Flowable fill was initially developed over 20 years ago as a solution to reoccurring problems originating from poorly compacted soil or granular fill. Originally, flowable fill mixtures were comprised of large amounts of fly ash and water. Unfortunately, this type of mixture, although it has its advantages in certain applications, experienced large volume changes due to the evaporation or dissipation of the mix water. Over the past decade, extensive research and technology advancement with flowable fill mixtures has led to its use in numerous applications. These include bedding and backfill for utility trenches, paving sub base, bridge abutment and retaining wall backfill. Recent laws passed by the Environmental Protection Agency concerning below-grade fuel tanks have resulted in an increased use of flowable fill in other void-filling applications such as abandoned tanks, basements, tunnels and mines, sewers, and other underground structures.
Flowable fill mixtures are usually comprised of combinations of cement, water, fine aggregate, and fly ash or slag. Due to the nature of the product, many materials that do not meet the quality standards for use in concrete have been used successfully in flowable fill mixtures. Chemical admixtures specifically developed for use in flowable fill mixtures have also become more commonly-used as a result of their unique benefits on the performance properties of the mixture. Your local ready-mix producer may suggest mixes varying from these components, depending on their availability and the specified project requirements. The versatility in design of the flowable fill mixtures offers numerous advantages to the specifying engineer.
Volumetric mixers have performed flowable fill work almost as long as the concept has been around. Most manufacturers offer this feature on the mixer as part of their equipment or as an option to achieve the low strengths required for flowable fill projects.
One of the first and largest uses of volumetric mixers for a flowable fill projects took place in California. AT&T had a small fleet of mixers geared to produce a low strength fill to cover up fiber optic lines to be used for the internet. Since that time municipalities and utility departments all over the world have come to see the many advantages volumetric mixers can provide in this arena.
Not only are the mixers capable of producing low strength fill, but they can with the turn of a lever or switch be quickly adjusted and used as a concrete mixer again. This feature allows many owners of volumetric mixers to not only pick up the work for the flowable fill but enable them to complete the concrete work with the same machine.
Many flowable fill projects call for massive amounts of fill as well vs. the usual street or utility cut. Again the volumetric industry has manufactured equipment that not only achieves the mix design but more importantly is able to achieve the production rates required. The amounts vary and can literally go into the thousands of yards per hour needed for various projects. Typically this is where one will see a large ten or twelve yard volumetric mixer that produces over one hundred yards per hour of flowable fill.