David Jones

The selection of an appropriate long-term laboratory aging protocol for implementing balanced mix design remains a concern within the asphalt industry. In this study, two different loose mix aging protocols were applied: medium-term oven aging (MTOA) and rapid medium-term oven aging (RMTOA). MTOA protocol involved loose mix aging for 20 h at 100°C, whereas a temperature of 135°C was considered for RMTOA for a quick turnaround time. To determine the equivalent aging duration for RMTOA to produce similar aging effects as MTOA, six different asphalt concrete mixes containing reclaimed asphalt pavement (RAP) and recycling agents (RA) were oven aged at different aging temperatures and durations. Aging indices of the binders extracted from these mixes were applied to the Arrhenius equation to obtain the activation energy for different mix types. Despite the mixes differing in binder sources (four refineries) and RAP contents (up to 40%), the activation energy ranged narrowly between 68.1 kJ/mol to 72.9 kJ/mol. The use of Arrhenius activation energy suggested that around 3 h at 135°C laboratory aging (RMTOA) simulated the similar aging parameters of MTOA protocol (20 h at 100°C). In the second stage of this study, indirect tensile IDEAL cracking tolerance tests were performed on both RMTOA- and MTOA-aged specimens. Seven different AC mixes containing RAP and RA were considered to characterize the age-related cracking performance after MTOA and RMTOA. The ANOVA analysis indicated that two aging procedures (MTOA and RMTOA) are expected to produce similar effects on the IDEAL-CT results.

The construction of two partial-depth (cold in-place) recycling projects was monitored and the quality control results were analyzed to compare gradations produced with single- and multi-unit recycling trains, to assess the effect of recycling train forward speed on gradation, and to compare differences between emulsified and foamed asphalt recycling agents in partial-depth recycling applications. The research highlighted the challenges with variability and small sample sizes associated with in-place recycling but provided representative and consistent results between projects. The results showed that there was no discernable difference in the density and strengths of partial-depth recycling layers produced with the single- and multi-unit recycling trains. The main benefit of the multi-unit train was better control of maximum aggregate size by the on-board screens and crushing unit, however, the crushing unit did not appear to change or improve the finer portion of the gradation (i.e., material passing the 5-mm sieve), which has a known larger influence on compaction density, air-void content reduction, strength, moisture resistance, and stiffness. The recycling train type and forward speed did influence the coarse portion of the gradation (i.e., >5 mm), but did not appear to influence compaction density. Test results indicated that the sections treated with emulsified asphalt had higher densities than those treated with foamed asphalt. However, there was considerable variability in materials and pavement structure along the length of the project, which probably had an influence on all results. The findings permit agencies to specify that recycling train and recycling agent choice can be the contractor’s decision.

This paper summarizes a multiphase research project to standardize cold recycling specifications in California. The specifications for partial depth recycling (PDR), or cold in-place recycling, and cold central plant recycling (CCPR), using emulsified asphalt (EA) or foamed asphalt (FA), were developed independently, resulting in different mix design and quality control (QC) procedures for the two recycling agents. Most specifications require the use of Marshall stability for EA materials and indirect tensile strength (ITS) for FA materials. The specifications allow the use of gyratory compaction with 30 gyrations or Marshall compaction with 75 blows per face. This paper showed that, based on a review of 92 PDR and CCPR projects, the specification for EA produces mixes with lower binder and active filler contents. The two compaction methods were compared using field projects and laboratory testing, with the results showing that Marshall compaction overestimates density and, therefore, strength and stability, resulting in mixes with lower equivalent binder and active filler contents compared with gyratory, and facilitating passing minimum strength/stability criteria for QC. The comparison of the testing methods, using two different recycled asphalt pavement sources, and comparing the results with dynamic modulus results, indicated that Marshall stability does not produce a meaningful measure of the material properties. This paper recommends standardizing specifications for the EA and FA by removing Marshall compaction and Marshall stability, and adopting gyratory compaction and the ITS for mix design and QC testing. Attempting correlations between the different compaction and test methods is not considered appropriate.

Cold recycling is a proven alternative to traditional remove-and-replace maintenance and rehabilitation strategies. However, implementation by road agencies across the United States has been limited, which can be attributed to a number of reasons. One primary reason is believed to be that cold recycling strategies do not readily fit current agency flexible pavement management frameworks, which have been established around remove-and-replace maintenance and rehabilitation strategies, or fit the traditional definitions for routine, preventative and corrective maintenance, and minor and major rehabilitation. These frameworks, and the decision processes associated with them, need to be updated to better accommodate new strategies, in conjunction with traditional strategies, to make the best use of available resources to cost-effectively and more sustainably extend the life of pavement networks and improve the overall condition of the network. This paper summarizes developments in cold recycling, identifies barriers to implementing cold recycling in agencies, and suggests a roadmap framework for updating policies and procedures to make better use of cold recycling. When designed and built appropriately, cold recycling strategies can be more cost-effective and more sustainable than traditional remove-and-replace strategies. Optimizing the recycling depth to remove distresses and provide the required structural support for the asphalt concrete surfacing for a given set of traffic, climate, environmental, and resilience criteria is critical for a successful outcome. Attempting to align recycle depths within maintenance and rehabilitation categories that are specific to traditional remove-and-replace strategies will yield less-than-optimal results and the potential for significant benefits will not be realized.

Drying shrinkage cracks in pavements rehabilitated using full-depth recycling with cement can reflect through asphalt concrete overlays. The effect of shrinkage cracks on the fatigue life of pavements increases with increasing crack width because of the loss of load transfer across the cracks. Wide shrinkage cracks can also lead to pumping of the subgrade material, leading to faulting of the base, loss of support, and eventual deterioration of the stabilized layer. One method to mitigate drying shrinkage cracks is with microcracking. Microcracking creates a network of hairline cracks in the stabilized layer which serve to relieve initial stresses during early hydration of the cement, thereby limiting the wider and more severe block cracks typical of cement stabilized layers. The process involves applying several passes of a steel drum vibratory roller, at maximum vibration frequency and amplitude settings, over the stabilized layer within a set time window after construction. The effectiveness of microcracking was investigated through regular crack monitoring and deflection testing over a four-month period on a 37-cell test road at the University of California, Davis. The results showed that microcracking does not prevent the development of shrinkage cracks but, in conjunction with selecting a mix design seven-day strength between 2.0 and 4.1 MPa (300 and 600 pounds per square inch), it can mitigate its effects by creating additional, thinner, more closely spaced, shrinkage cracks that are unlikely to reflect through the asphalt concrete surfacing.

In this research, field slab samples were taken from an asphalt pavement in a hot climate with a design life of 40 years at 2, 4, and 7 years of pavement age to determine aging behavior with time at different depths. The asphalt binders were extracted and recovered to evaluate the aging and fatigue performance of the intermediate layer of the asphalt pavement. The chemical and rheological aging properties of the binders were evaluated using Fourier transform infrared spectroscopy and a dynamic shear rheometer, respectively. The pavement included 25 mm (1 in.) open-graded and 75 mm (2 in.) polymer-modified layers over three intermediate layer lifts. The intermediate layer aging is mostly governed by the connecting air voids and binder content of the asphalt mixes. The variation in aging deeper than 100 mm (4 in.) below the pavement surface was minor because of the absence of heat and access to air, with similar aging parameter values observed for different years of field sampling indicating little to no aging after construction. A good correlation ( R ² values of 0.79–0.92) was observed between the chemical property (carbonyl index) and rheological properties of the extracted binder for the dense-graded asphalt concrete layers between 25 and 100 mm depths. The fatigue performance of these extracted binders was evaluated by performing linear amplitude sweep tests. The results indicate that binder fatigue performance initially improved with moderate levels of aging but started to decline under extreme aging conditions.