SHRP METHOD/SUPERPAVE SYSTEM The Strategic Highway Research Program (SHRP) of the U.S. National Research Centre deals with, amongst other things, road construction and between 1987 and 1993 new classification methods, tests and design techniques of bituminous materials were introduced. Based on this, a new system known as SUPERPAVE (Superior Performing Asphalt Pavements) has been developed by the Federal Highway Administration (FHWA) who were appointed to implement the research made under SHRP. This system is dedicated to the classification of materials, the design of the mix and forecasts of the pavement performance. The system defines new test methods, relative equipment and acceptance criteria. In practice it is a program of the design of bituminous mix so as to reach levels of performance and durability compatible with traffic levels and climatic conditions. These performance levels refer to the prevention and control of: - permanent deformation - formation of fatigue cracks - formation of cracks due to low temperature Of course the importance of these three problematics is affected by the local environmental conditions. The SUPERPAVE program, whilst based on test methods adopted by AASHTO as provisional standards, is still not completely used over all the advanced levels. This is probably due to the high cost of the instrumentation used and the need for further practical experience, also in the U.S. at this time except research centres. However, we think it is important and worthwhile to give a brief introduction of the basic test introduced by SUPERPAVE which represents a new and more realistic approach to the problem of studying bituminous mixes on the basis of their visco-elastic and plastic characteristics. Therefore, we illustrate the volumetric mix design method using the Gyratory Compactor, a method that is now being used in both the U.S. and Europe. Historical data/statistics of traffic levels and temperatures of the last ten years are recorded in "maps" which have been introduced in the SUPERPAVE software. - Traffic levels are classified by Single Equivalent Axles (ESALs) that pass a particular road lane

during the pavements programmed lire. An equivalent axle corresponds to a load of 80 kN. - The climatic conditions refer to the average temperature of the hottest seven days of the year

and the lowest temperature of the year. The mix design and compaction method are made on the basis of structural models which correlate directly with the aforementioned conditions to which the bituminous pavements are subjected. The traditional empirical test methods have been abandoned; these include, amongst others, the Marshall test for mixes. In substance the Marshall test is considered an empirical, not really relied to performance whilst gyratory volumetric test is not empirical but performance oriented.

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DESIGN OF BITUMINOUS MIXES SUPERPAVE divides the design complexity of bituminous mixes into two levels determined by traffic volume expressed in Esals which the pavement must support within its programmed life span. However, a fundamental of both levels of design is that the pavement mix must resist permanent deformation, cracks at low temperature and cracks due to fatigue. The levels are: - Base Level up to 106 Esals. Only the volumetric mix design method is used (i.e. gyratory

compactor). - Advanced Level aver 106 Esals. The mix is firstly designed using the volumetric method and

then defined in its optimum version by selecting between a few acceptable mixes, using rheological tests based on foreseen performance models.

Base Level Volumetric Method (AASHTO TP4-93/prEN 12697-9/SUPERPAVE Demonstration Project 101) Basically, the aim of the volumetric method is to select the best mix, on the basis of the traffic data, ambient temperature data, requisites such as filling the voids between the aggregate with bitumen (VFB), and percentage of voids of the aggregate material (VMA) (table III-8, III-7), so that upon compaction the final percentage of voids is 4%. The test is performed using a gyratory compactor (Controls model 77-B0250, 77-B0251 and 77-B0252).

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The machine compacts a cylindrical sample, normally dia. 150 mm (or, more rarely 100 mm) to a height corresponding to approximately 0.8 of its diameter using two compaction actions: - a vertical load of about 600 Kpa produced by a vertical ram; - a gyratory action of the mould with respect to the centre of the upper face of the sample so as to

maintain parallelism of the compaction platens and confer a rotational angle from the sample axis of 1.25o at a speed of 30 rpm.

These two simultaneous compaction actions simulate the compaction produced by roller compactors (in particular rubber wheeled and static compactors) much better than the compaction achieved using a Marshall compactor. With each gyration of the sample mould, the height of the sample will decrease, knowing the internal dimensions of the mould and the weight of the sample, with appropriate sensors and a PC connected to the compactor it is possible to monitor the density of the sample in real time during the compaction process. Naturally, once the data of the mix has been entered into the software (aggregate density, Gag, bitumen density, % aggregates, % bitumen) the % voids, compaction % against theoretical max. density, % VMA and % VFB are indicated with relation to the number of gyrations. For the volumetric test, the machine provides the density data relative to three stages of compaction (fig. 3.4, table 3.2) corresponding to the initial number of gyrations Ninit, design gyrations Ndesign and the maximum number of gyrations Nmax, that is the number of gyrations corresponding to three adjacent points on a curve which is practically a straight line, known as the densification curve, which shows the relationship between the theoretical % density and log number of gyrations. The mix must be compacted with an energy (nr of gyrations) determined by the traffic intensity, in equivalent axles, which the pavement must support during its life span, and the maximum average ambient temperature as shown in table VI.

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In practice we have a family of densification curves defined by two relationships which bind the three numbers of gyrations as follows:

Log Ninit = 0.45 Log Ndesign Log Nmax = 1.10 Log Ndesign

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Mixes made with aggregates having good aggregate structure, that is high angularity (fig. 3.5) present a densification curve with a sharp slope, whilst curves with little slope indicate a mix which is easy to compact because it has a poor aggregate structure with low internal friction, practically round, with a low angularity value. When using the same type of bitumen, the first type of mix will be more resistant to permanent deformation than the second. About this, it is interesting the standard prEN 12697-10 “Compactability of bituminous mixtures”, that defines the compactability factor “K” that represents the slope of the compactability curve, with the relative density or the percentage of voids on the Y axis and the compaction energy on the X axis. In gyratory tests the compaction energy is given by the number of gyrations, so that:

V% (Ng) = V% 1g - K log10Ng

Where: V% (Ng) = voids % at N gyrations V% 1g = Voids % at 1 gyration Ng = Number of gyrations If “V% (Ng)” and “V%1g” are replaced by “% GMM(Ng)” and “% GMM1g”, it becomes:

%GMM (Ng) = %GMM1g + K log10Ng

where, in this case, the coefficient K shows the concept of compactability as slope of the gyratory test result curve, as considered by Superpave.

The SUPERPAVE program defines that, in order to prevent this effect, the densification curves must present a percentage of initial compaction against max. theoretical density:

Cinit ≤ 89% where number of gyrations , Ninit varies from about 7 to 10. and a percentage of final compaction:

Cmax ≤ 98% (Nmax) of the maximum theoretical density The volumetric method is articulated in the following steps:

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- Choice of type of bitumen (PG x y) based on historical data of temperature and traffic levels. - Definition of temperatures of mixing and compaction depending on the viscosity of bitumen

which must be 0.17 Pa.s and 0.28 Pa.s respectively (e.g. by experimentation, far a PG 58 -34 bitumen, the mixing temperature will be 160 to 172°C and the compacting temperature 151 to 157°C).

- Choice of aggregates, that is checking their compliance with compulsory characteristics which

are: - angularity of coarse aggregate (percentage with one or two fractured faces). - angularity of fine aggregate (percentage voids in receiver container of fine aggregates falling

from a special funnel). - Shape characteristics of aggregate (determined with a proportional calliper with 1:5 ratio). - Clay content (sand equivalent value). The acceptance limits are given in appropriate tables in relation to the traffic levels.

- Definition of at least three different mix graduations with the calculation of the percentage

components so as to approximate to a typical recommended grading curve given in function of the maximum aggregate size.

It should be noted that respect of the angularity requisites ensures respect of permanent deformation, whilst that of shape, fatigue cracks. The prevention of cracks due to low temperature is given to the type of bitumen used. Three mixes are prepared, two samples for each mix, relative to the three graduation curves with a provisional percentage bitumen content calculated on the basis of effective density of the aggregate, an estimation of the bitumen absorbed by the aggregate with the formula based on the surface area of the aggregates used in the mix. The compaction should be made with the number of gyrations foreseen by the traffic level and maximum ambient temperature data (table VI-13).

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For each gyration, the compactor reads the decrease in sample height and stops the test once the preset number of gyrations has been reached. The software package calculates the values of the density up to the final gyration (i.e. corresponding to Nmax). It should be remembered that the density given is estimated and should be corrected with the measurement obtained from hydrostatic weighing (the ratio between the corrected density of the compacted sample and the corresponding estimated density can be used to correct all the estimated density values calculated during the compaction procedure as foreseen by our software package) (table VI-14 and fig. VI-7).

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For each mix a table is prepared showing the compaction data along with the densification curve. The most suitable mix between the three grading curves complying with the final criteria is selected, far example, far max aggregate size 19 mm the criteria is as follows: - Percentage voids compacted mix 4% (Nmax) - Percentage voids mixed aggregates VMA 13% - Percentage voids aggregate mix filled with bitumen VFB from 65 to 75% (this value is

dependant on traffic level, table III-8) - Percentage density of sample against maximum theoretical density at Ninit ≤ 89% - Percentage density of sample against maximum theoretical density at Nmax ≤ 98%

The selection is made using a second series of samples made with the gyratory compactor, mixed with a bitumen content recalculated with successive approximations using an empirical formula for the mix to achieve a voids content as close as possible to 4%. Consequently the values of % VMA, % VFB, % Gmm at Ninit and Gmm at Nmax are recalculated. At this point it is possible to select the aggregate mix most suitable on the basis of the criteria for a given traffic level and maximum aggregate size. Optimisation of the bitumen content is made (table VI-25, table VI-26, table VI-27, fig. VI-15, fig.

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VI- 16, fig. VI-17, table VI-18) by using a further 4 series of samples made with the selected aggregate mix and with a percentage of binder which differs from the calculated binder content by -0.5%, 0%, + 0.5% and + 1%.

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The final selection of the bitumen content is made after making the four densification curves, the comparative table of the percentage compaction in function of bitumen content at Ninit, Ndesign and Nmax and the diagrams that show: - Percentage voids final mix against bitumen content - Percentage VMA against bitumen content - Percentage VFB against bitumen content The influence of humidity on the final mix is checked by using the indirect tensile test as per AASHTO T283 (see Controls model 76-B78/A), one examines the reduction in percentage of resistance to traction of three samples saturated under vacuum with respect to three similar samples not saturated. In conclusion we can see that the difference in behaviour of the mix is caused above all by various resistance values due to the internal friction of the sample during the test. Thus it is evident that for research purposes it would be advantageous to have the possibility to measure the shear force that the gyratory compactor imparts on the sample during the test, the graph of this shows how this additional information helps in the selection of mix design. Our 77-B0251 model includes a load cell which allows the measurement of shear force (kN/m2) for each gyration. Each model we produce is supported by a macro program far bituminous materials (77-B0250/8) for test processing in Microsoft Excel format for 150 and 100 mm diameter samples. The macro program can be used to correct the data gathered by the standard software program and establish the mix design by processing the single test to evaluate the best aggregate graduation and the evaluation of optimum binder content based on the curves of up to four tests on each of up to four different bitumen contents. Later, on site the individual curves of the asphalt in production can be immediately checked to verify its conformity with the design mix. Therefore we can conclude that the use of Gyratory Compactor is dedicated to mix design and to the rapid verification that the bituminous mix to be laid in situ corresponds to the requisites fixed by the mix design itself. In fact we say rapid as we have noted that one test run requires not more than 5 minutes, against one day for Marshall test! We detail now a mix design procedure for bituminous mixtures using the Gyratory Compactor. Introduction The mix design procedure described hereunder follows the method described in SHRP A-407 with the following adaptations: - The fused aggregates are as defined by the Italian Authorities for base, binder and wearing

courses. - The reference densification curves are taken from Autostrade S.p.A. along with the relative

acceptance criteria for base, binder and wearing courses made with natural bitumen or modified soft or hard bitumen. If the above curves, which have already been tested in Italy, are not used, reference should be made to the tables given by Superpave. Here the characteristic number of gyrations of the gyratory compactor are defined (N initial, N design, N max) for the definition of the curve, dependent on the life of the pavement expressed in ESALs, and the maximum average temperatures. In this case the criteria are:

Compaction at Density as % of max. theoretical density

N initial C < 89 N design C = 96 N max C < 98

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Selection of aggregate mix grade 1. With the aid of one software for the rapid percent composition of available aggregate sizes,

three mix grades are determined, one close to the upper limit, one intermediate and one close to the lower limit

1.1. A provisional bitumen content is calculated for each of these three aggregate grade mixes as follows:

1.2. For each percentage fraction of aggregate, calculate the specific weight (aggregate oven dried) and its specific weight in SSD (Saturated Surface Dried) conditions.

+++

+++=

nGP...

2GP

1GP

)P...P(PG

sa

n

sa

2

sa

1

n21sa

+++

+++=

nGP...

2GP

1GP

)P...P(PG

sb

n

sb

2

sb

1

n21sb

Where: P1, P2…. Pn = fractional percentage of aggregate mix Gsb = Specific weight of aggregate in SSD conditions Gsa = Apparent specific weight of aggregate G1, G2…. Gn = specific apparent weight or fraction of aggregate

1.3. For each mix calculate the total effective specific weight (in terms of absorption of bitumen by the aggregate):

Gse = Gsb + 0.8 * (Gsa – Gsb) Where: Gse = Effective specific weight Anyway for bigger accuracy it could be directly measured by the use of vacuum pycnometer according to ASTM D2021 – Maximum specific gravity of bituminous paving mixtures.

1.4. Estimate the percentage volume of bitumen absorbed by the aggregate (Vba):

−=

sesbsba G

1G1*WV

Where: Ws = percentage weight of aggregate mix given by:

( )

se

s

b

b

ass

GP

GP

V1*PW+

−=

Where: Pb = Percentage weight of bitumen assumed as 0.05 (in decimal), that is 5%

bitumen as supposed as provisional dose Ps = Percentage weight of aggregate assumed as 0.95 (in decimal) Gb = Specific weight of bitumen, assumed as 1.02 Va = Percentage volume of voids, fixed at 4%

1.5. Estimate the effective percentage bitumen volume, excluding the absorbed bitumen:

Vbe = 0.176 – 0.0675 log Sn Where: Vbe = Effective percentage bitumen volume. Sn = Maximum nominal diameter of aggregate in mm.

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1.6. Calculate the total percentage of bitumen:

( )( ) sbabeb

babebbi WVV*G

VV*GP++

+=

Where: Gb = Specific weight of bitumen Ws = Percentage weight of aggregates

1.7. Two compactions are made on the gyratory compactor for each of three mixes mentioned above. The sample is oven conditioned at 160oC for one hour before compaction. The number of gyrations is selected on the basis of the type of conglomerate and the technical specifications given by Superpave or local agency.

1.8. At this point, once the average values of the three mixes have been processed, the macro

program installed in the PC calculates the average results of the three mixes to estimate the necessary bitumen content to reach exactly 4% voids content, in trial and error procedure. Consequently the following parameters are also automatically calculated, namely VMA, VFB, Gmm at Ninitial and Gmm at Nmax. - Gmm at Ninitial < 89% - Gmm at Nmax < 98% - VMA at Ndesign from 15 to 10.5% dependant on max aggregate size (Table III-8) - VFB at Ndesign between the limits defined by traffic level expressed in equivalent axles

(Table III-7) 1.9. Select the aggregate mix which most closely respects the above mentioned criteria.

Determination of optimum binder content 2. Having selected the aggregate grade mix, four samples are prepared with this aggregate grade

with the following binder content: - Percentage estimated automatically by macro program (see 1.8) - Estimated percentage less 0.5% - Estimated percentage plus 0.5% - Estimated percentage plus 1%

2.1. At least two samples from each batch are compacted in the gyratory compactor (after

having been oven conditioned as described above). 2.2. The macro program is there used to elaborate the certificates of the individual tests, the

certificate of the average values of the groups of tests with different binder contents and the abstract certificate containing the graphs of the parameters V%, VMA%, VFB% with respect to bitumen content and the average densification curves for the test groups.

2.3. On the basis of the voids percentage required at Ndesign (4%), the design binder content can

be established checking that it complies with the volumetric and density requisites established in the Superpave tables as mentioned before (see 1.8 above).

2.4. If the local agencies (as Soc. Autostrade in Italy) have already defined the specifications, in

term of number of gyrations, aggregate gradation and aggregate specifications, the abstract graph could be used to check compliance with the acceptance limit in terms of void percentage (V%) at Ninitial, Ndesign and Nmax of the mix with the selected bitumen content.