Christiane Raab

Mechanical Testing of Interlayer Bonding in Asphalt Pavements

Steadily increasing requirements on pavement performance properties, in terms of bearing capacity and durability, as well as new innovative developments regarding pavement materials and construction, are observed worldwide. In this context interlayer bonding at the interfaces of multi-layered bituminous systems is recognized as a key issue for the evaluation of the effects, in terms of stress-strain distribution, produced by traffic loads in road pavements. For this reason a correct assessment of interlayer bonding is of primary importance, and research efforts should be addressed in order to improve the lack of correlation and/or harmonization among test methods. Following this principle RILEM TG 4 organized an interlaboratory test in order to compare the different test procedures to assess the interlayer bonding properties of asphalt pavement. The results of the experimental research are presented with a preliminary overview of basic elements, test methods and experimental investigations on interlayer bonding. Then the RILEM TG 4 experimental activities, based on the construction of three real- scale pavement sections, are presented in detail. Each pavement section was composed of two layers, and three different interface conditions were chosen. The first pavement was laid without interface treatment and the others with two different types of emulsion. Fourteen laboratories from 11 countries participated in this study and carried out shear or torque tests on 1,400 cores. The maximum shear or torque load and the corresponding displacement were measured, and the shear or torque strength was calculated as a function of the following parameters: diameter, test temperature, test speed, stress applied normal to the interface and age of the specimen. The results of this study are presented in terms of precision and correlations regarding the parameters which results in useful information on asphalt pavement interlayer bond tests.

Numerical and Experimental Analysis for the Interlayer Behavior of Double-Layered Asphalt Pavement Specimens

Bonding characteristics between pavement layers have an important influence on responses of pavement structures. This paper deals with this subject by analyzing the interlayer behavior of double-layered asphalt DLA specimens taken from an in-service motor- way using a coaxial shear test CAST and a layer-parallel direct shear LPDS test. To analyze and model the influence of the interlayer condition, finite-element simulations of CAST DLA specimens were conducted. In the models, both idealized fully bonded and no- bonding assumptions were used to characterize the interlayer behavior of DLA specimens. Experimental results presented for CAST and LPDS tests demonstrate that there is a strong influence of temperature in the interlayer bonding mechanism. At lower temperatures, there is an increase on the bonding strength produced by the binder stiffening and aggregate interlocking. Further, it was observed that comparisons of CAST experimental and numerical results indicate that the interlayer can be close to the fully bonded condition at low temperatures below 20°C while the interlayer, at higher temperatures, behaves between fully bonded and no-bonding idealized conditions. DOI: 10.1061/ASCEMT.1943-5533.0000003 CE Database subject headings: Asphalt pavements; Finite element method; Numerical analysis; Experimentation. Author keywords: Interlayer; Double-layered asphalt; CAST; LPDS; Finite element.

The influence of age on interlayer shear properties

The paper focuses on the question of the influence of age on the interlayer bonding properties of two-layered specimens. The evaluation combines traditional data evaluation and artificial neural network (ANN) analysis of different data sets of pavements from Switzerland, United Kingdom and Italy. The results show that it is possible to use ANN techniques to derive models from data sets and to predict interlayer bond strength. The findings demonstrate that age has a positive effect on the interlayer bond of asphalt pavements and that long-term oven ageing can lead to similar results as in situ ageing. In addition, it was found that the positive effect of ageing is greater when a tack coat is used.

Comparison of Interlayer Bond Behavior Due to Ageing

The research focuses on the question of the influence of age on the interlayer bonding properties of two layered specimens. The evaluation compares and discusses the behavior of different datasets from laboratory and in situ pavements from Switzerland, United Kingdom and Italy using traditional data evaluation and Artificial Neural Network (ANN) analysis. The results show that it is possible to use ANN techniques to derive models from datasets and to predict interlayer bond strength. The findings demonstrate that age has a positive effect on the interlayer bond of asphalt pavements and that long term oven ageing can lead to similar results as in situ ageing. In addition it was found that the positive effect of ageing is greater when a tack coat is used. According to the results of the investigation a linear model from all data would predict that maximum strength may roughly increase by 1 % per month over a period of 10 years.

INNOVATIVE ASPHALT RESEARCH USING ACCELERATED PAVEMENT TESTING

Accelerated pavement testing is one key for innovations in the field of road pavement structures and materials due to its power of assessing mechanical performance in close to reality situations. This paper discusses two types of mobile load simulators for applying unidirectional wheel loads at comparatively high frequencies, the MMLS3, a one third scale device, and the MLS10, a full-scale device. Through examples it is demonstrated that these devices are suitable for testing complex systems, such as mechanical resistance of light reflectors on pavements, traffic resistance of bituminous plug expansion joints for bridges, performance of grid reinforced pavements as well as water resistance of porous asphalt against traffic induced pumping effects. Moreover, an example of MLS10 ultimate bearing capacity tests on a real test section is shown focusing on the discussion of strain gauge, acceleration, rut depth and falling weight deflectometer (FWD) measurement results.

Long-term monitoring of highway deformations

We report on the development of a measuring system for dynamic long-term monitoring of highway pavement; i.e. the relative vertical displacements within the pavement layers. Beside the knowledge of weight and frequency of vehicles it is more and more of interest to know the vertical deformations within the pavement layers due to the traffic loads. Our displacement measurement is based on the magnetostrictive principle. Several positions along one measuring axis corresponding to the different layers of the pavement are monitored. The development of the measuring system, the calibration, the embeddment in a test pavement and first test results will be presented.

Cracking and interlayer bonding performance of reinforced asphalt pavements

Abstract Reinforcement of asphalt pavements has become a valuable constructive method for preventing reflective cracking and prolonging service life of roads. Although the overall advantages of these reinforcements seem beyond doubt, there is still lack of information about the actual benefit of different systems on the prolongation of the road’s lifespan. Further, it is not clear how they affect the bonding between layers along the pavement’s lifetime. This paper investigates the effect of three types of reinforcement systems on the performance of two layered asphalt pavements, mainly considering: (1) Reflective crack propagation. (2) Shear bonding strength. To that end, model and real pavements were constructed in the laboratory and the field using selected reinforcement systems. Designs with various mesh opening sizes and different bond coatings were considered. Afterwards, the pavements were loaded with a down-scaled (MMLS3) and a full-scale (MLS10) traffic load simulator and their performance was compared to pavements without any type of reinforcement. These devices simulate the effect of traffic-similar loading in a compressed period of time. During the loading phase, the crack formation and propagation was monitored through visual inspections and indirectly by measuring the deformation of the pavements with sensors. The interlayer bonding strength was evaluated by coring the pavements and testing the specimens with a Layer Parallel Direct Shear apparatus. Results showed that two reinforcement systems were generally successful in delaying reflective crack propagation. Only one of the systems presented at least similar or even worse performance than the same pavement without reinforcement. On the other hand, it was found that the bonding properties depend on the reinforcement system type, but that in most cases interlayer bond strength was according to the standard requirements.

Inter-laboratory Shear Evaluation of Reinforced Bituminous Interfaces

Over the last decades, the use of grids between asphalt layers has been gaining interest. Several test methods have been proposed in order to simulate the complex mechanical behavior of reinforced pavements and to assist practitioners in the selection of the appropriate reinforcement product. For this purpose, the Task Group 4 (Pavement Multilayer System Testing) of the RILEM technical committee TC 237-SIB (Testing and Characterization of Sustainable Innovative Bituminous Materials and Systems) organized an inter-laboratory experiment, constructing one trial test section to obtain double-layered asphalt pavement samples for the participating laboratories. The experiment placed two grid types (a glass fiber reinforced polymer grid and a carbon fiber/glass fiber pre-bituminised grid) between two asphalt layers, thereby creating two reinforced double-layered systems. As a control, an unreinforced interface was also realized. This paper presents the overall results of interlayer shear tests carried out by five participating laboratories using five different shear testing methodologies. The objective is to show the effect of two grid types on the shear behaviour of reinforced double-layered systems and to compare the findings which emerged from using different test devices and methods under different testing conditions (e.g. sample geometry, temperature, loading time, normal stress). Consistent and reliable results have been obtained through the various methodologies adopted. It has been observed that grid-reinforced samples provide lower interlayer shear strength compared with unreinforced samples. Glass-fiber grid system, which is of greater thickness and greater torsional stiffness, displayed less shear strength than carbon fiber/glass fiber-reinforced grid systems.

Use of APT for Validating the Efficiency of Reinforcement Grids in Asphalt Pavements

Reinforcement of flexible asphalt pavements with interlayer systems containing grids is increasingly used to extend service life of roads. Typically, reinforcements are placed between layers. They can be utilized to prevent the propagation of reflective cracking, prolonging the service life of asphalt pavements. Many research and practical experiences have shown the advantages of using reinforcements to increase the life-span of a pavement. However, only few tests were done using Accelerated Paving Testing (APT), where the application of loads is carried out with controlled passing of truck tires. Further, there is still insufficient research carried out to predict the actual effect on the extension of the pavement’s life-span. This information is critical for pavement design purposes and to improve the design standards. This research focuses on the effect of different reinforcement systems on the life-span extension of two-layered slabs and an asphalt pavement, using scaled and full scale APT with the Model Mobile Load Simulator MMLS3 and the Mobile Load Simulator MLS10 respectively. Laboratory and full-scale results with MMLS3 and MLS10 show that the different grids used in the test will extend the life-span of the tested pavements up to two times. The full-scale tests show, that the effect of the construction of a reinforcement system combining a grid with a stress absorbing membrane (SAMI) with the grid, increases the rutting of the pavement.

A LTPP study with a new device for vertical deformation measurements

On the occasion of rehabilitating the heavily travelled Swiss motorway A1 between Zürich and Bern with SMA, a long-term pavement performance study was initiated in 1998, combining permanent in situ measurements, periodic inspections and selected laboratory tests on pavement samples. An in situ measurement station was installed to record traffic loads, vertical deformations and temperatures within the cross section of the pavement. This paper presents the traffic loads and the deformation response under traffic as well as the development of pavement conditions between 1998 and 2001 according to evenness, skid resistance and FWD measurements. Temperatures within the pavements cross section are compared to other motorway sections with different pavement structures.