Keeping the gaps open: here's a practical approach to pervious concrete research

Concrete Producer, The, March, 2005 by Rick Yelton

Producers first wanted to know what permeability values could be expected from pervious concrete. Crouch started by reporting information on reference permeability values for various unbound aggregates. For example, rip rap has a permeability range of 100 cm/sec. Permeability tests on piles of clean gravel would range from 1.0 to 100 cm/sec., depending on gradation. And clean sands would range from .001 to 1.0 cm/sec.

Permeability values

TTU's research next focused on developing permeability values of these local materials bound together with cement. They discovered permeability values vary greatly depending on many material selection factors. It's a function of effective void content and aggregate gradation. As shown in Figure 1, permeability can differ when using different materials of the same gradation based on a mix design of 0.30 water/cement ratio and 600 pounds of portland cement. The research also found that there are some combinations that won't work for pervious concrete, as evidenced by the blank spaces in the grid floor.

[FIGURE 1 OMITTED]

The researchers followed up by answering the question on how different effective void contents can be achieved through mix designs of differing aggregates. They found that effective void content is a function of aggregate properties, such as gradation, shape, and texture, along with paste amount. But how compaction affected each recipe was just as important.

Using a standard mix, researchers used a Marshall hammer to compact samples of various mixes using local aggregates into a 4x8-inch steel mold in the laboratory: They measured the way each batch mix responded after a set number of Marshall Hammer blows.

It wasn't surprising to discover that as batch mixes were subjected to repeated blows, they experienced losses in the effective voids. But what was surprising was how similar compaction efforts affected different materials of the same gradation.

As shown in Figure 2, a gravel batch mix had lower effective voids than a limestone mix after the same number of Marshall hammer blows. "The smoother texture and rounder shape of gravel allow it to compact to a higher density and lower voids than crushed limestone at the same compactive effort," says Crouch. This result strongly indicates the importance of the trail batch as a tool to help the contractor predict how the mix will perform during compaction.

[FIGURE 2 OMITTED]

TCA producers wanted to know if lab pervious concrete mixes had similar testing results as field-batched and placed concrete. Crouch's research revealed that they are very similar. "Laboratory compressive strengths are typically a little higher due to better curing and more consistent compaction," he says. Thus, laboratory pervious concrete with almost any desired effective void content can be produced to model field pervious concrete behavior as shown in Figure 3.

[FIGURE 3 OMITTED]

Crouch's TTU team soon will report to TCA on another practical question for producers: Can pervious concrete be more workable and still retain the required permeability and compressive strength?