Rajib B. Mallick

Using Warm-Mix Asphalt Technology to Incorporate High Percentage of Reclaimed Asphalt Pavement Material in Asphalt Mixtures

The use of reclaimed asphalt pavement (RAP) helps save natural resources and money. The percentage of RAP that can be utilized successfully in hot-mix recycling is primarily dictated by practical considerations. To avoid deterioration of the aged binder, RAP should not be exposed to relatively high temperatures. This study investigated the feasibility of using a warm-mix asphalt (WMA) additive, Sasobit H8, in successfully recycling hot-mix asphalt (HMA) with 75% RAP at a lower temperature. A control HMA was prepared with extracted aggregates and PG (performance grade) 64-28 binder at 150°C. Another HMA was produced with PG 52-28 binder at 135°C. Two WMA mixes were prepared with Sasobit H8 at 125°C, one with PG 52-28 and the other with PG 42-42 binder. Samples with design asphalt content were compacted by using 75 gyrations of the Superpave gyratory compactor. Their voids, tensile strength at −10°C, rutting potential at 60°C, and moduli at 0°C, 25°C, and 40°C (at different times) were determined and compared. The moduli samples were subjected to 60°C in between the tests. The results show that it is possible to produce mixes with 75% RAP with similar air voids as virgin mixes at lower than conventional temperatures using 1.5% Sasobit. The addition of a significantly lower grade of binder, PG 42-42, at a rate of 1.5% by weight of mix produced a mix that is most comparable with a virgin mix.

Effects of Warm-Mix Asphalt Additives on Workability and Mechanical Properties of Reclaimed Asphalt Pavement Material

The soaring cost of liquid asphalt binder and anticipated stricter environmental regulations have driven highway agencies to maximize the amount of reclaimed asphalt pavement (RAP) used for pavement construction. However, because of already aged and stiffened asphalt binder in RAP, the use of high percentages of RAP in hot-mix asphalt (HMA) presents many challenges. Problems with workability and compactability during construction need to be resolved first. This study investigated the feasibility of using 100% RAP HMA as a base course with warm-mix asphalt (WMA) additives (Sasobit H8 or Advera zeolite) at a lower temperature (125°C). Mix samples (control set with 100% RAP; a set with 100% RAP plus Sasobit H8 at 1.5%, 2.0%, and 5.0%; and a set with 100% RAP plus Advera zeolite at 0.3%, 0.5%, and 0.7%) were compacted with 50 gyrations. Their workability, bulk specific gravity, indirect tensile strength at 0°C, and moduli at 0°C, 26.7°C, and 50°C were determined. The effects of different amounts of WMA additives were compared. The results showed that workability of the 100% RAP HMA improved with the addition of Sasobit H8 or Advera zeolite at temperatures as low as 110°C. At temperatures less than 80°C, the addition of Sasobit H8 or Advera zeolite tended to stiffen the mix, as also reflected in increased seismic moduli and indirect tensile strength. Seismic modulus of the mixes was also found to be dependent on bulk specific gravity. The addition of Sasobit H8 proportionally increased bulk specific gravity of the mixes. The effect of amounts of Advera zeolite on bulk specific gravity was less well defined. It seemed that stiff asphalt binder in the RAP also affected compaction by preventing asphalt foam from fully forming, as it would when Advera zeolite was mixed with a virgin asphalt binder.

Review of very high-content reclaimed asphalt use in plant-produced pavements: state of the art

Asphalt is the most recycled material in the USA at a re-use rate of 99%. However, by average only 10–20% reclaimed asphalt pavement (RAP) is used in a given mix design and large part of the RAP is degraded for use in lower value applications. The amount of RAP in asphalt mixtures can be significantly increased with the application of good RAP management practice, readily available modern production technologies and advanced knowledge of mix design. This paper summarises the state-of-the-art approaches for increasing the amount of RAP in asphalt mixtures above 40%. The production challenges and common pavement distresses of very high RAP content mixtures are identified and methods to optimise the mix design as well as production technology in order to allow manufacturing of such sustainable mixtures are described. The best practices for RAP management and economic benefits of high RAP use are also discussed.

Harvesting energy from asphalt pavements and reducing the heat island effect

A rise in temperature of asphalt pavements contributes towards the urban heat island effect, causes problems with air quality and increases the power requirement for cooling buildings. A high temperature would also lead to the potential of rutting failure in asphalt pavements. The concept of mining heat from asphalt pavements, utilising an appropriate fluid flowing in pipes installed within the pavement, has been proposed. Theoretical considerations and results of laboratory testing and modelling simulation have been presented. The results indicate that the concept is feasible, and that the efficiency of heat mining can be improved by selecting appropriate surface layer and aggregates for pavement materials. The use of this proposed method would lead to a significant reduction in pavement and near-surface air temperature, and extension of asphalt pavement life.

EFFECT OF MIX GRADATION ON RUTTING POTENTIAL OF DENSE-GRADED ASPHALT MIXTURES

A study was conducted to evaluate the effect of mix gradations, both complying with and violating the Superpave restricted zone, on rutting potential of hot-mix asphalt. Superpave gyratory samples of mixes with granite, limestone, and gravel aggregates were used. The following gradations were used: gradation above the restricted zone, gradation through the restricted zone in close proximity to the maximum density line, and gradation below the restricted zone. Rut tests were conducted at 64°C with the asphalt pavement analyzer under 689-kPa contact pressure and 45.2-kg wheel load. Rut depths were measured at the end of 8,000 cycles. Repeated shear tests at a constant height were also conducted with the Superpave shear tester. Statistical analysis of rut data generally indicates a significant difference between rut depths obtained in mixes using different aggregate types and gradations. The gradations violating the restricted zone did not necessarily give relatively higher rut depths compared with the gradations in compliance with the zone.

Use of Superpave Gyratory Compactor To Characterize Hot-Mix Asphalt

A method was developed to identify unstable mixes with the help of Superpave gyratory data. The basis of this method is that the primary difference in behavior of a stable mix and an unstable mix is that a stable mix gains in strength with densification and retains it through further compaction and ultimately resists lowering of voids below a particular value, whereas an unstable mix initially gains in strength but loses it beyond a certain densification point and becomes susceptible to shear failure. Hence it is proposed that the strength of a mix be evaluated at two void levels: at 5 percent, where a mix is expected to have an increased shear strength compared with postconstruction voids, and at 2 percent, where a well-performing mix should have an increased or at least similar shear strength compared with shear strength at 5 percent, and where an unstable mix is expected to have a reduced shear strength compared with shear strength at 5 percent voids. U.S. Army Corps of Engineers and Superpave gyratory compaction data were obtained for five mixes from wearing courses, and the data were evaluated with the help of in-place rutting data. The gyratory ratio, a ratio of the number of gyrations required to achieve 2 percent voids and 5 percent voids, was found to be suitable for characterizing hot-mix asphalt. Theoretical calculations and in-place rutting data show that a gyratory ratio of 4 can be used to differentiate between stable and unstable mixes—mixes with a gyratory ratio less than 4 can be expected to be unstable.

Construction and Maintenance Practices for Permeable Friction Courses

This report recommends design, construction, and maintenance guidelines for permeable friction courses (PFCs). It presents recommended practices for (1) design and construction of PFCs and (2) PFC maintenance and rehabilitation. The report will be of immediate interest to public and private sector engineers with responsibility for the specification, construction, and maintenance of PFCs.

AN EVALUATION OF SUPERPAVE BINDER AGING METHODS

The objective of this study is to evaluate the rolling thin film oven (RTFO), and the pressure aging vessel (PAV) binder aging methods recommended by superpave. Test sections for this study were set up in 1992 and 1993 using six highways in five states: US 280 and AL 86 in Alabama, I-90 in Idaho, I-40 in New Mexico, I-385 in South Carolina, and STH 67 in Wisconsin. Asphalt binder was recovered from loose mix obtained during construction and from cores obtained after first, second and third year. Dynamic shear rheometer (DSR) and bending beam rheometer (BBR) tests were conducted on the recovered binder and on asphalt binders subjected to RTFO test (RTFOT) and PAV method. Comparison of DSR test results of asphalt binder recovered from loose mix and DSR test results of RTFOT conditioned asphalt binder indicated that asphalt binders are aged more in the plant mix process than in the RTFOT process. However, statistical analyses indicate that the RTFOT method seems to be capable of simulating short term aging of asphalt binder. Complex shear modules (G*) and phase angle (δ) values from asphalt binder extracted from in-place mixes were used to develop time versus property models. G* values were found to increase and δ values were found to decrease with time. Low temperature stiffness (S) values from asphalt binder extracted from in-place mixes were used to develop S versus time models. Stiffness values were found to increase with time. G*, δ, and S values after 10 years were predicted on the basis of the G*, δ, and S versus time models. The predicted fatigue factor (G*sin δ) and S values were then compared to G*sin δ and S values obtained from tests on laboratory aged asphalt binders. The comparison indicated that the PAV method is capable of simulating long term aging of asphalt binders.

Evaluation of Performance of Full-Depth Reclamation Mixes

The full-depth reclamation (FDR) process consists of reclaiming all of an asphalt-bound section along with a predetermined amount of underlying base, with some additive. The objectives of this study were to determine the suitable compactive effort for designing FDR mixes, evaluate the benefits of using different types of additives in terms of improvement of pavement life, and determine suitable structural numbers for pavements recycled with different types of additives. The scope of work for this part of the study consisted of conducting falling-weight deflectometer (FWD) tests, testing an existing pavement before FDR, sampling of materials during FDR, determining the density of in-place material after compaction, compacting of loose mix in the laboratory, determining the density of the compacted samples, conducting FWD testing on the finished pavement, determining the resilient modulus of in-place cores, and analyzing the data to determine the suitable number of gyrations, improvement in pavement life, and structural numbers. It was concluded that samples should be compacted to 50 gyrations during mix design and that a minimum of 98% of density of in-place loose mix samples, compacted to 50 gyrations, should be achieved in the field at the end of compaction. Cost comparison showed that for the options considered in this study, recycling with emulsion (3.4%) and lime (2%) is the most cost-effective option. A visual evaluation of recycled sections after one year showed no significant distress in any section except the one with water as additive, in which a moderate amount of edge cracking was noted.

Use of Accelerated Loading Equipment for Determination of Long Term Moisture Susceptibility of Hot Mix Asphalt

Stripping of HMA with moisture susceptible aggregates, under high temperature and aircraft loading has been a persistent problem in some areas of the Logan International Airport in Boston, Massachusetts. These problem mixes generally meet the retained tensile strength criteria, after freeze–thaw conditioning. This study was conducted to evaluate the use of accelerated loading equipment for identifying moisture susceptible mixes, and also to evaluate the effect of lime. Mixes with three different aggregates were prepared with PG 76-28 asphalt binder according to specifications. Specimens were conditioned by three different methods: (1) multiple cycles of freeze–thaw, (2) wet trafficking with the model mobile load simulator (MMLS3) and (3) cycles of stress with the moisture induced stress tester, (MIST). Thereafter, the respective indirect tensile strengths were determined. The results showed that accelerated loading can provide useful information for evaluating resistance of HMA to moisture damage under traffic at high temperature. For mixes to have adequate resistance to moisture damage by volumetric expansion–contraction, acceptable tensile strength tests were needed after at least six freeze–thaw conditioning cycles. The evaluation of resistance to moisture damage under traffic at high temperature using moisture stress conditioning proved to be highly promising. The methods should be investigated further. The use of hydrated lime, improved the resistance of HMA against moisture induced stress damage at high temperatures. On the basis of these conclusions, it is recommended that testing protocols, consisting of both freeze–thaw and accelerated loading/moisture induced stress testing at high temperature, be used for evaluating the most cost-effective anti-stripping agent (liquid or solid).