Marek Pszczoła

Investigation of low-temperature cracking in newly constructed high-modulus asphalt concrete base course of a motorway pavement

The paper presents the issue of low-temperature transverse cracks which have developed in newly constructed base courses made of high-modulus asphalt concrete mix (HMAC). Numerous transverse cracks developed in the analysed HMAC base during the winter season before the pavement was actually completed, both at the transverse joint locations and in the areas between them. This had not happened so far on such a scale during road construction projects in Poland. The research included both field examination and laboratory testing of high-modulus asphalt concrete used for construction of the base course of the A1 motorway pavement, followed by computational analysis to investigate the causes and mechanism of the analysed pavement distress. The first part of this paper describes the pavement structure, gives the main requirements applicable to HMAC, and presents the results of the cracking survey, visual assessment and laboratory tests carried out on cores cut from the analysed pavement. Later in the article, the causes of cracking are analysed, taking into account the temperature of air and pavement, performance grade assessment of the bitumen used in the works, homogeneity of the constructed layer and calculated thermal stresses induced in HMAC layers. The examinations and analyses allowed to conclude that the main causes of cracking included too stiff mix, taking into account the climate of Poland, deviations from the pre-defined installation practice during the construction of the base course and leaving the HMAC surface unprotected by the overlying layers: binder and wearing courses during winter. Finally, compliance with the best construction practice during placement of HMAC mixes and use of softer or polymer-modified bitumens are indicated as being critical to avoiding distress of HMAC pavement courses.

Evaluation of Thermal Stresses in Asphalt Layers Incomparison with TSRST Test Results

The paper presents the results of calculations and laboratory determination of thermal stressesat low temperatures. The modified Hills and Brien’s method was used to calculate the thermal stresses in asphalt layers of pavements and the results were compared againstthe values obtained at a laboratory with the Thermal Stress Restrained Specimen Test (TSRST) method. The laboratory investigations were conducted using plain grade bitumen, modified bitumen with SBS elastomer modification and multigrade type bitumen. It was found that the type of bitumen binder in asphalt concrete is of significant importance to the value of the calculated thermal stresses. For thecooling rate of 10oC/h the lowest value was obtained for asphalt concrete produced with the use of multigrade type bitumen. This fact can be an indication of a better resistance to low temperature cracking. The thermal stresses were had the highest values for asphalt concrete produced with plain bitumen. A good correlation was obtained betweenthe thermal stresses calculated withthe Hills and Brien’s procedure and the values of thermal stresses determinedwith the TSRST method.

Field investigation of low-temperature cracking and stiffness moduli on selected roads with conventional and high modulus asphalt concrete

High Modulus Asphalt Concrete (HMAC) was introduced in Poland as a one of the solutions to the problem of rutting, type of deterioration common in the 1990s. After first encouraging trials in 2002 HMAC was widely used for heavily loaded national roads and motorways. However some concerns were raised about low-temperature cracking of HMAC. This was the main reason of the studies presented in this article were started. The article presents the comparison of performance of pavements constructed in typical contract conditions with the road bases made of HMAC and conventional asphalt concrete (AC). The field investigation was focused on the number of low-temperature cracks, bearing capacity (based on FWD test) of road sections localized in coldest region of Poland. Also load transfer efficiency of selected low-temperature cracks was assessed. FWD test confirmed lower deflections of pavements with HMAC and two times higher stiffness modulus of asphalt courses in comparison to pavements constructed with conventional AC mixtures. Relation of stiffness of asphalt layers and amount of low-temperature cracks showed that the higher stiffness modulus of asphalt layers could lead to increase of the number of low-temperature cracks. FWD test results showed that the load transfer efficiency of low-temperature cracks on pavements with HMAC presents very low values, very close to lack of load transfer. It was surprising as section with HMAC road base were aged from 2 to 5 years and presented very good bearing capacity.

Evaluation of Asphalt Mixture Low-Temperature Performance in Bending Beam Creep Test

Low-temperature cracking is one of the most common road pavement distress types in Poland. While bitumen performance can be evaluated in detail using bending beam rheometer (BBR) or dynamic shear rheometer (DSR) tests, none of the normalized test methods gives a comprehensive representation of low-temperature performance of the asphalt mixtures. This article presents the Bending Beam Creep test performed at temperatures from −20 °C to +10 °C in order to evaluate the low-temperature performance of asphalt mixtures. Both validation of the method and its utilization for the assessment of eight types of wearing courses commonly used in Poland were described. The performed test indicated that the source of bitumen and its production process (and not necessarily only bitumen penetration) had a significant impact on the low-temperature performance of the asphalt mixtures, comparable to the impact of binder modification (neat, polymer-modified, highly modified) and the aggregate skeleton used in the mixture (Stone Mastic Asphalt (SMA) vs. Asphalt Concrete (AC)). Obtained Bending Beam Creep test results were compared with the BBR bitumen test. Regression analysis confirmed that performing solely bitumen tests is insufficient for comprehensive low-temperature performance analysis.

Influence of Bitumen Type and Asphalt Mixture Composition on Low-Temperature Strength Properties According to Various Test Methods

In regions with low-temperatures, action transverse cracks can appear in asphalt pavements as a result of thermal stresses that exceed the fracture strength of materials used in asphalt layers. To better understand thermal cracking phenomenon, strength properties of different asphalt mixtures were investigated. Four test methods were used to assess the influence of bitumen type and mixture composition on tensile strength properties of asphalt mixtures: tensile strength was measured using the thermal stress restrained specimen test (TSRST) and the uniaxial tension stress test (UTST), flexural strength was measured using the bending beam test (BBT), and fracture toughness was measured using the semi-circular bending test (SCB). The strength reserve behavior of tested asphalt mixtures was assessed as well. The influence of cooling rate on the strength reserve was investigated and correlations between results from different test methods were also analyzed and discussed. It was observed that the type of bitumen was a factor of crucial importance to low-temperature properties of the tested asphalt concretes. This conclusion was valid for all test methods that were used. It was also observed that the level of cooling rate influenced the strength reserve and, in consequence, resistance to low-temperature cracking. It was concluded that reasonably good correlations were observed between strength results for the UTST, BBT, and SCB test methods.

The Impact of Homogeneity of High Modulus Asphalt Concrete Layer on Low-Temperature Cracking

During winter season of 2012 numerous transverse cracks developed in high-modulus asphalt concrete (HMAC) base of newly constructed motorway. Pavement cracked both in transverse joint locations and in the area between them. Research which was conducted during investigation of the causes and mechanisms of cracking consisted of: field examination, laboratory testing of specimens cored out of the existing pavement, computational analyses and effect of pavement homogeneity on transverse crack frequency. This paper focuses mainly on impact of homogeneity of asphalt layer on number of transverse cracks. The field investigation of analyzed motorway section includes visual assessment of homogeneity and number of cracks. Laboratory test conducted on specimen cored out of the pavement allowed to assess volumetric properties: binder content, voids content and compaction degree, and mechanical properties: indirect tensile stiffness modulus and strength. Analyses of HMAC layer properties revealed their impact on the number of transverse low-temperature cracks observed in field. It was found that a less effective compaction contributes to increase in the number of cracks. Quality and homogeneity of pavement courses have a considerable effect on mechanical properties of HMAC. As analysis showed, intensity of cracks is well correlated with mechanical properties and homogeneity of asphalt layer.