Road Base Types: A Contractor’s Selection Guide

The main road base types are unstabilized granular bases (dense-graded aggregate/crusher run), stabilized treated bases (cement-treated base, asphalt-treated base, lean concrete base), recycled bases (crushed concrete, reclaimed asphalt pavement), and clean drainage stone. Each category is chosen based on the project’s required stiffness, drainage performance, and constructability under local conditions.

Quick application matches:

• Driveway or access road: Dense-graded aggregate (crusher run / #411) compacted in lifts
• Local or county road: Crusher run or recycled crushed concrete with verified gradation
• Heavy truck route or highway: Cement-treated base (CTB) or asphalt-treated base (ATB) for high stiffness
• Drainage behind retaining walls: Clean washed stone (#57 or pea gravel) with geotextile filter fabric
• Rigid (PCC) pavement support: Unstabilized granular base or lean concrete base, matched to slab design

The FHWA guidance on bases and subbases for concrete pavement confirms that unstabilized granular bases are the most widely used base type under concrete pavements and perform well when properly designed and constructed to meet material criteria similar to AASHTO M 147.

Key Takeaways

Matching road base type to application, load, and drainage conditions is the single most important decision in base construction, and it must be confirmed against the project’s spec sheet before any material is ordered.

Table of Contents

• What are the commercial names for road base materials?
• How do unstabilized granular bases perform structurally?
• What are stabilized bases (CTB, ATB, LCB) and when should you use them?
• Are recycled materials a reliable road base option?
• How do stiffness, flexibility, and permeability affect base selection?
• How do you choose the right road base for your project?
• What installation steps and QC checks produce a durable base?
• Quick reference: Type 2, Type 5, Class 5 vs. Class 6, and is road base better than gravel?
• Olearydevelopmentgroup delivers the right road base material to your San Antonio project site
• Sources
• FAQ

What are the commercial names for road base materials?

Supplier tickets and spec sheets use regional shorthand that can be confusing. The same material often carries three different names depending on whether you’re buying in Texas, California, or Ohio. Here’s how the common commercial names map to engineering categories.

Unstabilized granular / dense-graded aggregate (DGA):

• Crusher run, quarry process (QP), dense-graded aggregate, #411, 21A, 21AA, GAB (graded aggregate base), road base, flex base
• These are all broadly the same category: a well-graded mix of crushed stone or gravel with fines that compacts tightly

Clean drainage stone (open-graded):

• #57 stone, #67, #3, pea gravel, washed river gravel
• Minimal fines, high void space, used for drainage layers and wall backfill, not structural base

Recycled bases:

• RAP (reclaimed asphalt pavement), asphalt millings, crushed concrete (CC), recycled concrete aggregate (RCA)
• Performance depends heavily on stockpile quality and gradation consistency

Treated / stabilized bases:

• CTB (cement-treated base), ATB (asphalt-treated base), LCB (lean concrete base), soil-cement
• Produced by mixing binder into aggregate or native soil; higher stiffness than granular bases

Glossary of abbreviations used throughout this guide:

Regional naming matters. TxDOT Item 247 classifies flexible base by Type (A through E) and Grade (1 through 5), while Caltrans uses its own aggregate base and treated base designations. Always cross-reference the project’s spec sheet before ordering from a supplier using generic names.

Key field reality: DGA, crusher run, quarry process, and GAB are functionally equivalent in most markets, but project specifications from TxDOT, Caltrans, or a local DOT will dictate exact gradation limits and test requirements. Never assume a supplier’s commercial name satisfies a spec without confirming the gradation data.

How do unstabilized granular bases perform structurally?

Dense-graded aggregate is the workhorse of road base construction. It consists of a continuous particle-size distribution from coarse aggregate down to fine material (passing the No. 200 sieve), and that blend of sizes is what makes it work. The fines fill voids between larger particles, and the resulting interlock creates a stable matrix that distributes load to the subgrade below.

Key performance traits of DGA/crusher run:

• Bearing capacity: High when properly compacted; California Bearing Ratio (CBR) values typically exceed those of unimproved subgrade by a significant margin
• Compaction behavior: Responds well to vibratory roller compaction when moisture content is near optimum
• Frost susceptibility: Moderate to high if fines content is elevated; fine-grained material holds water and can heave in freeze-thaw cycles
• Drainage: Limited. The fines that create structural interlock also restrict water movement, so DGA is not a drainage layer
• Pumping risk: In saturated conditions, fines can migrate upward under repeated loading (pumping), which is why subgrade drainage matters

Common gradation labels you’ll see on spec sheets include AASHTO M 147 Class A through E, TxDOT Grade 1 through 5 under Item 247, and Caltrans Class 2 Aggregate Base. Each sets sieve limits and plasticity index (PI) requirements to control fines quality.

1. Verify the project spec’s gradation table before ordering.
2. Request the supplier’s gradation certificate and check percent passing the No. 200 sieve (typically capped at 12–15% for structural bases).
3. Place material in lifts no thicker than 6 inches compacted depth for most vibratory roller equipment.
4. Compact to the project’s target density, usually expressed as a percentage of Modified Proctor (ASTM D1557).
5. Proof-roll the finished surface with a loaded dump truck before placing the wearing course.

Pro Tip: Never place a single thick lift and expect adequate compaction throughout. A 12-inch loose lift compacted from the top will be dense at the surface and loose at the bottom, creating a weak zone that shows up as rutting after the first heavy rain.

What are stabilized bases (CTB, ATB, LCB) and when should you use them?

Stabilized bases add a binding agent to aggregate or native soil to create a stiffer, higher-strength layer than granular material alone can achieve. The three main types each use a different binder and suit different project conditions.

Cement-treated base (CTB): Portland cement is mixed into aggregate or soil, then compacted and cured. CTB produces a semi-rigid layer with compressive strengths that vary by mix design and cement content. The Caltrans Highway Design Manual, Chapter 660 recognizes CTB as appropriate for both rigid and flexible pavement designs where high stiffness is needed.

Asphalt-treated base (ATB): Aggregate is mixed with asphalt binder, similar to a lower-grade hot-mix asphalt. ATB is flexible, resists moisture damage better than untreated granular material, and can be placed quickly. It suits projects where construction schedules are tight and where some flexibility under load is preferable to brittleness.

Lean concrete base (LCB): A low-cement-content concrete placed as a rigid base layer. LCB provides the highest stiffness of the three and is used under heavily trafficked rigid pavements. Its rigidity also means it offers the least tolerance for differential settlement.

Trade-offs to weigh before specifying a treated base:

• CTB and LCB are brittle. Reflective cracking through overlying PCC slabs is a documented risk when the base and slab move differently.
• FHWA guidance notes that excessively stiff bases can increase slab stresses in concrete pavements, which is why base stiffness must be matched to the slab design.
• ATB is more forgiving under flexible pavements but costs more than granular base per ton.
• Treated bases require curing time before traffic loading, which can conflict with tight project schedules.
• Poor subgrade drainage accelerates deterioration of CTB; avoid treated bases over saturated or highly plastic subgrades without addressing drainage first.

Compressive strength target for CTB: Industry practice and state DOT specifications commonly target 7-day unconfined compressive strength (UCS) in the range of 150–300 psi for CTB used under flexible pavements, with higher targets for rigid pavement support. Always confirm the project spec’s exact UCS requirement, as values vary by state DOT and application.

Are recycled materials a reliable road base option?

Recycled aggregates, including RAP, crushed concrete, and slag, are legitimate road base materials when properly managed. The ForConstructionPros guide on road base materials lists crushed concrete and asphalt millings among the contractor-recognized base options alongside virgin crushed stone.

Common recycled base materials and where they work:

• RAP (reclaimed asphalt pavement): Works well as a base or subbase when gradation is consistent. The residual asphalt binder gives RAP some cohesion, which can be an advantage in dry climates. Avoid RAP in applications where the binder might soften under sustained heat and cause rutting.
• Crushed concrete (RCA): Performs similarly to crushed limestone when clean and well-graded. Watch for contamination with rebar, wood, or gypsum board, which can cause long-term problems. TxDOT Item 247 permits recycled concrete as a Type D or E flexible base material under specific conditions.
• Asphalt millings: A cost-effective base for private driveways and low-volume roads. Millings compact well and self-bind over time. They are generally not accepted on state highway projects without specific approval.
• Slag: Steel slag and blast furnace slag both perform well as structural base materials. Slag is dense, angular, and resistant to abrasion. Olearydevelopmentgroup supplies slag for industrial and heavy-traffic applications in the San Antonio area.

Quality-control steps for recycled bases:

• Test each stockpile for gradation and plasticity index before accepting delivery.
• Segregate stockpiles by source material; never blend RAP with crushed concrete.
• Check crushed concrete for sulfate content if the material will be used near Portland cement concrete structures.
• Maintain consistent moisture during compaction; recycled materials often have variable absorption rates.

Pro Tip: On-site pulverization of existing asphalt pavement (full-depth reclamation) can produce a usable base layer in a single pass, cutting haul costs and reducing project time. Confirm the pulverized material’s gradation before relying on it as a finished base without additional stabilization.

How do stiffness, flexibility, and permeability affect base selection?

Base selection is ultimately a structural decision, not just a material-name decision. The two competing demands are stiffness (to distribute load and prevent rutting) and flexibility (to accommodate movement without cracking). Getting that balance wrong in either direction causes premature pavement failure.

A MnDOT research report on base versus subbase performance demonstrates that aggregate quality, gradation, and layer thickness interact directly with pavement performance. Lower-quality local aggregates can be used effectively when designers compensate with increased layer thickness, but that trade-off must be calculated, not assumed.

Stiffness vs. flexibility in practice:

• Overly stiff bases (CTB, LCB) under PCC slabs can create stress concentrations at joints and cracks, leading to reflective cracking. FHWA guidance specifically cautions against this mismatch.
• Overly flexible or poorly compacted granular bases under heavy traffic will rut, especially when subgrade moisture is high.
• ATB sits between the two extremes and is often specified where moderate stiffness with some flexibility is needed.

Permeability and drainable bases:

The FHWA recommends two approaches for drainable base design: permeable base layers with edge drains, and free-draining daylighted bases. Recent best practice, as reflected in FHWA’s concrete pavement base guidance, favors moderately permeable but structurally stable drainable bases in the range of 500–800 ft/day rather than very high permeability open-graded layers that sacrifice stability.

Permeability note from FHWA guidance: Drainable bases with permeability in the 500–800 ft/day range offer a practical balance between drainage function and structural stability, avoiding the rutting risk of very open-graded materials while still removing moisture from the pavement structure.

Specification systems that drive material choice:

• TxDOT Item 247: Types A–E by material source; Grades 1–5 by gradation and confinement. Grade 1 is the tightest spec for high-traffic applications; Grade 5 is for lower-volume or confined uses.
• Caltrans: Class 2 Aggregate Base for most flexible pavement applications; treated bases (CTB, ATB) for rigid pavement or heavy-load situations.
• AASHTO M 147: Referenced by FHWA for granular base material quality; sets gradation and plasticity limits.

Pro Tip: When specifying CTB or LCB, include a maximum compressive strength limit, not just a minimum. A CTB that cures to 800 psi instead of the target 250 psi behaves like a rigid slab and can cause the cracking problems you were trying to avoid.

How do you choose the right road base for your project?

Work through this checklist before you call a supplier. Skipping even one item is how projects end up with the wrong material on site.

1. Define the load and traffic type. Passenger vehicles, delivery trucks, and fully loaded semi-trucks each require different base stiffness and thickness.
2. Assess subgrade conditions. Soft, wet, or expansive subgrade (common in parts of Texas) may require geotextile separation fabric or a subbase layer before the structural base.
3. Check drainage requirements. If the site has poor surface drainage or a high water table, a drainable base or edge drain system may be needed.
4. Evaluate frost susceptibility. In northern climates, high-fines DGA can heave; specify low-PI material or a non-frost-susceptible subbase.
5. Confirm local availability and cost. Crusher run limestone is the dominant base material in the San Antonio area; crushed granite is common in central Texas. Availability affects both price and lead time.
6. Check whether recycled material is acceptable. State and local specs vary; confirm with the project engineer before substituting RAP or RCA.
7. Pull the project spec sheet. The spec governs everything. A supplier’s commercial name means nothing without a gradation certificate that matches the spec’s sieve limits.

Typical compacted thickness ranges by application:

Thickness ranges above are general guidance. Always verify against the project’s pavement design and the applicable state DOT compaction and base construction requirements.

Before placing an order, ask your supplier for:

• Gradation certificate (sieve analysis) for the specific stockpile
• Percent passing the No. 200 sieve
• Plasticity index (PI) of the minus No. 40 fraction
• Source material type (limestone, granite, recycled concrete, etc.)

What installation steps and QC checks produce a durable base?

Field execution determines whether a well-specified base actually performs. The ODOT construction manual identifies compaction control, lift thickness, and testing frequency as the primary acceptance measures for base construction. Here’s the sequence that holds up in the field.

1. Prepare the subgrade. Proof-roll with a loaded vehicle, cut soft spots, and recompact. Install geotextile separation fabric if the subgrade is fine-grained or saturated.
2. Place the first lift. Spread material to a loose depth that will compact to no more than 6 inches. Thicker lifts are the single most common cause of inadequate base density.
3. Condition moisture. Check moisture content against the material’s optimum (from Proctor testing). Add water with a water truck or allow drying as needed before compacting.
4. Compact with appropriate equipment. Vibratory smooth-drum rollers work well for granular DGA. Pad-foot rollers suit cohesive or treated materials. Make the required number of passes per the project spec.
5. Test density. Use a nuclear gauge or sand cone test to verify you’ve reached the target percent of Modified Proctor density. Most state DOTs require 95–100% for base courses.
6. Repeat for each lift. Place, condition, compact, and test before adding the next lift.
7. Finish grade and proof-roll. After the final lift, proof-roll the entire surface. Any deflection or rutting under the loaded vehicle identifies weak zones that need correction before paving.

Common installation mistakes:

• Over-watering the base, which reduces bearing capacity and causes pumping
• Placing lifts thicker than equipment can densify, leaving a soft zone at the bottom
• Allowing delivered material to sit in a stockpile for days before placement, where rain can alter moisture content and segregation can occur
• Mixing fine-grained subgrade material into the base during spreading

Pro Tip: Coordinate your delivery schedule so trucks arrive as the crew is ready to spread. A stockpile left open overnight in wet weather can absorb enough moisture to push the material well above optimum, requiring a full drying cycle before you can compact.

Quick reference: Type 2, Type 5, Class 5 vs. Class 6, and is road base better than gravel?

These are the questions that come up most often on supplier calls and in the field. Here are direct answers.

Type 2 base: In most state DOT contexts, Type 2 refers to a processed aggregate base with a specific gradation, often a crushed stone or gravel meeting a defined sieve curve. The exact gradation varies by state. In California, Caltrans Class 2 Aggregate Base is the standard structural base for flexible pavements.

Type 5 base / #5 stone: Crushed stone grade #5 refers to stone sized at approximately 1 inch minus, used for base layers under pavers and as a compactable base material. It is not the same as a dense-graded structural base; it has fewer fines and less interlock than crusher run.

Class 5 vs. Class 6 (Minnesota and similar states): Class 5 is a recycled or natural aggregate base with a maximum aggregate size of 1.5 inches, commonly used for driveways and low-volume roads. Class 6 is a finer-graded material (3/4-inch maximum) used where a smoother surface or tighter gradation is needed, such as under concrete flatwork or as a final base course.

Is road base better than gravel? Road base (dense-graded) and clean gravel serve different purposes. Road base compacts into a stable structural layer because its fines fill voids and create interlock. Clean gravel (#57, pea gravel) stays loose, drains freely, and is the correct choice for drainage applications, wall backfill, and septic fields. Using dense-graded road base where drainage is needed can trap water and cause failure; using clean gravel where structural support is needed will rut immediately.

• For structural support under pavement: use road base for driveways or DGA
• For drainage behind walls or under slabs: use clean washed stone
• For decorative finish: use clean gravel or crushed fines

Pro Tip: When a vendor quotes you a “Type 3” or “Type 4” base without a gradation chart, ask for the percent passing the No. 200 sieve before accepting delivery. 200 will drain poorly and may not meet your spec’s PI requirements.

A note on procurement and scheduling from the field

One of the most consistent problems on base construction projects isn’t the material itself — it’s the ordering process. Contractors often call a supplier with a vague description (“I need some road base”) and receive whatever is in the nearest stockpile, which may or may not match the project spec. That mismatch only surfaces after compaction, when density tests fail or the surface pumps under proof rolling.

The fix is straightforward: specify the gradation before you order, not after. Request the gradation certificate for the specific stockpile, confirm the percent passing No. 200, and verify the PI. If the supplier can’t provide that documentation, find one who can.

Scheduling matters just as much. Right-sizing your deliveries to match your crew’s daily placement capacity avoids the open-stockpile problem entirely. A load that sits on site for two days in Texas summer heat or after a rain event is a liability. Same-day and next-day delivery options, like those available through Olearydevelopmentgroup’s online booking system, let you pull material as you need it rather than building a large stockpile that degrades before placement.

Olearydevelopmentgroup delivers the right road base material to your San Antonio project site

Getting the right material on site, on time, is where most procurement headaches actually live. Olearydevelopmentgroup provides same-day and next-day dump truck hauling and bulk material delivery across Greater San Antonio and surrounding cities, with a material lineup that covers the full range of road base options discussed in this guide: 2 Road Base, manufactured sand, crushed limestone, and specialty aggregates.

Every delivery comes with transparent per-load pricing, GPS-tracked trucks, and an online booking system that lets you schedule loads around your crew’s placement schedule rather than the other way around. Contractor accounts include net-30 invoicing and volume pricing for multi-load projects. Whether you’re building a residential driveway, a private access road, or a heavy-haul truck route, you can request a quote or book a delivery directly from the website and get material moving the same day.

Sources

These are the primary technical references used throughout this guide. Each covers a specific aspect of road base design and construction.

• Bases and Subbases for Concrete Pavement
• CHAPTER 660 — Base and Subbase (Caltrans Highway Design Manual chapter excerpt)
• Pavement Design: Performance of Base versus Subbase (MnDOT research report)
• ODOT construction manual (Section excerpts on base course and compaction)

For Texas contractors: TxDOT Item 247 is the governing specification for flexible base on state highway projects. Confirm your material’s Type and Grade against the project plans before ordering, and always request a gradation certificate from the supplier tied to the specific stockpile.

FAQ

What is Type 2 road base?

Type 2 road base is a processed aggregate base meeting a defined gradation, most commonly associated with Caltrans Class 2 Aggregate Base for flexible pavement construction. Exact sieve limits vary by state DOT specification.

What is a Type 5 road base?

Type 5 or #5 crushed stone is approximately 1-inch-minus material used as a compactable base under pavers and light structures. It has fewer fines than dense-graded crusher run and is not a full structural base for roadways.

Is road base better than gravel?

Road base and gravel serve different purposes. Dense-graded road base compacts into a structural layer for pavement support, while clean gravel drains freely and is used for drainage applications, wall backfill, and decorative finishes.

What is the difference between Class 5 and Class 6 road base?

Class 5 is a 1.5-inch maximum aggregate base used for driveways and low-volume roads, while Class 6 is a finer 3/4-inch maximum material suited for concrete flatwork subbase and finish base courses. Both are common in Minnesota and similar northern-state specifications.

When should you use a stabilized base instead of granular base?

Use CTB, ATB, or LCB when the project requires high stiffness under heavy loads, tight construction schedules, or when the subgrade is too weak to support adequate granular base thickness. Avoid stabilized bases over poorly drained or expansive subgrades without first correcting drainage.

Recommended

• # 2 Road Base | Oleary Development
• Limestone Boulders Delivery | San Antonio TX | ODG
• Slag | Oleary Development