In an effort to chase one of the better, cheaper and faster elements of any project we’re doing, curing is the slowest part of the job. Waiting for a liner to cure is similar to watching grass grow or paint dry. Each resin used in the CIPP lateral lining process has a work time and a cure time. Curing begins as soon as you mix the components of the resin. The clock begins at that time and depending on exposure to UV light and the temperature, the cure time can range from minutes to hours, sometimes many.
It’s worth noting that if you’re using a “30-minute” resin, it can be cured in 45 minutes if the temperature is raised to 52C. If the ground is 13C or less, the cure time is going to jump to seven hours or more. Just 3C change from 16C to 13C can add three hours to the cure time. Each type of resin you get from a supplier should be accompanied by a cure schedule. If it’s not printed on the container, ask for the printed version. It’s important for you to know the estimated schedule rather than wait and worry.
So, you wonder after consulting a cure time chart, why anyone would want to do an ambient cure if you can cut the time from seven hours or more to 45 minutes with a particular resin system. The answer is equipment. If you have a one-off, or if you seldom install very long liners, it’s beyond your standard equipment capacity to heat it. It makes more sense to leave a tech watching the liner and tending the compressor to make sure the line stays inflated while you carry out other work for the rest of the day.
When we talk about a 45-minute cure at 52C, we’re actually talking about the liner having the ability to stand on its own without collapsing on itself. It takes four full hours at 82C for most resins to reach full cure – similar to a concrete highway where concrete takes 28 full days of curing before traffic is allowed to drive on it unless special concrete formulas are used. When you leave after the liner is standing, but not fully cured, you may see the liner modulus not reaching a full cure for several months, but it will carry the loads and perform during this curing process.
When estimating cure time in ambient conditions, it can mean the actual temperature the full length of the liner is only guesswork, unless temperature monitors are placed every few feet down the liner. A typical failure we have is buckling due to a part of the liner not being cured while the rest of the liner is cured and groundwater is the most obvious cause of this type of failure. Groundwater is often 5C to 10C colder than the surrounding ground. So, if your ground temperature in that area is much colder than expected, the liner must be held under inflation to the coldest temperature, not the average temperature, and it’s always best to wait for that extra time, knowing that it’s better to be safe than sorry in this situation.