Cellular Concrete Annular Space Grouting
Cellular concrete is an excellent material for annular space grouting, and the most common grout CJGeo uses for the purpose. Whether an annulus is just an inch or a few feet, and regardless of host pipe size, cellular concrete is an economical, reliable annular space grouting material. CJGeo has grouted dozens of annular spaces on welded HDPE, corrugated HDPE, mechanically-joined HDPE, concrete, spiral wound, corrugated metal and DI pipes throughout the Eastern United States.
Mix Design for Annular Space Grouting
The typical cellular concrete mix design is 30PCF wet cast density. This yields an average compressive strength of 125psi at 28 days. Each cubic yard in place contains 512 pounds of Portland cement, 256 pounds of water, and 20.3 cubic feet of preformed Aerlite foam. In situations where higher compressive strengths are required, higher density cellular concrete may be used. If exceptionally high densities are needed, sanded mixes can be used, however sanded cellular concrete mixes don’t flow as well as neat mixes, which can require additional placement steps.
For higher strengths, 68PCF cellular concrete achieves 1100psi, so is frequently used when 1000psi is required for the annular space grout. It should be noted, however, that high strength isn’t usually required for annular space grouting.
Wet cast density is important for both strength and buoyancy control. Increasing strength requires increasing density, which is accompanied by increases in buoyant forces on the carrier pipe. When specifying a grout mix, the lowest density possible helps to ensure the lowest risk during grouting.
Placement Method for Annular Grouting
In all the but the rarest cases, CJGeo performs all pipe grouting by pumping. While some contractors use gravity placement, pumping ensures uniform material and a complete fill of the annulus. Our continuous generation equipment has integrated pumps so there’s no need for additional equipment. Cellular concrete requires far fewer injection points than traditional grouts.
Cellular concrete is economical for this type of grouting because the majority of the mix is atmospheric air (which is free). Because cellular concrete can be easily pumped long distances, it is easy to install in hard to access locations, very tight annuli and over long stretches.
The lightweight mixes used for pipe grouting generally have expansion ratios of greater than 3 times. The high expansion ratio greatly reduces the number of ready mix truck delivery trips. This reduces job site congestion and allows other trades to work uninterrupted. For applications adjacent to roadways, it can significantly reduce traffic control expenses.
Low Installation Pressure
Installation pressures, even in tight annuluses with long runs are typically below 10psi. As mechanical joint flexible relining pipes become more popular, the need for low pressure grouting has grown. Cellular concrete easily meets the challenge. Using cellular concrete greatly reduces the risk of collapsing or damaging carrier pipes compared to using flowable fill.
Carrier pipe buoyancy during grouting is an often over-looked, but very important concern. Using low density cellular concrete significantly reduces buoyancy of carrier pipes. To completely mitigate buoyancy, some pipes require partial filling with water to compensation for buoyancy. Grouting in lifts adds time to a job, but is sometimes required for reducing buoyancy.
Buoyancy is also controllable using blocking, spiders or centralizers. Enterable pipes usually facilitate installing blocking prior to sliplining or carrier placement. Non-enterable pipes require spiders or centralizers to counteract buoyancy if water filling isn’t possible.
Cellular Concrete Downsides
One of the beauties of cellular concrete is its ability to flow exceptionally far with minimal pumping pressure. This is usually an excellent characteristic; it can also be a problem in environmentally sensitive areas, or where watertight bulk heading isn’t possible. Also, if an annulus can’t be dewatered, if there’s large amounts of groundwater infiltration into the annulus, or the annular space volume is very small, or carrier pipe buoyancy is difficult to control, cellular concrete isn’t the most economical or practical grout for an annular space; polyurethane is often the better choice in these situations.
Polyurethane Annular Space Grouting
When performing annular space grouting with polyurethane, it’s very important to use a low exotherm undersealing polyurethane grout. Polyurethane grouts designed for lifting will easily crush carrier pipes, as they can exert up to 30psi of expansive force. Low exotherm undersealing grouts, such as NCFI 27-004 are formulated to gently flow after expansion is completed, and do not get so hot they soften HDPE and other plastic carrier pipes.
CJGeo has grouted around solid wall HDPE, corrugated HDPE, CMP, SSP & other structures using polyurethane grout. Because the grout cures very quickly (just a few minutes), leaks into sensitive environments aren’t a concern, the grout is not washed out by water infiltration, and logistically it’s very easy, in that a single box truck can carry up to 200CY of in-place product.
Ideal Situations for Polyurethane Grouting
- Short Runs
- Non-enterable pipes are usually limited to about 80lf when grouting with polyurethane. Because polyurethane grouts cure very quickly, they typically can’t be pumped long distances through sacrificial grout pipes like cellular concrete can.
- Non-Dewatered Annuluses
- When it’s impossible to dewater an annulus, particularly in man-enterable pipes, polyurethane grouting is ideal. The annular space grouting is just like performing back grouting & sealing of culverts. Because polyurethane grouts react & set quickly, they seal off water intrusion, and displace water without requiring anti-washout additives like cement grouts need.
- Remote Locations
- The smallest of CJGeo’s polyurethane grouting rigs carries 125CY of in-place material. So, a single box truck can arrive at a site and place more than the equivalent of 12 concrete trucks. For isolated sites, it’s much easier to get a single truck in than a pump truck, support trucks, and 12 ready mix trucks. When CJGeo grouted the Pinkerton Tunnel in Pennsylvania, only 2 semi loads of polyurethane grout had to be delivered to the site, and were turned into 2200CY of in-place material.