Comparing One-part vs Two-part Chemical Grout for Sewer Repair
We are often asked to discuss the differences between using one-part and two-part chemical grout for manhole and lateral line repair. Obviously, we are a bit biased when it comes to using polyurethane (two-part) vs permeation (one-part) grout, however we are a provider of both, but only in the right applications. That being said, we understand grout and we understand that each grout has its strengths and weaknesses and are situationally appropriate. This article will discuss the advantages and disadvantages of using both for the purpose of sewer repair. In order to address the applicative potential of permeation and polyurethane grouts, we look at each aspect of the polymers, from chemical characteristics to installation requirements to costs.
Chemical Characteristics and Definitions
Permeation grout is a direct pressure injection of a chemical fluid grout to fill spaces between soil particles without causing excessive movement or fracturing of the soil formation. The one-part grout travels through a soil profile to permeate and densify the contact soil mass with a slight expansion of 2-3% of initial volume. The permeation grout binds as an open cell formation.
Polyurethane grout is a hydro-insensitive plural component grout that expands as a consequence of a chemical reaction. The two-part grout sets and alters the physical properties of a geologic mass, traveling through a soil profile initially by permeation and then by densification due to an expansive exothermic reaction. The polyurethane grout binds as a closed cell formation and has expansive qualities of 15 to 22 times initial volume.
Chemical Characteristics of Permeation and Polyurethane Grouts Comparison
|Components||One-Part Permeation Grout||Two-Part Polyurethane Grout|
|Water Reactive Qualities||Hydrophobic, Hydrophilic||Hydro-Insensitive|
|Soil Densification Process||Permeation and Slight Expansion||Permeation, Exothermic Expansion of Closed Cell Formation|
|Expansive Qualities||2-3% Expansion||1500 – 2200% Expansion|
|Cellular Aspects||Open- Cell||Closed-Cell|
|Cure Time||Variable depending on moisture and catalyst||15 minutes to 90% Density
24 Hours to 100% Density
|Compressive Strength||N/A Free Rise
1,812 PSI (confined with Sand)
|30 to 90 PSA Free Rise
<8000 PSI (confined with Soil)
Permeation grout utilizes a box truck and airless sprayer to deliver the grout with a wheeled gas generator. The product is pumped from a drum/pail and requires available moisture and a catalyst to cure. The one-part grout is injected through 5/8” diameter steel tubing at a spacing of 3.0’ to 6.0’ centers.
The steel tubing is driven to the lowest targeted (termination) depth in a 90% vertical alignment. As injection begins, the tubing is extracted during the injection and may be reused. The first and final injections are at 1.5 gallons and intermediate injections are at 1.0 gallon per foot. Permeation grouting requires no monitoring for movement due to its low expansive qualities.
Polyurethane grout utilizes a fully self-contained injection rig with a 60kW diesel generator set, a 5hp 23 CFM air compressor and a refrigerated dryer/regulation system. The systems use double diaphragm feed pumps and an impingement-style gun.
The ½” diameter steel tubing is driven down to discrete injection elevations. The multiple injection elevations result in stacked spheres of polymer with the tubing being sacrificial upon completion of injection. The tubing can be angled for void fill or centerline foundation repairs. Due to the highly expansive qualities of the two-part chemical grout, monitoring for movement is required using a laser level.
|Permeation Grout||Polyurethane Grout|
|Installation Method||Extracting Tube||Discrete Point Injection|
|Angled Injection Capable||No||Yes|
|Injection Spacing||3.0’ to 6.0’||3.0’ to 6.0’|
|Water Sensitivity||Requires the right mixture of moisture and catalyst. Too much water can result in application not to cure.
|Hydro-insensitive, is not affected by water|
5 Gallon Drums
|Self-contained Injection Rig
60kW diesel generator
CFM air compressor
Double diaphragm feed pumps
What about Costs?
When it comes to cost, there really isn’t a comparison between permeation and polyurethane grout. This is for a variety of reasons, but the primary reason is that permeation grout takes nearly 200 – 400% more material than polyurethane grout, resulting in double to triple the cost. Other aspects have to do with the multiple methods for installing polyurethane grout, whether through lateral injections or in highly saturated environments.
Advantages of Permeation vs Polyurethane Grouting
Permeation grouting has a couple of advantages over polyurethane. The major advantage is that permeation grout can be used in fine sands and silty environments. This is because permeation is the primary method in which the permeation grout is utilized (hence the name), allowing the grout to saturate the layer and the time to saturate throughout the soils. Polyurethane grout is dependent on its expansion as a method for densification. Due to polyurethane’s limited ability to permeate fine sands and silts, the expansion pushes sandy soil layers opposed to binding, compression and stabilization. This limits polyurethane’s ability to densify in these environments.
The limited expansion of permeation grout can avoid over-lifting issues that polyurethane may cause. If polyurethane is overshot, then the polymer can damage sensitive structures, such as brick manholes or dilapidated CMP. This being said, there are several examples in which two-part grouts are used to both stabilize and re-grout compromised brick manholes and CMP, so perhaps this is a matter of contractor expertise
Soil density and PSI are more consistent with permeation grout. This allows designers to provide a more consistent and predictable pressure in the treated area. Predictability is important to those that need to meet specific can be important depending on the situation. This an additional benefit when using permeation grout for tunneling and excavation, as the concentration of the grout is highly predictable and allows for the area of influence to be calculable when trying to design an excavation zone that is properly shored.
Advantages of Polyurethane vs Permeation Grouting
Firstly (and to some most importantly), polyurethane is by far a more cost-effective option and has a great deal more adaptability than permeation grout. Although polyurethane is less effective in highly plastic and fine soils, it is highly effective in aggregates, soils with low granularity and where voids are present. In these environments, polyurethane can provide higher pressure and compressibility than permeation grout.
Polyurethane is also not as sensitive as permeation grout in regards to soil conditions. It is not dependent on moisture and is still effective in highly saturated environments. Additionally, polyurethane’s cure time is fast, setting up to 90% density in 15 minutes whereas permeation grout requires approximately 24 hours in optimal situations.
Adaptability is a major advantage of polyurethane. While permeation grout requires a vertical column in order to install, polyurethane can be shot both vertically and horizontally. This allows for injections to occur from inside of the lateral line, manhole or culvert. Additionally, the permeation and expansive stages of polyurethane help the polymer seek out any present voids and allow it to travel along structures to encapsulate and seal joint separation, voids around the structure and/or loose soils and aggregate. Also, polyurethane can be used to lift and realign structures based on specifically designed injection patterns that lead to upward or lateral movement.
Weight is another area where polyurethane has an advantage over permeation grout. The primary cause of this is due to the high expansion and close-celled nature of polyurethane. Permeation grout works by densifying the soil layer with high saturation, requiring a great deal of product to rehabilitate a single soil layer. The weight differences can range from 2-300% more than polyurethane. With this being said, it really depends on the weights of polyurethane that is being used as there are variations of 2lb all the way to 8lb polymers that are uniquely designed for the application.
Scorecard – Permeation vs Polyurethane Grout
|Permeation Grout||Two-Part Chemical Grout|
|Sandy and Silty Soils||Winner|
|Adaptability of Installation||Winner|
|Wet or Saturated Environments||Winner|
|Risk of Structural Damage due to expansion||Winner|
|Highly Granular Soils||Winner|
|Lifting and Leveling||Winner|
While on paper Two-Part Chemical Grout has more advantages than permeation grout, these advantages are still situational and to the preference of the owner/designer. The importance of these factors are only relatable to the needs of the project and to the value placed by the decision makers. Polyurethane is still a relatively new product being used for sewer rehabilitation applications, but it offers promising results when the benefits are understood and when the right contractor is involved in the design and installation. Also to note, both permeation and polyurethane grout have a wide range of formulations and installation techniques that may overcome some of their weaknesses.