Road Management & Engineering Journal
April 1, 1998
(U.S. and Canada)
Fax: (360) 335-6402
Last fall Dodge County tested ways of reclaiming asphalt pavement in demonstration sections on CTH V. They used four different reclaiming techniques for preparing the base, following all with a 2 « inch hot-mix mat. All sections looked good this spring, reports Bob Sindelar, Dodge Co. highway engineer. However, cores taken in July showed that an innovative asphalt foaming technique and a conventional milling and cold in-place recycling method both produced a strong base while pulverizing followed by a split lift compaction with no added asphalt produced a weaker one. The methods and their results are:
Foamed Asphalt Injection
This method improves on a European technique and involves pulverizing existing asphalt pavement to full depth, reshaping and repulverizing the top four inches. It is simultaneously injected with a hot liquid asphalt cement (AC 120-150) that has been expanded 8-10 times by spraying 1% to 1« % water into it producing a foam. The coated aggregate is relaid and compacted by a grader fitted with a ski and automatic slope control. The aggregate remains workable for up to eight hours and when cured it appears much like standard asphalt.
This section produced strengths of 4000-4500 pounds at 72oF in modified Marshall stability tests made shortly after completion. Core results show the four inches of foam pulverized pavement appear dense and well bonded. It cost about $250 to pulverize, foam and shape 100 feet of two-lane pavement (one station), plus about $200 for the AC.
Emulsion Asphalt Injection
Existing asphalt is pulverized in place to a full depth of the existing pavement. After reshaping, on a second pass, the material is injected to a four-inch depth with asphaltic emulsion (HFE-300) at 160oF, then shaped and compacted. The material sets up quickly, giving it shorter workability time. Modified Marshall density tests at 72oF showed this section at about 1800-2000 pounds. The five-inch emulsion-injected section of the core appears less dense with poorer bonding than the foamed asphalt core. Pulverizing, grading and injecting emulsion cost $315 per station and the emulsion an additional $190.
Split Lift Compaction
This method pulverized the asphalt to full depth, 10-12 inches, then followed a new Wis-DOT requirement to compact the material in two lifts or layers with water added to each. The top level is graded to the side while the lower half is compacted. The loose material is then spread back on the road and compacted. The cost was $315 per station. The top 1.7 inches of the core showed pulverized pavement bonded to the new asphalt, the remaining eight inches were fine and gravely with very little bonding.
Milling and Cold In-Place Recycling
This widely used conventional milling and cold in-place recycling method involves milling asphalt to a maximum of six inches. The reclaimed asphalt is crushed into one inch, screened, and passed over to a paver which relays it. No emulsion or rejuvenation is added. Moving the material may produce excess in windrows and the system can be difficult to balance. The cost was $234 per station. In the core, the top three reclaimed inches looked very similar to the hot mix top. Beneath is 6.5 inches of fine, gravely, pulverized pavement.
A researcher from Marquette University is evaluating the pavement quality produced by these methods. The goal is to produce high-quality finished roads at the most economical cost. The foaming technique may save money because its strong base permits a thinner overlay mat, cutting per mile costs and stretching the new hot-mix over more miles. It can also be used to build up the base for heavier traffic loads.