In keeping with this month’s theme of questions we need to ask ourselves as professionals in the course of our work, this post addresses a question that’s currently being debated in the industry: the necessity of structural liners adhering to the host pipe in repair and rehabilitation projects.
There is one school of thought currently floating about that proposes the assumption of 100% adhesion of the liner product to the host pipe or manhole, in order to be considered “structural.” Here at Sprayroq, we couldn’t be more strongly opposed to this view, and here’s why:
The industry standard for structural soundness in manholes and water / wastewater / stormwater pipe is the factor of “external buckling analysis.” This refers to force on the structures, induced primarily by hydrostatic water pressure from around the structure.
A Basis in Fact
The formulas used to determine external buckling strength are standard engineering algorithms. They have been known since Archimedes’ time, and used successfully to calculate structural liner thicknesses since liners became available, regardless the material used to make them. These equations have been proven effective and accepted for decades, and are referenced in the ASTM standards for such products and applications.
One of the components of these algorithms is the assumption of zero adhesion of the liner to the substrate of the structure. The reason is that, once a liner made of any material has been applied, there is no economically feasible way to ascertain whether there is any adhesion at all, and if so, how much of the structure is actually adhered to. Short of hammer-tapping each part of an installation, there’s no way to measure that.
Because engineering is a science that deals in facts and not conjecture, anything that can’t be empirically measured is not a fact. And because people’s lives and safety depend upon the veracity of the facts about the elements of this algorithm, anything that is not a proven fact cannot responsibly be assumed.
Strong Standards, Strong Structures
Might adhesion actually be happening in any liner installation? Of course, even probably. But for safety’s sake, under current standards that have ensured long-term product performance and user safety for decades, any adhesion that does occur simply enhances the design performance. It’s still not required for structural performance and stability.
So, when some parties consistently float the idea that liner adhesion of any percentage should be required to deem a liner “structural” in nature, we just have to scratch our heads.
Our SprayWall product, whose installation thickness design is formulated with the assumption of zero adhesion to the host structure, has an unblemished record of performance, with not a single latent structural failure. We can’t help but ask: If it ain’t broke, why try fixing it? You really do have to question the motivation behind those leading the charge to replace current, conservative, proven safe standards with more liberal, lax requirements.
Not only would this be (perhaps criminally) irresponsible, there’s an excellent real-life precedent to show what happens when such a misguided effort succeeds. The American Water Works Association (AWWA)—the trade organization that issues standards for pipe installation and maintenance—has in fact run into this very scenario before, and…well, let’s just say it didn’t end well for anyone involved.
We’ve seen it all before.
The AWWA has issued standards AWWC-301 and -303 for the manufacture of concrete pipes. 301 applies to pre-stressed concrete cylinder pipe (PCCP), dictating the design, including type of concrete and amount of steel reinforcement in its construction. This reinforcement comes in the form of a steel cylinder and wire wrapping, which provides additional springiness or tension to withstand hydrostatic compression from without, and the force pressurization inside the pipe.
The C-301 committee formulated the correct ratios of steel reinforcement to concrete, and that measurement held up for decades. Then in the 1970s, manufacturers found a way to make ductile iron pipes up to 54” in diameter. These are inherently stronger than unreinforced concrete pipes, and the committee approved their use for this purpose; but to be price competitive, manufacturers began cutting down the amount of steel used in their pipe, and at some point, it became weaker than that originally approved.
Even the best made concrete pipes have incipient cracking, which over time will allow water to seep in and rust the interior steel pipe. Water hammer will also vibrate the pipe to the point where the embedded steel reinforcement will explode, and that began to happen with the weaker, lower quality pipes.
Of course, this led to many pipe failures and it gave the entire industry a black eye, even forcing some of the concrete pipe manufacturers out of business. It remains a serious problem today with pipes laid in that era, and there have been many lawsuits against the manufacturers. And all because there was pressure from one segment of the industry to weaken manufacturing standards.
A Re-Run No One Wants To Watch
This could happen to structural rehabilitation liner manufacturers if we begin fooling with established norms that we know for a fact result in sound, long-term solutions with reliable performance. So, regardless of industry debate, assuming ANY amount of adhesion between structural rehab liners and host pipes — much less 100% — is simply foolhardy. And weakening standards that we know are sound is reckless.
The pipes we’re working on are not in a controlled laboratory test environment. These installations are in actual, working sewer pipes. We can clean them and prepare them as thoroughly as possible, but when the installation is finished, they’re subject to pressure from without and within; to deleterious gases; to vibration from traffic and seismic movement; and endless freeze/thaw cycles.
Even if an application begins as 100% adhered to a substrate, it’s unlikely to remain that way throughout the life of the liner. To assume it will is nothing short of irresponsible.
Choices have consequences, and we must make wise ones to maintain the track record of positive performance from concrete pipes that we have enjoyed for so long in this industry.