By the time someone confronted Seattle-based Stoneway Concrete and Greg McKinnon, its quality assurance and marketing manager, about the company's unorthodox flowable-fill production process, they were way beyond the most common problems with producing flowable fill and were solving contractors' problems profitably.

The process Stoneway used aroused skepticism even among its foaming agent supplier. Ignoring the supplier's instructions, Stoneway was modifying one of its geotechnical foaming agents and truck-batching the new concoction to generate foam. Instead of using a foam generator, Stoneway used its truck mixers' rotation to impart a high air content to the materials in the drum to produce low-density fill.

Stoneway's experimentation led to a new product designed to make flowable fill a viable market for producers. Stoneway's foaming agent supplier, Cellular Concrete LLC, now offers a new foaming agent that, like Stoneway's home recipe, generates an aggressive foaming action in a mixer drum. The foam bubbles provide fluidity without excess water, and, best of all for the producer, don't weigh anything.

Two problems are common in placing flowable fill around soldier piles for buildings, common projects Stoneway supplies.

Excavation of some of the fill is necessary. Testing often showed compressive strengths high enough to make excavation difficult, to say the least.

Density control is another problem. In "standard" flowable fill, heavy materials would sink to the bottom and force water up, often giving the material compressive strengths of more than 500 psi at the bottom of the hole and lower strengths toward the top.

Flowable fill also presents trucking challenges: the producer may not be able to batch enough to justify the use of trucks, and possible spillage of such a fluid material is always a concern. For these reasons, Stoneway and Cellular Concrete designed a controlled-density-fill mix using the manufacturer's new product, and Stoneway branded the mix Stoneflow. The driver dispenses the agent into the drum once onsite, and turns the drum at mixing speed for 5 minutes. McKinnon reports that by the time the drum stops turning, the fill's air content is between 25% and 30%, and slump increases from 1 1/2 inches to 8 or 9 inches. The slippery property of the cement-coated air bubbles produced by the foaming action is the key factor in increasing the slump.

Although compressive strength is not the ideal way to measure the material's service properties, McKinnon notes that it is a measurement with which engineers are familiar. Typical compressive strength for Stoneflow is well within the 300-psi limit for future excavatability as recommended in ACI 229, "Controlled Low-Strength Materials."

McKinnon can maximize payload on his mixers, which can haul 12 cubic yards by volume. When the foaming process is completed at the jobsite, the drum still holds 10 yards by weight but 12 yards by volume because the foam doesn't weigh anything.

A major reason for the reduced weight is a lower sand content. Stoneway also reduces costs by reducing or replacing fly ash with the foaming agent on projects that require only sand and cement mixes.

"The reality is, we can keep the same price, increase our profit margins, solve their problem, which is already costing them money, and everybody wins," McKinnon says.