PAVEMENT PERFORMANCE FOR HIGHWAY AND RUNWAY

A. Definition Pavement performance is defined as the ability of a pavement to satisfactorily serve traffic over time (ASSHTO, 1993). The performance or functional specifications principle is to define requirements in respect to asphalt layer or surface characteristics. The Swedish performance specification can be classified as a combination of performance (performance over time), performance based (fundamental mechanical properties) and end-result specification (end-product properties) (PIARC Report, 2000). B. Indicator These performance indicators addopted from Swedish National Road & Transport Research Institute (VTI). The goal of all pavement design methods is to provide a pavement that performs well. Performance is generally described in terms of structural and functional performance: • structural performance, a pavement’s ability to carry the imposed traffic • functional performance, a pavement’s ability to provide a safe and comfortable ride. 1. Structural requirements The most important design-related variables for ensuring a certain level of structural performance are as follows (Piarc Seminar Mexico 2009, Paper No. 36): a. Stiffness modulus Flexibility and load distribution capacity are two important characteristics of bitumen bound layers. High stiffness bituminous layers protect underlying layers by better stress distribution resulting in less stress applied to the underlying pavement layers. Low stiffness bituminous layers are flexible and desired in thin pavement structures with low traffic loading, where the purpose of the asphalt layer is not primary to increase the bearing capacity of the road, but rather to increase riding comfort and safety and to protect underlying layers. Stiffness of bituminous layer is one of the most important parameters in analytical pavement design. b. Fatigue cracking Fatigue failure of a bituminous layer means the development of cracks in the pavement layer caused by repeated traffic loading. Fatigue testing is time consuming and it is known that the fatigue property of asphalt concrete is well correlated with the stiffness of the material. Therefore fatigue testing is only recommended when using new type of mixes (not tested before) or if there are particular reasons. c. Flow rutting In spite of the cold climate in Sweden, flow rutting is one of the most frequent types of distress in high volume roads. This is primarily due to the use of softer binders. d. Wear rutting Rutting caused by passenger cars with studded tyres is one of the major causes of pavement deterioration on heavily trafficked roads in Sweden. Therefore the choice of aggregate type and mix design are important parameters to limit this type of rutting. Wearing resistance of asphalt concrete. e. Water sensitivity Durability of bituminous layers especially against water and moisture effect is one of the most serious factors contributing to the degradation of asphalt pavements in Sweden. Freeze-thaw conditions have also the potential to lessen the cohesive strength and stiffness of the asphalt layers. 2. Functional requirements These requirements based on surface characteristics are mainly correlated with traffic safety and riding comfort. Indicators influencing functional performance are follows: a. Friction (Skid Resistance) Skid resistance is the force developed when a tire that is prevented from rotating slides along the pavement surface (Highway Research Board, 1972). Skid resistance is an important pavement evaluation parameter because: 1) inadequate skid resistance will lead to higher incidences of skid related accidents 2) most agencies have an obligation to provide users with a roadway that is "reasonably" safe. 3) skid resistance measurements can be used to evaluate various types of materials and construction practices. Skid resistance depends on a pavement surface's microtexture and macrotexture (Corley-Lay, 1998). Microtexture refers to the small-scale texture of the pavement aggregate component (which controls contact between the tire rubber and the pavement surface) while macrotexture refers to the large-scale texture of the pavement as a whole due to the aggregate particle arrangement (which controls the escape of water from under the tire and hence the loss of skid resistance with increased speed) (AASHTO, 1976). b. Roughness The roughness of pavement surface is determined from longitudinal profile by calculating IRI (International Roughness Index) values. This Standard Practice calls for the use of a longitudinal profile measured in accordance with ASTM E-950 as a basis for estimating IRI. C. Method 1. Structural requirements a. Stiffness modulus Stiffness modulus is measured on cylindrical cores from asphalt layers using Indirect Tensile Test and according to Swedish standard (FAS method 454 or EN 12697026 Annex C). The effect of age has been found very significant especially during the first year after laying. The following relationship can be used to calculate the stiffness modulus of asphalt concrete layer in respect to age. This relationship is based on a number of cores taken from pavement layers at different occasions over a five-year period (Said, 2005). Where, is the stiffness modulus at t2 (30 days) in MPa is the stiffness modulus at t1 in MPa t1 and t2 are the age of the bituminous layer in months The stiffness modulus of a 300 day-old asphalt layer has been taken as the initial stiffness modulus in evaluation of bituminous layers. The structural functional characteristics requirements in respect to stiffness modulus of pavement layers are shown in Table 1. At least 6 cores must test per 40000 m². Table 1. Stiffness modulus requirements in MPa Layer Temperature °C +5 +10 +20 Surfacing <9000 - - Binder course <11000 5500-9000 - Base course AADT Lane-heavy>1000 11000 5500–9000 >1500 AADT Lane-heavy>1000 11000 4500–7000 >1500 AADT Lane-heavy>1000 9000 2200–7000 >1500 b. Fatigue cracking Fatigue resistance is measured on cylindrical cores from asphalt layers at 10°C using Indirect Tensile Test according to EN 12697024 Annex E. Requirements on fatigue resistance of asphalt layers are related to traffic volume and presented in Table 2. The allowed tensile strain at specified traffic volume is calculated from fatigue criterion of bituminous mixtures that depends on the stiffness modulus of the asphalt concrete layer. The fatigue criterion in the Swedish specification is based on laboratory measurements on cores and calibrated with the field-based criterion. The fatigue relationship can be used in stead of requirements in Table 2. Table 2. Requirements on tensile strain (µs) at 106 loading 10 C with respect to fatigue cracking as a function of design traffic Traffic Base course Binder course surfacing AADT Lane-heavy>1000 >80 >60 >80 AADT Lane-heavy>1000 >100 >60 >80 AADT Lane-heavy>1000 >130 >60 >80 c. Flow rutting Resistance of asphalt concrete layer to flow rutting is measured on cylindrical cores of pavement layer using Repeated Axial Creep Test according to Swedish Standard (FAS Method 468). The structural functional characteristic requirements in respect to flow rutting resistance of the asphalt pavement layers are shown in Table 3. These requirements are based on earlier experiences with creep tests on cores. As is the case with the stiffness modulus, the effect of the age has been found very significant on the creep results. Therefore a similar relationship has been determined for creep tests. The following equation can be used to calculate the creep deformation of asphalt concrete layer in respect to age. Where, is the permanent strain at t2 in µɛ is the permanent strain at t1 in µɛ t1 and t2 are the age of the bituminous layer in months Table 3. Requirements on creep deformation as a function of design traffic AADT Lane-heavy Permanent strain in µɛ Surfacing Binder course Base Extreme load <15000 <12000 <18000 AADT Lane-heavy>2000 <18000 <15000 <21000 AADT Lane-heavy>1000 <21000 <18000 <25000 AADT Lane-heavy>500 <25000 <21000 <30000 AADT Lane-heavy>100 <30000 <25000 - AADT Lane-heavy>100 - - - d. Wear rutting Wearing resistance of asphalt concrete is measured on cylindrical cores from pavement layers using Prall method according to Swedish Standard (FAS Method 471). Bituminous layers with a thickness less than 25 mm shall be tested on specimens compacted in the laboratory. Table 4 shows the requirements on wearing resistance of bituminous layers as a function of design traffic, which is AADT adjusted for studded tyre share, traffic speed and lateral position of passenger cars during winter period. One test is recommended per 20000m². These requirements are calculated with a wearing model (Jacobson et al 1998). For more precise prediction of wear rutting with time the use of the model is recommended in the specifications. Table 4. Requirements on wearing resistance as a function of design traffic for surfacing layer AADT adjusted Prall-value in cm3 >7000 < 20 3500 – 7000 < 24 1500 – 3500 < 28 500 – 1500 < 36 <500 < 50 e. Water sensitivity Water sensitivity of asphalt concrete is determined by testing cylindrical cores of pavement layers using indirect tensile test. The Indirect Tensile Strength Ratio (ITSR) (adhesion value) is obtained by the ratio of tensile strength of conditioned samples to unconditioned samples according to the Swedish Standard (FAS Method 446). The ITSR value shall be larger than 75 percent for bituminous layers. 2. Functional requirements a. Friction (skid resistance) Skid resistance is generally quantified using some form of friction measurement such as a friction factor or skid number. Friction factor (f) where, F = frictional resistance to motion in plane of interface L = load perpendicular to interface Skid number (SN) It is not correct to say a pavement has a certain friction factor because friction involves two bodies, the tires and the pavement, which are extremely variable due to pavement wetness, vehicle speed, temperature, tire wear, tire type, etc. Typical friction tests specify standard tires and environmental conditions to overcome this. Table 5 shows some typical Skid Numbers (the higher the SN, the better). Table 5. Typical Skid Numbers (from Jayawickrama et al., 1996) Skid Number Comments <30 Take measures to correct ≥ 30 Acceptable for low volume roads 31–34 Monitor pavement frequently ≥35 Acceptable for heavily traveled roads Skid testing in the U.S. may occur in a number of ways, this section covers some of the more common methods including: 1) The locked wheel tester 2) The spin up tester 3) Surface texture measurement b. Roughness The reference method based on AASHTO Standard Practice for Determination of International Roughness Index for Quantifying Roughness of Pavements, AASHTO PP 37-04. This Standard Practice calls for the use of a longitudinal profile measured in accordance with ASTM E-950 as a basis for estimating IRI. The maximum allowable IRI values, depending on road category is normally between 0.9 to 1.5 mm/m over 4000 m road section.