Q: Our contractor-customer asked us to provide a mix design for an upcoming concrete paving project. The high-volume road is planned for an area where the county engineer wants to minimize tire noise. The engineer has specified a rather thorough tining program on the concrete surface and has asked us to develop a mix design that would help. Do you have any guidelines?
A: Just after the concrete pavement is placed, contractors texture the surface to help reduce tire noise. Tining is the most common texturing method for concrete pavement. There are a host of patterns such as longitudinal and transverse tining that can be combined with other variants such as a skewed or wave pattern.
For the last five years, the National Concrete Pavement Technology Center (NCPTC) at the Institute for Transportation at Iowa State University has conducted a Surface Characteristics program on concrete pavements, evaluating more than 133 miles of pavement.
Paul Wiegand, transportation research engineer at the center, believes it is possible to construct quiet concrete pavements without using new concrete mixes. From his study, Wiegand suggests producers establish a greater level of control over all of the project elements, including the mix design, at the start of a quiet pavement project.
It all starts with the mix design. The consistency of the mix is critical to the tined surface texture, both in optimizing the pavement for low noise and providing a consistent texture quality. Adjustments in operations must be developed to address the environmental conditions that are present and changing throughout the day.
Changes in wind, sun, and temperature will impact the concrete mix. So you must adjust the timing of operations to maintain the quality of the texture imparted to the pavement surface.
It is vital that the mortar portion of the concrete be high-strength and exhibit low permeability to maintain the tined surface over the design life of the pavement. This is important because tining causes the hardened mortar ridge to become the road's wearing surface.
The mix's fine aggregate should be a siliceous sand, not only to increase the pavement's durability, but to also increase friction. The fine aggregate particle shape may also be important, as an overly sticky mixture can lead to displacement and shearing of the surface mortar. This, in turn, can distort the intended texture. Coarse aggregate should also be hard, durable, and polish-resistant.
Concrete mixes with a low water-cement ratio are ideal. Using supplementary cementitious materials is also recommended. This can help increase the mixture's durability and workability.
While potentially contradictory to meeting the objective of a durable mixture, researchers suggest the concrete in the vicinity of the surface should not possess too many intermediate-size aggregates. A more gap-graded mixture near the surface will allow for the 1/8- to 3/16-inch depth of mortar required for the grooves imparted by the tines to maintain their intended shape. If too much aggregate is present, the tines will constantly work around and displace the particles, resulting in a more aggressive and potentially noisy texture.
Discuss the mix design with the paving contractor. Because the surface has an increased percentage of mortar, there is also an increased potential for shrinkage (increased crack potential) and higher permeability (reduced durability).
The NCPTC has published “How to Reduce Tire-Pavement Noise: Interim Better Practices for Constructing and Texturing Concrete Pavement Surfaces.” It is available at the center's Web site, www.cptechcenter.org.