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Cement Additives for Permeation Grouting - Foam (Cellular) Grout

 Foam (Cellular) Grout

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Figure 1: Foam used for for grout mixture (Cellular Concrete, 2014)

Introduction

Foam or cellular grout is a cement grout mixture that contains foaming agents (surfactants) (Bruce, 2005). The foaming agents create many small air voids in the mix that reduce the unit weight and improve flow of the mixture. Foam grout density ranges from about 30-80 pcf (480-1300 kg/m3) which result in 28-day compressive strengths of 50-1200 psi (350-8300 kPa). The density and compressive strengths of the mix are tradeoffs: the higher the density, the higher the compressive strength. To achieve a specific compressive strength, different mix designs should be tested to find a minimum density to achieve the desired strength (Henn, 2003). Figure 2 shows the tradeoff between density and compressive strength for cellular grouts with varying degrees of foaming. With increased quantities of foam the density (unit weight) decreases resulting in a decrease in compressive strength.


Figure 2: Variation of compressive strength with unit weight and cure time for foam grouts (Vipulanandan, 2000)

Advantages

  • Easy to level

  • Free flowing (easy to pump vertically and horizontally, fills small voids)

  • Self-leveling

  • Does not require compaction (fills voids)

  • Frost resistance

  • Good thermal insulation

  • Good water absorption

  • Fast and inexpensive

  • Can pick desired density and strength (Barnes, 2009)

  • Good energy absorption (Vipulanandan, 2000)

  • Can endure deformations (Vipulanandan, 2000)

  • Requires low pump pressure (Midwest Mole)

Disadvantages

  • Low strength (McGillivray, 2012)

  • High compressibility (McGillivray, 2012)

  • If placed below the water table, the foam grout must be dense enough to displace the water (Henn, 2003).

Applications

Foam grout is typically used as a low cost option when strength is not a requirement (McGillivray, 2012). The air voids in foam grout allow the material to be somewhat compressible and therefore a good material for increased energy absorption. This property makes foam grout a good option for seismic areas, highways, and airport runways (Henn, 2003). It can also be used as backfill for tunnels and pipelines and fill material. It is also used to fill in the ring between the outside of a pipe and it’s surroundings (backpack grouting) (Vipulanandan, 2000).

Sliplining has been used with increasing popularity to replace existing concrete sewer pipes. In this process a new pipe is introduced within the existing pipe and the annular space between the pipes is filled with grout to support the new pipe and control infiltration. Poor grouting mixes and practices have resulted in many problems when it comes to sliplining including “unwanted buoyant uplift, excess deflection or collapse of the new liner pipe.” To avoid these problems it is very important to use a lightweight grout with good flow properties which is why foam grouts are typically used for these applications (Vipulanandan, 2000).

Although foam grout is generally used as a lightweight material to fill voids, it can also be used for stabilization purposes such as protecting slopes against earthquakes or preventing liquefaction. Typically, to stabilize a soil with grout the void space is completely filled with a particulate or chemical grout but often the same soil could be adequately stabilized by grouting the particle contacts without filling all the void space. Ali and Woods show how particle contact grouting can be accomplished with the use of a foam grout. By introducing bubbles through the foaming process, sand specimens were able to be grouted to various degrees of cementation. A micrograph showing cemented particles surrounded by void space can be seen below in Figure 3. For large scale remediation projects, the amount saved by not completely filling the void space can be significant (Ali, 2009).


Figure 3: Micrograph showing open pores and pendular elements formed between Ottawa 20-30 sand particles (Ali, 2009)

 

 

Case Study

Sinkhole remediation in Hillsborough Florida (McGillivray, 2012)

In 2003 a sand/cement/foam grout was used by McGillivray et al. to successfully treat sinkholes in Hillsborough Florida. The foam used for this project was a synthetic material which came in a concentrate form. Using a foam generator, shown in Figure 4, a foaming agent with small stable bubbles was created and dispensed directly into mixing trucks, shown in Figure 5. The mixing trucks, which originally contained a partial load of sand/cement/fly ash grout mixes the foaming agent with the grout mixture to create a pre-formed foam grout, shown in Figure 6. For this application, the grout was only required to be slightly stronger than the surrounding soil so a target strength of 3 MPa was used. Laboratory compressive strength tests of the foam grout used for the project had an average strength of 3 MPa with a standard deviation of 0.6 Mpa. Through experimentation it was shown that a 60% to 40% grout/foam mixture would result in a 20 to 25% savings on cost for a typical sinkhole remediation project compared to using traditional grouts.


Figure 4: Foam Generator Setup (McGillivray, 2012)

Figure 5: Foam being added to mixing truck (McGillivray, 2012)

Figure 6: Foam grout after pumping (McGillivray, 2012)

 

 

 

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