Pervious concrete has great potential but its widespread adoption has lagged. Partly due to the natural caution of owners, engineers, and contractors to use what they consider an unproven technology, but also due to some failures. Nothing derails a new technology like an unexplained failure—even when 95% of the projects are successful.
Virtually all of the problems with pervious have been related to raveling of the aggregate—where it breaks free of the pavement matrix. The causes are typically poor curing practices or inadequate compaction, but there can be problems with the mix that lead to these issues: too wet or too dry, too much fine material, or too much cement can put even an experienced contractor in a position where a successful installation is impossible.
In the past, there has been no standard method specific to pervious to test the mix onsite. ACI 522.1-08 Specification for Pervious Concrete requires density testing in accordance with ASTM C 138, Standard Test Method for Density (Unit Weight), Yield, and Air Content (Gravimetric) of Concrete, but that test has not been completely successful for pervious. In October 2008, though, ASTM Subcommittee C09.49 released C 1688, Standard Test Method for Density and Void Content of Freshly Mixed Pervious Concrete. This is the first step in a series of tests for pervious that should give everyone more confidence the material they are getting is right for the application. But not everyone agrees the test ensures the industry of a good pervious mix.
ASTM C 1688
First, there are a couple of important things to understand about ASTM standard test methods:
- The purpose of standardizing a test is so anyone with a little experience and basic knowledge can follow the test procedure and get similar results.
- However, just like with concrete cylinders, the results don't necessarily reflect the in-place properties of the material, which is not really the intent.
With cylinders, when made and tested according to ASTM C 31, Standard Practice for Making and Curing Concrete Test Specimens in the Field and C 39, Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens, the compression test results indicate the potential strength of the material, not the strength of the concrete in the structure. This is also true of the density of pervious concrete measured with ASTM C 1688. “Some people think the test should match the consolidation in the field,” says Karthik Obla, managing director of research and materials engineering at the National Ready Mixed Concrete Association and chairman of ASTM Subcommittee C09.49. “But the challenge is that in the field there are so many different ways of consolidating—different equipment and different procedures. So if you're trying to match the field consolidation, which one do you match? Some were concerned that we not consolidate too much in the test because then we would be too far from reality, but I think the procedure we chose is right in between, and so that's good.”
Prior to publication of ASTM C 1688 in October 2008, the unit weight (or density) of pervious concrete was tested using ASTM C 138, which is the method for testing the density of conventional concrete. To compact the mix, this was combined with the “jigging” procedure in ASTM C 29, Standard Test Method for Bulk Density (this is what ACI 522.1 currently requires although this will be changed to ASTM C 1688 when the specification is revised). This method, though, had large variations in the results. The new test method standardizes the compaction technique and produces reasonably consistent results. “It's a very easy test to run,” says Obla. “It's pretty consistent and it's easy to use in the field. Say the pervious is specified at 120±5 pounds/cubic foot, you can do the unit weight test quickly and make sure that the concrete that comes down the chute is what you ordered.”
To conduct the ASTM C 1688 test, what ASTM calls the “measure”—typically the bucket used for an air test under ASTM C 231, Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method—which has a volume of 0.25 cubic feet. A sample is collected from various locations in the batch (see ASTM C 172, Standard Practice for Sampling Freshly Mixed Concrete). The container is filled with pervious concrete in two equal lifts and each lift is compacted by 20 blows with a Standard Proctor Hammer. The pervious concrete is then struck off flush and the bucket is weighed both with the pervious inside and empty. The concrete's weight is divided by the volume (multiplied times 4) and the result is the density.
Void content is calculated as the difference between the theoretical air-free density (which can be determined from the material weights on the batch ticket) and the measured density.
Is it the right test?
“This test is less important than other critical issues,” says David Mitchell, president of Bunyan Industries, West Jordan, Utah. “It will produce erratic results when there are only very slight differences in the paste rheology.”
Mitchell describes the ideal pervious mix as a skeleton of 3/8 -inch aggregate with point-to-point contact between the aggregate that are coated with a thin layer of cement. “If the paste lacks the ability to move away from the critical point-to-point contact between the large aggregate, then the aggregate becomes separated by a margin of paste,” he explains. “It's a condition that I call fluff. When mechanical compaction is applied to that mix, the surface will compact but the body of the slab remains just as it was delivered.”
This inability of the paste to flow typically is caused by the paste being too dry. The cement paste can't form the paste bridge that connects the aggregate allowing it to carry the loads. “When pervious concrete is brought to the critical moisture content,” says Mitchell, “the unit weight test is quite consistent and reliable. Because the specimen is tested in a slightly drier condition, the results vary greatly.”