A CONVERSATION WITH ASPHALT INSTITUTE SENIOR RESEARCH ENGINEER PHIL BLANKENSHIP
What is the difference between Cold In-place Recycling (CIR) and rubblization?
Cold In-place Recycling (CIR) is the process of recycling an asphalt pavement without heat. Rubblization is the process of recycling a concrete pavement. In rubblization you’re recycling the concrete into an aggregate base for a new pavement—typically asphalt. CIR is just as it sounds. You simply take up the asphalt pavement and put it back down. You recycle the existing hot mix within an inch of the bottom of the layer with a milling machine, crush & screen it (as needed) run it through the mixer, pave and compact. There you go; a recycled pavement.
What makes a pavement a good candidate for CIR?
You’re looking for a pavement that is cracked or has other surface irregularities. It?s normally discussed when it is time to say “let’s cover this up,” but you have a concern that if you cover it up that you’re going to get cracks that come back through. You’d be putting a bandage on it. It’s a pavement where the materials are pretty decent materials—and you think “too bad we can’t use them again.”
What testing is needed to determine CIR suitability?
The existing pavement is usually investigated by coring to examine the in-place materials and base conditions. You can extract the asphalt to see how hard or soft it is in comparison with current standards. Then you ask, “Is this a good candidate for CIR? If so, can I adjust the asphalt emulsion and gradation to make the pavement like new?
What are some issues of concern for CIR?
Some issues that were identified with CIR early on were cracking and raveling. You would CIR a pavement and could have problems with the top layer raveling. Climate has a big play in that. Naturally, the same things we deal with for hot mixes apply. With warm climates, you have more consideration for rutting, and with cold climates, you have more concerns about cracking.
You must also consider the season in which you are doing the CIR. Since it is usually a water-based (asphalt emulsion) process, you have to be aware of how late in the paving season you go. Late season paving with CIR is just not advisable because you need time for the emulsion in your pavement to break and it can?t do that when your temperatures drop below freezing.
What are other circumstances when you might rule out CIR?
CIR might not be a good fit when there are many hot mix asphalt plants in the area. It may be more cost effective in those areas to mill up the old pavement, take the pavement to the hot mix plant and run it as traditional Recycled Asphalt Pavement.
Where CIR proves beneficial is when you get into the Midwest U.S. where hot mix plants are spread out. Simply put, if CIR is more cost effective than hot mix—it’s a good fit, plus it is one of our industries greenest processes. If CIR is more expensive than hot mix—it may not be feasible. Location and customer need are key; you don?t want to use it if it doesn’t fit.
In rural areas of states like Kansas, Oklahoma and Colorado, CIR has proven to be a good choice over mobilizing a hot mix plant.
What specific testing is needed with CIR?
Traditional methods have been just to look at gradation. Next you would mix an emulsion with it and you may have looked at the viscosity or penetration values, like you would with hot in-place recycling. But the problem with traditional techniques have been that you sometime end up with raveling or cracking.
Some of the newer techniques I’ve seen in the last 10 or 12 years are taking more core samples and looking at the road with a little more of a scientific hat. We have to understand that the road changes every quarter mile or less with gradation or on a continuous grading. You have to consider that since the materials in a road may change every quarter mile depending on how much maintenance the road has had, one will find various asphalt contents, stiffnesses and gradations.
Some of the techniques in the design process have been taking multiple cores and building a gradation band to understand “What’s the coarse side of this road?” and “What’s the fine side of this road?”
On the binder side, we ask “How do I adjust my emulsion I’m putting in so this pavement can meet a certain level of performance?” To do that, we conduct an Indirect Tensile test (IDT) or we may add a Resilient Modulus Test. We can understand a basic structural value coming from the modulus test and we understand at what temperature it’s going to crack from the IDT test. Working from these results is often referred to as Engineered CIR.
Other tests that have been added are a wet-track abrasion test. In this test, you see how well a pavement would do under raveling. That?s a basic test to see how much you can scratch the surface of a mix and how it would perform in a raveling simulation.
Is there a certain type of mix for CIR?
You need a specialized mix for CIR. You are trying to use what’s in place—this is not a process where you usually add aggregates. This is a process where about all you can do is adjust, crush and screen the aggregates and add emulsion. You may add about two or three percent emulsion and you are also adding water in the milling process.
We simulate that process in the laboratory using a bucket mixer process. We take the millings from the CIR and mix that with water for one to two minutes in a bucket mixer then add the emulsion. Next we compact the CIR mixture to a specified number of gyrations in the Superpave Gyratory Compactor. After compaction, the sample is ejected and measured for bulk specific gravity and ready for further testing like IDT or resilient modulus.