence of water/ice in the pavement layers and the subgrade, and the variation of temperature throughout the year. These two factors interact with each other, such as during the freezing of pore water in frost-susceptible subgrades, which results in heaving. Furthermore, they interact with traffic loads, for example during springthaw conditions, when the base or subgrade layers can be sufficiently weakened by the presence of pore water to fail under the action of heavy axles (often, secondary roads need to be posted with lower load limits under these conditions). Another example of the interaction of traffic and environmental factors is the problem of pumping in jointed portland concrete pavements. This consists of rapid movement of base/subgrade pore water and fines near and through the joints under the high pressure being built by the rapid movement of truck axles, resulting in erosion and settlement of the downstream slab edge, called faulting , as described in Chapter 9.
The importance of adequate design provisions to control the effect of these environmental factors cannot be overemphasized. Proper drainage and the ability to predict pavement temperatures are
paramount in ensuring proper structural behavior of the pavement layers over time. As discussed next, the latter is important to both asphalt concretes and portland concretes.
It should be noted that a number of pavement environmental problems are prevented through the proper selection of materials. Good examples are the selection of asphalt binder PG grades to prevent transverse cold-temperature-induced cracking by pre- scribing sufficient strength at the lowest temperature expected (see Chapter 5). Another example is the use of antistripping agents, such as lime, to control water from eroding the bond between binder and aggregates in asphalt concretes. Nevertheless, it should be under- stood that no structural layer thicknesses, nor material selection, can compensate for the lack of proper drainage in pavements.
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Water in Pavements
- Drainage
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Water in Pavements
PRINCIPLES
The effect of water and the need for drainage are two of the most often overlooked aspects of pavement design and construction. Drainage follows Darcyrsquo;s law, expressed as either:
Q = k i A (10.1a)
or:
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