Q: During a recent ICF pour, I received some “bad” concrete that caused defects such as voids that I found after removing the window bucks. Are there any special guidelines to follow when pouring an ICF foundation?
A: Rick Arnold, author of and a frequent presenter at JLC Live, responds: The typical concrete mix for a foundation wall includes 3/4-inch aggregate. But when you’re working with ICFs, it’s better to order the concrete with 1/4-inch aggregate, also referred to as “pea stone.” It is the same size aggregate usually used for flat work. ICFs ordinarily have a complex network of webbing and rebar installed inside the forms, and the smaller aggregate moves more easily in and around that maze to ensure better and more complete placement with no voids.
In my experience, most ICF wall defects are caused by placement practices; in other words, human error. But if your problems were indeed caused by a problem with the concrete, my first guess would be that one or more of the loads delivered were “hot.” It is common practice in my area for concrete trucks to leave unused concrete from an earlier job in the truck. That concrete then is mixed in with the new batch and delivered to the next site, creating what is known as a “hot” load. Depending on the amount of “old” concrete and how much time has passed since the first load of the day for that truck (there could be remnants of a couple of earlier batches in the truck), the concrete that you pour into the forms could set up in less than 20 minutes. If you didn’t know about the hot load and figured that you had plenty of time to vibrate and straighten the walls, you’d be in big trouble.
As a concrete contractor, I always asked the driver if there was any “old” concrete in the batch, especially if timing on the pour was critical. Most drivers will volunteer the information without being asked. If the load is “hot,” you can still place it correctly without defects; you just have much less time in which to do it.
You can also ask the ready-mix company to add a plasticizer, which will help the concrete flow more easily without the addition of water and is fine for ICF work. By the way, adding water can make concrete flow more easily, but it will reduce the strength of the concrete.
That said, here are some things to keep in mind when working with ICFs:
- Construct or alter the window bucks so that you have plenty of access through the sill section. This lets you both fill and visually check that problem area.
- Never fill more than a third of the wall height at any time.
- Always start at the corners and work toward the middle of the wall.
- Always have some sort of vibration method on hand, even if it’s just a couple rubber mallets.
- If you use a mechanical vibrator, don’t use it too long in one spot.
- Always plan to have extra crew on hand so that you can pour continuously. Moving too slow can cause cold joints between batches, which can reduce the overall strength of the wall.
- Make sure to have on hand plenty of extra equipment, such as concrete chutes, shovels, and the like.
Finally, schedule the truck arrivals ahead of time to keep the pour progressing at an even pace. Scheduling is largely under the control of the dispatcher, but a few words to the ready-mix office ahead of time can help to ensure timely delivery and the correct interval of time between truck arrivals.
