“The point of the NGCS surface is to limit positive or upward texture,” Scofield explains. “The NGCS texture is designed to develop good macro-texture through ‘negative’ or downward texture (grooves). We want to have both good microtexture (the texture on top of the lands), and good macrotexture, which is developed primarily through the grooves.
“The NGCS is quieter because it relies on negative texture and not positive texture,” Scofield continues. “Since it is more of a manufactured surface, it can control the fin profile to a greater degree than previously possible. Purdue University determined that the fin profile is the critical element in noise generation.”
A new reality
The epiphany in research was soon confronted by reality, Scofield says. Research showed that the flush-ground-then-grooved texture could produce a quieter pavement. But the research could not verify whether such a texture could be constructed with conventional equipment in the field.
Next, in response to industry representatives, researchers developed two methods of reproducing the NGCS. The first was a grinding head configuration that used three smaller blades stacked between two taller blades. That pattern was repeated across the grinding head. That way, in one pass the head could grind the surface smooth and also groove it on approximately ½-inch centers in one pass of the machine. The smaller blades would flush grind the specimen and provide microtexture while the taller blades created grooves.
The second grinding configuration used the same smaller blades to flush grind the pavement in one pass. Next a second pass using taller blades with spacers created the grooves, similar to what was constructed with the single-pass operation.
That way, contractors could choose either option – the single pass or the double pass – in field construction. Some industry representatives thought the single-pass operation would cause excessive blade wear and have the potential for ruining the head and blades. Many believed the two-stage process would be required. Today, this is not a concern. NGCS pavements have been placed with a single-pass or double-pass operation, and both work equally well.
The opportunity to construct field test sections became a reality when the Minnesota DOT allowed construction of test sections at the MnROAD Low Volume Road Test Cell Number 37 as part of an FHWA pooled fund effort. The two Purdue surfaces were to be compared to a conventional diamond grinding surface to assist in determining the benefit achieved by controlling the fin profile. So, there was a need to build three test surfaces.
Findings validated that the newly-developed surface was quieter, at the time of construction, than the conventional diamond ground texture. And, findings showed that the Purdue TPTA results could be reproduced in the field using conventional equipment. But because those were not full-width text sections, the next step was to construct a full-width test section using a conventional diamond grinding machine.
The first opportunity to construct a full lane-width test section occurred on Interstate 355 in the Chicago area. In October 2007, both a conventional diamond-ground test section and an NGCS were built on the I-355 tollway. The sections were 1,200 feet long and one lane wide.
Scott Eilken is the owner of Quality Saw and Seal, the diamond grinding contractor for the NGCS section at I-355. He is an ACPA member, a board member of IGGA, and was instrumental in writing the specifications for NGCS.
“On I-355 we did one pass to flush grind the surface, and the second pass as surface grooving,” Eilken recalls. “When we first tested it, the surface produced just 99 dB(A). We were one of the first concrete pavements in the nation to get below 100 dB(A), so it worked pretty well.”
The next opportunity to build test sections occurred at MnROAD’s Interstate 94. A two-lane wide by 500-foot-long section of NGCS was constructed in a single-pass operation on a 14-year-old random transverse-tined pavement in October 2007.
With the successful placement and performance of the two mainline sections, the ACPA officially named the texture as the Next Generation Concrete Surface (NGCS). The name describes a category of textures that evolve for both new construction and rehabilitation of existing surfaces.
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