Joint gaps in concrete pipe are a necessity for a number of reasons, but the allowable size of that joint varies depending on which state you work in.
The joint gap refers to the gap between two pieces of pipe once they are assembled in the field.
'Often we get calls from customers asking what is the allowable joint gap opening,' says Randy Reimer, Director of Sales for Hamilton Kent. 'What they are really asking is ‘the joint is open, so how much of an open joint can be allowed before we lose compression on the gasket?''
Lost compression in a joint means a potential joint failure or leak, but the allowable joint gap depends on the pressure requirement of the joint.
The national standard, ASTM C443, requires a joint to withstand 10psi while it is under maximum deflection — meaning the joint is essentially as open as it can be without being broken. Most states follow this standard, which is handy for infrastructure contacting firms doing work in multiple states.
However, there are some US states that require contractors to follow stricter standards, and one of those is the state of Florida. The Florida Department of Transportation (FDOT) has a more stringent open gap pressure rating of 5psi.
Florida's average elevation is just 100 feet above sea level, meaning the groundwater table is much closer to the surface than other regions — sometimes it's less than three feet below the surface. FDOT has the strictest requirements for these types of drainage and infrastructure projects, holding producers and installers to higher standards, including video inspection of every installation and proven watertightness to handle the greater potential for leaks.
Regulators don't come by these numbers by chance. The allowable joint gap is based on a mathematical calculation based on the specification, safety factors, the maximum opening and the pressure rating required when joint gaps are present.
A contractor will ignore these minimum standards at his or her peril, because doing so increases a joint's potential to fail.
'Often during installation, if a contractor is new or not at the top of their game that day, they could leave larger joint gaps, which could lead to a lot of problems,' says Reimer.
Many State DOTs now insist on video inspection after installation, and open gaps can be easily seen with this method. Sometimes leaks are spotted as well. This is something every contractor wants to avoid at all costs, explains Reimer. He adds that if open gaps or leaks are present post-installation, there are two options: grout the joints – an expensive and difficult task, especially in smaller diameter pipe (imagine crawling 100 feet through a 24-inch pipe to grout a joint after it has been backfilled and buried); or completely excavate the site and reinstall the pipe properly, which is a prohibitively expensive solution.
'These kinds of problems lead to big claims that are often disputed between producers, contractors and owners,' Reimer says. 'Sometimes even the gasket companies are called in, which is why all of Hamilton Kent's gaskets are designed with the specification in mind.'
These gaskets are designed to meet pressures from 5 psi soil-tight applications all the way up to hundreds of psi. Hamilton Kent designs different shapes, using different materials and means of confining the gasket in the joint so they fit any application.
Under pressure, rubber can become like a sheet of paper and be forced right out of the joint (Reimer says to think of a mouse getting in your house through a tiny hole that seems much smaller than his body).
'Keeping this in mind, we meet the specifications that the joints are designed for and the specification requires,' he says. 'Sometimes this is a balancing act; you want a safety factor in case the joint gaps are present post installation, but you don't want such a large safety factor that you're putting so much rubber in the joint that you drive the price up. The balance is performance versus safety factor versus cost.'