{ QUESTION } We are working with a local paving contractor on a project originally bid last year but it is now just being awarded. We were able to convert the parking lot project from asphalt to roller compacted concrete (RCC).

We have a pretty good handle on what the total price of our delivered material will be. But how do we ensure that we don't get caught in “yield trap” during the actual construction phase and, more importantly, regarding final pay? The contractor plans to order his daily requirement by weight (tons) while we are set up to batch the material by volume (cubic yards). The contract calls for pay based upon field parameters.

Also, how does our QC technician use the information given by the proctor to calculate the batch water's requirements on an RCC mix recipe?

{ ANSWER } These questions are quite common. The fact that the contract document states that the RCC material's volume will be calculated after it is compacted makes things a bit different than normal concrete applications.

Christopher Tull, an engineer in Indianapolis who has assisted producers on several RCC paving projects, recommends designing RCC mixture recipes without air content, trying to avoid even entrapped air. By targeting the trial mix to the theoretical density the producer should be able to achieve proper yield and have the contractor meet performance requirements even at 100% compaction (no voids).

To illustrate his point, Tull has provided a sample mix design for a recently placed RCC project (see table). On this project, Tull's client provided a saturated surface dry (SSD) mix design of 4164 pounds per cubic yard. Note the increased weight per yard, as most non-air mixes typically range about 4000 pounds per cubic yard. Because of this increase, Tull advises producers to plan for additional material costs when calculating bid material prices.

When calculating the amount of water to place in an RCC mix, Tull advises producers to take a lesson from the techniques used in testing soils. The optimal moisture content is calculated as a percentage of the dry weights. This is a different approach, as concrete producers normally use SSD values to pull weights.

In Tull's example, note that the total dry weight of the material is 3984 pounds per cubic yard. If the optimal moisture content is 4.5%, the total water content required for optimal moisture is 179 pounds per cubic yard (3984 x 4.5%). Tull reminds us that part of this water will be absorbed into the aggregates.

So in his example, Tull estimates that 26 pounds will be absorbed, leaving an SSD water content of 154 (rounding) pounds per cubic yard. Also, Tull estimates there will be some water introduced to this mixture by aggregate moisture contents above SSD. In this example, 107 pounds per cubic yard of water will be supplied by the aggregates, leaving only 47 pounds per cubic yard to be added by the plant.

“It's important to remember that the aggregates will bring a large portion of this water,” says Tull. The example shows that the free moisture in the aggregates will provide about 70% of the total required SSD water. “Monitoring moisture content from all sources in RCC mixes is critical to successful placement and durability,” he says. You can e-mail Tull at christull@sbcglobal.net.