The Engineering Behind the RPS Tapered Adhesive Joint

RPS Composites began fabricating the Tapered Adhesive joint (TA joint) in the 1960s as a way to improve productivity over butt & wrap joining both in the shop and in the field. (If you missed reading the commercial argument for using the TA joint, you can read it here).

From the beginning the engineering team set out to design a joint that had a burst pressure of at least 10x the rated pressure of the joined pipe, which was, and still is 150psi for all RPS standard piping systems. Special attention was paid to the tapering angle because of how this affected the area of adhesion, which in turn affected the joint strength.


Since those early days, the tapered adhesive bond has been used in hundreds of installations, but some end users have been reluctant to utilize it, as their personal operating experience has been limited to the butt & wrap joint.

In an effort to qualify and quantify the performance of the TA joint, RPS conducts burst tests and makes these results publicly available. In 2012, six samples of the P-150 and six of the A-150 were subjected to burst tests according to ASTM D1599-99(11). The P-150 is custom corrosion resistant FRP pipe; the A-150 series is erosion-corrosion resistant and is often used in more aggressive service conditions.

Each pipe was pressurized to failure, and each far exceeded the 10x the rated pressure of the pipe, or 1500psi. The mean failure pressure of 6” diameter RPS P-150 Tapered Adhesive joints was 2308 psi. The mean failure pressure of 6” diameter RPS A-150 Tapered Adhesive joints was 2317 psi. These test results demonstrate a significant margin of safety.

The TA joint has also proven its abilities in aggressive environments. One of the major applications for FRP pipe in the chemical industry is HCl service. Depending on the HCl concentration and temperature, the piping is either fabricated from a bisphenol A epoxy vinyl ester or an epoxy novolac vinyl ester resin. The pipe and fittings include either a 0.11″ or 0.20″ thick corrosion barrier. Many users have concerns that adhesive joints may be more readily attacked in hot concentrated HCl service, so RPS contracted Dow Chemical Co to provide detailed and controlled data on the comparison of tensile strength retention of novolac vinyl ester joints in hot HCl using a modification of ASTM D-638.

RPS made panels that represented a) typical hand lay-up laminates, b) secondary bonded joints typical of a tapered butt joint and c) Tapered Adhesive joints. These panels were immersed in three combinations of temperature and concentration of HCl at Dow’s laboratories in Freeport, Texas. At intervals of 1, 3, 6 & 12 months, panels were extracted, coupons cut and broken in tension. Dow assessed the validity of the data points and reported:

The main conclusion to be drawn from this data set is the good strength retention of the tapered adhesive samples compared to both the base laminate and the secondary bonds. Basing design values on the joint strength, Tapered Adhesive joint systems can be designed knowing that the joint will retain that strength better than the base laminate and that after a short period of time, the system strength will be governed by the laminate itself.

Clearly the RPS Tapered Adhesive bonded joint provides the same chemical resistance as a laminated butt & wrap joint. This makes sense, since the TA joint is designed with an overlap of the corrosion barriers so the liner is continuous throughout the piping system, the adhesive is formulated from the same vinyl ester resin used in construction of the liner, and the matched taper angle of the bell and spigot results in a very thin glue line reducing the chance of stress cracking, as may be experienced with resin putties used in butt joints.

End User Benefits

Design and testing give customers confidence in using the TA joint and since it is a fraction of the cost of the butt & wrap joint, it has become the clear choice in many environments. The benefits of the TA joint are numerous:

  • Continuous corrosion liner throughout pipe, fittings and joints
  • Hydro burst pressures 1500+ psi
  • A variety of resin choices
  • Fast, accurate field tapers (3” taper in 15 seconds)
  • Butt & wrap joints are labor intensive and add considerable time to your projected schedule (i.e., fabrication and curing of a 4″ dia. pipe requires 284 minutes, whereas a TA joint requires only 94 minutes)
  • Tapering tool is vacuum dust controlled and is available for purchase or rent
  • TA joints can easily be made in the field by maintenance departments, unlike the butt & wrap joints which require specific training
  • Pre-measured kits increase ease of use, eliminate the time required to accurately measure and possible measuring errors, also eliminate left-over materials
  • Heat cured joints can be placed into service immediately, no waiting
  • Labor and materials costs are considerably lower than butt & wrap joints

On your next project, specify the safety and reliability of RPS Composites’ 0.10” lined Tapered Adhesive joint system – available in corrosion resistant P-150, high temperature corrosion resistant H-150, and erosion/corrosion resistant A-150.


Download and/or print the TA Joint information bulletin here (including cost and time comparisons)

Download and/or print the RPS Piping Joining Systems comparison chart here.