CONCRETE PAVEMENT CONSTRUCTION:

SUBGRADES AND SUBBASESMichael E. Ayers

Global Pavement Consultants, Inc.

Presentation Outline Introduction and terminology. Subgrades and subbases as a design

element. Subgrade characteristics, construction

requirements and quality control/quality assurance (QC/QA) measures.

Subbase types, materials, construction requirements and QC/QA procedures.

Summary.

Introduction and Terminology

Support Layer Terminology

Subgrade (also used as a haul road on this project)

Subbase (stabilized in this case)

Definitions Subgrade – Usually the naturally

occurring soil on which the pavement is constructed. The subgrade is compacted to specification and trimmed to the elevation or grade shown on the plans.

Subbase – The material directly under the pavement slab. The subbase is also compacted to specification and trimmed to the proper elevation before the concrete is placed.

Support Requirements Concrete pavements require a uniform

level of support for good long-term performance.

Abrupt changes in support can lead to cracking, poor ride quality and other pavement distresses.

Concrete pavements do not benefit significantly from very strong support layers.

Even repairs require uniform support

Support Layer Design Fundamentals

Design Methods There are a number of concrete

pavement design methods in common use.

Although each method looks at the support layers in a slightly different way, none of them are very sensitive to small changes in support.

In reality, the importance of a good foundation under a concrete pavement cannot be overemphasized.

How Support is Measured Support for concrete pavements is

expressed in terms of “k” or the modulus of subgrade reaction.

This value used to be determined by a plate load test but now is generally correlated with other strength parameters.

The “k” used in design varies by the design method, so be sure to check that you use the correct value.

How Important is “k” The easiest way to estimate the effect of

“k” is by looking at required slab thicknesses for a fixed set of design conditions.

Poor support requires an 8 inch slab

Very good support requires a 6.4 inch slab

Other Support Considerations

Support layers, particularly subgrade soils may be prone to the following: Frost heave. Expansion/contraction.

Designing for these conditions is not realistic. Therefore, remedial measures must be employed to resolve these issues prior to construction.

What Influences “k” During Construction

Material properties. Gradation. Mineralogy.

Moisture content. Compaction.

Equipment type. Equipment operation.

Subgrade Characteristics and Construction

Subgrade Characterization Subgrade soils are characterized in terms of

their engineering properties. Gradation (sieve analysis). Liquid limit. Plasticity index. Mineralogy is also important although not a part of

the classification scheme. The most common characterization for

highway use is the AASHTO Soil Classification System.

Soils classed as A-1 through A-3 are generally good subgrade soils.

Soils classified as A-4 through A-7 are marginal and may require remediation.

Desirable Characteristics for Subgrades

Subgrades must provide uniform support over the life of the pavement.

The subgrade should ideally be erosion resistant (effectively limiting the amount of fine particles (minus #200 sieve).

The soil should have very limited frost heave and shrink/swell potential.

It is neither economical or good practice to compensate for poor soils with increased slab and subbase thickness.

Compaction Requirements Soil compaction is strongly related to the

uniformity of the subgrade and strength/deformation characteristics.

Example of a typical moisture-density curve

Compaction Methods Depending on the soil characteristics,

different compaction equipment and methods may be required.

Photos courtesy of PCA

Remedial Measures for Poor Subgrade Soils

Depending on the soil characteristics, proper compaction can alleviate many problems. For instance, compaction at 1 or 2 points above

optimum moisture content significantly reduces the expansion characteristics of most soils.

If compaction alone is not sufficient, a number of options are available to remediate poor subgrade soils: Remove and replace. Chemical stabilization (Portland cement, lime and

fly ash are the most common). Cross hauling and blending.

Chemical Stabilization Effective stabilization requires the right

type of stabilizer placed in the correct amount, thoroughly blended and compacted.

Photos courtesy of Joe O’Grady (PCA)

Grade Trimming The subgrade soil must be graded to the

proper elevation following compaction. The tolerance for trimming varies by

project but has a direct bearing on the thickness of the subbase and potentially the concrete pavement slab.

Trimming operations can utilize survey hubs, stringlines or electronic guidance controls.

Grading Control Systems

Photos courtesy of Gomaco and ACPA

Quality Control/Quality Assurance (QC/QA)

The most common QC/QA measure for subgrade soils is relative density.

The density is usually expressed as a percentage of the maximum dry density as determined by a Standard Proctor test (AASHTO T-99).

Field testing typically relies on nuclear density gage results although other devices may also be used.

Thickness StabilityStiffness

Typical Testing Requirements for Subgrades

Requirements for subgrade depths can vary from as little as 6 inches (150 mm) up to 2 feet (0.6 m) depending on governing agency.

Measures in-place engineering values using structural layer stiffness, klbf/in (MN/m) and Young’s Modulus of a material, kpsi (MPa).

Modified subgrade MUST be stable before next pavement course is constructed! Proof-rolling is the most commonly accepted practice.

Subbase Characteristics and Construction

Subbase Types and Properties

There are 2 general types of subbases for concrete pavements. Unbound granular materials. Stabilized granular materials.

The primary purpose of the subbase is to provide uniform support and minimize erosion and faulting under traffic.

Subbases are not required for pavements with low traffic volumes but may be advantageous from a constructability standpoint.

Unbound Granular Subbases

A variety of materials may be used including, but not limited to, the following: Crushed stone. Crushed gravel. Recycled concrete or asphalt. Others depending on locally available materials.

Important material parameters include: Gradation (particularly the maximum aggregate

size and the amount passing the #200 sieve. Durability and abrasion resistance. Availability and cost.

Unbound Granular Subbase Construction

Unbound subbases are generally 4 to 6 inches thick.

Thicker layers are rarely justified due to performance issues and higher material and placement costs.

Conventional thicknesses are placed in one lift and compacted to either AASHTO T-99 specifications or the more stringent Modified Proctor specification (T-180)

Stabilized Granular Subbases

Stabilized granular subbases can generally make use of the same materials as the unbound subbase.

The most widely used type of stabilized subbases include the following: Portland cement (commonly termed CTB or cement

treated base). Asphalt cement (commonly termed ATB or asphalt

treated base). Stabilized subbases dramatically increase the

“k” value and may reduce slab thickness.

Cement-Treated Subbase (CTB) Construction

Road mixed. Place cement with spreader. Mix with single-pass or multiple-pass

mixers. Central-plant mixed.

Blend granular material, cement, and water uniformly.

Spread to the proper depth and width. Compact, finish to grade, cure.

Construction methods

Spreading, Placing, and Grading CTB

Compacting CTB

Typical Testing Requirements for CTB

Gradation Moisture Density

A common gradation requirement is for 100% to pass the 1.5-inch (38 mm) sieve and a minimum of 60% to pass the No. 4 (4.75 mm) sieve (ASTM C136).

The density requirement is to be a minimum of 95% of the established laboratory standard Proctor density (ASTM D558).

A common moisture requirement is to be within 2% of the laboratory established optimum moisture content (ASTM D558).

Summary

Subgrades and Subbases Summary

Research and experience have shown that properly constructed subgrades and subbases can add substantially to concrete pavement longevity and performance.

It is neither cost effective or practical to compensate for support deficiencies with increased slab thickness.

Proper materials characterization and/or selection are required.

Subgrades and Subbases Summary (cont.)

A stable subgrade is necessary for adequate compaction of the subbase layer.

The pavement support layers provide a “working platform” during construction.

Stabilized subgrades and subbases can provide enhanced pavement performance.

Thank You! For additional information, please contact

Michael Ayers, Ph.D. Global Pavement Consultants, Inc.

Cell 2127-621-3438 mayers@globalpavements.com