These questions and answers were generated during Breakfast with the Experts at World of Concrete 2016, sponsored by BASF. Watch a video of Nmai and Bellamy discussing how to influence a job specification by clicking here.
Q. What can you tell us about internally cured concrete, and what other options are there besides lightweight sand?
A. Charles Nmai: The ultimate goal of internal curing is to eliminate all curing, but we’re not there yet. Nothing completely eliminates the need to cure. Today, internal curing is being used in applications where autogenous shrinkage may be too high — high-strength concrete or when using silica fume is used in the concrete. The current practice is to use lightweight aggregate — replacing a portion of the sand with saturated lightweight aggregate. The idea is that you have a reservoir of water that helps with hydration. Strength gets a boost and you will get lower shrinkage. The other technique or material is to use superabsorbent polymers—but that is not in widespread use today. The Expanded Shale, Clay & Slate Institute has more information on this.
Q. What kind of results are you seeing with lithium-based materials to mitigate ASR?
A. Charles Nmai: Alkali-silica reactivity, or ASR, is a destructive reaction that occurs when you have aggregate that contains reactive silica. The alkalis come from the portland cement itself. So if you have reactive aggregate and high alkali content in the cement then the reaction can occur in the presence of humidity. But since concrete is made with water, you have everything you need for the reaction. That reaction forms a gel that absorbs water and expands. As good as concrete is in compression, anything that happens that creates expansion after the concrete has hardened is bad news, whether that is corrosion or ASR, and it leads to cracking.
The most cost-effective way to mitigate ASR is to use supplementary cementitious materials (SCMs), such as Class F fly ash or slag cement, to mitigate the reaction. But if you don’t have access to those materials, the only other way is to use lithium-based materials. This is a proven technology that’s been used all over the country. The dosage rate depends on the alkali content of the cement and how reactive the aggregates are. If it’s used in combination with SCMs, that helps reduce the dosage.
Robin Bellamy: To have an ASR situation on existing concrete, you must have three things: reactive aggregate, an alkali solution, and moisture. The only thing you can do to mitigate that with existing concrete is to remove one of those factors. You can’t take the aggregate out and you can’t take the alkalinity out because then you will have a corrosion issue, so the only thing you can do is take out the intrusion of water. We can do that with penetrating sealers. Silane or siloxane blends on the surface of concrete are highly successful at limiting the amount of moisture. We’ve had some real success with this approach to mitigate the ASR reaction.
Q. What’s happening today to improve concrete performance?
A. Robin Bellamy: We’re starting to focus on education at sessions like this. We must get the message out that it’s not just a product, it’s not just an application, it’s not just a methodology. It’s a matter of understanding all the factors.
Will Gold: We are at a point where our collective understanding has improved to where we are no longer looking at how to innovative a product, but rather how to bring what we already know to the field and get it into codes and standards. For example, with concrete repair, ACI now has a code for repair and that brings a lot of collective knowledge together. We are taking what we know and standardizing it and making sure that everyone is aware of it.