Joint designs play a pivotal role in ensuring concrete components act as one.
By Mike R. Miller, Jim Westhoff and Pardeep Sharma
Mike R. Miller is the international sales manager for Press-Seal Gasket Corp. in Fort Wayne, Ind.; Jim Westhoff is president of A-Lok Products Inc. in Tullytown, Pa.; and Pardeep Sharma is a product engineering/technical services manager with Hamilton kent in Toronto.
According to ASTM C822, “Standard Terminology Relating to Concrete Pipe and Related Products,” a joint is defined as a connection of two pipe, manhole or box section ends made either with or without the use of additional parts and/or materials. More generally, joints are the connecting points for modular precast concrete components. In most cases, sealing the joint space is necessary to prevent infiltration of groundwater and exfiltration of liquids conveyed by the structure.
The focus here is on rubber gaskets for concrete pipe joints, but rubber gaskets are certainly not the only means of sealing precast concrete joints in pipe or other structures.
Since the 1800s, rubber gaskets have been used to seal concrete pipe joints. Although advances in materials and manufacturing methods have long since enabled the concrete pipe industry to deliver reliably jointed products in a wide variety of types and sizes with proper attention to design and manufacture, most in the industry agree that the performance standard for sealing will only become more stringent. This will require the continued improvement of concrete products in all areas, especially joint sealing. Making this happen will require the understanding and cooperation of all concrete producers and gasket manufacturers.
Here are some rules of thumb for successful concrete pipe joints.
- Consistency of product (pipe, gaskets, installation) makes sealing simpler.
- Variation in the process causes defects and must be identified and eliminated.
- Gaskets can compensate for some process variation.
Concrete pipe and other conveyance products have been around for a long time and have demonstrated their strength and durability in applications around the world. No other material possesses such an optimum combination of low cost, ease of manufacture and superior strength. In fact, concrete is the most common type of pipe and has been a part of virtually every sewer and drainage system in the world.
But the challenge for any pipe regardless of material has always been the joint. Concrete pipe was one of the first types of pipe made and installed, and in the early years it was installed without any seal. This caused both infiltration and exfiltration in many of the early installations. Today great strides have been made to improve the performance of concrete pipe through specification development as well as manufacturing technology and joint design. Through the use of confined and unconfined gasket design, concrete pipe can provide watertight installations designed for the service conditions required. Although pipe is typically used in horizontal applications, many of these designs can be adapted or utilized in vertical manhole applications as well.
The most critical part of the pipe is the joint. Making pipe with a reliable geometry on both ends and free from defects is a necessity. Typical joint geometries are measured in thousandths of an inch and are calculated in total tolerances of a few hundredths of an inch. The other pipe dimensions may vary by much more.
It’s sometimes hard to remember that the ends require an entirely different level of accuracy. It’s also hard to remember that water will find that 0.1 percent of the joint that isn’t perfect and will inevitably cause a leak there, regardless of how wonderful the other 99.9 percent of the joint is made.
Proper equipment and materials notwithstanding, concrete pipe joints may become compromised by:
Out-of-roundness. Whenever a joint becomes out-of-round, it inherently has areas of higher and lower gasket compression. Leaks can happen at the areas of lower compression, and bell fracture may happen at the areas of higher compression.
Mixing of joints. Borrowed equipment or company consolidation sometimes creates a “one brown shoe and one black shoe” pair. Headers and pallets may look identical but may not match geometry.
Poor surface quality. Apart from cracks, leaks can start at the interface of the gasket and pipe. Rough texture increases joint installation force and abrades the gasket surface. Bug holes create areas of reduced gasket compression.
Joint surface slurrying. Slurry or other required joint-coating materials can radically alter the geometry. A one-sixteenth inch thick coating, even just on one side of a joint, can dramatically change the gasket compression within the joint.
Poor assembly and installation. Even though somebody else is doing it, you will still get the call when your customer makes installation mistakes, because it will be your pipe leaking. There is no substitute for working directly with your customer and making sure that every job is installed properly.
Gaskets for concrete pipe
Concrete pipe joints seal by deforming a rubber gasket between two concrete mating surfaces. Rubber is not compressible. Like water and oil, it occupies a fixed volume and this does not change, only the shape (deformation) of this volume changes. This deformation is created through displacement of the original rubber shape. This creates pressure between the rubber and both pieces of concrete, sealing all three components and allowing for reasonable movement of the joint.
The types of rubber used in concrete pipe joints are defined as thermoset polymers. During the manufacturing process, the rubber itself undergoes a permanent change in its properties that causes it to retain its process shape regardless of the deformation applied. This means that it seals well for a long time and that it adjusts to changes in its deformed shape as the pipe joint settles over time.
To work properly, a rubber gasket must be properly designed, extruded and cured, and cut and spliced. Proper design means that the gasket is specified to meet the requirements of the joint mating surfaces, the manufacturing methods and the installation crew.
There is a wide variety of concrete pipe joint designs, reflecting the local nature of specifications, processes and equipment that exist in our industry. Unique design developments have created new joint requirements, yet conventional designs steadfastly retain requirements dating from the early 1900s. This is truly an industry in which variety is the norm.
Gasket design merits its own discussion, but some basic information may be of help. To seal properly, every concrete pipe joint has two critical variables: the Gasket Annular Space (GAS) and the Control (or the Concrete-to-Concrete) Annular Space, known as CAS (see the sidebar “Making a Good Seal”).
The GAS is defined as the distance between the surfaces of the pipe where the gasket sits in an assembled joint. The larger the GAS, the larger – and therefore more expensive – the gasket that is required. A larger gasket (of any design) will absorb a wider range of variation from all factors. That is why larger pipe sizes usually have fatter (or higher-volume) gaskets.
The CAS serves to center the pipe joint and prevent over- and under-deformation of the gasket by acting as a positive stop to axial eccentricity, which is what occurs when pipe settles. The geometric relationship between GAS and CAS determines the range of sealing and tolerance compensation available in the joint.
Beyond the design of the gasket is the actual manufacture. Virtually all concrete pipe gaskets are made from extruded rubber that is cut to length and spliced into a continuous ring. Each part of the process has its own requirements.
In extrusion, raw (uncured) rubber is forced through a metal die and then subjected to heat, which activates chemical additives in the rubber. These additives cure the rubber permanently. It is virtually impossible to return rubber to its original state once cured. That gives rubber its permanence of physical properties.
Rubber for gaskets can be cured in several ways, including steam, oils and heated glass beads that can be heated reliably to the required 300-500 F (149-260 C) range, but usually a combination of processes is used, such as microwave, hot air and/or “salt” (sodium nitrate/sodium nitrite). Any process that consistently heats the rubber to the proper temperature for the correct time may be used.
The cured rubber is then processed into gaskets by being cut to the required length. This may be done manually or by automated equipment, depending upon the manufacturer and the profile being processed. Marking of the gaskets is also required so they can be traced and their applications can be determined at all stages.
The ends of the cut gasket are coated with cement (usually a substance containing uncured rubber polymer) and placed into close-fitting heated molds. The molds raise the temperature of the area of the splice to a point where the uncured materials in the cement become cured, forming a continuous rubber gasket whose physical properties are consistent throughout. Other methods of splicing are to inject uncured rubber into the splice area or to use a thin film of uncured rubber to bond the ends. Virtually all gaskets use a form of rubber to splice the ends. Although many adhesives (notably cyanoacrylate adhesives) bond rubber well initially, none are suitable for permanent installation.
Concrete pipe gaskets are remarkably durable in transportation and storage. As long as they are kept out of direct sunlight, they will keep for many months or years with no change in properties or performance. Despite this, gaskets should always be stored in a cool, protected environment. Also, some polymers (nitrile most notably) change physical properties when subjected to extremely cold temperatures.
Concrete product joint designs are an important part of the overall structure being assembled. Given the ever increasing focus on preventing both infiltration and exfiltration into and out of water and wastewater collection and conveyance systems, it is important to ensure a high-quality joint design.