Section 3: Flexible (Unbound) Base Selection
The base course in a pavement structure serves multiple functions, but the primary function is to supply foundational support and capacity to the pavement structure, to provide a stable course to minimize flexural tensile stresses in surface layers, and/or to dampen stresses induced by traffic loading to weaker underlying subgrades. The selection of an appropriate base material for a pavement structure is dependent on the overall interaction of the base course with the entire pavement structure, which encompasses interaction factors such as:
- Availability and cost. The availability and cost often dictates the selection of base materials in various areas with limited alternatives.
- Surface thickness. The thinner a surface layer, the more stress is transferred to the base course requiring higher performing strength properties: as the base course function as a load bearing layer increases.
- Subgrade stiffness and strength. Base materials with high quality stiffness and strength properties minimize the base course thickness required to protect weak subgrades.
- Amount of lateral support (shoulders). Base material with low unconfined strength, but good confined strength can benefit from the stability produced by the lateral confinement of shoulders.
- Traffic volume and loading. The amount (ADT) and magnitude (ATHWLD) of traffic loading will determine the stresses induced to the pavement structure. High amounts of stresses induced by traffic loading may require a base course with higher performing strength properties.
- Subbase function. Base material used as a subbase typically is not a load bearing or structural layer, but will need to provide uniform and durable support and low compressibility properties to load bearing surface layers, such as Portland cement concrete (PCC) or thick hot mix asphalt concrete (HMAC) layers.
In Item 247, “Flexible Base,” there are only four different grades of flexible base from which to select. Each grade serves a different purpose given the function in which the base material will be used. Grade 1 and Grade 2 base materials are the primary base material grades for base course performing as a structural layer in a pavement structure. Comparatively, there are differences in the material requirements specified by Item 247, but depending on the use of the base material, similar pavement performance can be obtained from both Grade 1 and Grade 2.
Grade 1 provides a more uniform gradation given more controls on the particle distribution. Grade 2 is more gap-graded relative to Grade 1, as it lacks materials in the coarse-sand gradation band (material passing No. 4 sieve to retained on No. 40 sieve). Grade 2 does promote a coarser gradation on the coarse-aggregate section of the gradation band (material retained on No. 4 sieve and larger). This promotes good frictional strength properties, but requires adequate lateral support (shoulders, backfilled edge slopes) and/or overburden (surface thickness) to promote these frictional properties and to supply stability to compensate for the reduced cohesive properties, which is indicated in the lower 0 psi lateral strength requirement for Grade 2. If Grade 2 material is given adequate confinement, this material can achieve and possibly exceed the strength level of Grade 1 material. Grade 1 material does provide more cohesive strength properties, given the higher 0 psi lateral strength requirement. This offers more stability to the material in the presence of minimal lateral support or overburden, such as roads with narrow pavements or thin surfaces.
Grade 1 requirements also contain more stringent plasticity property specifications and less allowable fines (material passing No. 40 sieve), that can result in base material with less moisture susceptibility and less variable mechanical properties (strength) relative to Grade 2 material. It is recommended to perform moisture conditioning in Part I of "Tex-117-E, Triaxial Compression for Disturbed Soils and Base Materials," triaxial testing to reveal the effects of moisture on the strength properties of either grade.
Grade 3 base material is not recommended for base courses in pavement structures. This grade of material is primarily used for non-load bearing subbase courses or maintenance uses, such as backfilling pavement edges, rehabilitation, or shoulder work.
Many times Grade 1 and Grade 2 specifications do not meet the needs required for specific pavement situations. Grade 4 presents the flexibility to engineer a specification to address unique pavement and material design situations. Adjusting material requirements in Grade 4 should be done for the following reasons:
- To improve the performance of mechanical properties (strength) given an increased demand for performance from a base course. Additionally, stringent gradation, plasticity, and hardness requirements can be written into a Grade 4 specification to increase the strength and durability performance of the base material. This can occur when significant increases in traffic loading are anticipated and/or weak subgrades will be encountered in the pavement structure. Often it is more economical and practical to increase the quality of the base than to increase the thickness and stiffness of more expensive surface layers, especially when these high quality materials are available. Also, higher quality bases dampen stresses more efficiently than regular bases, which better protect soft underlying subgrade soils. In addition, higher quality bases regularly retain these improved mechanical properties more readily than conventional bases.
- To design non-load bearing subbase materials in pavement structures for specific uses. Some of these particular uses can include drainable or permeable subbase layers, separation layers, or PCC (rigid) pavement subbase layers.
- To allow the acceptance of marginal local material in
particular circumstances. At times, Grade 1 and Grade 2 performing
base materials are not available in certain areas or a pavement does
not require the performance of Grade 1 or Grade 2 base materials
for specific pavement uses. In these cases, material specifications
can be modified for this exceptional situation. The goal of modifying
material requirements should be to meet the minimum base course
performance for the pavement structure. If the required pavement
performance can not be attained by local base material via Grade
4, then modification of the local base material with additives (cement,
fly ash or lime) or specifying other base material sources must
be considered. When drafting a Grade 4 specification, it is recommended:
- not to remove material testing, as outlined in Item 247, unless there is satisfactory data or reason to do so
- not to mix and match Grade 1, 2 or 3 requirements to create a Grade 4 specification because ordinarily material requirements are not compatible when combined in this manner
- to establish minimum material requirements meeting the mechanical properties (strength/stiffness) required and dictated by the traffic loading and pavement structure.