RCC: $20 to $25/ton less than asphalt for a base course

About 20 years ago, the village began allocating general revenues directly for its 100 miles of streets. Public works uses the $1.5 million generated every year to try out processes – like RCC – that haven’t yet been approved by the state. Public Works Director Matt Mann says “we needed another tool in our tool box and other states have used it successfully.” They’ve learned that feathered edges create a weak joint, so a wheel loader cuts a straight edge along an intersection. That ensures the material on both sides of the joint is full thickness and properly compacted to provide maximum strength.

Today’s work picks up where crews left off (note the RCC’s cross-section). The old pavement was 6 inches of asphalt over 6 inches of stone; the new pavement is 4 inches stone, 6 inches RCC ($58,362) topped with 2 inches hot-mix asphalt ($23,839). Saw-cutting and control joint sealing are contracted out ($3,816). Total: $86,017. New asphalt streets are 7 inches hot-mix asphalt base and 1.5 inches surface layer for $110,359 in material costs. Public Works Director Matt Mann says the 15% to 20% savings over asphalt makes RCC a “no-brainer” for a base course.

Placement begins. The mixture is spread along the straight edge that was prepared earlier. Note that the RCC is placed about 2 inches thicker than the material from yesterday so that it can be compacted that amount to end up at the same elevation.

Prairie Material of Bridgeview, Ill., delivers the mix – 400 pounds of cement and 125 pounds of fly ash with BASF Construction Chemicals’ Delvo hydration stabilizer – to a Blaw-Knox asphalt paver. The mixture reaches 6,000 to 10,000 psi at 28 days. “Materials producers are getting better and better at producing a mix that won’t fail,” says Public Works Director Matt Mann. The reason: They’re using a new type of equipment – ribbon mixers -- instead of pug mills.

A crew member sprays a mixture of water and BASF’s Confilm along the edge to keep it moist until the adjacent lane can be placed.

Next step: compacting the new RCC to the same thickness as the previously placed material. Four passes with a 10-ton Ingersoll Rand DD-90; two with vibration, two without. Prairie Material uses ASTM’s D1557 modified Proctor test on both mix design and materials to determine optimum moisture and density.

The village rented a Lee-Boy kettle to spray asphalt emulsifier on the compacted concrete as crews move on to second half of the street. This is being used as a curing agent to seal the concrete and prevent moisture loss. The asphalt binder also acts as a primer coat for the eventual asphalt overlay.

The finished product. BASF’s Confilm evaporation retarder will be sprayed on joints to keep them from drying out.

Tim Dunne from materials consulting firm Rubino Engineering Inc. of Elgin, Ill., uses a Troxler 3430 surface moisture density gauge to test the pavement. He says “they’ve been getting better” than the required 98% density.

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