Roller Compactor Cum Rut Analyzer (RCRA) an Alternative Compactor for Bituminous Mix Design

Transcription

1 Roller Compactor Cum Rut Analyzer (RCRA) an Alternative Compactor for Bituminous Mix Design Dr. B.V. Kiran Kumar, Dr. H.S. Jagadeesh, 3 Dr. R. Sathyamurthy³ Assistant Professor in Civil Engineering, Govt. SKSJTI, Bangalore Professor, Department of Civil Engineering, BMSCE, Bangalore 3 Visiting Professor, Department of Civil Engineering, BMSCE, Bangalore Compaction plays a vital role in performance of a bituminous mixes. In India Marshall Method is adopted for designing bituminous mixes where specimens are prepared using Marshall Hammer. Compaction effort generated by Marshall Hammer doesn't simulate the field compaction effect which leads aggregate degradation during mix design. Other draw back of Marshall Procedure is the number of blows given to compact the specimen is fixed and during compaction the densification data of mixes cannot be recorded, this data plays a vital role in determining Theoretical Maximum Density (TMD). Recent advancement in bituminous mix compaction is Superpave Gyratory Compactor (SGC) a product of Strategic Highway Research Program (SHRP), which simulates the field compaction effect during specimen preparation for mix design at laboratory. Further it also records the densification data during compaction of mix. With these methods available to compact the bituminous mix at laboratory during mix design, another compacting equipment known as Rolling Compactor cum Rut Analyzer (RCRA) (Patent Pending) machine which is indigenously developed by the authors was used in this project, where it compacts the specimen in laboratory by generating field compaction effect of a roller as well as records densification data of the mix during compaction. Later the same is used to evaluate the performance of mix in terms of rut potential. In this project an attempt was made to evaluate the performance of bituminous mixes designed using three types of compaction equipments, based on the results obtained suitability of the compaction equipments to be adopted during bituminous mix design at laboratory has been suggested. It is well known fact that the quality of bituminous pavement to a greater extent depends upon the degree of compaction. Depending upon the degree of compaction, the strength, durability and stability of the bituminous pavement vary with variation in compaction. The objective and sufficiently accurate control on the degree of compaction appears to be the most important factors. In India procedure followed at laboratories during compaction of specimen for mix design does not represent the field conditions this leads to ambiguity in the results obtained by testing such sample. In laboratory the specimens prepared by Marshall Method of compaction do not match the field compaction condition. In Marshall method of mix design, the specimen is compacted by confining the mix in all direction using a metallic cylindrical mould, where the mix is compacted by using an hammer which has specific weight and height of fall. A total of blows are given to compact the specimen. Further the compaction method leads to degradation of mineral aggregate, as the aggregates are confined in all direction and has no scope to realign and reorient during the compaction of mix by dropping hammer. Whereas in field aggregates which come under the roller will be subjected to kneading action and will be compacted with less chance of aggregate degradation since they have the scope to realign and reorient them self. Hence aggregate degradation during laboratory mix design leads to increase in optimum binder content of the mix and optimum binder content thus obtained when transferred to the field leads to functional and structural failure of pavements. Bituminous Mix Design Practice in India In India, highway agencies still adopt Marshall method of bituminous mix design. The Marshall method entails a laboratory experiment aimed at developing a suitable bituminous mixture using stability / flow and density / voids analysis. The advantage of Marshall method is its attention to density and voids properties of bituminous mixes. This analysis ensures the volumetric proportions of mix materials for achieving a durable mix which is 8 The Masterbuilder February

2 backed by field performance data (Kiran Kumar et. al, ). The Marshall specimen used to determine the volumetric properties of a bituminous mix is prepared using a Marshall Hammer which is used to compact the specimen under a confinement by giving or 7 blows on both sides of specimen. The number of blows given to compact specimen depends on traffic. The weight of hammer used to compact is 44. N and height of fall being 47 mm. During this process the aggregate degradation takes place, aggregate structure which is carefully chosen to achieve maximum density is lost. Where in case of field compaction of bituminous mix under a roller, the material has certain scope for the movement or adjust itself under the roller with this, in field compaction the bituminous mixes are subjected to kneading action but in case of a Marshall hammer the blow given by hammer from a height of 47 mm damages the aggregate structure by producing dynamic loading effect rather then kneading action. Recent advancement in Marshall method of compaction is to use indented Marshall hammer, by doing so the compaction of specimen will be similar to the field compaction this is because the indent caused on face of Marshall hammer allows the aggregate to realign when it is compacted using Indented Marshall hammer. Introduction to Superpave Gyratory Compactor (SGC) Strategic Highway Research Programme (SHRP) developed a compactor known as Superpave Gyratory Compactor (SGC) which simulates field compaction effect at laboratory. The SGC is used to produce specimens for volumetric analysis during a bituminous mix design and it also records data to provide a measure of specimen density throughout the compaction. The unique features of SGC are a loading mechanism which presses against the reaction frame and applies a load to the loading ram to produce a. n/mm compaction pressure on the specimen. The SGC mould has a base plate at bottom and the mould which provides confinement during compaction. The SGC base rotates at a constant 3 revolutions per minute during compaction with the mould positioned at a compaction angle of.o (SP, ). Specimen height measurement during compaction is an important function of SGC. Specimen density can be estimated during compaction by knowing the mass of material places in the mould, the inside diameter of the mould and the specimen height. Using these measurements the specimen's compaction characteristics can be developed. Here since the base is rotated at an angle of. this accommodates the movement of the material without getting confined in mould and this also helps in reducing aggregate degradation. The constant pressure applied from the top and rotation of base compact's the mix by kneading action which is similar to that of field compaction of bituminous mix under a roller. Introduction to Rolling Compactor and Rut Analyzer (RCRA) The RCRA is an indigenously developed compactor by the authors. The unique features of this compactor are it can apply a constant pressure of. n/mm and if required the pressure can be applied up to 3 n/mm. Similarly the rolling speed in the compactor can be varied. It has a temperature control unit which maintains the pre selected compaction and rut test temperature. The equipment can record densification data of the mix while compaction. Using the densification data the specimen's compaction characteristics can be developed. The compactor is an hydraulically operated with twin non return valve system and has a Programmable Logical Circuit (PLC) which is inturn connected to vertical and horizontal transducers capable of recording changes of + mm. The following are the novel features of RCRA. RCRA produces field compaction effect (kneading effect) in laboratory during compaction of specimens in bituminous mix design. RCRA can record the densification data during compaction of specimen. RCRA can maintain a Pre selected temperature during compaction or rutting test. RCRA is a completely stable Machine and does not require foundation to install. RCRA is mounted on castor wheels and can be transported with easy and can be used as quality control machine at site or laboratory. RCRA can be operated with ease and it is maintenance free. RCRA has a capability of applying pressure up to 3 n/mm so that tyre and compaction pressure can be varied and its effect on mix can be studied. RCRA is capable of change in speed during rolling or rutting operations. The entire operation of compaction of slab and rutting test is automated and there is less human intervention to temper the test results. Formulation of Problem The issue addressed earlier regarding the compaction by Marshall Hammer doesn't simulate the field condition during compaction in laboratory and hence it is to be replaced by a compactor which can replicate or simulate field compaction during preparation of the sample in the laboratory. In order to cater for these requirements a design practice was developed by the United States Congress named has SHRP one of the main objectives of this programme was to develop a compactor which can simulate the field condition during preparation of specimen for mix design. But SGC developed under the SHRP was able to simulate the field compaction effect during laboratory specimen preparation to some extent. But the problems faced The Masterbuilder February 9

3 in this compactor were, it is an expensive compactor and very sophisticated equipment which needed a lot of maintenance. Another issue with SGC was that, it cannot prepare a specimen in the desired shape and size. For example SGC cannot prepare a Beam Specimen or a Slab Specimen which are required for various performance tests in order to evaluate the bituminous mix. These difficulties have led the researchers to find an alternative compactor which can simulate the field compaction and also be cost effective and versatile. In present study an ernest effort is made to over come the above difficulties by using RCRA. Where RCRA generates the field compaction effect (i.e., kneading action) with that it can produce different shapes and size of specimens for various performance tests on bituminous mix. In current study mix design was conducted on Bituminous Concrete (BC) Grade using three different compaction equipments viz., Marshall Hammer, Superpave Gyratory Compactor and Rolling Compactor cum Rut Analyzer. Optimum Binder Content (OBC) of the mix for three compactors was found out. Further BC Grade specimens were cast using different compactors at there respective OBC's, these specimens were subjected to performance tests such as Fatigue Test, Indirect Tensile Strength test, Rutting, Binder Recovery and Moisture Sensitivity test. Suitability of compactors for mix design at laboratory was evaluated using the above performance test results. Laboratory Studies Material Characterization Test on Bituminous Binder Bitumen of Viscosity Grade3 (VG3) grade was used for the present studies, tests on binder, results and codes of practice followed are indicated in Table.. Tests on Aggregates The physical requirements and test values obtained for the coarse aggregates to be used in BC Grade pavement layer as per Ministry of Road Transport and Highway (MoRT&H) fourth revision specifications are indicated in Table.. Grading of Commercial Available Mineral Aggregate The mineral aggregate is graded by performing sieve analysis. Sieve analysis of each aggregate fraction was done separately as well as for the aggregate blend, once the proportioning was finalized. Gradation sample is indicated in Table 3.. Table 4. shows MoRT&H specifications for BC Grade. Proportioning of the Mineral Aggregate Blend Aggregate must be blend in such proportion that, the final blend should be in the acceptable range as given in table 4.. Table. and Figure. shows proportioning mineral aggregate used for present study. Test Description & Code Penetration at ºC, g, s,. mm. (BIS 3978 reaffirmed 8) Softening Point (R&B), ºC, Min (BIS 978 reaffirmed 9) Viscosity at 3 C in CPS (ASTM 44) Specific Gravity (BIS 978 reaffirmed 8) Flash Point (º C) (BIS 9978 reaffirmed 9) Solubility in Trichloroethylene percent, Min. (BIS 978 reaffirmed 9) Viscosity Ratio on residue of Thin Film Oven Test (TFOT) Sample C, Max (BIS (Part)978 & both reaffirmed 9) Ductility at C, cm, Min, on residue of TFOT Sample (BIS & both reaffirmed 9) Table. Test Results for VG3 Bitumen Design Binder Content Results Requirment of VG3 as Per BIS After finalizing the aggregate structure, specimens for three different compactors were prepared at varying binder contents. Sl.No 3 4 Test Description Combined F&E Index (%) Specific Gravity Water Absorption (%) Impact Value (%) L.A. Abrasion Value (%) Soundness with MgSo 4 Table. Test Results of Aggregates Test Method IS:38 (Pt I93) IS: 38 (Pt IV 93) IS: 38 (Pt III 93) IS: 38 (Pt IV 93) IS: 38 (Pt IV 93) IS: 38 (Pt V 93) 4. 4 Result mm mm Stone Dust A minimum of four binder contents were tried and at each binder content minimum of three specimens were prepared. Mix properties were evaluated for the selected blend at the different binder contents, by using the densification data. The volumetric properties were calculated at different binder content. From the above data, graphs such as air voids, VMA and VFA versus binder content were plotted. The design binder content was The Masterbuilder February

4 Sieve Size 9 mm 3. mm 9. mm 4.7 mm.3 mm.8 mm µ 3µ µ 7µ Coarse (mm) 9 9 % Passing Fine (mm) Stone dust Lime 8 Table 3. Grading of Commercially Available Aggregates GRADING Nominal Aggregate Size Layer Thickness IS Sieve (mm) µ 3µ µ 7µ Table 4. MoRT&H Specifications 3mm 34mm Cumulative % by Weight of Total Passing established at 4% air voids and all other mixture properties such as VMA and VFB were checked at the design binder content to verify that they meet the criteria. Sieve Size (mm) mm 3 3 Proportioning of Mineral Aggregate Blend mm Stone dust 4 % Blending Lime 4. Combined Gradation MoRT&H Specifications Lower Limit Table 4. MoTable. Proportioning of the Mineral Aggregate BlendRT&H Specifications Upper Limit Figure. Shows Proportioning of Materials Bituminous Mix Design Mix Design using Marshall Hammer and SGC OBC of selected aggregate blend and VG3 binder for Marshall Hammer and SGC were obtained as per ASTM 9, 97 and SP standards respectively. Mix Design using RCRA Slabs of 3. mm height 7 mm wide and mm in length at varying binder content were prepared using RCRA. Cores of mm diameter were drawn from the slabs, OBC and other volumetric properties were found out using cores drawn. Figure. explains step by step process involved in bituminous mix design using RCRA. Step involves, mixing aggregate blend with varying binder content by weight of mix then heat the same and further place in RCRA for compacting into slab. Step involves drawing cores from compacted slabs in order to determine the density voids characteristics of the mix. Step3 determines the voids and density properties of the cores drawn. Based on the voids and density relation OBC for the mix is established. Step4 involves determination of Marshall Stability of the mix. This was done in order to ensure that the mix compacted using RCRA satisfies the Marshall Stability and Flow value criteria. Results of BC Grade Mix for three Compactors The compaction of the aggregate structure chosen for BC Grade confirming to MoRT&H limits was done using three compactors and table. and figure 3., 4., and. indicate BC Grade mix properties for the compactors. Evaluation of Moisture Sensitivity Specimens for BC Grade were cast at their design binder contents obtained for compactors and tested as per AASTHO T 83 moisture evaluation test, one subset of three specimens The Masterbuilder February

5 were compacted to approximately 7 percent air voids at design binder content and named as controlled specimens. One more subset of three specimens was cast with design binder content at 4 percent air void content. The evaluation for moisture sensitivity was done by thawing cycle only. The controlled subsets of three specimens are conditioned by subjecting the specimens to partial vacuum saturation that is, the specimens were kept at Step: Placing and Compaction of Mix Sl.No Properties OBC in (%) by Weight of mix Stability in (Kg) Retained Stability in (Kg) Flow in (mm) Bulk Density in (gm/cc) Volume of Air Voids in (%) Volume in Mineral Aggregate in (%) Volume filled by Bitumen in (%) Marshall Hammer Compaction SGC Compaction RCRA Compaction Table. Properties of BC Grade Mix for three Compactors Step: Drawing Cores from Compacted Slabs Step: 3 Measuring Height and Bulk Density of Core Specimen for Volumetric Analysis Figure 3. Optimum Binder Content for Three Compactor Step: 4 Measuring Marshall Stability and Flow Value Figure. Steps Involved in Mix Design by RCRA o C for 4 hours in water bath followed by hours in temperature controlled chamber at o C. The other subset of three specimens was kept in a temperature controlled chamber at C for hours and these specimens were called as unconditioned Figure 4. Voids in Mineral Aggregate for Three Compactors The Masterbuilder February 3

6 where, TSR = Tensile Strength Ratio (percent) For example Average tensile strength of conditioned sample = 9.7 kg/cm and Average tensile strength of controlled samples = 9.3 kg/cm then VG 3 Grade IDT ** Compactors IDT * TSR SGC RCRA MH Table 7. Indirect Tensile Strength Value of BC Grade Mix for three Compactors Figure. Voids Filled with Binder for Three Compactors The tensile strength ratio comes out to be 97.4 % where the value is higher than the criteria specified by MoRT&H for BC mixes. The criteria being 7% all the specimens compacted by various compactors at there OBC's qualified the test. Rutting Test Rutting test was conducted on the specimens cast at OBC's of different compactors and were conditioned at + C. A tyre Figure. Densities for Three Compactors specimens. All the specimens were tested for their indirect tensile strengths and the ratio of conditioned to that of unconditioned specimens indirect tensile strength was represented as Tensile Strength Ratio (TSR). Figure 7. indicate a typical comparison of TSR values obtained for BC specimens compacted at OBC's for three compactors viz., SGC, RCRA and Marshall Hammer using VG 3 grade binder. Table 7. indicates Indirect Tensile Strength values of BC Grade mix for three compactors and VG 3 grade binder. IDT* Indirect Tensile Strength of Conditioned Sample in kg/cm IDT** Indirect Tensile Strength of Unconditioned Sample in kg/ cm SGC Super Gyratory Compactor RCRA Rolling Compactor cum Rut Analyzer MH Marshall Hammer Figure 7. Comparison of Indirect Tensile Strength Value of BC Grade for Three Compactors pressure of. kg/cm was maintained constantly through out the test. It was observed that the specimen cast at OBC's of RCRA and SGC showed same deformation trend in the graph up to passes later RCRA sample showed some significant improvement in deformations up to passes then SGC and Marshall Hammer specimens, but later failure was rapid. Here specimen cast using OBC obtained from Marshall Hammer failed at passes. Figure 8. and 9. indicate comparison of rutting test results at + C for BC Grade with VG 3 binder and Rutting test in progress respectively 4 The Masterbuilder February

7 Fatigue Test Specimens were prepared using different compactors viz., Marshall, SGC and RCRA at their respective OBC's and was subjected to repeated loading at rate of cycles/min, at stress levels of 4% and % of Indirect Tensile Strength value obtained for each compactor earlier. It was observed that both SGC and RCRA specimens were able to carry load. times more that of Marshall Hammer specimen. Figure. shows the fatigue test results at + C for BC. Conclusion It was found that the OBC for BC mix at 4. % air voids design criteria where., 4.8 and 4.9 percent by weight of mix, when compacted using Marshall, Superpave and Rolling Compactor respectively. Here the lower binder content for the same Binder Extraction Test Binder extraction test was conducted in order to evaluate the aggregate degradation that would take place due to variation in the type of compaction while preparing the specimen. The tables 8. indicate that the RCRA compacted specimens results in a maximum of 3% aggregate degradation whereas SGC compacted specimen shows a maximum variation of % and Marshall Hammer compacted specimen show a maximum variation of %. The test was a clear indicator that RCRA compacted specimens where subjected to less aggregate Figure 8. Comparison of Rutting Test results at + C Figure 9. Rutting Test on Bituminous Slab using RCRA degradation this can be attributed to the kneading effect of compactor and creation of enough space in the mould (7 X mm) which provide enough scope for the aggregates to adjust and realign themselves when it is subjected to compaction. Figure. Comparison of Fatigue Test at + C aggregate structure was obtained by using SGC and maximum stability of 39kgs is shown by the specimen compacted using SGC wherein the aggregate and binder being the same for all the three compactors. In case of a Roller Compactor specimens exhibited stability value which was near to that of SGC this is because in case of RCRA and SGC the specimen is compacted to obtain the maximum theoretical density but in case of Marshall the number of blows are confined to 7 or on each side of mould based on the traffic conditions. The density of SGC compacted specimens were higher when compared to specimen compacted using other compactors and this has been substantially supported by the test results of moisture sensitivity test wherein the specimens compacted by SGC has more resistance to moisture damage than that of specimens compacted by other compactors. There is an improvement in the compaction characteristics of specimens compacted using Sieve Size in mm Gradation After Binder Extraction Test MH SGC RCRA Desired Gradation Table 8. Gradation after Binder Extraction Test SGC and RCRA compactors when compared to Marshall Hammer. In case of Rutting test specimens cast at OBC of RCRA The Masterbuilder February

8 performed much better than of specimens cast at OBC's of SGC and Marshall. RCRA specimens carried passes before causing a rut of mm, whereas SGC specimen sustained passes by causing mm rut. Whereas Marshall specimen sustained passes only. In case of fatigue test SGC and RCRA specimens carried. times more repetitions then that of Marshall hammer compacted specimen. Further binder extraction test has clearly exhibited that specimen compacted by using RCRA undergoes less aggregate degradation since the compaction provides enough scope for the aggregates to realign and adjust during compaction of specimen when compared to SGC or Marshall Hammer compacted specimen, where the aggregate structure and binder remain same for all the compactors. From the above study it can be concluded that RCRA is a compactor which produces compaction effect which is similar to field compaction and aggregate degradation does not happen. It can be used as quality control equipment and determine the rutting characteristics of various bituminous binders. Further it is required to take up a full fledge study by laying test tracks of BC layer with various OBC's obtained from these compactors and the test tracks needs to be evaluated at a constant interval. Acknowledgement The work reported herein was conducted as a research studies at Dayananda Sagar College of Engineering, Bangalore and B.M Srinivas College of Engineering, Bangalore. The authors acknowledge the efforts of several research assistants and graduate students who were involved in the research program. The research programme was sponsored by All India Council of Technical Education (AICTE), New Delhi and M/s. Tinna Overseas Limited, New Delhi. Disclaimer The contents of this paper reflect the view of the authors who are responsible for the facts, findings and data presented herein. Reference Anderson R.M., Turner A. P., Peterson L. R., and Mallick B. 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Superpave Level Mix Design, Superpave Series No. (SP and SP), Asphalt Institute, Lexington, 3 and. B.V. Kiran Kumar, Sridhar Raju, Sunil Bose and K.N. Vishwanath (), "Effect of Air Voids and Compaction Temperature on Bituminous Mix Design", Advance in Road Transportation National Conference Proceedings, National Institute of Technology, Rourkela, Orissa. Anderson, R.M., () "Using Superpave Gyratory Compaction Properties to Estimate the Rutting Potential of Asphalt Mixtures", Asphalt Paving Technology, Association of Asphalt Paving TechnologistsProceedings of the Technical Sessions, Vol 7, pp The Masterbuilder February 7