Flowing profits: using flowable fill in residential applications saves money

Concrete Producer, The, Jan, 2004 by Dean Plank

There are few things more exciting than creating a new application or market for an existing material or technology. I was trying to find a way to level a problem area at my home that would hold water whenever it rained. This area is used as an entrance and exit to my barn, so I wanted it to have good compaction.

The answer was flowable fill--a surprise material for this application if there ever was one. But using it in a residential application was a first. The story of how this collaborative effort happened is exciting.

We've been working with flowable fill at Superior Concrete, Frederick, Md., for about five years. We've used several designs, including Darafill from Grace Construction, and Rheocell, Master Builder's version. We've bad great success using these mixes. Once, we placed more than 500 yards for a utility project on Braddock Mountain. So we were no stranger to flowable fill.

While I was thinking about the entrance to my barn, I realized we were storing six skids of pavers at our yard that were slated to go into a patio project at the home of Ralph Kline, who owns Superior Concrete with his parents. After looking at those pavers, I started thinking about how to get the flowable fill to draw enough compaction to make this work.

I then approached one of our landscape customers with the idea of using flowable fill as a base for pavers. By using it over CR6 stone, we felt we could save much time in moving the material around a house, then tamping for compaction.

Eliminating callbacks

We also discussed the trouble of settlement in the areas of raised patios and swimming pool pads six months alter a project was completed. The landscaper believed that by using flowable fill in those areas, he could eliminate callbacks and complaints about sink spots, especially around pools and hard-to-reach areas that need good compaction.

A few days later, I talked with Tom Evans, promotion director for the Maryland Ready Mix Concrete Association, about the association's flowable fill committee projects. I told him this would be an excellent way to develop a new market for the material. We met several times the next few weeks to explore, ways to use all of the committee's research in an actual project.

We then tested different variations of flowable fill mix designs, including pumping some of them. Hap Powers, pump operator for Hollerbach Concrete Pumping, was very patient as we used the trial and error method to pump the mixes.

Finding the right mix

When we started with flowable fill, we worked with a manufactured sand for a base. We only wanted to use enough cement to reach 100 psi. Getting to a high compaction rating of 98% was important. We started with 100 pounds of cement and about 2000 pounds of sand. The trick was to get the proper percentage of air generated by either using Rheocell or Darafill.

Our first tests yielded lower than expected. We didn't have enough information to determine the amount of air we were entraining into the mix. So, using the above amounts of sand and cement as a control, we made several trial batches to work up a standard deviation. At the same time, we used the trials for test pours to see where the strength and compaction levels would reach. After several tests, we dialed in the air percentages and started working fly ash into the mix while lowering the cement content.

We discovered Rheocell entrained a little more air than the Darafill. Rheocell averaged 33%, Darafill 30%. We could not get either one to reach the 35% level that was supposed to be attainable. We figured the Loss on Ignition (LOI) percentage of the fly ash we were using could be the reason. Once we dialed everything in to yield a good mix, we used compaction testing equipment to determine our compaction level. It only took a few tests to find we could reach 95% in roughly six

hours with this mix. All three trial batches were at 98% the following day.

Our first trials resulted in strength levels higher than expected. Using the manufactured sand we stock, it took less cementitious materials than a natural sand to reach the strength level we wanted. (A spade shovel should be able to dig up a good flowable fill the next day. Our first trials required a backhoe for removal.) Our final mix design was: 2310 pounds of manufactured sand; 38 pounds of cement; 57 pounds of fly ash; 31.2 gallons of water; and one bag of Rheocell.

We still had to come up with a pumpable mix that would still meet all of the criteria we wanted. It took several tries with subtle changes to eventually get a good, affordable pumpable mix. We now have a few different designs ranging from the original to a blended mix, to a pumpable mix that can be used in a 2-inch trailer pump and a boom pump.

There were many downfalls. Most involved clogging the pump and others dealt with yields and costs. Sometimes, the mixes would be so coarse, they wouldn't get past the first elbow in the pipe. Other times, it wouldn't clog until it was almost through to the end of the pump. And a few became clogged somewhere in the middle. Each time, Hap would have to break down and clean out, so we could clog him up again. In some ways, it was a lot of fun. And in other ways, it was very painful and frustrating. But in the end, we gained a tremendous amount of experience with flowable fill.