The American Concrete Institute doesn’t change its logo lightly—the new 2014 logo is the first since 1964. The familiar globe was upgraded with some snappy new colors and a pledge to always “keep advancing.” Such an auspicious occasion calls for a unique unveiling, such as a decorative pervious concrete slab at the 2014 World of Concrete with the new logo embossed on the surface. This also allowed us to illustrate some of the subtle nuances of pervious concrete and the new practices we have adopted throughout the entire construction process.
First, the mixture. The mix design might seem unusual in two ways: powder is reduced to 500 pounds and the water/cementitious materials ratio (at 0.31) is higher than for pervious mixes of the past. These differences are another part of our message that pervious is advancing in mixtures and in methods.
The inexperienced producer will commonly design a mix that is overloaded with powder. This would force the quality control team and the contractor to starve water from the mix, which begins the malfunction of six critical processes that result in pavement that either ravels or chokes. Mixtures choked with excessive paste has led to pervious concrete pavements that did not survive freezing conditions.
Five years ago we discontinued the advice that pervious should be a zero slump mix. We have recognized the superior performance and manageability of a slumpable, flowable mixture. We have also shown that the powder must be reduced to make space for more water. This seems risky to producers who are already nervous about making concrete with voids. In some cases, this has required producers to reduce their total powder by 300 pcy.
Nevada Ready Mix supplied the ACI logo concrete, giving us exactly what we asked for. We hauled in special aggregate from a local quarry so we could use aggregate with higher void content. Since we were using Type F fly ash instead of Type C, it was bumped up to 20% of the total cementitious material. We also used 5% silica fume, one of the great advancements we have discovered for pervious. Fume gives increased density to the paste, important for resisting absorption of deicing salts. Silica fume also adds lubricity to the paste to help form the paste coating and reform into the critical paste bridge. A mix water conditioner was also used to increase the wet-out of the dry materials, to resist evaporation, and to retard the set.
Considering the time required to complete the stamping process, we added some hydration stabilizer. This was contrary to our desires to generate strength, but our need to be pretty meant we needed enough time to do the stamping before any the concrete set up.
Aggregate voids are the prime concern in proportioning a pervious concrete mixture. This is traditionally done using the ASTM C-29 test (Bulk Density [Unit Weight] and Voids in Aggregate) with oven-dry aggregate. We find useful information in an extra step that verifies the void content and we use aggregate that is in an SSD (saturated surface-dry) condition. The calculation uses the specific gravity of the SSD stone to determine the volume of solids in the sample. One might presume that the inverse of this volume would determine the volume of the voids, which is true, but we verify this volume by filling the sample with water to note the added weight of the water that fits into the same sample. This critical number drives the maximum paste volume that is allowed in the pervious concrete mixture. People are often surprised to discover how much this reduces the amount of cement and many presume it’s an effort to economize the mix. But this puzzle is studied with one objective: to increase moisture in the mixture without choking the 20% design void content.
The slab was made tough. We embedded #4 epoxy-coated rebars and included structural fibers at 5 pounds per cubic yard. We prefer Forta fibers because they disburse easily. The slab was placed with a Bunyan motorized roller screed and finished with a Bunyan skip float.
The logo art and lettering was formed from plastic templates that were laser-cut to shape and connected with small ribs to hold it all in alignment. Once positioned, the art was embedded to a depth of 3/8 inch using trowels. The thin plastic sheeting formed a slight radius as it stretched into the edges of the stamped zones. This plastic was removed and the surface was sprayed with The Bean as a cure and penetrating sealer. All of the formal practices of moist curing were observed, including installing 6 mil polyethylene sheeting. We did not use superabsorbent polymer. Many people are misguided about that being the essential component for successful pervious or as a shortcut in moist curing. The product should not be used as an excuse to skip moist curing. I would not consider it or any other VMA as an essential component to a successful pervious mixture.
Day two began with a white background created with white cement and white sand, with the pattern blocked out by the same plastic components of the art. The stamps were then removed and the inlaid colors began. We made a small part of the logo color with Black Eyed Peas, a ¼-inch cubic black basalt aggregate from Oldcastle, and the rest was crushed glass, including one containing glow in the dark glass. All of these mixtures used a clear polyaspartic binder with all of the color coming from aggregate.
Following the show, the ACI logo slab was moved without cracking and now serves in continued observation of abrasion and density at the Nevada Pervious Research Facility, a think tank for pervious gurus who continue to “always advance” pervious concrete.
David Mitchell is owner of Bunyan Industries, a Salt Lake City-based manufacturer of roller screeeds. For more, visit www.bunyanusa.com.