From the Laboratory’s Perspective: CIPP Test Samples

Testing is part of doing business in the construction industry. From soil to concrete to cured-in-place pipe (CIPP) — the list goes on.

Most sample preparation is done by the engineering or testing firm hired by the owner to monitor the project. CIPP tends to be different — it is usually the contractor’s responsibility to prepare these samples for laboratory testing. The results from the testing are of great importance to the contractor due to the fact that it impacts the project’s acceptance and his pay. It is vital that the people responsible for making these samples are adequately trained in the sample’s proper preparation to deliver a good and representative sample of the finished CIPP.

Below I will discuss the primary sample types we see for CIPP and the major items that can impact the quality of each sample type.

There are three types of samples that I have dealt with in our laboratory. They are flat plate, restrained and unrestrained samples. Plate and restrained samples are the two primary types. From my experience, plate samples tend to produce the highest physical property results followed by restrained and then unrestrained samples.

Plate Samples

A plate sample is ideally made by extracting a sample from the tube it represents after the tube has passed the calibration roller during the saturation (wet-out) process. More often, however, the plate sample is made by the saturation technician cutting pieces of felt to make up a representative sample and saturating it “off-line.” The saturated sample then is encapsulated in a plastic bag and clamped into a metal mold (two flat plates). The plate is placed inside the tube during the installation process and is exposed to curing medium (hot water or hot air). The main problems that could potentially happen with these samples are the plate is bowed or not well secured. Also during the sample’s construction, either an insufficient amount of resin is used or air is introduced between the felt layers, which causes the layers to act independently.

Restrained Samples

A restrained sample is made by letting the bag extend at either the upstream or downstream manhole through another piece of pipe (which has the same I.D. as host pipe) placed in the manhole. The host sample pipe should be kept level and surrounded by a heat sink such as sand bags. Also, the sample tube length must be long enough for the minimum length required in the testing — typically, 16 times the thickness of the CIPP plus enough length to account for issues at the entrance and exit of the sample mold.
The installation crew needs to have an indication where the full saturation stopped and the resin run-out began. One big issue I have seen when this is not well communicated is that the tube material is resin-starved. Excessive inversion pressure also can force resin from the felt matrix of the tube either by it being squeezed or just not enough liner with full saturation being pulled through the restraining pipe; it leads to a bad sample sent to the laboratory resulting in lower than normal or failing test results.

One thing that is helpful, particularly for a down stream sample, is a heat sink. This can be achieved by placing sand bags around the restraining pipe to more closely represent the pipe being in the ground. If not, you can have a variation in temperature between the top and bottom halves of the sample, which could give you two different results due to loss of heat during the cure process.

One of the things to look for when a sample is cut to send to the laboratory is to look at the cut line closely. It you have a colored resin and you see a lot of white spots at the edge you have some lack of resin in the sample. If you have a fairly large starvation of resin, the sample will have a lot of felt fibers showing along the cut line. When you have good saturation and curing it should look like a solid piece of plastic at any cut line.

Unrestrained Samples

Occasionally an unrestrained sample will be taken due to lack of a restraining pipe or a very large diameter pipe that at the last minute was decided to be tested for some reason. Since there is no restraining pressure, the tube stretches excessively and the finished thickness is less. Also heat is lost at a much higher rate to the ambient air. The unrestrained area is very susceptible to “resin boiling” and thus the finished cross-section typically becomes very porous and non-representative of the actual CIPP. If a sample needs to be taken in such an area, we would recommend that you choose the location that has received the most restraint, say from the portion of the tube that was on the invert (bottom). When the sample is removed look at the cut lines the same as you should with a restrained sample. The appearance here should give you a reasonable idea if there will be decent results from the physical testing.

Additional Comments

In addition to the sample’s preparation, proper transportation and handling is key to ensuring that your sample makes it to the lab in a condition that reflects its insitu condition. Tossing it into the pickup bed, excessive prying of the sample to extract it from the mold and not shipping it by “fragile” type methods (i.e. using impact prohibiting Styrofoam peanuts or similar) can result in fine fractures that will diminish the samples’ physical properties.

The samples should be handled with proper care at all times. The key items with all the sample preparation is to take your time and pay attention to the material being placed in between the plates or in the host pipe. Always try to allow for enough wet-out area when you know testing will be done.

Doing these things will increase the chances that representative samples are being sent to the laboratory for testing.

Brian C. Sain is laboratory supervisor at Boyle Consulting Engineers, Charlotte, N.C.