GYROPAC BULK DENSITY DETERMINATION ISSUES IN SOUTH AUSTRALIA

Transcription

1 GYROPAC BULK DENSITY DETERMINATION ISSUES IN SOUTH AUSTRALIA HUGO VAN LOON SENIOR ASPHALT ENGINEER, DTEI: TRANSPORT JOHNNY TRAN ASPHALT ENGINEER, DTEI: TRANSPORT RAY THOMPSON SENIOR PROJECT MANAGER, BARDAVCOL ABSTRACT Asphalt supplied to the State Road Authority (SRA) in South Australia is designed to the Austroads Publication Series 4B (Austroads, 2006) requirements with several adjustments by the SRA specification. Monitoring by the SRA involves assessing air voids obtained by contractor production testing against target design air voids, and SRA also conduct comparison testing on audit samples to monitor contractor laboratory results. This paper details the outcomes of auditing undertaken by the SRA, and more specifically the discussions centred on the determination of the bulk density of asphalt by both the contractor and SRA and their respective laboratory requirements. A review of SRA laboratory procedures and recommendations for improvements to Australian Standards relating to bulk density determination including sampling and gyratory issues is provided. BACKGROUND The South Australian SRA presents a specification based primarily on Austroads Publication Series 4B (Austroads, 2006). Application is strengthened by use of a single target design air voids to remove ambiguity. The expectation is for contractor production test results to achieve this target within tight specification tolerance limits and this data be monitored to ensure appropriate binder content is used to deliver this target. This process is as described previously (van Loon, 2006). The manufacture of asphalt is strengthened with the contractor s quality assurance procedures and processes that are documented and carried out in practice. In addition, laboratories require equipment calibration and proficiency capabilities of technicians through NATA requirements. An important task for a SRA is the establishment of commonality between contractor production laboratory test results and the SRA laboratory auditing results of both product asphalt and in situ air voids checking. This paper documents some auditing experiences of the South Australian SRA in its role of ensuring all laboratories deliver a similar outcome, concentrating on the determination of bulk density of asphalt compacted samples including sampling and gyratory compaction issues. 1

2 METHODOLOGY This paper briefly states the auditing outcomes for a project where a difference in bulk density between the contractor and SRA was identified and further investigated, including the use of an independent auditor. To put the bulk density test into perspective, a discussion on test method and its variation is provided, including uncertainty of measurement and repeatability & reproducibility data from a range of proficiency exercises conducted. A range of testing was undertaken demonstrating the variability of the bulk density test and also the possible source of difference between SA SRA and contractor. Tests on hot asphalt, a revised test procedure, effect of Gyropac mould size and variability of bulk density test without sampling and compaction influences is then provided. The laboratory practices and operating imperatives are discussed for both production and auditing laboratories as background information for possible differences for the bulk density test results. A discussion is provided on the bulk density test for specific areas where the possibility existed for different interpretation of testing requirements including differing practices for production and auditing laboratories. Finally a discussion and set of recommendations is presented to minimise the variability in determination of bulk density test results including sampling and gyratory compaction issues. Suggested changes for both SRA test procedures and Australian Standards are given. AUDITING OF AN AC20 MIX The AC20 mix is defined by the SA SRA as requiring gradings such that 10% is retained on the 19mm sieve. This mix could therefore be classified as being an AC28 according to AS2150. In auditing the AC20 for a project, a difference between contractor and SA SRA became evident, and independent testing on 3 audit samples with production of approximately 12 asphalt samples provided the same mean as that of the SRA. BULK DENSITY INVESTIGATION To determine possible reasons for the identified difference between contractor and SRA, a meeting was held with all participants calling for possible reasons for the difference and a joint comparative testing program as set out in Table 1 was decided upon. 2

3 Table 1: Bulk Density Comparative Testing Program Variable Test 1 Test 2 Test 3 Test 4 Test 5 Mould size 100mm 100mm 100mm 100mm 100 / 150mm Sample heating Hot Hot Hot Hot / Hot Reheat Sample splitting Scoop Scoop Scoop / Scoop Scoop C&Q Sample spading Nil Nil / spaded Nil Nil Nil Sample drying Own chamois DTEI chamois DTEI chamois DTEI chamois DTEI chamois. BULK DENSITY TEST VARIATION FROM PROFICIENCY EXERCISES Bulk Density Test Requirement The bulk density test used by SA SRA is performed according to Australian Standard AS : Determination of bulk density of compacted asphalt Presaturation method, and as amended by SA SRA test procedure TP428. The bulk density is obtained from the following formula: where: ρ bulk = m 1 x ρ w / (m 3 m 2 ) m 1 = mass in air of the sample (g), m 2 = mass in air of the saturated sample (g), m 3 = mass in water of the saturated sample (g), ρ w = density of water at the test temperature (t/m 3 ). Bulk Density Test - Measurement of Uncertainty A spreadsheet has been prepared to calculate the minimum test variability due to equipment errors etc and is provided in Appendix A. This suggests that for a 95% confidence interval the measurement of uncertainty (UM) for the bulk density test is t/m 3. Bulk Density Test - Repeatability and Reproducibility The major exercises that have been carried out recently in Australia are: 1. The NATA proficiency exercise of 2001 which is estimated to provide a t/m 3 reproducibility value (NATA, 2001). 3

4 2. The ARRB exercise in 2003 provided estimates of repeatability and reproducibility of the bulk density test as and t/m 3 respectively (Oliver, 2003). 3. The first South Australian exercise of 2006 provided a t/m 3 reproducibility value (Butcher, 2006a). The results appear to reflect the scope of each exercise, with the largest result for NATA 2001 being an Australian wide exercise. The middle result was the ARRB exercise and even though this encompassed only 10 laboratories, it would have been a multi-state exercise. The smallest result is the DTEI exercise which was restricted to laboratories in South Australia. A further exercise with a single pair of moulds provided a repeatability value of t/m 3. A representation of variability for the bulk density test is provided in Figure 1, showing minimum variability from equipment (UM), a step change where sampling and Gyropac influences are included up to a repeatability value of t/m 3 for consideration of all laboratories in Australia. R = 86 NATA 2001 R = 72 ARRB 2003 R = 40 DTEI 2006 R = 16.4 DTEI 2006 (No mould influence) R = 14.1 DTEI 2006 UM = 3.0 Bulk Density with Sampling and Gyropac Influences Bulk Density without Sampling and Gyropac Influences Figure 1: Bulk Density Test Variability Diagram Bulk Density Test - Component Variables The bulk density test can be separated into separate phases which affect its final value and these are: Gyratory machine influence (s g ) Gyratory mould influence (s m ) Bulk density test (s t ) Material Sampling (s s ) 4

5 Where influence s g, s m, s t and s s represent the standard deviation or variability of the component. The bulk density variability (s b ) is presumed to be related as follows: s b 2 = s g 2 + s m 2 + s t 2 + s s 2 Two additional proficiency exercises were conducted in South Australia where the same set of homogenous subsamples was used as for the first exercise. The second exercise (Butcher 2006b) used two low usage hardened steel moulds that were rotated through the laboratories in place of laboratory production moulds. The value of Gyropac (IPC:1993) mould variation (s m ) is estimated by the differences in variance (standard deviation) as t/m 3. As part of the first proficiency exercise, several asphalt pats were sent to each laboratory and the variance was t/m 3 (s t ). Thus with the bulk density variability at t/m 3 (s b ), sensible values for above equation suggest both (s g ) and (s s ) to be in the order of t/m 3. This suggests that a lot more care needs to be taken on the issue of mould wear and the current maximum value of 0.5mm wear Clause 4.5, T211(Austroads T211, 2007) may need to be reduced if repeatability values for the bulk density test using the Gyropac are to be reduced Unrestricted Same pats Same moulds 2.42 Bulk Density (0.001t/m 3 ) Sample Sample 1 Figure 2: DTEI 2006 Proficiency Testing Summary Youden Diagrams. Figure 2 presents the Youden diagram to show the effects of the first two DTEI exercises including the sharing of pats. A feature to note with Youden diagrams is that the minor axes tend to reflect repeatability or within laboratory variations, while the major axes tend to reflect reproducability. The third exercise involved the same set of homogenous subsamples as second exercise but used for the Servopac (IPC, 1996) gyratory machines and sent out throughout Australia and New Zealand. The results are provided in Table 2 (Butcher, 2009b). 5

6 Table 2: Bulk Density Proficiency Exercises (t/m 3 ) Exercise Norm. IQR* (Std Dev) NATA NATA DTEI NZ * Normalised Inter Quartile Range Equipment Marshall Gyropac Servopac Servopac The value of the Servopac mould variation (s m ) is estimated by the differences in variance as t/m 3, not too dissimilar to the Gyropac mould variation. BULK DENSITY VARIABILITY TESTING Bulk Density Test Hot Mix Test Results A matrix was developed from Table 1 that concentrated on a comparative testing program for spading verses no spading and riffle and scooping verses cone and quartering. In addition the method of ensuring asphalt temperature was at the correct temperature, being either within mould or on a tray was tested. Note that terminology in Table 3 re conditioning does not represent heating sample for an hour as required in Australian Standard but time required to achieve required sample temperature. The results from testing are average of four samples for each box and is summarised in Table 3. Testing was carried out by DTEI Asphalt Laboratory at Walkley Heights and samples were provided by truck coming to laboratory and sampled hot and asphalt maintained hot for testing. Tests were carried out on the hot material on the one day except where indicated otherwise. Additional material was sampled and reheated to complete testing. Values in Table 3 provide the differences in results in terms of air voids with the lowest number minus the higher number in columns and rows between the bulk density results. Thus top or left most sample minus next means that as the bulk density was higher for first test over second, lower air voids were obtained and thus a minus number is shown. 6

7 Table 3: Matrix Test Results (Average Bulk Density t/m 3 ) Testing Method No Spading Voids Diff Spading to DTEI Method Voids Diff. Sample Splitting using Divider and scooping and * condition sample in mould Voids Difference Cone and * Quartering condition sample in mould Spading to Australian Standard Voids Difference Cone and * Quartering condition sample in tray Voids Difference Sample Splitting 2.367* * * using Divider and scooping and condition sample in tray * These test results apply to reheated samples The following conclusions could be made from the data. The riffle influence seemed to be excessive and unexplained, Spading to Australian Standard produces the most variance, Difference between standard contractor procedure (2.412 t/m 3 ) and standard state road authority procedure (2.400 t/m 3 ) was -12 kg/m 3. The general consensus from matrix testing was: sample size of four samples was too small to provide meaningful results, Need to tighten test procedures and testing requirements, Coarse mix made tests more variable than for finer mixes. Despite all the efforts made by both head and asphalt contractors and SRA with trucks sampled at plant and at SRA laboratory prior to placement at the field and altering testing procedures, no clear conclusion was reached for the difference in bulk density data between SRA and asphalt contractor. Bulk Density Test Test Procedure A testing procedure was prepared summarising the state road authority test procedures and Australian standard requirements. Part of the procedure is provided below to give an indication of the type of changes made to strengthen it. (a) Remove the mould from the oven and place on balance. Remove the baseplate, lower wearing disc from the oven and place in the mould followed by a circular paper disc and zero the balance. 7

8 (b) Scoop the required amount of asphalt in a single swoop through tray of sampled asphalt, and transfer the approximate quantity into the mould as a single lot, ensuring that the last 10% approximately is slowly added and halted at required mass. Remaining material is to be discarded. Using a single 6 mm rod 250 mm long with both ends rounded, insert approximately two thirds depth for four times around the perimeter and four times around the interior. Note: On completion of rodding ensure surface is level. (a) On reaching 155 o C or higher, remove mould from oven and place on bench and monitor temp fall. (b) Place temperature probe into mix in mould. (c) When temperature reaches 150 o C ± 1 o C record time and temperature and proceed. (d) If temperature falls below 149 o C, remove material from mould, place mould in oven, place in a glass bowl, heat in microwave, and repeat steps 7(h) on. (e) Remove probe and place a circular paper disc and then the upper wearing disc and the upper platen on top of the asphalt in the mould and place the filled mould and assembly to the gyratory compactor. (f) Immediately compact the specimen in the gyratory compactor to the set number of revolutions of the compactor (see Note 2). (g) Remove the specimen from the mould using the specimen extractor, placing a heat-resistant board on each end to assist in handling the specimen. Allow the specimen to cool in front of a fan until temperature reduced to below 50 o C (record time till bulk density testing). NOTE: Care should be taken to ensure that the specimen has cooled sufficiently to ensure that it does not deform when removed from the mould. It is clear from the above procedure that strengthening of many clauses is required to obtain more robust and consistent bulk density test results. Bulk Density Test Effect of Mould Size The SA SRA allows the use of a 100mm diameter mould to be used for compaction of asphalt samples. This is a practical consideration, as 150mm diameter moulds require a greater compaction angle, and changing of Gyropac angle is considered to provide more potential errors than use of a 100mm diameter mould. This could be overcome by either purchase of additional Gyropac machines dedicated to use of different angle, or purchase of a Servopac where angle is able to be adjusted electronically. During the testing for Table 3, some pats were visually bonier, had a greater height and therefore have a much larger volume for the same mass as shown in Figure 3. This has the consequence of a lower bulk density than the visually more homogeneous pats. This is due to a variety of reasons including poor sampling procedures and disturbance of sample from excessive spading. 8

9 Figure 3: Asphalt Samples Coarse and Fine Texture Comparison resulting in Height Difference Bulk Density Test Variability without Sampling and Compaction Influence Three separate mass measurements (m 1, m 2 and m 3 ) are required to calculate bulk density. These measurements can be compared to determine and evaluate any differences between the results from the two laboratories. This will provide any difference in the bulk density test without any influence from sample preparation and compactions in the Gyropac. Contractor production testing provided asphalt pats that were obtained and retested by the DTEI laboratory, and data comparisons for the three components of the bulk density test are provided in Figures 4 through 6, and actual bulk density comparison is provided in Figure Contractor Pats - Dry Mass Comparison AC20M320H3--T475 & AC20M3203--T Dry Mass - Company Bulk =0 Voids = 0% Average Dry Mass (DTEI) = /11/2008 Figure 2 Dry Mass - DTEI Figure 4: Dry Mass Comparison (Part of Bulk Density) with DTEI & Contractor 9

10 1260 Contractor Pats - SSD Mass Comparison AC20M320H3--T475 & AC20M3203--T473 SSD Mass - Company (1246.2, ) SSD Mass = 1 Voids = 0.2% Average SSD Mass (DTEI) = /11/2008 Figure 3 SSD Mass - DTEI Figure 5: SSD Mass Comparison (Part of Bulk Density) with DTEI & Contractor 750 Contractor Pats - Mass in Water Comparison AC20M320H3--T475 & AC20M3203--T473 Mass in Water - Company (735.2, 735.8) Average Mass in Water (DTEI) = /11/2008 Figure 4 Mass in Water - DTEI Figure 6: Wet Mass Comparison (Part of Bulk Density) with DTEI & Contractor 10

11 Contractor Pats - Bulk Density Comparison AC20M320H3--T475 & AC20M3203--T Bulk Density - Company (2.427, 2.431) Bulk =0.004 kg/m 3 Voids = 0.15% Average Bulk Density (DTEI) = t/m /11/2008 Figure 1 Bulk Density - DTEI Figure 7: Bulk Density Comparison with DTEI and Contractor The data shown in Figure 7 shows a difference of 4kg/m 3 at the average bulk density, which equates to 0.15% air voids. The offsets at midpoint from Figures 5 and 6 being 0.2% and 0.1% air voids respectively is not cumulative as shown in Figure 7. The r 2 values are 0.908, 0.936, & for Figures 4 through 7 respectively. The greatest difference is when obtaining m 2 (surface saturated dry mass) where the amount of drying is subjective and improvements are suggested below. GENERAL DISCUSSION OF LABORATORY PRACTICES Production Laboratory Practices A production laboratory must carry out a range of tests on production asphalt as prescribed in the specification at a prescribed frequency. The purpose is two-fold: firstly to provide the contractor with up-to-date information on plant performance to make appropriate plant adjustments so as to maintain specification targets, and secondly to satisfy the client that the specification has been met. The production laboratory invariably has time constraints, firstly due to frequency stipulated in specification, and also to provide quick feedback on plant production performance. The fast pace of a production laboratory must be acknowledged by the SRA. The production laboratory is required to test the asphalt produced for air voids, as well as binder content and maximum density. For efficient operation within the laboratory, asphalt to be placed in a Gyropac mould must be hotter than that required by the test procedure. Should the temperature fall below that required, substantial time is lost as temperature rise of asphalt in an oven is slow. For adequate temperatures to be maintained splitting is to be minimised and moulds and components must be hot. 11

12 To meet the South Australian SRA specification, approximately 24kg of AC20 needs to be sampled from a truck usually in two containers: one for SRA auditing and one for immediate production testing. The two samples should be riffled and split to form SRA audit and production samples, but this is often not done. The production sample is taken to the laboratory where sufficient material is scooped in a single movement of a quantity of approximately 1250g to compact a single pat of asphalt. Rodding of some form is carried out and thermometer is placed inside mix to monitor temperature. Once temperature drops to 150 C (+5 C) then compaction occurs on first sample, and similar process is adopted for second sample. Bulk density reported is average of the two pats. The rest of material is split for maximum density and binder content determination. The second sample is riffled to split in two and placed in two tins for auditing by SRA. SA SRA Auditing Laboratory Practices An auditing laboratory is not subject to the same time constraints as a production laboratory. An auditing laboratory always has to reheat an audit sample, and is nearly always done from ambient temperature. To ensure samples are tested in a representative manner, cone and quartering is applied to the two audit tins (approximately 12kg) once the mass is heated to approximately C. Cone and quartering continues until portions of approximately 1300g suitable for Gyropac compactions are obtained. The portions are placed in separate trays in an oven with temperature gauges inserted or in a mould and allowed to reheat to approximately 10 C above recommended. Compactions are carried out similar to production laboratory procedures. Testing of binder content, gradings and maximum density are carried out on other portions prepared from the audit sample. Independent Auditor s Assessment on Laboratory Practices An independent auditor was invited to review laboratory practices between the two laboratories and some of the issues raised included the following: Air voids difference identified was not likely to be due to random error and there was a systematic difference between the bulk densities from the two laboratories, Operators in both laboratories had less than one year experience in testing asphalt and both had been trained in-house, The SRA operator strictly adhered to 150 C compaction temperature. However, he needs to initiate compaction immediately after compaction temperature has been achieved, The SRA process for removing the compacted sample from the loading platen needs revision. The process of sliding the sample onto another plate before inverting introduces another step that could go wrong. There is a need to increase the height of the extractor post to allow the operator s fingers to be able to reach under the sample Charging of the mould was one of the main issues. Trickling in material to obtain the correct mass can introduce segregation which will affect the compactability of the mix. The Contractor operator typically added about 1000 g of asphalt to the mould with little to no segregation. The Contractor operator would then trickle in the remainder to obtain the correct mass (1250 g). The SRA operator would start to trickle in the asphalt almost from the beginning. This had the effect of 12

13 loosening the mix and allowing particles to roll past each other, with larger particles ending up predominantly in the bottom third of the sample. SRA operator should ensure that samples are centrally located on the balance top pan. When asphalt samples are placed in the water bath small air bubbles that adhere to the sides of the sample should be removed. The pat dry process for the two operators was different. The Contractor operator gave the sample a little shake over the water bath to shed water and then patted the sample dry. The SRA operator ensured that the chamois was wrung out every couple of samples so that he started with a drier (and more absorbent) chamois, however his drying was less severe in practice. The contractor operator was more positive in the rodding of the sample. Note that the rodding process involved eight rodding actions not the 25 described in AS The Contractor operator packed the asphalt around the temperature probe in the mould to achieve a more consistent temperature. In conclusion, the differences in air void content were likely due to (in no particular order): 1. Compaction temperature both SRA and Contractor need to be consistent with their work practices to ensure that the mix at the time of compaction is under control. This includes controlling any delays subsequent to monitoring the temperature. 2. Charging the mould any practices which allow particles to tumble individually into the mould will introduce some segregation and are to be avoided. 3. Measurement of bulk density any adherent bubbles should be removed prior to determining the buoyant mass. The pat dry process needs to be refined to remove operator sensitivities. This may mean a change in cloth type and/or saturation level of the cloth. BULK DENSITY PRACTICES FOR SOUTH AUSTRALIA The following discussion is provided on suggested improvements to various specifications and standards. To conduct the bulk density test, asphalt samples need to be subject to a form of preparation. There are three main areas that require standardisation: The Bulk Density test (without sampling and compaction influences) Sample splitting & preparation issues and, Gyropac pat production issues. Bulk Density Test Without Sampling and Compaction Influence Figures 1 through 3 selected from testing by both Contractor and SRA, illustrate the three separate measurements used to determine a value for bulk density. It is evident that the surface saturated dry mass determination has the greatest difference. This mass determination also has the most operator influence. The risk in over drying a sample is to increase the probability of a greater value of bulk density resulting in the calculation of a lower air voids value. To minimise the operator influence the following practices and equipment are recommended when testing an asphalt specimen: 13

14 A temperature controlled water bath or container, in preference to using a calculated temperature density adjustment based on ambient temperature, Bath or container to be of sufficient size to contain samples under water that can be transferred directly onto weigh scale while still submerged, Water to be visibly clean, The preferred orientation of placing the specimens in the water bath is on their curved surface, such that the specimen has the least contact with the bottom of the bath, That an effort is made to reduce the majority of visible bubbles adhering to the specimen prior to any weighting action while remaining submerged, Specimen on being withdrawn after weighing in waster, is placed straight onto a moist chamois (or towel) that has been placed on a flat bench in preparation of the Surface Saturated Dry condition. The specimen is allowed to drain into the bath as part of the following five second drying period. This specimen is not to be shaken on extraction from the bath. A damp chamois (or towel) to be used that is rinsed and wrung out at start of testing, and is continuously monitored for consistent moisture condition that is neither too dry or excessively wet, Samples to be placed on chamois and patted dry and this action stopped after a five second period. Bulk Density Test - Sampling Issues To carryout production testing of a South Australian AC20 mix and provide an audit sample for the SRA, a sample size of approximately 25 kg is required and two methods are considered acceptable. 1. For hot asphalt manually sampled from a truck a number of containers are generally required, and these are usually compatible with receivers (or trays) for a riffle box or sample splitter. These trays need to be combined and split using either Cone & Quartering or the riffling process to produce at least two samples, one for production testing and one for austing. 2. For hot asphalt sampled from a truck utilising an automatic sampling device, a single container is usually all that is required. This sample can then be Riffled or Cone and Quartered to obtain at least two samples, one form production testing and one for auditing. If the production sample is produced as a result of riffling, sub-sampling to obtain the bulk density test samples directly from the riffle box tray must not be permitted as there is a high risk that a non representative sample will result. Bulk Density Test - Gyratory Compaction Issues Production Laboratory In the case where the production sample is produced as a result of riffling, the process can be continued according to AS to produce individual splits for each required test group. Alternatively the riffle box tray with the production bulk sample must be emptied on a flat impervious surface or suitable steel plate, mixed to from a cone which is then partially flattened to form a pad from which a square shaped scoop can be used to obtain a sample of approximately 1250g in a single scooping motion, keeping the base of the scoop in contact with and moving along the bottom of the plate or surface. An experienced technician should be able to obtain close to the required mass in a single action with a suitably sized scoop. It is preferred that a slight excess 14

15 is sampled and that asphalt is removed from the Gyropac mould rather than dribbling in asphalt to make the specimen up to the required mass. Note that an acceptable mass range needs to be decided upon, possibly different for each mix size. Alternatively, where the bulk sample is not made up of multiple numbers of trays or bins (as in the case of single samples obtained mechanically) the initial bulk sample can be emptied onto a flat impervious surface or suitable steel plate, mixed to form a cone which is then partially flattened to form a pad from which a square shaped scoop can be used to obtain a sample of approximately 1250g in a single scooping motion, keeping the base of the scoop in contact with and moving along the bottom of the plate or surface. An experienced technician should be able to obtain close to the required mass in a single action with a suitably sized scoop. It is preferred that a slight excess is sampled and that asphalt is removed from the Gyropac mould rather than dribbling in asphalt to make the specimen up to the required mass. Note that an acceptable mass range needs to be decided upon, possibly different for each mix size. The scooped material is to be placed directly into the Gyropac mould, as closely as possible in a single action from the scoop to minimise any sample disturbance. Rodding or spading of the specimen in the Gyropac mould is generally not recommended. Should rodding be deemed a requirement, it is suggested that the use of a 6mm diameter steel rod or similar is applied four times around the outside (at a radius of approximately 40mm) and twice around the inside (at a radius of approximately 20mm) of the specimen mass. The remaining sample pad material, after extraction of the scooped sub-samples for the determination of the bulk density, is to be remixed and further cone & quartered for any additional testing or leaded back into a riffle tray to be riffle split for any additional testing. A further aspect that requires some attention is the measurement of sample temperature that is either in the mould or on a tray. The temperature measurement should be taken in the centre of sample and placed approximately two-thirds into sample and not disturbed once in place (otherwise encapsulated air may produce a lower temperature reading). Temperature should be monitored such that it rises to its peak and compaction carried out once temperature falls to that required. Bulk Density Test - Gyratory Compaction Issues Audit Laboratory The SRA usually does not have the same imperative that the contractor has, of maintaining sample temperature above that required, so as to prevent loss of time due to reheating. Generally an audit sample will require reheating from ambient to just over target temperature. Thus splitting techniques can be used to create a homogenous sample once asphalt reaches a softening temperature, in an environment free from time contraints. Scooping from a bulk sample can be undertaken before sample is split into component samples for binder content and maximum density. Note that this is a departure from full splitting using either cone and quartering or riffling techniques to obtain all the bulk density, maximum density and binder content test samples previously. It is suggested that this adoption will minimise differences in comparing values of bulk density between contractors and SRA. The reheating is expected to 15

16 create a small difference in bulk density but will apply to all asphalt production laboratories in the state. The remaining procedures of scooping, placing in mould and temperature measurement needs to be done the same as for the production laboratories. BULK DENSITY SPECIFICATION ISSUES The following discussion is provided on suggested improvements to various specifications and standards. Australian Standard Clause 3.3(f): The balance requires a capacity with a limit of performance of ±0.5g in place of ±5.0 g. A mass of 5 grams equates to 1% air voids variation, which is unacceptable. Clause 3.3(j):.spatulas requires a definition of this piece of equipment. It suggests a flat ended bar on a rod, while all the work in South Australia is based on a 6mm rod with possibly a handle. Clause 4: Conditioning and Compaction Temperatures: Unless otherwise specified, asphalt shall be conditioned and compacted at the following temperatures. The statement that the asphalt SHALL be conditioned and compacted only applies to asphalt mix prepared in the laboratory. Samples obtained from an asphalt plant do not need to be conditioned, and thus this clause needs to be clarified. Further additional temperatures need to be added for other conditions such as polymer modified asphalt, stone mastic asphalt and warm mix asphalt. Clause 5.2 (c) Discard the asphalt if its temperature is outside the range -10 O C to +20 O C of the compaction temperature. This is very restrictive and does not allow for enough loss of temperature where a sample is adequately split using AS methods of riffling or cone and quartering. Any cone and quartering will reduce temperature by 20 to 30 O C. Note that many companies have been preparing asphalt samples by scooping to ensure limited loss of temperature this is not allowed in any Australian Standard. Clause 5.2 (d) Cool, heat or leave the asphalt to stand over a period of not more than 30 minutes to ensure that the temperature of the asphalt is within ±5 O C of the required compaction temperature. This also is very restrictive, and in a production sense should be extended to an hour and for auditing a four to five hour window for reheating is required. Clause 5.2 (d) and (e) remove if required from both clauses. Clause 5.2 (d).and transfer as a single lot. Spade the asphalt vigorously with a heated spatula 15 times around the perimeter and 10 times over the interior. It appears that there is a conflict in placing asphalt in a single lot, suggesting minimal disturbance, and then spading vigorously. It is suggested that if rodding or spading is adopted, then simple rodding four times round the exterior and twice the interior with a 6mm rod to settle the mix rather than wholesale disturbance is more appropriate. 16

17 Clauses 5.1 & 5.2: Both these parts do not allow for audit work where reheating is carried out, and once heated how to prepare subsamples from a larger audit sample. No conditioning would be carried out on audit samples as they have been through the rigours of an asphalt plant, and temperature ranges and time frames as discussed earlier above do not allow for the audit situation. South Australian Test Procedure 425: Sampling of Asphalt This test procedure to be deleted. South Australian Test Procedure 428: Compaction of Asphalt Test Specimens Using a Gyratory Compactor The following revisions are required for this test procedure. Clause 3: Leave coning and quartering equipment requirement but remove flat hot plate requirement. Clause 5.1: The current clause clarifies conditioning requirements and is to remain. Clause 5.2: Delete any reference to TP425 (it has been deleted). Clause 6(b): Leave current wording in place. Clause 6(d): Complete rewording is required, including differentiating between production laboratory and audit laboratory requirements, and use of test procedure similar to that described elsewhere in this paper is required. Clause 6(e): Leave current wording in place. Clause 6(g): Leave current wording in place. CONCLUSION This paper presents findings and discussions on learnings as gained from the auditing of a coarse graded asphalt production mix by the local SRA. The bulk density test is an important asphalt test that is heavily relied upon by the SRA. The bulk density test variances have been discussed and summary data presented from both internal and external proficiency exercises, which provide perspective to the results obtained. The influence of Gyropac mould wear has been highlighted through proficiency testing using the same hardened steel moulds. Three influences on variability for the bulk density test have been shown, being the bulk density test itself without sampling & compaction influence, asphalt sampling issues and finally gyratory compaction issues. The differing requirements of an audit laboratory compared to a contractor production laboratory have also been discussed. A range of recommendations are provided for improvements to the relevant Australian Standards and the SRA Test Procedures, and is provided as a learning for all in Australia that require asphalt to be provided to Austroads 4B requirements. 17

18 REFERENCES AUSTROADS (2006) Publication Series 4B (Previously: Design and Selection of Asphalt mixes: Australian Provisional Guide, APRG Report No.18, 1996). AUSTROADS (2007) T211 Verification Procedure for a Gyropac Gyratory Compaction. BUTCHER, M.J. (2006a). Asphalt Proficiency 2006 Report. Department for Transport, Energy and Infrastructure, South Australia. Report No. # BUTCHER, M.J. (2006b). Asphalt Gyropac/Servopac Proficiency 2006 Report. Department for Transport, Energy and Infrastructure, South Australia. Report No. # BUTCHER, M.J. (2009a). Asphalt Servopac Proficiency 2009 Report. Department for Transport, Energy and Infrastructure, South Australia. Report No. # BUTCHER, M.J. (2009b). Asphalt Gyropac/Servopac Precision 2009 Report. Department for Transport, Energy and Infrastructure, South Australia. Report No. # INDUSTRIAL PROCESS CONTROL PTY LTD (1993) Gyropac (Asphalt Compaction Machine) Operating and Maintenance Manual. INDUSTRIAL PROCESS CONTROL PTY LTD (1996) Servopac (Servo Controlled Gyratory Compactor) Operating and Maintenance Manual. National Association of Testing Authorities, Australia (1990). Bituminous mixtures proficiency test program October PTAC Report No.66. National Association of Testing Authorities, Australia (2001). Bituminous mixtures proficiency test program. OLIVER, J.W.H. (2002) Precision of the asphalt bulk density test higher void content results. ARRB TR Contract Report. OLIVER, J.W.H. (2003) Precision of the measurement of the air void content of Gyratory Compacted Asphalt specimens. ARRB TR Contract Report. STANDARDS AUSTRALIA (1995) Methods of sampling and testing asphalt - Sample preparation - Mixing, quartering and conditioning of asphalt in the laboratory. AS STANDARDS AUSTRALIA (1995) Methods of sampling and testing asphalt - Sample preparation - Compaction of asphalt test specimens using a gyratory compactor. AS VAN LOON, H (2006) Maintaining Asphalt Mix Designs in South Australia. 22nd ARRB Conference, Canberra. 18

19 ACKNOWLEDGEMENT The authors thank the Executive Director of the Department for Transport, Energy and Infrastructure (DTEI), Transport Services Division for permission to publish this paper. In addition the authors wish to recognise the positive role the asphalt contractor, the main contractor and the State Road Authority in working together to achieve the outcomes and learnings of the paper. In particular thanks go to the laboratory staff: Luke Hansford, Ben Richards, Paul Gutschmidt and Karen Henley. The views expressed in this paper are those of the authors and not necessarily those of DTEI. 19

20 APPENDIX A Measurement of Uncertainty Bulk Density of Asphalt AS/NZS :1993 ρ bulk m ρ 1 w = m m 2 1 Test Parameters Symbol Test Data Sensitivity Factor c i Mass of asphalt in air, (g) Mass of asphalt in water, (g) Mass of asphalt SSD (g) Density of water (t/m 3 ) m 1 m 2 m 3 rw bulk density c1 c2 c3 c4 ρ w c1 = m m 3 2 ρ w c = m 2 ( m m ) ρ w c = m 3 c = 4 ( m m ) ( m m ) 3 m 1 2 Measurement Source Uncertainty Distribution Type Mass of asphalt in air, (g) Mass of asphalt in water, (g) Mass of asphalt SSD (g) Var. in pat dry mass (g) balance (g) balance (g) balance (g) procedure u m1 u m2 u m3 u p Rectangular Rectangular Rectangular rectangular U value Dist. Factor Std D of F Weighting u ic i (ui(y)) 2 Uncertainty u (xi) vi c i E E E E-06 Density of water (t/m 3 ) thermometer u temp rectangular E-07 SUM E-06 u c(y) k 2 U REPORT: Bulk Density t/m 3 With an expanded uncertainty of t/m 3 At a confidence level of 95% With a normal coverage factor of 2 The uncertainty of measurement value shown does not include any estimate of the effects associated with sampling. Test results should be assessed using precision in terms of repeatability and reproducibility, measurement uncertainty and sampling effects. 20

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