Summary of Findings and Conclusions

This research investigated the possibility of using asphalt mixture stiffness and strength as performance indicators to verify rutting resistance of marginal mixtures.

The major objective was to develop a performance criteria or threshold value to be used as a quality control/quality assurance tool in the hot mix asphalt construction projects. The research concentrated on investigating asphalt surface mixtures used by Indiana Department of Transportation.

The stiffness of hot mix asphalt is dependent on the amount and type of raw materials and additives. In addition, the compaction effort and type of compaction equipment affect the mixture stiffness and strength characteristics. Therefore, three sets of samples were included in the laboratory testing as follows:

• Laboratory fabricated and compacted mixture using raw materials,

• Asphalt plant loose mixture samples compacted in the laboratory, and

• Pavement cores:

ƒ 1^st set of cores obtained right after pavement construction, and

ƒ 2^nd set of cores obtained approximately one year later.

Laboratory testing comprised of mixture stiffness and strength testing to find the best performance parameters and test conditions for criteria development. Mixture stiffness was measured using SST shear tester to obtain |G*| and IPC servo-hydraulic testing machine was used to measure dynamic modulus |E*|. Testing was conducted at 40°C and 54.4°C temperatures. The strength of the mixture was studied by measuring triaxial shear strength at 54.4°C using 7.5 and 50 mm/min ram loading rate, and indirect (IDT) tensile strength at 35°C using 0.06 and 0.36 mm/min ram loading rate.

A novel method of using horizontally stacked specimens to measure dynamic modulus of thin pavement cores was developed as an alternative for the SST testing of pavement cores. However, it gave significantly higher stiffness values compared to the

SST shear testing. Therefore, until these deviations can be explained satisfactorily, this method is not recommended to be used in the HMA production.

Evaluation of Test Conditions and Parameters Stiffness Testing:

Ranking of mixtures changed significantly when tested at 40 and 54.4°C temperatures. Identified possible reasons are change in relative binder stiffness from low to high temperature; increased damage accumulation in the specimen at higher test temperature; and/or different mixture behavior due to increased aggregate influence at higher test temperatures.

Confinement in the axial stiffness |E*| testing reduced the binder influence, which relatively increased the stiffness of the softer mixtures. Furthermore, the use of stiffer binders in the SMA mixtures seems to compensate the need of using confinement to verify the mixture performance.

The theoretical relationship between axial and shear stiffness is not valid, which means that |E*| and |G*| are not interchangeable, and one cannot be obtained from another by using theoretical formulations. Since |E*| and |G*| of mixtures are correlated by stiffness of the asphalt binder, this provides a possibility to predict

|G*|mix based on |E*|mix or vice versa using the Hirsch stiffness prediction model.

The average testing variation ranged from 12 to 19%. Testing variation increased with increasing testing temperature and decreasing frequency.

Strength Testing:

For IDT strength testing there was a good correlation between the slower and faster loading rates (R² 0.84-0.90). In addition, a theoretical relationship exists between the IDT strength and triaxial shear strength. Therefore, for practical purposes, the IDT tensile strength can be used as a surrogate test to replace the triaxial strength test. If no information is available for the friction angle, a ratio of 1.80 of the cohesion to IDT tensile strength is a good approximation.

The average testing variation for the IDT strength was less than 10%, which is less than the testing variation for stiffness testing.

Selection of Parameters and Test Conditions

Overall, the dynamic modulus |E*| and SST shear modulus |G*| did not rank mixtures in the same way; although the rankings at 40°C temperature were quite similar, at 54.4°C rankings deviated substantially. The plant mixtures were ranked in a more similar way by axial and shear stiffness, while field cores gave the most different rankings. Unfortunately, at this point it is not possible to confirm if this reflects the real material behavior or if it is an artifact of the differences between the test methods.

However, all test methods and test conditions were generally able to identify the softest and the stiffest mixtures in a similar manner measured from the raw material or plant mixtures, which is illustrated in Figure 64. However, this is not the case for the field cores rankings, as Figure 65 shows.

The stiffness difference between the stiffest and softest mixtures ranged from 2.2 to 4.0. The strength difference between the strongest and weakest mixtures ranged from 1.8 to 6.4. This refers to mixture composition of steel slag, blast furnace slag, and dolomite aggregate combined with three binder grades ranging from PG64-22 to PG76-22.

The average mixture stiffness and strength ratios within a high temperature binder PG grade ranged from 1.4 to 2.4. This suggests that by changing aggregate type and gradation, and binder content, it is possible to double the stiffness of mixture without changing the binder grade. For field cores, the strength difference was up to 4.4, reflecting differences in the roller compaction during construction. The |E*| gave indicated higher stiffness ratios compared to the |G*| and IDT testing.

The largest difference between mixtures was measured from field cores taken approximately one week after construction followed by the gyratory compacted asphalt plant mixtures.

0

SR64 I74 SR66 SR56 I65 SR135 US30 SR161 US31 US24 SR15

|E*|, (MPa)

|E*| 40-10 Hz |G*| 40-10 Hz IDT06 ^{Plant Mix}

(a)

SR64 I74 SR135 I65 SR56 US30 SR66 US31 SR161 SR15 US24

|E*|, (MPa)

Figure 64. Plant Mixture Rankings (Normalized).

0

SR56 SR135 SR161 SR64 I65 I74 SR66 US24 US31 US30 SR15

|E*|, (MPa)

SR135 SR161 US24 I74 I65 SR56 SR64 US31 US30 SR15 SR66

|E*|, (MPa)

Figure 65. 1^st Coring Mixture Rankings (Normalized).

Many engineering materials possess different strength and stiffness properties.

Literature suggests that for asphalt mixtures the correlation of stiffness and strength is mix dependent. The dense-graded mixtures had a good to excellent correlation between stiffness and strength for raw material and plant mixtures, while the correlation for the SMA mixtures was negative, meaning that the stiffer mixtures were weaker than the softer mixtures. The confined axial stiffness |E*| did not correlate to strength properties.

Field cores had poor correlation between the stiffness and strength properties.

This suggests that the construction compaction produces samples which have mechanical properties significantly different compared to the properties obtained from the gyratory compacted specimens. Then, the internal structure of cores is dependent on the types of rollers used and compaction temperature and it differs from the structure produced by the gyratory compactor.

Based on the research findings, both stiffness and strength are needed to predict mixture performance. Although mixture response obtained at lower and higher testing temperature deviates, at this point the lower testing temperature is recommended mainly to minimize the testing variation. Both temperatures are identifying the softer and stiffer mixtures, which is a requirement for a good performance parameter. The need to use a higher testing temperature may arise from the need to use different low temperature binder grades, which may mask the influence of the binder stiffness at the lower testing temperature, especially for mixture with modified binders.

The plant mixtures and field cores obtained one week after construction seemed to follow the binder high-temperature PG grading in identifying the softest/weakest and stiffest/strongest mixtures. The stiffness test results from the one-year old cores indicated that mixtures had aged heavily, and there was a significant change in the relative rankings; mixtures with softer binders had aged more than the mixtures with stiffer binders. The higher amount of binder in the SMA mixtures with stiffer original binder grade may have slowed down their aging while the stiffness of some dense graded mixtures with softer binder doubled.

The IDT strength test results indicated some strength decay in the second cores for the SMA mixtures with stiffer binders. This may indicate a shift of ductile to brittle material behavior. The relatively higher stiffness and loss of strength may contribute to vulnerability to cracking in the SMA mixtures.

The SMA mixtures without fiber had very different properties compared to mixtures with fiber. It had the lowest stiffness of the SMA mixtures and after 2 years of field aging, it had the lowest stiffness among all mixtures. While the strength seemed to decrease for the SMA mixture with fiber, the US31 mixture strength increased 14%. However, because only one mixture was tested, these findings should not be considered significant at this point

These results verify that asphalt mixtures are at the most vulnerable stage for rutting right after construction, as expected. However, the possibility of failure depends on the time of construction, i.e., Spring or Fall and variation of construction materials, which affects the initial aging of mixtures and the amount of traffic. The fact that the plant mixture behavior deviates from the behavior of cores, and cores are giving the largest difference between studied mixtures, suggests that the performance assessment must be conducted from field cores instead of using gyratory compacted plant mixtures. Because, at this point, the composite specimen technique did not turn out to be a reliable way of obtaining mixture stiffness, the SST shear testing is recommended to be used to measure the stiffness of field cores.

Although dense graded mixtures had a good correlation between stiffness and strength for the plant mixtures, the stiffness and strength correlated for field cores was very poor. These findings suggest that both stiffness and strength must be used for performance assessment of asphalt mixtures.

Micro-texture Collapse

Micro-texture collapse was studied using confined dynamic creep testing. If low axial stiffness |E*| and large dilatation potential (εp ratio) are used as the criteria, the most vulnerable mixture is US24 followed by SRS161. Both mixtures had the

softest binder, and SR161 had 34% flat particles and small FAF Bailey Method ratio, which indicates a low amount of fines in the fine portion of aggregate gradation. US24 is the coarsest of all dense graded mixture having the lowest percent passing 4.75 mm sieve and lowest Bailey Method CA ratio. If high densification (cumulative εp) and low stiffness are used as criteria, the most vulnerable mixture is US24. These results, however, may be confounded by the confinement used in the testing and it is expected that different results may be obtained if testing is repeated using unconfined testing.

Criteria Development

The performance criteria for stiffness and strength were developed by using I74 forensic study data as a baseline, and criteria verification was done using findings from I70 SPS9A study. For stiffness criteria, two different criteria from the literature were compared against the measured mixture performance. These criteria were stiffness criteria developed by Pellinen using layered elastic analysis and rule of thumb criteria from Asphalt Institute. The IDT strength criteria was developed using IDT strength criteria developed by Christensen et al. as the baseline.

Indiana mixtures seem to comply well with the Asphalt Institute rule of thumb stiffness criteria, when it is applied for cores with 7.5% air voids content; while the criteria based on layered elastic analysis was too stringent for the roads with heavier traffic loading. The IDT criteria derived from the Christensen et al. criteria seems to comply well with the Indiana mixtures; while the I74 forensic study criteria seems to be too stringent.

It was evident from the I74 failure data that a factor of safety is needed when applying performance criteria to avoid failure. The needed factor of safety may be as high as 3 for the asphalt mixtures in Indiana. However, there is a delicate balance in preventing failure and at the same time rejecting good performing mixtures or accepting fair mixtures. However, at this point there is not enough data to analyze adequately the risks of acceptance or rejection. It can be speculated that the factor of

safety 2.2 may be adequate when tight quality control is exercised during HMA construction.