Civil Engineering Association

Analysis of the Federal Highway Administration’s (FHWA’s) Long-Term Pavement Performance (LTPP) data reveals that a pavement’s foundation (base or subbase and subgrade) is one of the most critical design factors in achieving excellent performance for any type of pavement.* For concrete pavements, the design and construction require ments of a roadbed or foundation structure may vary con siderably, de - pending upon subgrade soil type, environmental conditions, and the amount of anticipated heavy traffic. In any case, the primary objective for building a roadbed or foundation for concrete pavement is to obtain a condition of uniform support for the pavement that will prevail throughout its service life. Drainage considerations are also important in the proper design and construction of a roadbed or foundation for concrete pavement. It is important not to build a supporting layer system that holds water underneath the pavement slabs. This has been a common mistake in the design of concrete pavement structures, which has led to poor field performance of some concrete pavement sections. It is equally important not to over design the permeability of a subbase layer. Overzealous engineering of a permeable subbase will most likely lead to a foundation that does not provide the requisite stability for longterm pavement performance. Where stability has been sacrificed for drainage, concrete pavements have performed poorly and have experienced unacceptable numbers of faulted joints and cracked slabs within a relatively short period. Free-draining and daylighted subbases are the reasonable alternatives to rapidly draining permeable subbases with an edge drainage system that often lack stability for long-term perfor mance or cause other performance problems. In northern or cold climates, the influence of frost and freezing of the roadbed is an important consideration. Certain subgrade soils are particularly susceptible to frost action, which raises the foundation and concrete pavement layer(s) vertically during freezing periods (commonly referred to as heaving or frost heaving). Generally, frost heave is limited to areas of freezing climates with silty soils. If the heaving is uniform along a pavement section it is not detri mental, but if heaving is localized, it upsets the unifor mity of support provided to the surface pavement. Removing or treating these materials will be necessary to ensure that the pavement performs as expected. For nearly every pavement design there are many different subbases to choose from (i.e., unstabilized recycled concrete aggregate, cement-treated, lean concrete, etc.), as well as the decision of a natural or a treated subgrade. In some cases, as for most clays and some silty soils, it may be most economical and advanta geous to treat the subgrade soil and then to provide a unstabilized (granular) subbase as a construction platform. In the case of a road for a relatively low level of traffic it is likely that a natural subgrade may suffice, as long as it is evaluated to be acceptable as a roadbed. The optimal subbase and subgrade design or selection must balance both cost and performance consid erations. The same combination of subbase and subgrade treatment used for heavily-trafficked highways is likely not necessary for a low-volume roadway, even in the same area and subject to the same climate. Finally, it is likely that as this document is printed and distributed, some new and emerging technologies are advancing within the grading and paving industries. This guide captures the fundamental parameters, recommendations, and considerations for subgrades and subbases for concrete pavement. Emerging technologies, such as intelligent compac - tion and GPS-guided grading/placing equipment, are likely to become more commonplace in the future. These improvements to existing methods are not a replacement for the necessary consideration of the fundamentals. By the same token, we encourage agencies and contractors to advance their construction methods and improve the quality of their work using advanced technology.