Construction on the new Folsom Dam auxiliary spillway, also known as the Joint Federal Project, will be wrapping up soon – the project is scheduled for completion by October 2017. But how do those living in the Sacramento region know that the project will withstand the test of time as advertised?
Part of the answer comes from a quality control (QC) process implemented during the mixing of the concrete placed on the auxiliary spillway.
The U.S. Army Corps of Engineers Sacramento District and its contractors placed roughly 293,000 cubic yards of concrete in a span of 3 ½ years to build the approach channel, control structure, chute and stilling basin.
But to understand the QC process, it’s helpful to understand a few things about concrete itself.
Concrete is made up of various-sized stones known as aggregate, as well as sand, cement, water and other admixtures. That mixture becomes important when we start talking about temperature.
And while the average weekend warrior likely doesn’t pay much attention to the exothermic properties of their concrete or the size of the aggregate it’s made of, this is all very much a precise science for a civil engineer.
For the JFP, a collaborative team of experts from the Corps, Bureau of Reclamation and academia took very seriously the task of coming up with the right concrete mix and testing all of the concrete placed on the project.
One of the team’s first steps was understanding how all of those concrete elements would react with one another and their environment.
As concrete hydrates, or hardens, it changes in physical state and experiences an exothermic reaction, meaning it releases energy, or heat, into its surroundings.
“With these massive concrete structures we’re building, they develop a tremendous amount of heat,” says Bill Halczak, a Sacramento District civil engineer. “Using larger aggregates allow us to use less cement and less water, which in turn means the temperatures won’t rise as high.”
But increasing temperatures are only part of the consideration. It’s not so much the heating up of concrete that causes one of the biggest problems – cracking – but rather the cooling of the concrete. So, while engineers are controlling peak temperatures, they are conversely controlling cooling temperatures as well.
“As the concrete hydrates, it expands,” Halczak explains. “Once the hydration has completed, the concrete will start to cool and as it cools it will likely crack. Therefore the less it has to cool, the less chance there is of cracking.”
And that’s where the really fun part comes in – testing how much pressure the concrete can handle before it breaks.
Have you ever seen the internet video where they crush a can of silly string in a hydraulic press? That’s exactly how Halczak and his team tested the strength of the concrete used in the JFP. They placed cylindrical-shaped samples of hardened concrete, cured anywhere from a day to a year, in a hydraulic press and crushed them until they broke. Not quite into oblivion like the silly string can, but enough to test the breaking point.
“All structures are designed to perform under a desired strength, so the cylinder breaks confirm whether or not you’ve achieved that desired strength,” says Halczak.
Of course, there are tough quality control standards for any kind of structure, but Halczak says the concrete engineering community always takes it a step further to ensure there is an additional factor of safety above and beyond the standard.
With two concrete plants located on the JFP, the team tested at least one concrete batch every shift and completed more than 14,000 cylinder breaks over 3 ½ years.
“If you think about just how much concrete that is by itself, then you can get a sense of how big our project is,” says Beth Salyers, chief of the Joint Federal Project office.
“The control structure – the thing that’s holding back all the water in the Folsom reservoir – is a concrete structure, so you want to make sure its strength is sound and adequate,” she says. “Everyone living below the dam is basing their lives and their insurance bills on the quality of that structure.”
“Conducting 14,000 cylinder breaks on the JFP lets people know that we did our job – that we ensured the concrete placed on site is exactly as designed and is going to keep people safe,” she added. “That’s our assurance.”