G. Tebaldi

A mechanistic test to evaluate effects of interface condition characteristics on hot-mix asphalt overlay reflective cracking performance

Reflective cracking is a distress mode that is of particular concern for pavements with thin hot-mix asphalt (HMA) overlay as a maintenance and/or rehabilitation method. Traditionally, efforts have been made to mitigate reflective cracking by increasing overlay thickness, using anti-reflective cracking interlayer systems and special treatments on existing pavements and cracks. A new way to improve HMA overlay reflective cracking resistance is to use highly polymer-modified asphalt emulsion (PMAE) interface. Tests currently available are not appropriate to evaluate the effects of interface conditions on reflective cracking. This paper presents the second phase of a study devoted to develop a mechanistic test to study effects of interface conditions on pavement cracking performance. A new approach to represent reflective cracking (a teflon spacer introduced in an existing pavement layer) instead of the traditional notch method was introduced to the composite specimen interface cracking test developed for top-down cracking. In this study, effects of two types of interface conditions, conventional tack coat and PMAE, on reflective cracking were evaluated. HMA fracture mechanics was employed to identify the reflective cracking mechanism when the teflon spacer was installed in the composite specimen. Test results clearly indicated that the proposed test method with the teflon spacer was able to simulate reflective cracking mechanism and evaluate effects of interface conditions on reflective cracking.

Cold Recycling of Reclaimed Asphalt Pavements

Pavement engineers have in front of them multiple challenges linked to addressing issues related to social development and society’s expanding needs. One of the most substantial of these issues is perhaps how to effectively rehabilitate and/or maintain the existing road network while preserving and sustaining limited natural resources. The re-usage of existing pavement materials to reconstruct/rehabilitate our future pavements is the solution that is now more and more selected by the different road administrations around the world. However, upon closer inspection, one can find many areas and details, not negligible issues, that are simply extensions of HMA technology (i.e. mix design process in cold recycling) or empirical arrangements; in particular RAP still does not have an internationally recognized classification. So SIB – TG6 decided to develop a classification protocol of RAP, depending on its intended application. The objective has been followed by considering the procedures generally utilized to classify the natural aggregates: tests able to identify the main components (i.e. the geometrical and mechanical properties of aggregates and the characterization of recovered bitumen for RAP) and provide information on their behaviour under specific conditions, near to real life usage (e.g. the Los Angeles test for aggregates gives an idea of the potential behaviour of aggregates under the action of a roller compactor). The following sections illustrate and explain the actions of the TG in order to achieve the goals outlined above: the review of current standards, the protocol designed to classify RAP and the round robin tests carried out to validate the protocol.

Active fillers’ effect on in situ performances of foam bitumen recycled mixtures

Cold recycling is one of the most employed rehabilitation techniques for asphalt pavements and it is becoming more and more important as reducing emissions becomes a priority in the reduction of the greenhouse effect. The main advantages of asphalt cold recycling techniques are the use of reclaimed materials and the fact that there is no need of aggregate heating to make the mixtures. This paper describes the evolution with time of in-situ performances of different foam bitumen-stabilised mixtures made with different active fillers (cement and lime), monitored during the first year from construction. Results are part of a more extensive research programme aimed to investigate the effects of using lime as an active filler in cold-recycled mixtures. Mixtures have been laid down on a specifically designed trial section in Italy, close to Florence. Short-term bearing capacity, immediately after construction, has been evaluated using a light weight deflectometer while to evaluate the mid-term performances falling weight deflectometer (FWD) tests have been performed after 24 hours, 14 days, 28 days and 9 months from construction. During these 9 months the test road was not opened to traffic, so the mixtures experienced almost no traffic (only construction traffic loads). This fact allowed to have the curing process without any influences other than the temperature: it means same curing conditions for all mixtures. Subsequent FWD tests are still ongoing to evaluate the evolution over time of pavement bearing capacity due to traffic. Results obtained positively support the use of lime as an active filler in the foam bitumen-stabilised material and allow to underline the effect of different active fillers in the material behaviour, even if all the mixtures underline excellent performances under traffic loading. FWD tests are scheduled to be repeated every 6 months in order to monitor the stiffness evolution of the mixtures and evaluate the nature of traffic damage.

Active filler’s effect on in situ performances of bitumen emulsion recycled mixtures

This paper address the in situ performance over time of different bitumen emulsion stabilized mixtures blended with different types of fillers (cement, lime and mineral filler), monitored during the first year from construction. Results are part of a more extensive research program aimed to investigate the effects of using lime as active filler in cold recycled mixtures, both with bitumen emulsion and foam bitumen. A specific test track has been designed on a construction road near Florence in order to evaluate the evolution of mixture performance over time. Short term bearing capacity has been evaluated by means of LWD (Lightweight tests) after 4 h from compaction while FWD tests have been performed after 24 h, 14 days, 28 days and 9 months to monitor the mid-term performances. Up until mid-2014 tests road was not yet opened to traffic so the mixtures experienced only construction traffic loads. Results obtained positively support the use of lime as active filler in the bitumen emulsion stabilized material and underline the effect of different blends of fillers in the material behaviour. FWD tests are scheduled to be repeated every 6 months in order to monitor the stiffness evolution of the mixtures and evaluate the nature of traffic damage.

Effect of active fillers on cracking performance of bitumen-stabilised materials

A laboratory investigation was conducted to evaluate the effect of active fillers on the cracking resistance of bitumen-stabilised mixtures. Nine types of stabilised mixtures composed by 100% recycled aggregates, different blends of fillers (cement, lime and common filler) and bitumen emulsion or foamed bitumen as stabilising agents were used to build three consecutive experimental sections on a constructing road. Cores of the trial sections were tested in a laboratory according to a Superpave Indirect Tensile Test (IDT) procedure developed for quasi-brittle materials. An in-house-developed Digital Image Correlation (DIC) software code was applied. The results show the benefit of the active filler to the mixture’s cracking resistance in terms of increased tensile limits to failure. Significant damage has shown to be strongly more localised in mixtures containing active fillers. Among all the blends, the more promising blend combination consists in 1% of cement, 2% of lime and 1.5% of traditional filler.

Influence of SAMI on the Performance of Reinforcement Grids

Over the last decades, reinforcement grids have been used to prolong the service life of pavements. Although there is a broad consensus about their overall efficiency, there are still many open questions about the best alternatives regarding the materials to use, the shapes of the grid, their installation method or their position in the pavement among others. One controversial aspect is the use of Stress Absorbing Membranes Interlayers (SAMI) together with the application of a grid. Many manufacturers claim that the SAMI helps sealing any preexisting cracks in the underlying layer, retarding their propagation to the surface. This paper reports about the performance of a reinforcement grid installed with SAMI in a pavement with artificial cracks. One of the parameters analyzed to evaluate the performance, is the propagation of artificial cracks from the base course to the pavement surface. Other important consideration is the rutting development. To that end, the same cracked pavement, reinforced with a fiber grid and SAMI and without any reinforcement is loaded with identical accelerated trafficking, carried out with the traffic simulator MLS10. Results show that, although the grid with SAMI is able to slow down the formation of cracks in the surface, the density of cracks at the end of the tests is as high as if no grid was used. Moreover, resulting rutting in the reinforced pavement is higher than in the pavement without grid, indicating that the use of SAMI has a counterproductive effect on the rutting performance of the reinforcement.

Fracture Energy Evaluation of “Interstitial Asphalt Mixtures”

Having an adequate coarse aggregate structure in any asphalt mixture may not be enough to accurately distinguish the cracking performance of an asphalt mixture: indeed, it has been proved that the interstitial volume (IV) can affect asphalt mixtures cracking performance. The IV is defined as the volume with in the coarse aggregate structure filled with finer material, binder and air voids (interstitial components). Several surveying activities on pavement sections made with mixtures designed with Dominant Aggregate Size Range (DASR), which is the coarse aggregate that forms the structural interactive network of aggregate, have been developed in the past. This paper presents an experimental study, recently developed at the University of Florida, aimed at investigating how interstitial components are influenced by different variables, as types of aggregates, aggregate gradation and by binders. The DASR-IC model was used to identify a range of mixtures to be tested by first designing the coarse aggregate structure with adequate inter-locking and then varying the fine portion of the gradation. Laboratory test results from Superpave Indirect Tension Test (IDT) clearly showed that the IV characteristics have a significant effect on asphalt mixture fracture performance.