As the pervious concrete market grows, concrete producers, contractors, and regulators are learning first-hand what works, what doesn't work, and the challenges of working with the material.

Thanks to an unprecedented amount of research and field trials, the concrete industry has advanced this pervious concrete technology greatly over the past five years. Inroads have been made in mixture proportioning, construction techniques, and design applications.

Yet there remains one concern about pervious concrete—its surface durability. Specifiers want to be assured that a pervious mix design has strong resistance to raveling.

In the last few months, efforts have been made to help provide a new tool to assess pervious concrete's durability. If adopted, the method could help producers determine the durability of proprietary mixes, and given enough industry support, become a test method that could provide a quality standard.

In this study, researchers prepared lab-cast cylinders. They then subjected these cylinders to a modification of a common aggregate quality test method. The team compared the raveling resistance of pervious concrete mixtures using different aggregates, various cement contents, and basic chemical admixtures using this method. The results will provide the industry with beginning correlations between basic mix ingredients, and the surface durability of a finished pervious concrete pavement.

The problem

Pavement raveling concerns are not new. It's a common problem in asphalt paving. The Federal Highway Administration defines raveling as the wearing away of the pavement surface caused by the dislodging of aggregate particles.

While you don't normally encounter the problem with concrete paving, pervious concrete is much different than traditional concrete. Technically, you can describe a pervious concrete pavement as a prismatic mass of aggregate with each aggregate particle having a coating of cementitious paste that bonds it to adjacent coated aggregate particles. The bonding paste should be thick enough to give the concrete structural strength, but thin enough to allow the voids in the mass to remain open, even under compaction.

For a pavement to ravel, excluding aggregate failure, there must be sufficient stress imparted on the coated aggregate to cause the bonding paste to fracture completely. Three sources can generate sufficient stress. First, the force could be from a large load. Second, the bonding paste strength could be weak. Third, the area of contact between adjacent aggregate particles could be too small.

Engineers traditionally think raveling occurs from quality control issues associated with batching, handling, or curing. Unfortunately, pervious concrete pavements can ravel, even if batched with sufficient mix water, handled properly, and properly cured.