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The Case for Lightweight Fills in Reline Applications

Relining existing pipes and culverts is becoming an increasingly popular means of addressing aging utilities.  The advantages of sliplining include the following:

  • Minimal disruption of traffic
  • Cost-effective in many cases when compared to replacement
  • Potentially faster project completion
  • Lower environmental impact versus replacement

Conventional grouts generally weigh between 125 – 135 pounds per cubic foot (pcf).  Lightweight fills or low density cellular concrete (LDCC) grout can have densities as low as 30 pcf and are commonly used around 35-45 pcf.  The weight reduction has many advantages in the reline process.

In most reline applications, a new pipe is installed inside the existing line and the annulus between the two pipes is filled with grout.  The purpose of the grout is to fill the voids between the existing structure and the new reline pipe as well as to provide structural support for the new pipe.  The grout is typically delivered through a pumping operation.  Depending on the site conditions, the grout is either pumped through pipes of different lengths that are cast into a bulkhead at the ends of the reline pipe or it is pumped through grout ports that are installed throughout the length of the pipe being relined.  Regardless of how the grout is delivered, the mechanics of the filling of the void between the host and reline pipe are the same.  The grouting operation requires the grout to be fluid enough to flow around the annulus of the pipe and along the length of the pipe to fill the voids completely and provide uniform support for the reline pipe.  This assures positive transfer of loads from the host pipe to the reline after installation.

Flow characteristics of concrete are generally categorized through the use of a slump test.  This involves filling a cone of set dimensions with fresh concrete and measuring the amount of vertical movement the concrete exhibits when the cone is removed.  Most medium workability concrete and grout mixes have slumps on the order of 2” to 3.5”.  A high workability mix would typically exhibit a slump of 4” to 5”.

Lightweight grouts such as cellular concretes generally have slumps between 8” to 9”.  These materials can be pumped long distances and will flow for greater distances along the length of the pipes when filling the annulus just under gravity flow.  The increased flow characteristics of the lightweight grouts means that fewer access ports are required to fill a long reline project.  Additionally, it provides greater assurance that any voids that may have existed in or outside the host pipe are filled and that the reline pipe will have consistent, uniform support from the grout.

Another significant advantage with the use of lightweight grouts in reline applications is the decreased need for strutting to counteract the buoyancy forces that the grout imparts on the reline pipe during the grouting process.  Any grouting process will create forces that tend to lift (i.e. float) the reline pipe until the grout can harden due to curing.  Floatation generally needs to be avoided to maintain the desired flowline elevation.  Therefore, countermeasures must be installed in order to resist the floatation forces.  These countermeasures can include strutting, blocking, spacers or other techniques to brace the pipe and keep it in place during the grouting operation.  The floatation force that has to be restrained is a combination of the grout lift heights used during installation and the density of the grout used to fill the annulus.  By using lighter weight grouts, the strutting required for the pipe can be reduced or the lift heights using during the grouting operation can be increased.  Larger lift heights can help accelerate the installation process.

Some engineers express concerns about the lower compressive strengths of the lightweight fills as compared to heavier, more conventional grouts.  While there is no question that the compressive strengths of lightweight fills is significantly lower than that of conventional grouts, for most applications the strength is still more than adequate to provide proper support and load transfer to the reline pipe.  Most conventional grouts have a compressive strength of 2,500 psi or greater.  Lightweight fill materials may have compressive strengths from 40 – 120 psi.  However, even 40 psi translates to 5,760 psf – which represents a support level similar to a compacted granular backfill.  That would be sufficient for the vast majority of culvert installations with low to moderate cover levels.  Very large structures or those with deep installations may require more strength, but the vast majority of installations would be fine from a strength perspective with the use of lightweight grout materials.

With all the advantages they bring – it’s time to consider the use of lightweight fill materials on more reline applications.