Geographically located to the south of Mexico and north of the continent of South America, Central America (CA) has a population of 30 million people in an area of less than 600,000 square kilometers. Surrounded by the Caribbean Sea and the Pacific Ocean, the region includes the following countries: Belize, Guatemala, Honduras, El Salvador, Nicaragua, Costa Rica, and Panama. These last two countries are the most developed. Until the year 2000, research and specifications for asphalt materials were very limited. This was because each country reduced their efforts in adopting American Association for State Highway and Transportation Officials (AASHTO) specifications for viscosity graded asphalt and penetration. The only methodology used for designing hot-mix asphalt (HMA) was the Marshall Design procedure. In 2000, the turning point for the asphalt industry arrived when the Central American Manual for Highway and Bridge Construction (CA 2000) adopted the performance graded (PG) grading system, as well as the Superpave method as an option for HMA design and asphalt binder selection. In addition, in 2002, Costa Rica devoted one percent of the fuel tax in order to perform project auditing, applied research, and technology transfer through the Transportation Infrastructure Program (PITRA) of the National Laboratory of Materials of the University of Costa Rica (LanammeUCR). This resulted in a state-of-the-art laboratory. Current research focuses on developing specifications and material analysis which is representative of tropical weather conditions and weak soil foundations of the region. Examples of these research projects are as follows: calibration of the Witczak model for Central America's HMA mixture; development of a new moisture damage test for tropical weather; adaptation of chip and slurry seals for local conditions; nano-characterization of neat and modified asphalt binders; development of fatigue and rutting models for HMA mixtures; and developing an empirical-mechanistic pavement design guide for the CA region - the most ambitious project.
As is the case with most Latin American countries, Colombia is facing rapid economic growth. Yet, in terms of meeting the upcoming challenges and demands, the transport infrastructure of the country is lacking. In particular, the Colombian road infrastructure is one of the most under-developed in the region. The preparation of ambitious road projects has been motivated by the situation, and these are increasing the demand for high quality asphalt materials. The asphalt market in Colombia is currently controlled by a Colombian oil company called Ecopetrol. This is the only company, by law, authorized to produce and commercialize asphalt binder in the country. An average of 22,000 tons of asphalt binder are produced by month by this company in two different refineries; this amount equals almost half of the actual capacity of the refineries. Penetration is still the basis of specifications for asphalt binders in Colombia, and it is unlikely that a modification to performance-based specifications will occur in the short term. Nonetheless, there are two actors in the country that are becoming significant promoters of changes, as follows: universities and contractors, particularly those in road concession projects. World-class research is being conducted in the area of pavement engineering, and the expectation is that these results will have an impact on the paving industry. Yet, as an economical scheme that was developed for the first time in the 1990's with the goal of achieving better management of national roads, road concessions are assuming risks in order to optimize their operation by exploring the use of new materials and techniques. As a result, in the upcoming years, significant changes in the asphalt market and paving industry may be expected.
When it comes to asphalt technology in Chile, the state of the practice is very empirical. With traditional testing, binders are specified. Using the Marshall method, mixtures are designed. The majority of mixes used currently are dense graded. Using procedures based on American Association of State Highway and Transportation Officials (AASHTO) 93, flexible pavement design is carried out. With the expectation of important changes, important research projects are currently underway. One such project is a large one focusing on optimizing mixtures for overlays. Advanced rheological and performance characteristics of binders and mixtures are involved. Another such project is for the implementation of the Mechanistic-Empirical Pavement Design Guide (MEPDG) in Chile; this includes developing an initial database for traffic, weather, and material characterization for mechanistic pavement design. Modeling of indentation testing on asphalt binders, research on foamed asphalts, and distress model calibration efforts are among some other research initiatives.
One of the most significant factors leading to premature distress of an asphalt pavement can be poor workmanship. An understanding of the processes, procedures, and consequences of failing to observe proper practices is crucial for mix plant and paving personnel. The result of improperly prepared surfaces may be an inability to meet smoothness specifications, an inability to bond well to existing pavements (in the case of an overlay), or failure due to inadequate subgrade support. In many cases, milling is used to prepare existing surface. Details of proper milling are presented here, as well as what is involved in the proper planning of paving a smooth pavement.
The overall performance of flexible pavements is influenced by properly constructed longitudinal joints. Longitudinal joints are the first place, in too many cases, where durability-type failures occur. A cooperative study to identify the best methods for specifying and constructing longitudinal joints was conducted by the Federal Highway Administration (FHWA) and Asphalt Institute (AI). Best practices for constructing asphalt longitudinal joints in four ways were reviewed, as follows: a benchmark survey of the FHWA's 52 offices was conducted with regards to longitudinal joint construction methods, specifications, and performance; going back to the 1960's, an extensive literature review was conducted; interviews with ten nationally-acknowledged pavement experts were carried out; and representatives (department of transportation, contractors, and researchers) from five states that have implemented longitudinal joint specifications met with FHWA and AI. The focus of this presentation was construction techniques. The method deemed best for constructing a longitudinal joint is by paving in echelon such that the lanes are rolled together while hot to eliminate it. Yet, the opportunity for paving in echelon is limited by the need to maintain traffic. In the event that a joint had to be constructed, there was an even division between the experts interviewed as to whether it should be a notched wedge joint or the traditional butt joint. Joint performance can be affected by mix selection. Better joints tend to be the result of smaller nominal maximum aggregate sizes and finer mixes. Further details are elaborated here.
Two research projects to evaluate factors during compaction that affect pavement density were sponsored by the Wisconsin Highway Research Program. In order to identify relationships, both intelligent compaction and traditional density testing were used. The benefit of identifying soft spots in subgrade or underlying pavement is a benefit of intelligent compaction. In the first study, eight projects were tested. Primary factors, including gradation (coarse and fine), aggregate source (limestone or gravel), and base (rigid or flexible), were evaluated using a full factorial design. Two more factors, N sub design and nominal maximum aggregate size (NMAS), were varied to also understand their effect. Significant factors for fine-graded mixes were layer thickness and the interaction between layer thickness and base (flexible or rigid). While thicker lifts were easier to compact on flexible (crushed aggregate) bases, thinner lifts were easier to compact on rigid bases. Unfortunately, for coarse-graded mixes, several variables were confounded. In the second study, attempts were made to isolate the effects of temperature and pressure. Included in the study were thirty layers from 22 projects. Using roller (e.g. breakdown, intermediate and finish) and vibratory setting, densification was analyzed. Total passes had a dominant effect on achieved density, for breakdown roller; also important, but to a lesser degree, were temperature and use of vibratory compaction. A dominant factor as the temperature dropped below 200°F was mat temperature, and through all temperature ranges, passes remained an important factor. For finish rolling, relationships among factors were relatively weak. A significant increase was provided by use of vibration; total passes was a dominant factor; although practically less so, temperature was statistically significant. Densification was relatively easy to achieve at first, as expected, and as mat density increased for all rollers, it became increasingly difficult. Higher mat temperatures resulted in fewer passes, overall, but maximum recorded mat temperatures were 275°F or less. In all cases, regardless of varying all the aforementioned factors, 92% of G sub mm could be achieved in four to ten cumulative passes.
Making a lab mix similar to the plant-produced mix is the key to a good lab design. As the stockpile is being built, aggregates should be sampled. Representative samples from the perimeter of the pile can be difficult to obtain. Gradation standard deviation and trends need special attention. As the target gradation for each material in the mix design, be sure to use the latest trend. As a tool to evaluate aggregate packing, the Bailey method can be used. This method has resulted in improved field compaction and decreased mix design trials. A reduction in tender mixes has also been seen. Much of the difference between plant and lab mixes can be accounted for by aggregate breakdown. Additional fines and edges knocked off coarser aggregates are created in plant production. Lower voids and lower voids in mineral aggregate (VMA) is the result of both. In order to compensate, bag house fines should be added to lab mix. The best indicator of lab versus plant volumetric properties is VMA. Differences between lab and field VMA averages less than 0.1%, using these principles. Consistency is key for percent within limits (PWL) projects. Developing a paving plan and communicating between plant and field to plan plant starts and stops is important. Loading trucks on an incremental basis is also important, as well as building variability into design and maintaining a consistent paver speed.
The goal of National Cooperative Highway Research Program (NCHRP) 09-47A was to more precisely and completely quantify emission reductions and energy savings realized by warm mix asphalt (WMA) technologies. Based on ten technologies evaluated at five sites, fuel savings up to 22% were achieved. Also confirmed were fuel savings resulting from reduction in stockpile moisture content. Using Environmental Protection Agency (EPA) protocols, stack emissions were measured at two drum plants and one batch plant with comparisons between WMA and HMA. Producing mix at lower temperatures will undoubtedly reduce burner emissions and save fuel. Since 2001, prices for liquid asphalt have increased drastically. An excess of RAP is the result of mill and fill rehabilitation strategies. In order to correct gradation, fractionation is used. Film thickness, fatigue resistance and low temperature performance are improved by rejuvenators. Rutting is resisted by asphaltene rich reclaimed asphalt pavement (RAP) binders.
The research reported herein deals with the analysis of different rheometrical approaches to evaluate the damage-related properties of modified asphalt binders at high service temperatures. Static and repetitive creep experiments were performed to associate specific characteristics of the binders' time-dependent response to the materials damage resistance in conditions of incipient failure. The model parameter sub 0 was found to be the most important factor, which controls and predicts the binder resistance to the propagation of a viscous flow. Further data show that this condition applies even if non-linear domains are considered. The concept of intrinsic resistance to non-reversible deformation was consequently introduced and advanced performance-based criteria for technical qualification of modified asphalt binders were proposed.
Rolling thin-film oven (RTFO) and pressure aging vessel (PAV) aging methods do not necessarily reflect low-temperature oxidative aging of asphalt binders as it occurs in pavement structures constructed with modern asphalt materials, it has been shown. Development of an alternative laboratory aging apparatus employing a compact Attenuated Total Reflection spectrometer and airflow Aging Cell (ATRAC) equipped with a temperature controller has been targeted by this ongoing research. Asphalt mix samples are collected, concurrently, from several pavements of different ages constructed in Connecticut and Rhode Island with non-modified PG 64-28 (AC-20) and crumb rubber-modified PG 76-34 binders. A good correlation is shown by preliminary results between oxidation rates of binders aged by the ATRAC and oxidation rates of asphalt mixes collected from the existing pavements. In addition, it is indicated by comparison of ATRAC measurements on New England PG-graded binders and Strategic Highway Research Program (SHRP) core asphalts that there is a potential use for ranking performance of asphalt binders at high temperatures by oxidation rates produced by ATRAC.
The current design standard for asphalt mixtures provides guidance on selection of aggregates, asphalt binder and includes requirements for the amount of mineral filler to be included. The amount of filler that can be included is limited to a ratio in the range of 0.6-1.2 by mass of the binder. However, this range is based on experience rather than on scientific evaluation of the interaction between filler and binder. Although many researchers acknowledge the physico-chemical interaction between asphalt binder and the mineral filler, currently a procedure to quantify this interaction, and consider it in selecting favorable filler to binder ratio, is not available. In this paper, the effect of fillers on the glass transition temperature (T sub g) of the base binder was used to evaluate the physico-chemical interaction in mastics. The total reinforcement of the filler, which is measured in terms of relative viscosity of the filled binder to the unfilled binder, consists of two parts: mechanical and physico-chemical. The mechanical reinforcement part is calculated based on micromechanical models commonly used in the literature that take into account volume filling effects and particle-to-particle interactions. Physico-chemical reinforcement is estimated based on the change in the T sub g in both Williams-Landel-Ferry (WLF) and Arrhenius time-temperature shift models. The concept introduced in this study is evaluated by viscosity and dilatometric T sub g testing of three binders mixed with three different fillers, at different concentrations. Results show that the physico-chemical interaction between the mineral filler and the binder can be accurately estimated from the difference in the glass transition temperature of the mastics and the binder.
Surface free energy is a thermodynamic material property representing the work required to create new surfaces of unit area in a vacuum. Surface free energy has been used to quantify and screen both the cohesive bond energy of asphalt binders and the adhesive bond energy of asphalt binder-aggregate interfaces in wet and dry conditions. The bond energy is computed based on the surface free energies of the constituent materials. The total work of fracture is the cumulative effect of energies applied to the sample to create two new surfaces of unit area. These energies include the bond energy, calculated from surface free energy, dissipated plastic energy, and dissipated viscoelastic energy. This paper presents experimental results from a series of pull-off tests using asphalt binder-aggregate samples that demonstrate the relationship between bond energy and total work of fracture. In order to fully explore this relationship, temperature, loading rate, specimen geometry, and moisture content were varied.
Two high polymer mixtures (HPM) were placed at the 2009 National Center for Asphalt Technology (NCAT) Test Track as a field trial. These mixtures featured 7.5% polymer in contrast to the more typical 2-3% polymer contents. During construction, each mixture (one surface and one base mixture) was sampled for evaluation using laboratory performance tests. The results of the dynamic modulus test, asphalt pavement analyzer, flow number, bending beam fatigue, indirect tension creep compliance and strength test, energy ratio, and moisture susceptibility tests were compared with the results from comparable control mixtures placed in the same round of testing. The laboratory test results suggest HPM mixtures can be placed to develop more efficient (i.e., thinner) pavement cross-sections due to their enhanced fatigue and rutting resistance.
Aggregate packing concepts developed in the field of high-performance cement concretes, initially by Caquot (1937) then by contemporary researchers since the 1970s, were transposed to the field of asphalt concretes. These concepts, associated with the use of the gyratory compactor on aggregates only, enabled the development of a new laboratory design procedure of dense high-modulus asphalt concretes. These mixes are characterized by single or double gap-graded curves, great coarse aggregate interlock and no need for low penetration grade bitumens to fulfill the European EME2 specification requirements, in particular the 14,000 MPa stiffness modulus value at 15°C. In addition, the use of polymer modified binders (PMBs), at a content of about 4% up to 4.5%, combined with such an optimized aggregate packing leads to the design of the so-called high-performance asphalt concretes (HPAs) characterized by great compactability, very high stiffness modulus and high fatigue resistance in a single formulation, allowing for reduced pavement thickness and increased longevity. Moreover, the proposed mix design and the 4?4.5% binder content makes PMBs use affordable in base courses. Laboratory assessment of such materials consisted in the evaluation of compactability, moisture resistance, rutting resistance at 60°C, complex stiffness modulus at 15°C and fatigue resistance at 10°C. Apart from these results, the paper also addresses the successful application of this new material on different job sites, located mainly in France. The proposed HPA material may be potentially considered as a relevant solution for sustainable long life pavements that do not deteriorate structurally, needing only timely surface maintenance.
An aggressive adoption of new practices within the asphalt paving industry have been spurred by recent improvements in warm mix asphalt technologies. Among federal and state agencies, concerns have arisen regarding the effects on the performance of asphalt pavements of this line of products. While varying the loose mix aging time, the authors performed an investigation of the effects of lowering mixing, compaction and aging temperatures using Meadwestvaco's Evotherm 3G™ 09 chemical additive. Flow Number, Fracture Energy testing, Hamburg Wheel-Track Testing, and Dynamic Modulus testing were used to evaluate mechanistic properties of the materials. It was found that rutting related testing was influenced significantly by lowering production temperatures, and for dynamic modulus and low temperature fracture energy testing, the effect was negligible.
Commonly used to modify asphalt binder for use in hot-mix asphalt construction, ground tire rubber (GTR) is the focus of this study. For use as co-modifiers with GTR in asphalt binder modification for asphalt pavement construction, various additives have been recommended. Co-modifiers have been promoted more recently to be incorporated into hot-mix asphalt through dry addition into asphalt mixtures of GTR and additive. An improved modified asphalt paving binder is represented by these additives to be provided by incorporating a small amount of additive into GTR modified asphalt binder. This, in turn, leads to easier mixing, reduced cracking, reduced tackiness, lower life-cycle cost, longer service life, and less permanent deformation. The results of a comprehensive evaluation of trans-polyoctenamer (TOR) modified binder incorporated with GTR in modification of asphalt binder are reported in this paper. Evaluated in dense, open graded friction course (OGFC), and stone matrix asphalt (SMA) mixtures were four binders, as follows: GTR wet pre-blended; GTR dry added to mixture; performance grade (PG) 67-22; and Styrene-Butadiene-Styrene block copolymer (SBS).
A comprehensive study focusing on methodologies to improve the performance of high reclaimed asphalt pavement (RAP) content mixes is presented in this paper. One major concern that many agencies have is addressed, firstly, in this paper - that of RAP variability. In Texas, it was found that, within individual stockpiles, the RAP materials are consistent and have low variability in terms of aggregate gradation and asphalt content. In addition, when the authors evaluated the impact of RAP on optimum asphalt context (OAC), cracking resistance, and rutting/moisture resistance, they found that OAC generally increases with more RAP (and reclaimed asphalt shingles (RAS)) usage, but when the RAP content is below 20% the increase in OAC is small; and as measured in the Hamburg wheel tracking test (HWTT), increasing RAP content always improves rutting/moisture resistance. Yet, particularly when RAP content is 30% and above, and also when RAP/RAS combinations are used, it was found in the laboratory that cracking resistance always reduces with increasing RAP content. This study proposes a balanced RAP mix design approach. The authors recommend the use of the Overlay Tester (OT) to directly measure cracking resistance of RAP mixes and the HWTT for evaluating rutting/moisture resistance, recognizing the challenge of accurately estimating voids in mineral aggregates of the RAP mix. Furthermore, a maximum density of 98% is recommended to avoid over densification and potential bleeding for Texas dense-graded hot mix asphalt (HMA) mixes designed using the Texas gyratory compactor. The final asphalt content is selected after optimizing the mix density, HWTT, and OT requirements in the proposed balanced mix design approach. The mixing and compaction temperatures used for the virgin binder should be used for the RAP mix as well, as was recommended. Finally, through the construction of field test sections containing different levels of RAP, the balanced RAP mix design procedure is demonstrated and validated. The fact that cracking requirement in terms of OT cycles should vary, depending on the climate (cold vs. hot), traffic (heavy vs. light), and existing pavement condition (overlay over cracked pavements vs. new construction), is one of the interesting findings. A minimum OT requirement for overlays over severely cracked pavements of 300 cycles previously proposed was further validated with performance data from the RAP sections on Interstate Highway (IH) 40 near Amarillo, Texas. In order to develop criteria for different climatic zone and different pavement conditions, more work is needed.
A Federal Highway Administration (FHWA) funded study was conducted to investigate the influence of extraction methods on aggregate properties. The properties of the virgin aggregates were compared with those of aggregates extracted from laboratory-produced recycled asphalt pavement (RAP) from four different aggregate sources. The extracted and actual asphalt binder contents were also compared. The study investigated the influence of the extraction method on tendencies to under- or over-estimated certain mix design properties. The test results were also examined to determine the impact of the RAP aggregate properties on the voids in mineral aggregate (VMA) over different RAP percentages. Recommendations were made for the most appropriate method to estimate the RAP aggregate specific gravities based on acceptable levels of error in VMA for mixtures with varying levels of RAP.
The main focus of this study was to obtain plant produced Reclaimed Asphalt Pavement (RAP) mixtures, to document the mixture production parameters and to evaluate the degree of blending between the virgin and RAP binders. The effect of mixture production parameters on the performance (in terms of stiffness, cracking, rutting, and moisture susceptibility) and workability of the mixtures was evaluated. Eighteen plant produced mixtures were obtained from three locations in the Northeast United States. RAP contents (zero to 40%) were varied and softer binders were used. The data and analysis illustrated that the degree of blending between RAP and virgin binders is a function of production parameters. The stiffness of the mixtures increased as the percentage of RAP increased, but not when the discharge temperatures of the mixtures were inconsistent. The cracking resistance was reduced as the percentage of RAP increased. The rutting and moisture damage resistance improved as the percentage of RAP in the mixtures increased. Finally, reheating the mixtures in the laboratory caused a significant increase in the stiffness of the mixtures.
This paper presents the results of a study to evaluate the mixture properties of plant-produced asphalt mixtures containing up to 40% reclaimed asphalt pavement (RAP). Five sets of asphalt mixtures were tested to determine their dynamic moduli and low temperature tensile creep compliance and strength. The mixture moduli were analyzed to assess changes in mixture behavior over a range of temperatures (frequencies) and in combination with extracted binder properties to analyze the extent of binder blending in the mix. The low temperature test results were analyzed to predict the critical cracking temperature of the mixtures. The results suggest that, for these materials, up to 25% RAP could be added to the mix with no change in the virgin binder grade without detriment to the low temperature properties of the mix.