REFUSAL DENSITY CONCEPT FOR DESIGN OF DENSE
Due to heavy wheel loads on our roads, secondary compaction has led to premature plastic deformation in the mix. The voids in the mix (VIM) are an important parameter as far as heavy wheeled traffic is concerned. The failure of Bituminous Concrete (BC) mixes due to plastic deformation owing to the reduction of VIM to less than 3 per cent has become a common problem. The refusal density of the BC mixes need to be determined to accurately predict the life of the pavements subjected to heavy traffic while maintaining 3 per cent VIM.
In this study an attempt has been made to come up with a laboratory testing procedure, simulating the secondary compaction due to traffic loading by extended Marshall compaction. The laboratory testing procedure included the compaction of Marshall specimens to 300 blows using the Marshall hammer. The mixes were designed using four different binders viz 60/70, 80/100, CRMB-60, and PMB-40 modified binders. The values of Optimum Binder Content (OBC), were found to be 4.9, 5.1, 5.5 and 5.2 for 80/100, 60/70, CRMB-60, and ‘ Bvniey modified bitumen respectively.
The samples were prepared at varying compaction effort i.e. they were subjected to 35, 50, 75, 100, 150, 200, 250 and 300 blows. It was concluded that for determining the refusal density of the mixes the samples should be compacted to 250 blows to avoid degradation of aggregates. At refusal density the VIM of the specimens prepared using various binders was not less than 3 percent to resist failure due to plastic deformation. The refusal density concept in dense bituminous mix design resulted in reduced binder content.
Our country having a National Highway network spanning 52,000 kilometers needs an efficient transportation system. One such attempt is the National Highway .Development Program (NHDP), which covers 20,000 kilometers and in which about Rs. 60,000 crores as being invested. Majority of the roads covered under this project are of bituminous pavements subjected to heavy wheel loads. Plastic failure of an Asphaltic surfacing occurs when heavy vehicles compact the material to a point where the air voids content in the mix is too low.
1.1 Mix Design
Currently Marshall method of mix design is used in many countries, and it will be some time before other methods of mix design and performance testing such as those recommended in Super-pave’ will be used.
Swanson et al2 defines compaction of an asphalt concrete mixture as ‘...a stage of construction which transforms the mix from it’s very loose state to a more coherent mass, thereby permitting it to carry traffic loads.... the efficiency of the compactive effort will be a function of the internal resistance of the bituminous concrete. This resistance includes aggregate interlock, friction resistance, and the viscous resistance.’
As per Smith3 the reason for compaction of asphalt pavements is to make it watertight and impermeable to air. Blankenship4 has concluded that an increase in the mix’s density will result in a stronger mix but not necessarily a stronger pavement. However, there is a point of optimum density that correlates with the best combination of strength and durability.
In addition to this Epps5 reported that current methods of laboratory compaction are not suitable to simulate field conditions. The properties of the asphalt and aggregate based upon the long-term densification’ of a pavement must be taken into account; that is, consider the resistance of the paving mixture to compactive effort.
If the resistance to compactive effort is weak, the pavement will be sufficient only for low volume traffic; if the resistance is strong, the pavement will be suitable for higher traffic .The laboratory compactor needs to be able to simulate final density. Heavier the traffic (number of axle loads), more is the increase in density of pavement. Thus, equivalent single (18 kip) axle load (ESAL) is a convenient way to account for the effects of traffic volume on the pavement density. The number of blows of Marshall hammer required for bituminous concrete surface course at varying traffic in terms ofESAL’s as per Road Note 196 is given in the Table below:
Due to compaction the volume of air in a bituminous mix is reduced through the application of external forces. The expulsion of air enables the mix to occupy a smaller space thereby increasing the unit weight or the density of the mass. The compacted mass should have sufficient air voids to allow the asphalt to expand without filling the voids resulting in flushing. The voids should be high enough to allow for some subsequent traffic-induced densification during the first few years of service without the void content to fall below about 3-4 percent for dense graded mixtures.
Linden et. al7 have reported that if the void content in a dense graded asphalt mix drops below 3 percent, significant permanent deformation can occur. However, for dense graded mixtures, the pavement life is reduced by about 10 percent for each percent increase in voids above 7 percent. Historically, there have been three primary compaction methods that have been used in routine asphalt concrete mix design-impact compaction, kneading compaction and gyratory compaction.
1.3 Refusal Density
Under severe loading conditions asphalt mixes must be expected to experience significant secondary compaction along the wheel-paths. Severe conditions cannot be precisely defined but will consist of a combination of two or more of the following:
Failure by plastic deformation in continuously graded mixes can occur very rapidly once the voids in the mix (VIM) are below 3 per cent, therefore the aim of refusal density design is to ensure that at refusal there is still 3 per cent VIM. For sites, which do not fall into the severe category, the method can be used to ensure that the maximum binder content for good durability is obtained. This may be higher than the Marshall optimum but the requirements for resistance to deformation will be maintained. Where lower axle loads and higher vehicle speeds are involved, the minimum VIM at refusal can be reduced to 2 per cent8.
Researchers9'10 have concluded that mixes with an air void content below about 2 to 3 per cent in service are likely to become unstable and prone to rutting. Additionally Cooper et al9 have also reported that it has been shown that Asphaltic surfacings must retain a minimum of 3 per cent VIM after trafficking to prevent plastic deformation. On severely loaded sites in tropical environments the final density of the surfacing is likely to be very close to refusal density. With a target level of compaction of 95 per cent of refusal density this implies that the air voids content of the surfacing material after compaction by road rollers may be in the region of 8 per cent” (which is acceptable as per the guidelines issued by The Asphalt Institute). At this air voids content the surfacing will be very vulnerable to oxidation and ingress of water.
1.4 The Percentage Refusal Density Test
The percentage refusal density (PRD) is the ratio of the density achieved in the field over the density of the same material compacted to refusal in the laboratory. This test method was developed by TRRL after recognising that although other methods of control yield satisfactory results, there is scope for improving compaction, especially in the critical wheel path zones of road pavements12.
The PRD test was developed for the control of road base and base course materials. Powell et al13 are of the opinion that the PRD test is insensitive to material variables other than the level of compaction. The results are not affected by sample thickness, temperature, binder content or aggregate relative density but there is a unique relationship between the PRD and the number of passes during compaction in the field. The main advantage of this method is that it provides the direct knowledge of the state of compaction and means of assessing the potential for improvements.
The BS 598: Part 104:1989 defines Refusal Density as ‘The mass per unit volume, including voids of the specimen compacted to refusal in accordance with the test method’. Additionally it defines Percentage Refusal Density (PRD) as ‘The ratio of the initial dried bulk density of the sample to the refusal density expressed as a percentage’. Refusal Density can be determined by two methods
Researchers12'13 are of the opinion that the time required for a full PRD test cycle is some 2.5 to 3 days in order that the refusal density can be determined for control purposes. Briefly the method involves the removal of a core from the carriageway, determination of density followed by the re-heating of the core within the mould with vibratory hammer and finally the determination of this latter density. The main deficiency of this method was found to be the time delay for the testing cycles and the demands on the testing personnel. When the material composition significantly changes, any corresponding change in refusal density cannot be determined until three days later.
The initial dried bulk density of the core sample of bituminous mix is measured by weighing the sample in air and water, the sample being coated in paraffin wax to prevent the ingress of water. After removal of the wax coating, the sample is then heated and compacted with a vibrating hammer ‘to refusal.’ The final density of the sample is then measured by weighing the sample.
1.5 Extended Marshall Compaction7
In this method, the normal Marshall design procedure using 75 blows should be completed first to provide an indication that Marshall design parameters will be met. The binder content corresponding to 6 per cent VIM obtained in the Marshall test should be noted and additional test samples, prepared at each of the three binder contents, namely the binder content corresponding to 6 per cent VIM and also the binder contents which are 0.5 per cent below this value. These samples must be compacted to refusal.
The number of blows required to produce a refusal condition will vary from one mix to another. It is preferable to conduct a trial using the lowest binder content and compact using an increasing number of blows, say 200, 300, 400 etc. until no further increase in density occurs. Usually 500 blows on each face are found to be sufficient. By plotting a graph of VIM at the refusal density against binder content the design binder content that corresponds to VIM of 3 per cent can be determined. This value should be obtained by interpolation, not by extrapolation.
2.0 LABORATORY TEST
a. Aggregates: For the purpose of the study quartzite aggregates of sizes 20 mm, 10 mm, stone dust and lime as filler were obtained from Delhi. The physical properties of aggregates are given in Table 2. The gradation is given in Table 6. In the present study one type and one grading of aggregates was used to find the effect of refusal density on bituminous concrete mixes. Different types of aggregates or various gradation of aggregates will increase the number of parameters as four types of binders are selected in the present study. Further studies are required using various types of aggregates at varying gradation.
b. Bitumen: Four different types of bitumens, viz. IS 80/100, IS 60/70, CRMB-60 and PMB-40 modified binders were used in the present study. The physical properties of the binders are given in Table 3.
c. Marshall Mix Design: The Marshall test method, as described in The Asphalt Institute (TAI) manual (MS-2), was used to select the “optimum/design” binder contents for all the mixes investigated in the study. For mixes investigated, four different binder content percentages were selected for the Marshall Stability analysis. Binder percentages progressed in 0.5 percent increments to cover an air void range of 3 percent to 6 percent, with three replicates at each, binder content by giving 75 blows on each face. Bulk density, air voids, voids in mineral aggregate, voids filled with bitumen and the stability/flow values were calculated for each specimen and the average for the replicates at the same binder content were also calculated as shown in Table 4. OBC was calculated at the average of maximum bulk density, maximum Marshall stability and 4.5 percent air-voids (mid value of MoSRT&H Specified 3 to 6 percent).
Marshall specimens were prepared at OBC and tested for Stability, flow, air voids, voids in mineral aggregate, voids filled with bitumen for various binders and checked with the MoSRT&H Specifications, and the values obtained are given in Table 5.
refusal density could be achieved at 250 blows, without much degradation of aggregates. From Table 6 it can also be observed that the degradation of coarse aggregates was in the range of 1 to 2 percent for 250 blows compaction but the degradation was 3 to 4 percent for 300 blows compaction. The degradation of coarse aggregates has led to increase in the aggregate passing 75 microns to 6.1, 6.3 and 6.8 percent for 200, 250 and 300 blows respectively. This could be the reason for reduction in the density after 300 blows compaction effort.
To study the effect of air voids at refusal density, Marshall samples were prepared at OBC and ±0.5 percent from the OBC and compacted 35, 50, 75, 150, 200, 250 and 300 blows. OBC is the average at maximum stability, maximum density and 4.5 % air voids. At OBC, the designed air voids is lower than 4.5 percent. The air voids at 75 Marshall blows compaction and design binder content of 4.9, 5.1, 5.5 and 5.2 are 4.15, 4.30, 4.22 and 4.20 percent for 80/100, 60/70, CRMB-60 and PMB-40 bitumen respectively.
Figure 3 to 5 shows the effect of secondary compaction and bitumen content on the voids in mix for 80/100, 60/70, CRMB-60 and PMB-40 bitumen. For 80/100 bitumen, the design binder content is 4.9 percent and the voids in the mix is 4.34 percent, and it reduces to 3.62 and 3.46 at 150 and 250 blows respectively. Increase in number of blows led to degradation of aggregates in the mix as given in Table 6. From figure 2 it is also observed that to maintain a design air voids of 4.5 percent and air voids of not less than 3 percent after secondary compaction, 4.9 percent can be considered as the design binder content for 80/100 grade bitumen. As the bulk density of the mix was reduced after 300 blows, 250 blows can be considered as the refusal density.
Figure 3 shows the binder air voids relationship for 60/70-grade bitumen. If the secondary compaction under traffic is underestimated the resulting voids in mix can be reduced to less than the critical value of 3 percent and a high risk that plastic deformation will occur. From figure 3 it was observed that the air voids at design binder content was 4.36 percent and at 150, 250 and 300 blows were 3.27, 3.06 and 3.19 percent respectively. As the air voids reaches 3 percent at refusal density (250 blows) at 5.1 percent binder content, there is a potential for air voids to decrease due to secondary compaction. By increasing the design air voids to 4.5 percent in place of 4.36 percent, the OBC can be reduced to 5.0 percent to maintain 3 percent or higher air voids after 250 blows of Marshall hammer.
Figure 4 shows the relationship between binder content and air voids for CRMB-60 modified bitumen. The refusal density though depends on the type of binder, the degradation of aggregates depends only on the number of Marshall blows. From figure, it was observed that even after 300 blows the density of the BC sample increased when compared to 250 blow Marshall sample. This may be due to the modified binder, but the degradation effect showed no variation with change in type of bitumen. As the air voids reaches a value of 2.95 percent at refusal density (300 blows) at 5.5 percent binder content, there From Table 7 it can be observed that film thickness is with in the range of 6-84111 with the revised binder content, and the reduction in film thickness is in the range of only 2 to 4 percent.
3.0 DISCUSSIONS AND CONCLUSIONS
1. When secondary compaction under traffic is underestimated the resulting VIM can be reduced to less than the critical value of 3 per cent and a high risk that plastic deformation will occur. It has been recommended in this Road Note that for design traffic in excess of 5xl06 ESA, the design VIM should be 5 per cent at 75 blow compaction15. Cooper9 and Oliver10 concluded that mixes with an air void content below about 3 per cent in service are likely to become unstable and prone to rutting. From the present study it can be concluded that 250 blows of extended Marshall compaction can be considered as the refusal density for the gradation and type of aggregate selected in the study as 3 percent air voids was retained in the mix. The study can be extended for different types of aggregates and varying gradation. It can be concluded that the bituminous concrete mix design with refusal density as additional parameter helps in deciding the compaction level in the field.
The authors are thankful to Director, Central Road Research Institute, New Delhi, for his kind permission to publish this paper.