mixing and compaction temp of modified binders

8/12/2019 Mixing and Compaction Temp of Modified Binders

1/9


2/9

binders which are modified by suitable additives are expected to improve the mechanical

properties of the mixtures. The pavement constructed with modified binders shows higher

resistance to rutting, thermal cracking and decreased fatigue damage (Kandhal et al., 2011).

During the manufacture of bituminous mixtures, the binders are heated to hightemperature to ensure that the aggregates are well coated by the binder during mixing.

Immediately after mixing, the mix is transported to the construction site and compacted. The

loss of temperature during transport is minimized by appropriately insulating the truck so that

the mix during compaction will be workable and reach the desired level of compaction. The

various standards and specifications for hot mix asphalt construction (Asphalt Institute, 2003)

clearly stipulate the temperature during mixing and compaction for mixes prepared with

unmodified bitumen. The Asphalt Institute mix design manual defines the mixing and

compaction temperature for unmodified binder in terms of the viscosity of binder.The

temperature to which the bitumen is heated to produce a viscosity of 0.170.02 Pa.s and

0.280.03 Pa.s shall be the mixing and compaction temperatures respectively (Asphalt

Institute, 2003).Asphalt Institute (2003) also suggests that one should use the manufacturers

guidelines for determining the mixing and compaction temperature for modified binders. It is

well understood that if one uses the viscosities as mentioned here for modified binders, the

temperatures are going to be unusually high. IRC:SP:53(2010) suggests 165 to 185oC as the

range to be used for mixing and 130 to 160oC as the range for compaction temperature.

Most of the manufacturers suggest mixing and compaction temperature for modified

binders to be 20 higher than that for unmodified binders. If one goes by the manufacturers

recommendation, the mixing temperature for modified binder mixture will be 185 C and the

compaction temperature will be at 170 C. Such high mixing and compaction temperature can

lead to phase separation of some of the modified binders. Also, while it is possible to make

samples at such higher temperature in the laboratory, it is plainly not possible to subject the

material to such high temperature uniformly throughout in the field. One of the biggest

concern currently expressed by the highway engineers is the need to identify a method for

determining the mixing and compaction temperature.

Yildrim et al., 2000, used the same viscosity range that is used for the unmodified

binder. The viscosity-temperature chart and the temperature corresponding to the viscosity of

0.170.02 and 0.280.03 Pa.s was taken as the mixing and compaction temperature

respectively. Bahia et al., 2001, used zero shear viscosity to determine the mixing and

compaction temperature. The mixing and compaction temperature obtained using zero shear

viscosity was found to be less than that obtained from conventional viscosity chart. A test

methodology referred to as the phase angle method, which was based on Newtonian responseof the material was suggested by West et al., (2010) for determining the mixing temperature.

A frequency sweep test is conducted using a Dynamic Shear Rheometer at 50, 60, 70 and 80oC, and a master curve is constructed. Using empirical expressions, the mixing temperature

is determined for frequency corresponding to the phase angle of 86 .

Determination of compaction temperature is even more challenging when compared to

mixing temperatures. At these temperatures, the material exhibits transition from Newtonian

to a non-Newtonian fluid and hence the viscosity determination at any specific temperature

will be non-unique. The decrease in viscosity due to the shear thinning behaviour of the

modified binder due to shearing and the increase in viscosity due to temperature drop rather

complicates the issue. Currently, the compaction density is the only quality control parameter

used to check the compaction of pavement. However, it is possible that for a given density (orair voids), the possibility of a widely varying mechanical properties can exist and unless one


3/9

understands the issues related to compaction mechanics, it is not possible to arrive at an

unique compaction temperature range for a given binder and mix (Saradhi et al., 2008).

Added to the above issues is the aging that happens to the mix during mixing and

compaction. During the transport of mix from hot mix plant and during compactionconsiderable aging occurs to the binder and this also influences the manner in which

compaction could be carried out. To simulate the short term aging of the mix, the current

AASHTO protocol prescribes that the mix be kept in an oven for a specific temperature for 4

hours before compaction (AASHTO PP2, 1994). Prior to compaction, the sample is then

transferred to another oven which is kept at the compaction temperature for 30 minutes and

immediately thereafter the sample is compacted. While there is considerable discussion and

debate in the technical literature (Bahia et al., 2001; West et al., 2010) about compaction

temperature for mixes with modified binders, it is not clear whether the short-term aging

protocol for mixes with unmodified binders could be identically adopted for mixes with

modified binder.

As mentioned earlier, the modified binder shows transition from Newtonian to non Newtonian at mixing and compaction temperature. This transitory behaviour of modified

binder was characterized using the apparent viscosity from protocols established at IIT

Madras (Padmarekha et al., 2011). The current study focuses on the concept identifying the

Newtonian temperature for mixing. As mentioned earlier the compaction occurs in non

Newtonian regime of the modified binder. The temperature at which the effect of shear rate

increment and temperature decrement on the viscosity of binder compensates each other can

be identified as the compaction temperature.

In this investigation, the Newtonian non-Newtonian transition temperature of the

modified binders were mapped during mixing and compaction. Using this data, the mixing

and compaction temperature for fabricating modified binder mixtures were arrived at. A

control sample following Industry practice was bench marked with mixes made with the

temperature proposed here and the variation of volumetric properties were determined.

2. EXPERIMENTAL INVESTIGATION

As part of current study, a polymer modified binder (PMB40), a crumb rubber

modified binder (CRMB60) was used for binder characterisation.The CRMB and Plastomer

samples were prepared for the mixture tests. The median grading of bituminous concrete

grade-2 as per MoRTH (2001) were used.

2.1 Binder Investigation

Three different experimental protocols were developed by Padmarekha and Krishnan

(2011) to determine the on-set of Newtonian regime for unmodified binders and fourth

protocol was developed as part of current study is to quantify the shear thinning

characteristics of the modified binder. The readers can refer to Padmarekha and Krishnan(2011) for details related to the first three protocols. In order to characterize the rheological

response of the binder, the data collected from the tests conducted as per the three protocols

were analysed to determine the mixing temperature. The experiments were carried out on

unaged binder.

The results of experiments performed on PMB40 as per protocol 1 shown in Fig 1.(a).The apparent viscosity is expressed as a function of temperature. The binder was subjected to


4/9

three different shear rate increments of 0.01, 0.02 and 0.03 rpm/s. The temperature was

increased at rate of 2 oC/min starting from 90 oC . At this temperature, the binder is expected

to be non Newtonian. The transition temperature is identified as temperature at which shear

rate independent viscosity is obtained and from the graph in Fig 1 (a) it is seen that it occurs at137 oC. Fig 1(b) shows the data of experiments performed on PMB40 as per protocol 2. The

apparent viscosity is expressed as a function of shear rate. The experiments were conducted at

two different temperatures.The binder showed Newtonian response at 145 oC. The data of

experiments conducted on PMB40 as per protocol 3 is shown in Fig 1(c) The binder was

subjected to a steady shear at constant temperature. The plot in Fig 1 (c) shows the apparent

viscosity of the binder expressed as a function of time at 150oC. As mentioned earlier, the

temperature at which the binder shows shear rate independent viscosity is taken as transition

temperature. The temperature corresponding to the on-set of Newtonian response of the binder

is considered as the mixing temperature .The transition temperature obtained from analysis of

protocol I, II and III is shown in Table 1.

Fig 1. (a) Apparent Viscosity as a

function of temperature of PMB40 -

Protocol 1

Fig 1. (b) Apparent Viscosity as a

function of shear rate of PMB40 - Protocol

II

Fig 1. (c) Apparent Viscosity as a function of time of PMB40 protocol III


5/9

Table 1. Transition Temperature obtained from analysis of protocol I, protocol II and

protocol III

Binder Protocol I Protocol II Protocol IIIPMB40 137 150 160

CRMB60 183 185 (>200)

The fourth protocol results were studied to determine the compaction temperature. Fig

2 depicts protocol IV wherein the binder was subjected to a shear rate ramp and temperature

was allowed to drop according to the ambient conditions. The experiments were run at steady

shear, low shear rate increment of 0.0067 rpm/s and high shear rate increment of 0.1 rpm/s.

The consistency in temperature drop was ensured. Brookfield HA DV-II rotational viscometer

with the thermosel apparatus was used for binder characterization. The SC4- 21 spindle of

16.77 mm diameter and 35.15 mm effective length was used. The sample preparation wascarried out as per ASTM D4402, 2013. The viscosity as a function of shear rate at different

temperatures was recorded.

Fig 2. Angular velocity of spindle and Temperature as a function of Time Protocol IV

As mentioned earlier,the temperature regime at which the effect of shear rate

increment and temperature decrement on the viscosity of binder compensates each other can

be identified using protocol IV. The range of temperature over which the viscosity will not

change drastically can be obtained and thus reduce the compaction temperature

appropriately.The data obtained from the experiments on CRMB60 exhibited non

Newtonian behaviour to a large extent at 170 oC. The shift in apparent viscosity values as seen

between steady shear plot and high shear rate increment plot is due to shear thinning

behaviour of the binder. The combined effect of shear rate increment and temperature drop on

the binder will cause minimum change in apparent viscosity over a temperature regime, from170 oC to 150 oC as seen in Fig 3(a). The apparent viscosity of PMB40 as a function of

temperature is shown in Fig 3 (b) The shift in viscosity between steady shear and high shear

rate increment is less.


6/9

Fig 3.(a) Apparent Viscosity as function of

temperature (CRMB60)

Fig 3.(b) Temperature as a function of

time (PMB40)

Fig 3(a) and Fig 3.(b) shows the variation of apparent viscosity as the temperature is

dropped and the material was subjected to different shear rate sweeps. It is very clear that

CRMB60 shear thinned considerably at high shear rate (0.1 rpm/s) and hence the gain of

viscosity due to temperature drop was not as substantial when compared to steady shear

experiment or the low shear rate experiment (0.0067 rpm/s). This clearly gives one enough

information to make a choice of compaction temperature as the temperature drops drastically.

Based on different trials of experiments carried out on PMB40 (not reported here for want of

space), it was very clear that the plastomer and unmodified binder exhibited identical non-

Newtonian characteristics during compaction temperature regime. Hence, the compactiontemperature of plastomer was kept identical to that of an unmodified binder. The compaction

temperature of unmodified binder is 150 oC. On the other hand, from the analysis of the high

shear rate viscosity data for CRMB60, it is clearly seen that one can reduce the compaction

temperature to an extent in which the viscosity values remained more or less constant. Taking

into account the issues related to measurement accuracy of the Brookfield viscometer, it was

found that the compaction temperature could be reduced to 160 oC since viscosity variations

are minimal when compared to the starting temperature of 170 oC.

2.2 Mixture investigation

The bituminous mix of BC grade 2 was fabricated using plastomer PMB40 and

CRMB60. Two sets of bituminous mix samples were fabricated with different mixing and

compaction temperature mentioned against mix protocol I and mix protocol II in Table 2. All

the bituminous concrete mixes were prepared for a 4% target air voids with 5% bitumen

content. After mixing, the mix was kept in the 135 oC oven for 4 hours to simulate aging that

occurs due to transport of the mix from plant to site. The aging temperature for modified

binder mixture is not explicitly specified in any standards. The mix temperature after short

term aging is increased to compaction temperature before compaction. The target density of

the compacted block was reached at the programmed terminal air voids. The rectangular

blocks of 450 150 165 mm height were produced using shear box compactor..


7/9

Table 2. Mixing and Compaction Temperature as per Mix Protocol 1 and Mix Protocol 2

Protocol Binder Type Mixing

Temperature(oC)

Aging

Temperature(oC)

Compaction

Temperature(oC)

Mix

Protocol 1

CRMB 185 (binder)

170 (aggregates)

155 170

Plastomer 185 (binder)

170 (aggregates)

155 170

Mix

Protocol 2

CRMB 185 (binder)

170 (aggregates)

155 160

Plastomer 135 (binder)

150 (aggregates)

135 150

The cylinders of 100 150mm are cored out of the beam sample to determine the

volumetric and mechanical properties. The compaction termination criterion was achieving

the target air voids of 4%. The data collected by means of the UTS software in terms of

variation of air voids with the number cycles is as shown in Fig 4 (a) and Fig 4 (b). The

number of compaction cycles for mix protocol 1 and 2 was similar for CRMB60. The results

of PMB40 also showed less variation in the results of the two mix protocols.

Fig 4. (a) Air voids attained as a function

of number of compaction cycles forCRMB60

Fig 4.(b) Air voids attained as a function of

number of compaction cycles for PMB40

The cylindrical samples cored were sliced to three discs in order to determine the air

void distribution. Table 5 shows the air voids, bulk specific gravity (G mb), maximum specific

gravity (Gmm) of cored and sliced PMB60 samples as per mix protocol 2. The samples which

were fabricated as per mix protocol 1 as part of investigation carried out at IIT Madras,

showed similar air void content as obtained from mix protocol 2.


8/9

Table 4. Air voids of samples cored from the beam (Mix protocol 2)

Sample No. Gm Gmm Air voids (%)

PMB 1 2.4932.602

4.180PMB 2 2.497 4.001

PMB 3 2.512 3.438

Table 5. Air voids of samples sliced from the cylinder (Mix protocol 2)

Sample No. Gmb Gmm Air voids (%)

PMB 1 2.515 3.350

PMB 2 2.501 2.602 3.740

PMB 3 2.494 4.100

3. SUMMARY

In this investigation the non Newtonian to Newtonian transition temperature of PMB40 and

CRMB60 was determined. The temperature at which the binder exhibits Newtonian behaviour

was considered as the mixing temperature. The range of temperature over which the viscosity

did not change drastically was obtained from the results of the experiment conducted as per

protocol IV. The reduced compaction temperature of 150 oC for PMB40 and 160oC for

CRMB60 was identified from the results of the experiment run as per protocol IV. The

modified binder mixture samples were fabricated on two sets as per mix protocol I and II.

Though different sets of temperatures were used, similar volumetric properties were obtained

for both the binders using the new protocol when compared to the current industry practice.

Acknowledgements

The authors thank Department of Science and Technology for funding this

investigation. The grant number is DST/TSG/STS/2011/46.

References

1) AASHTO PP2. (1994). Standard practice for mixture conditioning of Hot-Mix Asphalt

(HMA). American Association of State Highway and Transportation Officials,

Washington D.C.

2) Asphalt Institute (2003). Superpave mix designs, superpave series no. 2. (SP-2).

Asphalt Institute, Lexington, KY, USA.

3) ASTM-D4402. (2013). Standard test method for viscosity determination of asphalt at

elevated temperatures using a rotational viscometer. ASTM International, West

Conshohocken, PA.

4) Bahia, H. U., D. I. H. M. Zeng, H. Zhai, M. A. Khatri, and R. M. Anderson (2001).

Characterization of modified asphalt binders in superpave mix design.Transportation

Research Board, National Research Council, Washington, D.C.

5) IRC SP: 53 (2010),Guidelines on use of polymer and rubber modified bitumen in roadconstruction, Second Revision, Indian Roads Congress,New Delhi.


9/9

6) IS 15462. (2004). Polymer and rubber modified bitumen - specification. The Bureau of

Indian Standards, New Delhi, India

7) Kandhal, P.S. and M.P. Dhir (2011). Use of modified bituminous binders in India:

current Imperatives,Journal of Indian Road Congress, 72(3), New Delhi, October

2011

8) MoRTH. (2001). Specification for roads and bridge work.4threvision, Ministry of

Shipping, Road Transport and Highways, New Delhi, India

9) Nagabhushana, M.N. (2009). Right grade of Bitumen for flexible pavements: Indian

PerspectiveNew Building Materials and Construction World, Vol. 15, Issue No.3,

September 2009, NBM Media Pvt. Ltd. New Delhi.

10)Padmarekha, A. and Krishnan, J. M. (2011), Experimental Investigations on high

temperature transition of asphalt, Construction and Building material, 25 (2011)

4221-4231.

11)Qiu, J., Li, N., Pramesti, F. P., van de Ven, M. F. C. and A. A. A. Molenaar (2012)

Evaluating Laboratory Compaction of Asphalt Mixtures Using the Shear Box

Compactor,Journal of Testing and Evaluation, Vol. 40, No. 5, 2012

12)Saradhi, K., Eyad, M. and Rajagopal, K. R. (2008). A thermomechanical framework

for modeling the compaction of asphalt mixes.Mechanics of Materials, 40(10), 846

864.

13)West, R. C., Watson, D.E., Turner, P. A. and Casola, J. R. (2010).Mixing and

compaction temperatures of asphalt binders in hot-mix asphalt.Transportation

Research Board, Washington, D.C.

14)Yildrim, Y., Solaimanian, M., Kennedy, T. W. (2000). Mixing and compaction

temperatures for Superpave mixes,Journal of the Association of Asphalt Paving

Technologist,69, 34-72

mixing and compaction temp of modified binders

Documents