Use of AB No-Fines Concrete Backfill has increased our ability to install reinforced retaining walls in locations where typical construction would not be possible because of property line constraints or limited excavation options. When using the Allan Block products with No-Fines concrete, the permeable concrete actually attaches to the back of the block and extends the depth of the retaining wall mass. This allows for taller retaining walls with less excavation than conventional geogrid reinforced retaining walls.
Typical geogrid reinforced retaining walls require an excavation depth of 60% or more of the wall height; while a No-Fines backfill retaining wall, with similar site conditions, requires only 30 to 40% of the wall height. Limiting the excavation depth will not only save time and money, but it might make the difference between getting the job or not.
There are additional advantages to using the No-Fines solution. Contractors are able to build with better production rates and with less manpower. The use of No-Fines Concrete Backfill also eliminates the need for compaction and compaction testing of the reinforced soil. It provides superior wall drainage since the entire mass is permeable; therefore eliminating the need for wall rock in the cores and behind the retaining wall. This pervious concrete backfill will provide a “solid” solution that can reduce the overall settlement behind the wall.
No-Fines concrete provides superior drainage
- No-Fines Concrete Backfill can be used with any of the Allan Block Retaining Wall Collections.
- No-Fines Concrete Backfill typically consists of cement, fly ash, water and coarse aggregate. The quantity of cementitious material is approximately 500 lb/yd³ (297 kg/m³ ) with a water/cement ratio of approximately 0.30 – 0.40.
- No-Fines Concrete Backfill is designed using 3/8 in.to 3/4 in. (9.5 mm to 19 mm) aggregate with an aggregate/cement ratio of 6:1.
- The density of this product will vary with the density of the aggregate used, but will typically range between 100 lb/ft³ – 135 lb/ft³ (1600 kg/m³ – 2160 kg/m³ ).
- No-Fines Concrete Backfill exerts pressure on the soil and the retaining wall similar to loosely poured aggregate until cured.
- When using No-Fines Concrete Backfill, the backfill zone will also serve as the required drainage or wall rock zone within the cores and directly behind the wall.
Pour No-Fines concrete into block cores and behind block
Refer to reinforced retaining wall installation for the complete installation steps when preparing the base trench and installing the first course of blocks. Once the first course of blocks are installed and leveled, follow these simple steps to place the No-Fines Concrete Backfill:
- The vertical height of a No-Fines concrete pour should not exceed 16 in. (406 mm) or two courses of block. Therefore, No-Fines walls should be built in no more than two-course lifts.
- It is recommended for straight wall sections that at least one of the back wings of the Allan Block units be removed to help secure the block face to the concrete backfill.
- Fill all voids in the block with No-Fines concrete. Rodding the material into the cores will ensure that all voids in the two-course lift are filled completely.
- Backfill to the specified depth with the No-Fines concrete up to level with the top of the two-course lift. Obviously, there are numerous ways to get the concrete mix to the back of the wall. Each site will be different.
- It is not required to let the No-Fines concrete cure between pours because it will start to cure soon after being placed. You can continue to pour the No-Fines concrete mix until the two course lift is complete. Additional courses of block can be stacked following the completion of the lower two-course lift.
Brush off top of blocks before installing next course of blocks
- Before allowing the No-Fines concrete to cure, excess material must be brushed off the top of the block to aid in the installation of the following courses of block. Install the next course of blocks ensuring that they are level. Place the No-Fines concrete backfill the same way as outlined in the previous step.
- Continue these steps until the wall reaches its designed height.
- If any No-Fines concrete spills onto the face of the block, it is important to remove it before it cures. Using a brush and clean water helps to remove the cement past.
- Use 8-12 in. (200-300 mm) of low permeable fill on the last lift to finish off wall.
- Place a horizontal layer of filter fabric above the No-Fines mass prior to placing low permeable fill above.
- Learn more information on ending and topping off the wall.