Technical Newsletter Issue 10Allan Block and No-Fines - No Problem |
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Woodland Heights, a 30-unit townhome association built in the
1980’s, needed to replace its second timber wall. However, numerous
site constraints did not allow enough clearance to install
geogrid reinforcement within the property limits. Based on site
constraints and project goals, Duffy Engineering and Associates
called for the use of ‘No-Fines’ concrete backfill with an Allan
Block AB Classic wall. Duffy utilized AB Walls 10 software’s alternative
reinforcement option to design the wall and confirm
that all safety factors were exceeded.
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No-Fines concrete has been utilized in the construction industry
for decades and the segmental retaining wall industry for more
than 15 years. Like most concrete mixes, No-Fines concrete includes
portland cement and coarse aggregate. However, sand
(fine aggregate) is omitted from the No-Fines mix to allow voids
to develop, thereby creating a permeable, yet very strong material.
After receiving approval from the City, The Lawn Ranger Inc. proceeded with construction in a series of phases to minimize potential water management and slope stability issues during construction. This turned out to be a good decision because, midway through construction, the site received 4 in. (100 mm) of rain over 48 hours. Fortunately, the contractor planned ahead and covered the entire work area with large tarps. The wall was built by placing No-Fines concrete in the block cores and 2.5 ft (0.76 m) behind the block retaining wall in two course increments. Once the placement of the No-Fines backfill was completed, the crew began stacking the next two courses of block. By the time the blocks were in place, the first lift of concrete had cured enough to place the next lift. This process was repeated until the top of the wall was reached. In the end, 40 cubic yards (30.6 cubic meters) were placed within the retaining wall structure. Since the footprint of the No-Fines concrete retaining wall was smaller than that of a geogrid reinforced wall, the expense of hauling the excavated soil offsite was minimized. This successful project outlines how No-Fines concrete can provide a cost effective way to build a segmental retaining wall when space behind the wall is limited.
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For more information on this story, check out the Woodland Heights - No Fines project profile. |
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No-Fines Concrete History:No-Fines concrete construction had its first boom in England after World War II where it was used in the construction of homes known as the Wimpey Houses. Since labor was in short supply and skilled masons were hard to come by, George Wimpey developed a system where unskilled workers could form up and pour No-Fines concrete walls for the multi-story homes.No-Fines concrete was used because it exerted less pressure on the forms while being more mobile than traditional concrete mixes. The term ‘No-Fines concrete’ became ‘pervious concrete’ once the most popular use became pavement applications. |
Wimpey House |
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No-Fines Concrete BackfillUse of AB No-Fines concrete backfill has increased our ability to install reinforced 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 wall mass. This allows for taller walls with less excavation than conventional geogrid reinforced walls.Typical geogrid reinforced walls require an excavation depth of 60 percent or more of the wall height; while a No-Fines reinforced wall, with similar site conditions, requires only 30 to 40 percent 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 drainage rock in the cores and behind the wall. This pervious concrete backfill will provide a “solid” solution that can reduce the overall settlement behind the wall Engineering Properties:
Installation Steps:Refer to the AB Commercial Installation Manual 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, following these simple steps to place the No-Fines Concrete Backfill:Placing No-Fines Concrete Backfill – Fill all the voids in the block and backfill to the specified depth with the No-Fines Concrete. Obviously, there are numerous ways to get the concrete mix to the back of the wall. Each site will be different.
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Permeable Concrete 
Superior Wall Drainage 
Use with Any AB Retaining Wall 
Fill Voids in Blocks and Backfill 
Remove Excess Material |
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AB Walls 10 Does It Again!Did you know the only segmental retaining wall design program that allows the design engineer to select No-Fines concrete as a reinforcing option is AB Walls 10? The best part is the program allows the design professional to examine multiple options by providing alternate reinforcement options. Once you are ready to design a section, the reinforcement button allows the designer to switch from conventional geogrid reinforcement to No-Fines concrete. |
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