Partial depth cold in-place recycling (CIR) has been performed throughout North America for many years. The process is used to eliminate surface irregularities, especially cracking. Besides providing a high quality base for a new asphalt surface course, environmentally friendly CIR also permits the preservation of existing asphalt pavement materials. In the typical CIR process, 4 in. of existing pavement is milled and mixed with an asphalt emulsion for stabilization. Once the recycled material has cured, it is then usually overlaid with an asphalt surface course or treatment. A chip seal may be selected for very low traffic applications, or a hot-mix asphalt (HMA) overlay may be used in heavier traffic conditions.
The recycled millings are normally stabilized with emulsified asphalt, though other recycling additives, such as portland cement and fly ash, also have been used. Both anionic and cationic asphalt emulsions have given good results. More recently, polymer modified asphalt emulsions have been used to take advantage of the greater durability, greater resistance to cracking and lower temperature susceptibility imparted by the polymers.
The Kansas Department of Transportation (KDOT) had used asphalt emulsions for CIR in many projects with good results. However, some emulsion CIR projects exhibited rutting and asphalt stripping problems, leaving KDOT, in 1992, to specify Class C fly ash as the only approved recycling additive for CIR. Fly ash was found to prevent early rutting and raveling, especially when traffic drove on the CIR before the overlay was placed. The fly ash CIR projects, however, were found to prematurely crack.
Concerns about early cracking with fly ash caused KDOT to construct a 19.05-mile experimental CIR project on U.S. 283 between Minneola and Dodge City, Kan., in 1997. The experimental project had approximately two equal sections of CIR, one with fly ash and one with both a solventless asphalt emulsion formulated for recycling and a lime slurry. The hydrated lime slurry was produced from the tank slaking of quicklime in the following exothermic chemical reaction: Quicklime (CaO) + water (H2O) = Hydrated lime (Ca[OH]2) + Heat.
For the fly ash section, 10% by weight of millings of Class C self-cementing fly ash was added to the recycled mixture. In the asphalt emulsion section, 1.5% by weight of millings of hydrated lime was added to the recycled mixture. The percentage of slurry added was dependent upon the slurry solids content (at 35% solids, the slurry was 4.3% by weight of millings; at 37% solids, the slurry was 4.1%, and so on.). The purpose of the hydrated lime was to improve the early gain in strength and resistance to moisture damage of the asphalt emulsion recycled mixture. Both of these benefits have been observed in earlier research projects.
Hooking up the train
The construction of both experimental sections used a recycling train. The train consisted of a full-width milling machine, a trailer-mounted screening and crushing unit and a mixing unit. The millings were sized such that 100% of all material passed a 1 1/4-in. screen. The asphalt mixing unit was equipped with a belt scale and computer for accurate addition of asphalt emulsion, lime slurry or water by weight of millings.
For the fly ash section, the fly ash was placed ahead of the recycling train on the existing pavement at the required amount using a calibrated dry spreading unit. The fly ash was initially incorporated into the millings during pulverization by the milling machine cutting mandrel. Water was then added and final mixing completed by the mixing unit. With asphalt emulsion, water was added at the cutting mandrel of the milling machine and emulsion added at the mixing unit.
The CIR mixtures were placed with a conventional asphalt paver equipped with a pick-up device for transferring the windrowed mixture. A heavy (27 metric tons, 30 short tons minimum weight) seven-tire pneumatic roller was used for the initial compaction. Intermediate and final rolling used a double drum vibratory roller, both with vibratory and static passes.
After proper curing, the CIR was overlaid with a 1 1/2-in. wearing course of Kansas BM-2A HMA manufactured with a PG 64-22 performance graded asphalt binder.
Cores were taken in both sections in 1998 to measure the performance of each of the CIR materials. The 150-mm diam. cores were tested for cold temperature properties, for the modulus and for rutting properties. Cores were taken for the fly ash section at mile marker 38.4 and for the emulsion with lime slurry section at mile marker 41.2.
Cold temperature properties were measured with the Superpave creep compliance and tensile strength tests. These tests were performed using Koch Materials Co.’s indirect tensile testing machine. The 150-mm diam. cores were cut to 50 mm in height and tested according to AASHTO TP 9 at 0°C, -10°C and -20°C. Tests also were run at -30°C because the estimated initial cracking temperature was below the -20°C of the TP9 protocol.
Creep compliance is approximately the inverse of stiffness. At very low temperatures, the stiffer an asphalt mixture, the more brittle it is. Inversely, a higher compliance of a mixture at cold temperatures indicates that it is less brittle. For a mixture with a given binder, the compliance at warmer temperatures is higher than the compliance at colder temperatures. Normally when comparing two mixtures made with the same aggregate, but different binders, the creep compliance measurements can be grouped by temperature. The data shows that the compliance of the asphalt emulsion lime slurry cores at -20°C is higher than the compliance of the fly ash cores at 0°C. This indicates a big difference in the thermal relaxation properties of the two materials, with the emulsion treated recycle mixture exhibiting less brittle behavior than the fly ash treated mixture. The pavement thermal stress properties were estimated from the compliance data.
Theoretically, at the temperature where a mixture’s tensile strength equals the pavement thermal stress, cracking will initiate. Since mixtures made with different binders will relieve tensile stresses at different rates, depending on their engineering properties, each mixture will have a unique pavement thermal stress curve. Intersection for asphalt emulsion and lime slurry occurs at -27°C (-17°F). These results confirm the fly ash mixture should crack before (at higher temperatures than) the asphalt emulsion and lime slurry mix.
Tests also were run using the Superpave shear tester, following the procedures outlined in AASHTO TP 7. The frequency sweep test at 20°C and 40°C compared the shear complex modulus values of the two materials. The stiffness, or shear complex modulus, of the fly ash cores at both temperatures was higher than the stiffness of the emulsion and lime slurry cores.
Cores were tested in the Asphalt Pavement Analyzer according to Georgia DOT Method GDT-115M. In this test, performed at 50°C, concave-shaped wheels travel back and forth over a stiff, pressurized rubber hose which rests directly on the specimen. Six 150-mm diam. cores from each of the experimental sections were subjected to a 100-lb wheel load and 100-psi hose pressure. GDOT failure criteria for conventional mixes tested at 40°C is no more than 7.5 mm rut depth after 8,000 cycles (16,000 passes) under dry conditions. SMA specimens and modified asphalt specimens are tested at 50°C according to GDOT specifications. The wheel speed is approximately 60 cm/sec.
A distress survey was performed in November 1999. Transverse cracks were measured in three fly ash sections and three emulsion with lime slurry sections. The three fly ash sections measured were at mile markers 34, 36 and 38; the full 24-ft width was evaluated for transverse cracking. The sections were 500 ft in length and went north for mile markers 34 and 38 and south for mile marker 36. The emulsion plus lime slurry sections measured were at mile markers 43, 45 and 47. The sections were 500 ft in length, the full 24-ft width of pavement, and went north for mile markers 43 and 47 and south for mile marker 45.
A total of 711 ft of cracking in the three fly ash sections was measured, and a total of 369 ft of transverse cracking was measured. The fly ash section has nearly twice the amount of cracking as the emulsion with lime slurry section, and it is believed that the excess cracking originated in the recycle fly ash layer. Of course, some of the cracking probably originated in the hot mix overlay, but it can not be separated from the cracking in the CIR layer below it.
No cracks were wider than 1/4 in., with most cracks about 1/16 in. wide. Superpave indirect tensile thermal cracking tests in the laboratory verified field transverse cracking.
The fly ash section also is showing longitudinal cracking throughout much of the length of the section; crack lengths were not measured in the field survey. In some cases, the cracking was in both wheel paths, and in other cases just in the outside wheel path. In some cases, there were two longitudinal cracks side-by-side in the outside wheel path. There was either no longitudinal cracking in the emulsion plus lime slurry section or short areas of cracking with lengths of 10 to 30 ft. These cracks also were low severity. The longitudinal cracks are believed to be load associated, especially since they are predominantly in the wheel paths.
The greater severity of longitudinal cracking in the fly ash section indicates a greater tendency for fatigue associated damage. The tendency of the fly ash section for greater fatigue damage is indicated by the greater stiffness as measured in shear modulus testing. There was slight to no rutting observed in the field and no notable difference between the sections. Laboratory measurements with the APA and shear modulus testing indicate low susceptibility for rutting of the recycle layers.