Concrete Pipe vs. Flexible Pipe
What’s the BIG DEAL? Pipe is pipe.
The “BIG DEAL” is that when flexible pipe is improperly installed and backfilled, it deflects beyond the limitations of its physical properties. This deflection causes the flexible plastic pipe to lose its longevity. In extreme cases, the pipe may deflect until the joints begin to pull apart. When any of this happens, any up-front savings are forfeited.
In today’s market, there is increasing pressure to save money on construction projects. Owners are requesting products they know little about, yet they have heard are cheaper. This puts design engineers in a precarious position. There is no measurable cost difference between concrete pipe and plastic pipe. The claims of costs savings with plastic pipe exist only when the manufacturer’s guidelines for proper plastic pipe installation are ignored.
If installation guidelines are followed, it will cost a great deal of money to properly design and adequately compact large quantities of aggregate backfill required for plastic pipe. The additional aggregate backfill required adds significant material costs compared with a concrete pipe installation. It also adds substantial manual labor costs to the project. The only way to save money when comparing the two materials is to install flexible pipe as if it were concrete pipe. When one hears, “Plastic pipe is cheaper,” think of it as code for shoddy installation practices. It encompasses lack of design, lack of soils tests, lack of proper backfill, improper compaction, and no post-installation testing.
So, how should flexible pipe be installed? First, a proper flexible pipe installation requires rigorous soils testing. The design engineer will use these soil borings to create a unique trench design for each soil type, trench depth, and pipe size, working through 21 different equations for each scenario. Assuming the soil boring analysis has been performed and the engineer has created a stamped design, the burden to create the structure now becomes the contractor’s responsibility.
In the field, proper trench widths must be excavated. Significant amounts of aggregate backfill must be used. The thickness of this aggregate backfill must mimic the design engineer’s trench detail. Aggregate must be placed and compacted in lifts. The aggregate backfill must also be knifed into the ribs of the flexible pipe. Small equipment is required to place aggregate backfill to avoid deflecting the flexible pipe beyond the limits of its physical properties. Vibratory compactors attached to excavators may not be used to compact aggregate backfill. The contractor must replace large equipment with jumping jacks or smaller, more labor-intensive tools to hand-compact the aggregate backfill.
The structural test for concrete pipe is the D-load test performed at the plant prior to shipment. During a D-load test, concrete pipe is placed under a hydraulic press, and the force necessary to create a .01” crack is measured. ASTM C76 stipulates the minimum D-load values RCP must pass. This value is expressed in pounds per foot of pipe, per foot of diameter. Because of this, we know every pipe we deliver meets or exceeds LRFD HL93 live load requirements.
Different test methods are required for plastic pipe, because it relies upon a field-created compacted aggregate structure. Laser profiling, or an AASHTO Section 30-approved 9-arm mandrel (or greater) provide the owner viable test methods to check deflection. All pipe that deflects beyond 7.5% needs to be removed and replaced. Pipe that deflects beyond 5% requires an engineer’s evaluation. Mandrels must be sized and inspected by the Project Engineer prior to testing. Testing cannot be conducted until the pipe has been installed for at least 30 days. Nearly all of the strength of the flexible pipe structure is created by the trench workman in the field using aggregates. Every foot of this labor intensive backfill is variably compacted. In contrast, concrete pipe is a constant structure. It is machine made, with every pipe consistently produced the same according to ASTM standards.
When all the necessary protocols surrounding thermoplastic pipe are followed, it is never cheaper to specify flexible pipe. In addition, concrete pipe will not burn during a grass fire and will not float. Furthermore, concrete pipe does not require a water-free trench for installation. And, it lasts a lifetime. Concrete pipe has been proven to last 100 years or longer, delivering the best performance and lowest lifecycle cost. That’s why, in today’s cost-conscious construction environment, there’s no better option than reinforced concrete pipe.