Pumping Tips for High-Volume Concrete Synthetic Fibers

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General Pointers:

For both low-volume and high-volume fiber projects, there are general pointers that will assist ready-mix producers and pumping contractors in minimizing project-site delays and problems.  Concrete that includes synthetic fiber reinforcement has historically been very pumpable, however following the simple precautions noted below will help ensure a successful project and a trouble-free jobsite.


Avoid adding fibers as a first ingredient to central-batch mixing systems or ready-mix trucks.  When fibers are added first, the odds for fiber mixing and balling problems increase dramatically.  Add fibers with other ingredients or at the end of the batch cycle.  When superplasticizers are used, it may be of advantage to add and mix the fibers prior to adding the admixture to allow for sufficient mixing friction.


When synthetic fibers are added to the mix, the visual slump (as measured by the slump cone) will be reduced, though actual flowability and workability will not be affected to the same degree.  Naturally, low slump mixes (less than 3”) can be a challenge to pump – with or without fibers.  Most synthetic fibers are hydrophobic – do not absorb water – and do not take free water out of the mix.  Fibers do, however, tend to act as a cohesive agent and bind the mix together, though actual pumpability is rarely affected.  In fact, in many cases, pumped fiber concrete projects have reported a more uniform resulting pump pressure and slightly less-than-normal pump pressure due to the reduced segregation in the fiber mixes.


It is important not to pile up the FRC (Fiber Reinforced Concrete) on the grate.  The ready mix truck operator should discharge the fiber concrete into the pump hopper only as fast as the pump auger can take it away.  The fiber-reinforced mix moves quite well, but is sometimes difficult to restart once it has become static.


The position and placement of the ready-mix truck chute is probably the most important aspect when pumping FRC.  In normal situations, the chute is typically placed at the very bottom of the pump hopper, and most often rests directly on the grate surface.  With fibers acting as a cohesive agent in the mix, this placement typically results in a FRC buildup that is difficult to move through the grate.  With FRC mixes, especially with long-length and high fiber dosages, it is important to raise the chute much higher (i.e. 12” from the grate surface) to allow for natural falling velocity to help the concrete pass through the grate.  Depending on the landscape and position of the pump truck on the site, this may require a temporary ramp buildup in front of the pump hopper to allow for the chute to be elevated, yet still retain enough slope to allow for concrete discharge flow down the chute.

It is also beneficial to place the chute nearer the back or uppermost part of the pump grate instead of near the bottom seam.  This higher position will give the FRC more opportunity and space to work its way down through the grate openings. Grates that are sloped are much better for FRC mixes than flat, horizontal grates.


Though FRC often looks stiffer than plain concrete, it still moves quite well under vibration.  A strong vibrator on the grate is important to minimize fiber buildup and maximize FRC flow.  On some pump trucks, the grate vibrator is activated only when the pump is in actual operation, making it important not to pile up excess FRC on the grate during the non-pumping/non-vibration periods. Vibrators mounted directly on the grate surface will be much more effective in moving FRC mixes than those mounted only on the hopper shell.