Introduction
No-fines concrete (NFC) is a specialized form of concrete made from coarse aggregate, cement, and water—without sand or other fine particles. The absence of fines creates an interconnected network of voids, allowing water to pass freely through while still providing substantial structural strength.
In segmental retaining wall (SRW) construction, NFC serves as a rigid, permeable backfill that can replace the reinforced soil mass created by geogrid systems. This approach changes the wall from a flexible soil-reinforced structure into a gravity wall, where stability is provided by the weight and depth of the concrete mass.
For contractors and engineers, NFC offers several advantages in specific situations, particularly where space is limited or drainage challenges exist. For homeowners or general readers: think of NFC as a “free-draining concrete backbone” behind your wall that eliminates the need for geogrid reinforcement and large excavation areas.
This post covers:
- When NFC is the right choice—and when it isn’t.
- Typical mix designs and specifications (with a summary table).
- How NFC changes SRW construction (only the modifications).
- Design considerations for engineers.
- Testing and verification during construction.
1. When to Use No-Fines Concrete Over Geogrid
1.1 Advantages of NFC
1. Reduced Excavation Depth
- Geogrid requirement: In a conventional SRW, geogrid lengths typically extend 60% of the wall height into the retained soil. For a 10-foot wall, that means 6 feet of excavation behind the wall face.
- NFC alternative: NFC masses can be designed at 30–40% of the wall height in depth. For the same 10-foot wall, this could be as little as 3–4 feet of excavation.
- Practical benefit: Less excavation means reduced site disturbance, faster installation, and fewer conflicts with property lines, utilities, or existing structures.
2. Space-Constrained Sites
- Ideal for walls adjacent to buildings, roads, parking lots, or steep slopes where full geogrid length isn’t possible.
- Allows building directly adjacent to easements without needing costly engineering workarounds.
3. Improved Drainage and Hydrostatic Pressure Control
- The void structure of NFC provides continuous drainage through the mass.
- Eliminates the need for separate wall rock zones or drainage pipes in most designs.
- Significantly reduces the buildup of hydrostatic pressure, which is one of the most common causes of retaining wall failure.
4. Faster Construction
- No soil compaction is needed between lifts—NFC is simply placed, rodded lightly into place, and left to cure.
- Reduces labor and equipment requirements.
- Ideal for tight schedules where speed of construction is critical.
5. Alternative to Large Precast “Big Block” Walls
- NFC can provide the mass and stability of large block systems while allowing the use of standard SRW blocks.
- Easier to source, transport, and place compared to oversized precast units.
1.2 When NOT to Use NFC
1. Highly Seismic Regions
- NFC walls are rigid and have less flexibility than geogrid-reinforced walls, which can absorb some seismic movement.
- Special engineering is required to accommodate dynamic loads.
2. Poor Foundation Soils
- NFC increases the weight of the wall. If the foundation soil has low bearing capacity, it may require stabilization before NFC is used.
- Example: Soft clay foundations may experience settlement under the heavier NFC mass.
3. Temporary Structures
- NFC is intended for permanent installations; it’s not cost-effective for walls that will be removed or relocated.
4. Sites Needing Future Modifications
- Once in place, NFC is monolithic and difficult to remove or alter compared to soil backfill and geogrid.
2. Typical NFC Specifications and Mix Designs
NFC has no “universal” specification—local materials, climate, and project requirements can change the mix. However, decades of field use have identified typical ranges that deliver consistent performance.
Key differences from conventional concrete:
- No sand or other fine particles—only coarse aggregate.
- Lower cement content than structural concrete.
- Higher void content (15–25% air voids).
- Lower compressive strength compared to standard concrete (but still adequate for SRW applications).
2.1 Standard NFC Mix Table (English Units)
| Parameter | Typical Value / Range |
| Aggregate size | 1/2″-1″ (No. 57 or No 6 stone per ASTM C33) |
| Aggregate type | Clean, durable, angular gravel or crushed stone |
| Aggregate-to-cement ratio | 4.5:1 – 6:1 |
| Water-to-cement ratio | 0.3-0.5 |
| Cement content | 250–300 lb per cubic yard of NFC |
| Unit weight (cured) | 110-130 lbs/ft |
| Air void content | ~24% |
| Compressive strength | 900–1,400 psi at 28 days |
| Internal friction angle | 75–77° (design value) |
Read the full Specification Guide here.
2.2 Practical Mix Notes
Workability:
- The mix should be wet enough to coat all aggregate surfaces but dry enough to maintain open voids.
- Overwatering reduces strength and drainage capacity.
Climate Adjustments:
- Hot weather: Use cool water, shade aggregate, and work in smaller batches to avoid premature setting.
- Cold weather: Use warm water and protect fresh NFC from freezing for at least 48 hours.
Strength Adjustments:
- Lower aggregate-to-cement ratio (e.g., 4:1) increases compressive strength but decreases drainage.
- Increase cement content for higher load requirements.
Aggregate Quality:
- Avoid rounded river gravel—it reduces interlock and strength.
- Angular, crushed stone is preferred.
3. How NFC Changes SRW Construction
For crews experienced with standard SRW installation, most procedures remain the same. NFC modifies the reinforced soil zone portion of the build.
3.1 Key Modifications
1. Block Preparation
- Remove one rear alignment wing from each block to allow NFC to flow into the cores.
- This bonds the face units to the NFC mass, creating a single integrated structure.
2. Lift Height and Placement
- Place NFC in lifts no higher than three block courses (~24 in.).
- This helps prevent segregation and ensures the mix fills all spaces.
3. Consolidation
- Rod or tamp* lightly—just enough to settle aggregate into voids.
- Avoid vibration, which can cause paste to settle and voids to close.
4. Continuous Operation
- Avoid long delays between lifts; cold joints reduce monolithic performance.
- Organize crew so block placement and NFC mixing/placement are synchronized.
5. Surface Cleaning
- Before placing the next course, sweep block tops clean.
- Remove splatter from faces with a stiff brush* before it sets.
6. Top Treatment
- Install a strip of nonwoven filter fabric* over the top of the NFC zone.
- Place 8–12 in. of low-permeability soil or topsoil to keep surface water from entering directly.
4. Design Considerations for Engineers
When an SRW is designed with NFC, the design process shifts from internal stability (geogrid) checks to external stability (gravity wall) checks.
4.1 Structural Behavior
- Mass-based stability: The wall resists loads through its own weight and width, not through tensile reinforcement in the soil.
- Depth ratio: Typically 30–40% of wall height is used as the NFC mass depth.
- Bearing pressure: At ~100 pcf, NFC is lighter than soil backfill (120 pcf), slightly reducing base pressure.
4.2 Stability Checks
- Sliding: Verify that horizontal loads from retained soil and surcharge are resisted by the base friction and wall weight.
- Overturning: Check that the resisting moment from the wall’s weight exceeds overturning forces.
- Bearing capacity: Ensure foundation soils can support the increased mass without excessive settlement.
4.3 Material Properties for Design
- Unit weight: ~100 pcf (field-verified).
- Compressive strength: 900–1,400 psi (28 days).
- Internal friction angle: 75–77° (design assumption).
4.4 Drainage Considerations
- NFC is permeable enough to eliminate traditional drainage aggregate and piping.
- Protection at the top surface is critical to prevent direct infiltration of surface runoff.
5. Testing and Verification During Construction
Testing ensures that NFC performs as designed. Field ch ecks are quick and inexpensive when planned.
5.1 Common Tests
- Visual Workability Check
- Rocks should be fully coated but voids remain visible.
- Overly wet mixes should be rejected.
- Unit Weight Test
- Use a container of known volume; weigh the NFC to confirm ~100 pcf.
- Compressive Strength Test
- Cast cylinders (6×12 in.) or cubes; cure and test at 28 days.
- Air Void Content
- Estimate visually during placement or test a hardened sample in a lab.
- Aggregate Gradation Check
- Verify supplied aggregate is within the specified size range before mixing.
Conclusion
Using no-fines concrete (NFC) in segmental retaining walls (SRWs) is an increasingly popular method – it offers a creative solution to traditional reinforcement challenges and can result in a long-lasting, well-drained, and strong retaining wall. By understanding when to use it, how to properly design and build it, and what to watch for during construction, contractors and engineers can confidently employ NFC on their projects. As always, consult manufacturer guidelines (Allan Block, Anchor Wall, NCMA, etc.) for specific details related to the block system being used, and don’t hesitate to reach out to those technical resources if unusual conditions arise. With the knowledge from this guide, you should be well prepared to plan and execute a successful no-fines concrete retaining wall – combining the “no fines” concept with fine results for your next retaining wall project!
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