JUST THE FACTS CONCRETE PIPE VS. CSP

Transcription

1 JUST THE FACTS CONCRETE PIPE VS. CSP

2 Just the FACTS! BURIED PIPELINES PERFORMANCE AND DESIGN COMPARISON CONCRETE VS. CSP Contents Current Standards Manning n Deflections and Deformations Long Term Pipe Durability Joints Flotation Salvage Value of Pipe Materials

3 Just the FACTS! CURRENT STANDARDS In Ontario, design and construction standards used are generally a combination of the Ontario Provincial Standards (OPS) and Canadian Standards Association (CSA). For corrugated steel pipe, an American Society for Testing and Materials (ASTM) standard is also referenced for specific corrugation patterns. A review of the current standards related to the materials used in the manufacturing as well as the installation of corrugated steel pipe provides designers and specifiers with the requirements necessary to undertake project design. A COMPILATION OF THE CURRENT STANDARDS FOR CORRUGATED STEEL PIPE INCLUDES: OPSS 1801: Material Specification for Corrugated Steel Pipe Products this standard provides general guidance as to appropriate standards for materials and quality assurance and references other standards. CAN/CSA-G401-93: Corrugated Steel Pipe Products this standard applies to the material and fabrication of corrugated steel pipe and the uses of the products in various applications. Products from 200 mm to 3600 mm diameter are addressed. ASTM A760M90: Standard Specification for Corrugated Steel Pipe, Metallic-Coated for Sewers and Drains this standard covers requirements and test methods for corrugated steel pipe to be used for drainage. OPSS 1801 references this standard for spiral rib pipe corrugations. 2

4 A COMPILATION OF THE CURRENT STANDARDS, FOR CONCRETE PIPE PRODUCTS INCLUDES: OPSS 1820: Material Specification for Circular Concrete Pipe this standard provides general guidance as to appropriate standards for materials and quality assurance CAN/CSA-A257.1: Circular Concrete Culvert, Storm Drain, Sewer Pipe and Fittings this specification covers nonreinforced circular concrete pipe and fittings 100 mm to 900 mm in diameter CAN/CSA-A257.2: Reinforced Circular Concrete Culvert, Storm Drain, Sewer Pipe, and Fittings this specification covers reinforced circular concrete pipe and fittings 300 mm to 3600 mm in diameter CAN/CSA-A257.3: Joints for Circular Concrete Sewer and Culvert Pipe, Manhole Sections, and Fittings Using Rubber Gaskets this specification covers the design and performance requirements for flexible watertight joints Designers and specifiers are encouraged to become familiar with these standards prior to undertaking project design. This is required to ensure the drainage product specified is appropriate for the project envisioned. 3

5 Just the FACTS! MANNING n Concrete Pipe-Smooth Interior Wall Laboratory tests on corrugated steel pipe reflect a wide variation of the coefficient of roughness due to the variety of corrugated steel products. Selection of the proper value for the coefficient of roughness of a pipe is essential in evaluating the hydraulic requirements of culverts and sewers. An excessive value is uneconomical and results in oversizing of pipe, while a low value can result in hydraulically inadequate pipe. Laboratory tests on corrugated steel pipe reflect a wide variation of the coefficient of roughness due to the variety of corrugated steel products. The MTO lists a Manning n value range from to 0.033, depending on the corrugation design. Generally, with larger diameter pipe, corrugation increases in size and/or the direction of corrugation becomes annular, resulting in higher Manning n values. This larger Manning n value consequently leads to larger corrugated steel pipe diameters when compared to concrete for the equivalent application. Designers and specifiers must take this into account as part of the design process. Numerous laboratory studies have been conducted in an effort to determine Manning n. Results of these laboratory studies conducted with concrete pipe found that the Manning n values ranged from to 0.010, in sizes from 450 mm to 900 mm. CSP-Corrugated Interior Wall 4

6 The Ministry of Transportation for Ontario has provided guidance for designers in selecting appropriate values for Manning n in MTO Drainage Management Manual, 1997, Design Chart 2.01: Manning Roughness Coefficient. Using the MTO design methodology Design Chart 2.29: Nomograph: Circular Pipe-Flowing Full, with a discharge rate of 5 m3/s and a sewer slope of 1%, the concrete pipe design diameter, with Manning n of is 1350 mm, while the corrugated steel pipe with a Manning n of is 1650 mm. A difference of two standard pipe diameters. DEFLECTIONS AND DEFORMATIONS The many types of deformations that show up in the finished installations often indicate the difficulty of installing corrugated pipe. According to Prevost and Kienow: CSP after removal Installation can control the deformation, depending on how it is carried out. It is a matter that is resolved in the field, not at the designer s desk. Installation encompasses the bedding of, and the nature and condition (compaction) of, the backfill around the pipe. Installation is critical: it determines the soil pressures on the pipe, particularly at its sides, and thereby its shape. Installation can elongate the vertical diameter, as well as flatten the pipe, far in excess of the design (calculated) deflections. Installation can control the deforma- CSP showing deformation 5

7 Just the FACTS! tion, depending on how it is carried out. It is a matter that is resolved in the field, not at the designer s desk. There are thus important random deformations that ought to be controlled, but which cannot be calculated or modeled. Internal Rifle Barrel In considering the durability of corrugated steel pipe, the designer must address the corrosive and abrasive potential of the installation. In an effort to control corrugated steel pipe deflection during installation, Contractors have, in some cases, installed temporary struts, particularly on large diameter pipe, during the backfilling operation to maintain the pipe shape. This process typically slows the installation operation, costing owners time and money. In contrast, the installation of concrete pipe is not as reliant on the pipe/soil interaction and is not susceptible to the same type of deformations created by poor installation. In addition, the weight of concrete pipe resists lateral movement during backfill operations. LONG TERM PIPE DURABILITY CSP vs Concrete Pipe In considering the durability of corrugated steel pipe, the designer must address the corrosive and abrasive potential of the installation. Abrasion is a result of prolonged exposure to coarse materials passing over the metal surface. Eventually, the abrasion leads to the removal of the protective coatings, 6

8 thereby permitting corrosion to occur. Corrosion of metal is defined as an electro-chemical process, initiated on the surface of the pipe, and propagates through the movement of electrical currents causing metal ions to go into solution. This results in a loss of metallic material from the pipe. Abrasion and corrosion must be considered during the design phase as these characteristics present severe limitations to the service life of corrugated metal pipe. Concrete pipe is designed to crack in the installed condition. The cracking permits the steel to perform structurally and to act in retaining pipe shape. These cracks are not a sign of a defect or a problem. In fact, the cracks will generally heal themselves, with time, due to physical characteristics of the concrete known as autogenous healing. Concrete pipe provides adequate cover over its reinforcement protecting it from corrosive elements. All concrete pipe provides a minimum of 25 mm cover as a protective coating. Corroded CSP Invert Concrete pipe is designed to crack in the installed condition. The cracking permits the steel to perform structurally and to act in retaining pipe shape. As a reference, the U.S. Army Corps of Engineers has investigated design and service life for various drainage products. U.S. Army Corps of Engineers EM Conduits, Culverts and Pipes provides the following information: Corroded CSP Invert 7

9 Just the FACTS! Stucturally Designed and Tested A water tight joint may be accomplished through the use of rubber gaskets contained between the bands. c. Product service life. Products made from different materials or with different protective coatings may exhibit markedly different useful lives. The service life of many products will be less than the project service life and this must be considered in the life cycle design process. A literature search (Civil Engineering Research Foundation 1992) reported the following information on product service lives for pipe materials. In general, concrete pipe can be expected to provide a product service life approximately two times that of steel or aluminum. However, each project has a unique environment, which may either increase or decrease product service life. Significant factors include soil ph and resistivity, water ph, presence of salts or other corrosive compounds, erosion sediment, and flow velocity. The designer should investigate and document key environmental factors and use them to select an appropriate product service life. Organizations wishing to specify drainage products would be well advised to review design and service life research prior to proceeding with design. JOINTS 8 CSP-Soil Tight Joint Corrugated metal pipe jointing is accomplished with coupling bands. These bands generally provide a soil

10 type joint. Joints are susceptible to differential movement as they are a weak point in the conduit, and may move during the backfilling operation. Care must be provided during the backfill operation. A water tight joint may be accomplished through the use of rubber gaskets contained between the bands. However, the same concerns related to this weak point in the conduit must be recognized. As with the soil tight joint, the quality of the joint is dependent on the installation of the soil/pipe envelope for structural integrity. Concrete Pipe - Watertight Seal Concrete pipe uses a rubber gasket, watertight joint which is initiated when the spigot is homed into the bell. This style of joint (male/female) provides a positive joint across the pipe conduit. Hydrostatic testing of concrete pipe is used as a quality assurance test to demonstrate the watertight nature of concrete pipe joints. There have been documented cases where the corrugated steel pipe has washed downstream during periods of high runoff. FLOATATION Floatation needs to be addressed during the design of buried pipelines. The lack of weight and stiffness of corrugated steel pipe makes this an important design consideration in stream culvert design and areas with a high water table. There have been documented cases where the corrugated steel pipe has washed downstream during periods of high runoff. If corrugated steel CSP-Lightweight Material 9

11 MATERIALS The basis of corrugated steel pipe is steel. Steel pipe must be galvanized or coated to meet a long range design life. Although the coatings provide protection for the pipe, mishandling may lead to scratches and gouges, reducing the effectiveness of the coating. Steel corrosion will propagate from these scratches and gouges, limiting the life of the steel pipe. CSP-Formed with Rolled Steel Reinforced concrete pipe utilizes conventional materials with known and understood properties. Aggregates, cement and reinforcing steel are the basic raw materials used in reinforced concrete pipe. There are variations within these basic materials, but the engineering properties of each are well known and may be easily measured. Reinforced concrete pipe utilizes conventional materials with known and understood properties. Aggregates, cement and reinforcing steel are the basic raw materials used in reinforced concrete pipe 11

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13 Just the FACTS! Notes: 12

14 Also available: JUST THE FACTS CONCRETE PIPE VS. HDPE For all the facts, visit 13

15 Just the FACTS! pipe cannot be restrained with overburden, anchoring devices should be used. Due to the heavy weight and strength of concrete pipe, floatation is usually not an issue. SALVAGE VALUE OF PIPE RCP-Structurally Heavy Material Salvage value of pipe is closely related to its inherent strength and ability to survive the abuse of installation and removal. Pipelines, and in particular culverts, are often used in temporary applications to facilitate drainage during construction stages. While designers often try to minimize the cost of these facilities, one of the overlooked components is the salvage value of the pipe. Salvage value of pipe is closely related to its inherent strength and ability to survive the abuse of installation and removal. Flexible pipes show considerable wear and tear when removed after a temporary installation. The difficult task of digging up a buried culvert is complicated when heavy equipment is used to accomplish the job. The rigid nature of reinforced concrete pipe (RCP) is ideal for removal and replacement. RCP is able to handle abuse of the type that would be expected in a removal job. Concrete Pipe Stock Pile In terms of life cycle costs, it is prudent to account for the salvage value of the pipe when planning a temporary line for drainage. 10