Bituminous layers

7.4.1 The Resilient Modulus (MR) of bituminous mixes is another input for the analysis of stresses in flexible pavements. Its magnitude depends upon the grade of binder, the temperature, Frequency /

loading time, air void, volume of bitumen, texture, gradation of aggregates, maximum size of aggregates, state of packing of aggregates, orientation of the test sample while testing and viscosity of the binder. VG40 is most suitable for the hot climate in plains of India. Modified binders also are suitable both for hot and cold climate. The modulus of mixes with modified binders varies a great deal depending upon the modifier, duration of blending and the extent of air blowing of base bitumen. It may vary from a low of about 1600 MPa to a high of 3000 MPa at 35°C, lower modulus is not a reflection on quality of the modified binder. Though an elastomer modified binder gives a lower modulus of the bituminous hot mix, its fatigue live, which controls the thickness of the bituminous layer, is much higher. Lower modulus is due to large recoverable deformation in elastomer binders. It is to be noted that the modulus values change continuously with a change in temperature and it can be as high as 5000MPa at lower temperatures at midnight to less 1000 MPa at 65⁰ C at noon when the bitumen becomes too soft. Indicative values of resilient modulus values of the bituminous materials at 35⁰Cwith different binders are given in Table 7.1. These are based on l a b o r a t o r y tests at IIT Kharagpur in Indirect Tensile mode as per ASTM 4123 now revised as ASTM: D7369-09(12). The European standard equivalent to ASTM 4123 is EN 12697-26 and it is available commercially. A loading pulse of 0.1 second followed by a rest period of 0.9 second is adopted for Indirect Tensile Test. These findings are in agreement with those given in some other literature as well for similar binders e.g., Monismith et al. (20), AUSTROADS and others. VG30 bitumen may have a viscosity from 2400 poise to 3600 poise at 60⁰C while for VG40, the viscosity range is 3200 to 4800 poise. Hence modulus values of both VG30 and VG40 may be close to each other in some cases when viscosity overlaps.

The guidelines, therefore recommend that the modulus values of mixes with normal and modified binders should be determined from a repeated Indirect Tensile Strength test. Figure 7.2 shows a schematic diagram of indirect tensile test as per EN 12697-26 ( similar to ASTM D4123).Total recoverable horizontal deformation is measured and the modulus computed considering a Poisson’

ratio of 0.35, a value commonly recommended in the method. The test is simple to carry out as compared to that that prescribed in ASTM: D7369-09(12) which gives inconsistent result for 150mm DBM 1 mixes because of larger size aggregates.

Figure 7.2 Indirect tensile test Dynamic Load

LVDT

Marshall sample

1 2

Resilient modulus of a Marshall specimen is given as

𝑀𝑟 =

𝑡𝐻_𝑟 7.4 Where p= cyclic load (10% of the Indirect tensile strength),

µ= Poison’s ratio (range 0.35 to 0.40), Hr= Recoverable horizontal deformation across the diameter (sum of recoverable deformation of LVDTs 1 and 2 in Figs 7.2) and t=thickness

If the test parameters are Load=1500 N, diameter of specimen(D)=102mm, height(t)= 60.75mm, Recoverable horizontal deformation (Hr) = 4 micron (0.004mm), assuming µ=0.35 Mr= 3827 MPa.

Higher Poisson’s ratio gives higher modulus. A regression equation (49) given below for 102mm diameter specimens can be used for the determination of M_R value of the dense graded bituminous mixes if test facility is not available. For 102mm diameter DBM 2 mix, Aggregates retained on 26.5 mm should be replaced with those passing 26.5mm and retained on 19mm for mix design if 150mm diameter mould and compaction hammer are not available.

MR(MPa)=2.6975 x ITS + 724.46 for mixes with neat bitumen -- 7. 5 R² = 0.84

MR(MPa)=1.1991 x ITS + 1170 for mixes with modified bitumen …. 7. 6 R²= 0.89

Where, ITS =Indirect Tensile Strength in kPa, MR= Resilient Modulus in MPa. 102mm diameter specimens give marginally higher ITS value than 150mm sample for the same aggregate size. Higher aggregate size gives higher modulus values. ITS is to be determined in the Marshall loading frame at a speed of 50mm/minute as explained in Appendix IX. Over 30 number of 150mm DBM samples from the construction sites containing VG40 bitumen from different parts of India were tested at 35⁰C at IIT Kharagpur and the results along with the regression equation are shown in Figure 7.3. It can be seen that most Mr values are greater than 3000 MPa.

The regression equation is valid for ITS values from 460 kPa to 650 kPa. Large scatter is due to VG40 from different sources used by contractors in different parts of the country

When a number of 150mm diameter DBM samples were cast in IIT Kharagpur laboratory with V40 bitumen from a single source (Haldia refinery), 68% of the Mr values were between 3090 and 3700 MPa and 95% of the values were between 2710 MPa and 4030 MPa. 10 to 15% variation in the modulus of bituminous mixes has a minor effect on the pavement design. A change in modulus of ± 500 MPa may result in thickness variation of about 5 mm per 100mm thickness of the DBM layer having a modulus of 3000 MPa. Examples are solved in Annexure II to illustrate the effect of modulus on thickness of bituminous layers. An additional thickness of about 10 mm of the DBM layer is to be provided to take care of undulations in WMM layer considering that 10mm variation in surface irregularity of the WMM/WBM layer is permitted in MORTH. A small portion of DBM acts as a profile correcting course. Maximum modulus of the DBM layer may be taken as 3000 MPa with VG40 bitumen even if higher modulus is obtained in laboratory tests, and for VG30 bitumen, the recommended upper limit is 2000 MPa. Moduli at 35⁰C of DBM mixes can be determined in the laboratory or from the regression equation shown in Figure 7.3. For temperatures less than 35⁰C, Equations 7.5 and 7.6 can be used for pavement design. The values in Table 7.1 agree with majority of laboratory results.

7.4.2 Diameter of Test samples for ITS and Resilient Modulus Test: The DBM mix is the main structural layer of a flexible pavement and forms bulk of the thickness of a bituminous layer for high volume roads with granular bases and sub-bases. Unlike wearing course, it ages slowly since it is situated at 40mm to 50mm below the surface. Its modulus plays an important role in lowering the tensile strain in its bottom and vertical strain on the top of the subgrade. Since the maximum nominal size of aggregate in DBM can be 40mm and the largest dimension may be still higher, 150mm diameter test specimen shall be adopted for the Mr test. Higher aggregate size gives higher modulus of the bituminous mixes because of increased interlocking.

y = 11.088x - 3015.8

Figure 7.3 Mr of DBM with VG40 vs Indirect Tensile strength for 150mm marshall specimen at 35⁰C

7.4.3 Modified Bitumen for wearing course and Resilient Modulus: The mixes with modified bitumen are known to (i) age slowly (ii) give improved fatigue lives and higher rut resistance notwithstanding lower resilient modulus values obtained in the laboratory for some polymers and CRMB. Even if the modulus of the wearing course such as SMA/Gap Graded Rubberized bitumen mix with modified bitumen (IRC:SP:79, IRC:SP:107) is lower, the modulus assigned to the DBM layer may be adopted for the wearing course also for the stress computation because (i) the wearing course is under direct compression below the wheel loads and the dynamic modulus values at a speed of 50 to 60 kmph (corresponding to the frequency of 10 hz) under the compression will be higher and (ii) the wearing course will age faster and the modulus will increase. As per the international practice (MS2), only modified binder such as PMB40 or rubberized bitumen (Equivalent to PG76 or higher as per US practice) are recommended in Stone Matrix Asphalt. In case of gap graded mixes with rubberized bitumen, binder containing at least 20% crumb rubber ( IRC:SP:107) shall be used.

7.4.4 DBM Mixes with Modified Bitumen: The mixes with modified bitumen can have a fatigue life that can be three to ten times of mixes with neat bitumen as illustrated in Fig. I-1 in Annexure I.

While the wearing courses with PMB/CRMB can be milled and recycled easily since they become brittle due to ageing due to exposure to sun, the DBM I and DBM II mixes with modified bitumen can be used in lower layers if their recyclability can be established from the experience in India or abroad. Mixes with elastomeric binders may cause problems in milling and recycling due to stickiness, and pavement rehabilitation can pose a serious problem for lack of technology.

Sustainability issues have to be kept in mind.

7.4.5 High viscosity binder is recommended for the DBM mixes to limit rutting and loss of riding quality. Their fatigue lives depend upon the test method whether the tests are done under constant strain or constant stress modes. Stiffer mixes have higher fatigue lives under control stress mode and lower fatigue lives under control strain mode. For thin bituminous layer, constant strain mode is applicable while for thicker mixes, constant stress mode is more appropriate. Fatigue life of mixes can be further improved by reducing the air voids and increasing the volume of binder. Low air voids and higher bitumen content may, however, induce bleeding and rutting if softer binders are used.

These guidelines, therefore, recommend that these factors should be taken into account while designing bituminous mixes.

7.4.5 The Poisson’s ratio of bituminous layer depend upon the pavement temperature and a value from 0.35 to 0.40 recommended for temperature up to 35°C and value of 0.50 for higher temperatures. Stress computation is not very sensitive to minor changes in Poisson’s ratio. Fatigue equation at any pavement temperature from 20°C to 40°C can be evaluated by substituting the appropriate value of the resilient modulus of the bituminous mix, air void and volume of bitumen.

Examples of typical designs are given for a temperature of 35°C. It is to be noted that the bituminous mixes harden with time and modulus may increase due to ageing than what is given in the Table 7.1.

Hence field performance is to be periodically recorded for future guidance. Annex. VII gives various

considerations for the selection of binders and mixes in the light of international experience.

Table 7.1 Typical values of Resilient Modulus and of Bituminous Mixes, MPa*

Mix type Temperature °C

20 25 30 35 40

BC and DBM for VG10 bitumen 2300 2000 1450 1000 800

BC and DBM for VG30 bitumen 3500 3000 2500 2000 1250

BC and DBM for VG40 bitumen 6000 5000 4000 3000 2000

BC and DBM for Modified Bitumen

(IRC: SP: 53-2010) 5700 3800 2400 1600 to

2800

1300

BM with VG 10 bitumen 500 MPa at 35°C

BM with VG 30 bitumen 700 MPa at 35°C

RAP treated with 3-4 percent bitumen emulsion/

foamed bitumen (2-2.5) percent residual bitumen and 1.0 percent cementitious material.

800 MPa at 35°C

*Thickness of bituminous layer is affected by about 5mm per 100mm of the layer for a variation of about 15% in modulus of 3000 MPa

7.4.5 Quality issues in DBM and BC: Though mix design is done in the laboratory much ahead of the start of the bituminous construction, it is to be ensured that the material that is being laid has the necessary parameters as per the mix design because of inherent variability in a large scale

construction. In case of two or three lane paving, at least two sets of loose samples from locations close to paver from each lane shall be taken during the paving and a part of the sample shall be used to prepare three specimens in Marshall moulds in the field itself with 75 blows compaction on each face to form the reference density for comparing the density of cores of the compacted mass at the same location. Another part of the loose samples may be used for the determination of the maximum theoretical density and the grading so that remedial measures are taken early. This should be done at every 250m.