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View Full Version : Thrust Block Debate (LONG... You might want to ignore.)


PurpHaze
11-21-2006, 12:32 PM
Found this on the LASCO support site. It even has part of the ASTM specifications in it.

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Thrust Blocking

The question occasionally rises, "Do solvent cemented systems need to be Thrust Blocked?" To answer this question we must first understand what thrust blocking is. Then we need to understand the differences in design between Solvent Joined, and Gasket (O-ring) systems.

The force of water within piping can be great enough to cause separation of certain types of joints. Thrust blocking is the procedure of placing a solid backing (such as concrete) behind a joint in order to hold the joint together. The joint is held in place once the pipe, joint and concrete is buried in the ground.

In a solvent cement system, the joined parts are cemented together. This means they are fused and/or bonded together into a self-restraining entity. The same happens when steel pipe and fittings are arc welded together. The American Society for Testing and Materials (ASTM) socket dimensions used in Schedule 40 and Schedule 80 fittings provide in excess of twice the socket depth necessary to constrain the thrust forces generated by the system pressures. Therefore, thrust blocking is not required on a solvent cement joint.

In a Gasket or O-ring joint system the parts are not cemented or fused together. They remain separate entities. An O-ring is required to halt seepage of water from the joint. Because they remain separate entities the only restraining forces that are available are friction and burial. However, neither of these forces is capable of counter balancing the force created by the internal hydraulic pressure of the system. It is for this reason that thrust blocking is required on a Gasket or O-ring system joint.

Some references to this subject are noted in the ASTM Standards* (D 2774 & F690).

ASTM D 2774 - Standard Practice for Underground Installation of Thermoplastic Pressure Piping.

5.3.1 Thrust transmitting joints such as heat fused or solvent cemented, should be capable of restraining maximum anticipated pipe pull-out forces which may be generated by internal pressure or pipe expansion/contraction, or both. Gasket and other non-thrust transmitting joints should be restrained by means of a properly engineered external restraints or joint restraint devices (see 7.3)

7.3 Thrust Restraint-- When installation piping systems with joints which cannot transmit the anticipated maximum longitudinal thrust, thrust restraint may be necessary at certain points in the system, such as changes in direction, or terminal ends, to prevent joint disengagement.

ASTM F 690 - Standard Practice for Underground Installation of Thermoplastic Pressure Piping Irrigation Systems

6.Thrust Blocking

6.1 When installing piping systems that include joints that are self restraining (for example, elastomeric seal type) thrust blocking may be necessary at certain points in the system, such as changes in direction, in order to prevent possible disengagement of the fitting from the pipe.

PurpHaze
11-21-2006, 12:46 PM
Since I'm currently working on updating our specifications and installation detail drawings I've now included: "In accordance with ASTM D-2774 and F-690 specifications PVC solvent weld fitting main lines are not required to have concrete thrust blocks installed at change of direction points."

If you do contract work for us you can put away the Redi-Crete when it comes to solvent weld installations. :)

Dirty Water
11-21-2006, 07:51 PM
Hayes, I've seen both a 3" line and a 4" line (both seeing around 100 psi) push out of fittings.

Both had been in the ground about 10 years, so I doubt it was faulty installation.

But perhaps it was.

PurpHaze
11-21-2006, 09:05 PM
Jon,

I'm not saying that it can't happen, just that in my opinion, based on experience and now backed up by those that know a whole lot more about the issue than I ever could, it's a waste of time and energy to routinely thrust block solvent weld fitted piping. There are many variables that can go wrong, some the installer has control over while others absolutely no control.

We have a middle school site (I've mentioned before) where we've replaced 25-30 fittings since it opened up about 13 years ago. I took one of the removed fittings (shortly after failures started showing up within six months of installation) and had a local college professor send it in for analysis as to what was going on. The analysis showed a chemical inferiority in the heavy bodied fast set gray PVC cement that was used during installation. Putting chemicals together is just like any other manufacturing problem. Sometimes doo-doo happens.

In another case, about five years ago we totally replaced the existing system for our main football stadium's field. I drew up the design and did a material take-off before heading out for a three week vacation in summer. The materials were gathered when I was on vacation and when I got back we started at 3:00 am with stadium lights in order to beat the summer heat. The large trencher they rented for the 4" main line (this was prior to the district purchasing the Bobcat) had too small of a chain on it so the trench wasn't as wide as I'd have liked. We had a problem with one joint (bell) where it looked like the pipe had pushed out a little. Instead of immediately replacing it I decided to take a chance on it but I remarked to the guys, "I'll probably be back in here replacing that bell." It lasted a year before the leak showed up. We coordinated its repair with re-sodding the field as to not disturb things too much. It was actually this issue that helped prompt the district into buying the Bobcat.