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The paper proposes a new method for calculation of equivalent axle load factors based on the analysis of strain energy of distortion induced in road pavements by traffic loads. The main advantage of the method is the more accurate calculation of the effects of multiple axles and super single versus dual tyres. The methodconsiders the location of critical points, at which strain energy of distortion reaches extreme values. When single axles are considered, the function of equivalent axle load factor takes on the form of the well-known power equation with the exponent ranging from 2.7 to 5.3. It was proved that the damaging effect of triple axles on asphalt pavement is several times higher than the damaging effect of three single axles carrying the same load, but at a greater distance to each other. Due to this fact, traffic load may be significantly underestimated in many pavement design methods.

Axle load spectra constitute a crucial part of the data for pavement design and pavement distress analysis. Typically, axle load spectra represent static load from vehicles and do not include dynamic loads generated by vehicles in motion. While dynamic loads can significantly contribute to faster pavement distress, this fact is mostly omitted in pavement design methods. The paper presents a methodology for consideration of dynamic loads in axle load spectra for mechanistic-empirical pavement design. Calculations of dynamic axle load spectra for pavements of various evenness (expressed by IRI) and various vehicle speeds were performed and discussed. The effect of dynamic axle load spectra on pavement performance was analysed. M-EPDG calculations performed for three selected flexible pavements show that dynamic loads have a minor effect on pavement performance if the pavement is smooth and IRI is close to 1.0 mm/m. The detrimental effects of dynamic axle loads increase rapidly with the pavement evenness deterioration, resulting in faster (up to 25%) development of pavement distresses for IRI = 4.0 mm/m and a vehicle speed of 60 km/h. The analysis proved that thinner pavement structures are more sensitive to dynamic loads than thicker pavement structures. The investigation of vehicle speed impact on vehicle dynamic loads and pavement performance showed that at low vehicle speed (30 km/h) dynamic loads have a minor effect and pavement distress results mostly from a decrease in stiffness modulus of asphalt mixture and increase in permanent deformations, while for vehicle speeds higher than 90 km/h dynamic loads significantly contribute to pavement distress and adverse dynamic effects are not compensated by an increase in stiffness modulus of asphalt mixtures. The results also emphasise the significance of proper pavement evenness maintenance, especially on high speed motorways.

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.