Section 3: Flexible Base Selection
Anchor: #i10135983.1 Introduction
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 to dissipate stresses induced by traffic loading to subbases and 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.
The “ Flexible Base Selection Guide”1 may be used to assist department personnel with the selection of appropriate base grades. The descriptions that follow are contained in this guide; these bases are also most frequently prescribed by the standard specification Item 247. Other information, such as a background for establishing the current specification, structural notes, base selection factors, and base selection example, is also included.
Anchor: #i10136163.2 Flexible Base Description
There are four different grades of flexible base in Item 247, Flexible Base. Each grade serves a different purpose based on the application for which it will be used.
Grade 1-2 and Grade 5 are the primary bases to be used as a structural layer in a pavement; Grade 4 may be used when the other grades do not provide the material needed for the specific application intended.
3.2.1 Grade 1-2
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- When there is little confinement, a base material must provide its own stability. Grade 1-2 base is recommended for conditions that do not provide stability from the pavement structure. When there is no lateral support or confinement, the base may become unstable as vertical loading is applied. In this case, the base must provide its own cohesion and stability. Anchor: #GROVQAMY
- When there is no lateral confinement, inherent stability is necessary. With an unstable base, the pavement will deflect under traffic loads that are well below legal limits. When thinner (< 3 in.) HMA/bituminous surfacing is used, higher stresses are transmitted into the base material; the likelihood of base rutting and surface fatiguing will increase under these conditions. Anchor: #LVPTOIJS
- In the case of low confinement or little lateral support, Grade 1-2 is more likely to protect the HMA from failure at all levels of traffic. Grade 1-2 is best used with thin surfaces, little or no confinement (no shoulders), and for moderate to high traffic levels. The gradation for Grade 1-2 will almost always meet the requirements of Grade 5.
3.2.2 Grade 3
Grade 3 base material is not recommended for base courses in pavement structures. This grade of material is primarily used for subbase courses or maintenance uses, such as backfilling pavement edges, rehabilitation, or shoulder work.
3.2.3 Grade 4
Grade 4 (properties shown on the plans) presents the flexibility to customize a base specification to address unique pavement and material design situations. Consider adjusting material requirements in Grade 4 for the following reasons:
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- Roadways with low traffic loading (<500,000 ESALs). Surfacing consisting of a seal coat or HMA three or more inches deep and with or without shoulders can specify Grade 4 material with the gradation, plasticity index, liquid limit, and wet ball mill requirements of Grade 1-2. A strength requirement can be waived for these situations, as long as the available local sources have history of acceptable performance. Anchor: #JGRQQRAY
- To improve the performance of
mechanical properties (strength). An increased demand
for performance from a base course may require increased restrictions.
Additionally, Grade 4 specifications can add more stringent gradation,
plasticity, or hardness requirements to increase the base strength
and durability.
This scenario can be useful for anticipated significant increases in traffic loading and when encountering weak subgrades. The use of higher quality can also reduce hot-mix surface thickness and dissipate stresses more efficiently than regular bases, creating better protection of soft underlying subgrade soils.
Anchor: #EMDFKVGT - To design subbase materials in pavement structures for specific applications. Some of these applications can include drainable or permeable subbase layers, separation layers, or PCC (rigid) pavement subbase layers.
Consult with the Maintenance Division, Pavement Asset Management Section, before adjusting material requirements for reasons other than those listed above.
3.2.4 Grade 5
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- Grade 5 material allows harder rock with a lower fines content. Fines may be less cohesive than those found in Grade 1-2. The material that meets this specification may have difficulty providing its own stability; therefore, it is recommended for conditions where stability is provided from the pavement structure and roadway features (shoulders, surface thickness, or other material placed over it). Anchor: #HOMDWSHX
- Grade 5 base is a modification of a Grade 1-2 base and has most of its characteristics, except for the unconfined compressive strength (UCS). The ability of the material to meet the UCS requirement is dependent upon the gradation and the constituents of the binder material. Since the Grade 5 base has the potential for having non-cohesive fines but has strengths equivalent to Grade 1-2 base when confined, a 3 psi lateral confinement is used for Grade 5 base requirements. Unless Grade 5 base is used as a subbase under an appropriate base and with appropriate thickness, it is not recommended for high traffic roadways with thin surfaces or for roadways with no shoulders.
3.3 Base Selection
Many factors are considered beyond material testing and acceptance in accordance with the characteristics identified in Item 247. Selection factors often considered are:
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- availability and cost Anchor: #IRXCXJAO
- surface thickness Anchor: #TRMOPLPB
- subgrade stiffness and strength Anchor: #NNOHILMR
- lateral confinement Anchor: #QWQNHDDR
- traffic volume and loading.
From these factors, a number of different selection matrices may be generated. The applicability of these factors may be weighed quite differently in each district.
An example of a decision matrix for the selection of base grades is contained in Table 3-8. Grades recommended in a chart similar to Table 3-8 should be chosen based on local experience and historical performance.
|
Shoulder Width |
HMA Surface Thickness |
Traffic (Design ESALs)1 |
Base Grades |
|---|---|---|---|
|
< 3 ft. |
Surface Treatment |
< 500,000 |
4* |
|
> 500,000 |
1-2 |
||
|
HMA < 3 in. |
All traffic levels |
1-2 |
|
|
HMA > 3 in. |
< 500,000 |
4* |
|
|
> 500,000 |
1-2 or 5 |
||
|
> 3 ft. |
Surface Treatment |
< 500,000 |
4* |
|
> 500,000 and < 3,000,000 |
1-2 or 5 |
||
|
> 3,000,000 |
1-2 |
||
|
HMA < 3 in. |
< 500,000 |
4* |
|
|
> 500,000 |
1-2 |
||
|
HMA >3 in. |
< 500,000 |
4* |
|
|
> 500,000 |
1-2 or 5 |
||
|
1Percentage of trucks in addition to design ESALS should be taken into consideration. *Grade 1-2 requirements without minimum strengths. |
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