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During a recent Contech reline webinar and following question and answer session, I realized there were several great questions asked about Reline Grouting. It made sense for me to address some of those here, hoping the responses provided might prove helpful to an extended audience. Hopefully, you agree!




    Cellular grout is becoming more and more popular, and the geographical areas of availability is ever expanding. Many cementitious and polyurethane grouts will work as long as they are able to be placed effectively. Strength-wise, the minimum 28 day compressive strength offered by cellular grouts (200 psi and up) is usually plenty. That strength level is easily achieved by conventional mixes. Keep in mind, the purpose of the grout is to fill the void between old and new and to provide similar load transfer characteristics as conventional structural backfill. We prefer to see slumps greater than 10” for easy flowability. Some reline products essentially act as a grout form and rely on a structural grout with a compressive strength requirement of 5000 psi or higher, as designed. All of Contech’s reline offerings work with a 200 psi cellular grout except in rare situations. It should be noted that exposed ends in areas subject to freeze-thaw, or in situations where significant impact loads from debris are expected, might warrant a structural concrete capping of the grout at the upstream and downstream ends. Designing said cap should follow procedures used for designing a normal headwall. Regarding the minimum annular space, there are practical minimum limits depending on grout placement locations, length, slope and the presence of water. Generally, the more the space the better, but a new liner can touch the host pipe in spots and grout can still fill all voids if proper decisions are made on mix design and placement techniques. This needs to be examined on each particular application. In some situations and when the hydraulics of the end condition warrant, a 3” minimum space should be the goal. Don’t panic if that isn’t possible. If the host pipe or the new pipe has a corrugation or open profile then fluid grout can move into tight spots from below or above, if the slump and mix design is right. Many reline applications use 2” diameter grout ports, hoses and grout tubes as the ideal size. Although I didn’t witness it, I’ve had a credible source describe the successful use of ¾” copper grout tubes that had been ovalized to give them a shorter height. Certainly the longer the pumping distance the greater the potential for the foaming agents in cellular grouts to break down. The mix design needs to consider the placement challenges such as pumping distance, size of grout tubes, void space, expected temperatures, number of lifts, lift volumes, and overall grouting plan. Grouting contractors and the manufacturers of cellular grouting equipment and the foaming agents should be consulted if less than 3” of space is expected, or other unusual conditions will be encountered. Aerix Industries is a leader in this area.

    Buckling pressure during grouting is one set of loading circumstances that should be considered, but not the only one. It can develop in different ways and is often misunderstood. Contech is occasionally asked what the buckling pressure is for a sliplining pipe product, and our first response is, “What will your grouting process look like?”. If the new pipe is to be floated to the top of the host and then grouted evenly and all at once, buckling pressure is an important aspect. The pressure may be from the bottom, it may be at the grout port being used, or it may be at the downstream end where fluid pressures could be the greatest. One should distinguish between collapse pressures and local buckling pressures.

    What we often provide is a pressure that generates 1% or 2% deflection, but that pressure is rarely seen. Instead, we lobby for letting gravity move the grout during all but the last lift. A grout that is very fluid (10” or more for slump) is preferred. We also look at buoyant loads that must be countered in order to maintain the position of the new pipe (for drainage pipes the invert grade and elevations are usually trying to be preserved so floating to the crown of the host isn’t an option).

    As a rule of thumb we don’t like to see live grouting pressures (during pumping) at the nozzle greater than 5 psi. At the pump it can be higher but keep in mind the pressure level anywhere other than that felt by the new pipe, can be unimportant. Also, many grout pumps don’t have a pressure.. For any reline situation involving grout, allowable buckling (or deflection pressure) is only part of the set of limits to be evaluated.


    There’s no short answer, but it’s not as difficult as it might seem. It depends on the type of pipe, the diameter, the size of the void space, the slope, the length, and the design of the new liner. Generally, cellular grout or conventional cementitious grout is pumped via a grout pump through grout tubes in the bulkheads that potentially run longitudinally along the crown of the host pipe to strategically determined distances, or through grout ports that are built into the wall of the new liner pipe. Each situation requires a specific plan that addresses grout material, bulkhead construction, grout placement techniques, buoyant load management, and quality controls. An experienced grouting contractor is a terrific resource for the fine details, but owners and engineers should have a good understanding of the grout plan so they can be satisfied there won’t be any ‘uh-oh’ items to deal with after the work is done. Some questions to ask yourself as you review a grouting plan: Are the bulkheads temporary or are they part of the final product? Will the mix design and the proposed methods allow the grout to reach the furthest and tightest points? Will any grouting hardware attached to the new pipe be left in place, and what is the impact?

Hopefully these provided some clarity to concerns and possible options you were considering. Any other questions? Please free to leave a comment, and I will make certain to respond!


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