Concrete Pipe and Boxes

Design engineers who are involved in building healthy communities and safe infrastructure recognize concrete pipe for its consistent strength and variety of designs. It comes in a wide range of standard sizes from 150 mm to 3,000 mm diameter, and up to 2.44 metres in length. It also provides ease of installation and long-term performance.

The structural strength of concrete pipe allows the pipe to resist up to 90% of the total load that is imposed upon it in a buried condition, and it can compensate for construction difficulties such as high fill heights and trench depths. Precast reinforced concrete boxes are used in standard and customized designs for culverts and sewers, retention tanks, and pedestrian and conveyor tunnels.

Production becomes more computerized, more specialized

There was a time when high quality production and a wide variety of standard products were sufficient for any project. Today, the applications are more demanding and specialized because of factors such as environmental legislation, new regulations, performance-driven standards, and health and safety codes.

The widespread use of computer technology, in-house and commercial software, and the internet has added a new dimension to the design of concrete pipe. The product can be modified easily to meet clients’ performance specifications. The internet facilitates the speedy download of design programs and the exchange of engineering drawings — a capability that few producers possessed until the mid 1990s.

Canadian concrete pipe plants are highly automated and many producers have moved to semi and fully robotic production facilities. These plants produce pipes and boxes with consistent design tolerances. From batching and mixing, through the fabrication and placement of the steel reinforcement, to labeling and shipping, all units for a specific order have the same uniformity in their specification and concrete mix. Each unit is designed as an integral piece of an overall system, so a pipeline or culvert acts as a single structure. Production runs are planned to build complete systems, including special pieces and accompanying maintenance holes and other appurtenances. In many applications, the units are coded for accurate field assembly.

In Canada, the most comprehensive quality control program for the concrete pipe industry is the Plant Prequalification Program. This program was established in 1965 to pre-qualify concrete pipe and production facilities for supplying pipe to municipal and provincial highway projects.

“Standard Installations” design

Through research, the concrete pipe industry has found that steel reinforcement in its products is being over designed in many applications. There have been changes to the Canadian Highway Bridge Design Code (CHBDC) and Canadian standards to allow for these reduced steel requirements and greater flexibility in wire and rebar design.

“Standard Installations” is a term for a new design method for reinforced concrete pipe. Standard Installations Direct Design (SIDD) is included in the Canadian Highway Bridge Design Code and was adopted by the Ministry of Transportation Ontario in 2002. The method was developed through the evolution of finite element analysis tools that allow designers to evaluate the soil-pipe interaction on a much more sophisticated basis.

This new soil-structure interaction analysis and design system allows for more accurate procedures for determining the structural response of the pipe to applied loads. The analysis also provides more accurate methods for determining the most effective reinforcement requirements and configurations, and it allows more effective designs for concrete pipe to be used in new standardized bedding and embedment soil configurations. In addition, the standard installations approach leads to a reduction in the volume of imported fill on construction projects, thereby reducing the disposal of native materials and reducing costs for the concrete installation.

Pipe joints must always be considered in specifying pipe material. In Canada, all concrete pipe is supplied to job sites with gasketed joints. Gaskets also exist for many sizes of boxes, and are readily available.

Basics of design and performance

The hydraulic capacity of all types of pipe depends on the smoothness of the interior pipe wall and is represented by Manning’s Roughness Coefficient, commonly called Manning’s “n.” A greater volume of water will flow through a pipe as the Manning’s “n” value becomes lower. Engineers use a value of 0.012 or 0.013 for concrete pipe, taking into account the differences between laboratory testing and actual installed conditions (bends, manholes, debris, or other obstructions) of various sizes of pipe as well as allowing for a factor of safety. Research has shown that in most cases hydraulic designs using concrete pipe can be downsized by at least one size because of its smooth walls and joint transitions.

The performance of concrete pipe is influenced by concrete compressive strength, density, absorption, water to cement ratio, cementitious content and type, and aggregates.

Compressive strengths for concrete pipe normally range from 28 to 41 MPa and are a function of many factors, including aggregates, cementitious material, manufacturing, curing and mix design. Most concrete design strengths refer to 28-day compressive strengths. Typical design strengths exceed 40 MPa, and 28-day compressive strengths can exceed 60 MPa.

Concrete pipe densities typically range from 2,150 to 2,650 kilograms per cubic metre. Usually, the higher the density, the greater the concrete durability. Absorption is primarily used to check the density and imperviousness of the concrete. As with compressive strength, the absorption can be greatly influenced by both the aggregates and the manufacturing process. Canadian and U.S. standards specify maximum allowable absorption levels, depending on the test method.

Low water-cementitious (W/C) ratios are one of the trademarks of quality concrete pipe, with corresponding high compressive strength as a function of the low W/C ratio. Typical precast concrete pipe has W/C ratios of approximately 0.34. Cementitious content, which has always been a topic of concern with engineers and manufacturers, includes cement and fly ash or slag. The key to proper cementitious content is the proper design of the mix, taking into consideration all material properties, manufacturing and curing processes. All types of cement have been used in the manufacture of concrete pipe, but generally Type II cement is used.

Concrete pipe aggregates, both coarse and fine, meet specific industry and government standards. Both natural and manufactured aggregates are suitable for use in concrete pipe. All aggregate properties should be considered when taking into account strength, durability and performance. These include gradation, absorption, specific gravity, hardness, petrographic number, and in some cases alkalinity.

Wire reinforcement adds significantly to the inherent strength of structural concrete.

Concrete protects the steel both physically and chemically. Physically, it restricts ingress of basic components required to initiate corrosion (water, oxygen, chlorides). Chemically, the pore solution in concrete typically has a very high pH, which leads to the formation of a protective iron oxide film around the steel reinforcement.

Due to recent advances, concrete pipe and boxes have entered a renaissance as more engineers become aware of the diversity of applications and long-term performance of these products.

Sal Iannello, P.Eng. is executive director of the Ontario Concrete Pipe Association in Burlington, Ontario. E-mail: sal.iannello@ocpa.com