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EARLY HARDENING OF BITUMEN AND DETERIORATION OF HIGHWAY STRUCTURES IN HOT AND ARID REGIONS (SAHARA)

By

Mohamed Khalifa Ali Mohamed O. Imbarek

(Department of Civil Engineering, Al-Fateh University) Tripoli, LIBYA

Meor Othman Hamzah

(Department of Civil Engineering, Universiti Sains Malaysia) MALAYSIA

ABSTRACT Hot arid regions are characterized by their substantially high maximum temperatures and low precipitation rate. North Africa and Middle East are typical examples of regions experiencing this climatic type. In Libya, where most of its area lies within this region, a large network of highways and numerous airfields were constructed during the last four decades using asphalt concrete. An experimental program has been carried out to evaluate the process of hardening by monitoring the loss of volatile components of the bitumen and recording the temperature variations in the region. Test samples were taken from selected sites where this phenomenon is experienced, to find out the effect of hardening due to aging and exposure to solar radiation at different depths from the surface and at different ages. Test samples were obtained from a number of highways in the region. The samples ranged in age from zero to eight years. Each sample was sawn into slices of 15 mm thick through its depth to represent the change of exposure to environmental effects. The results showed that, the concentration of the bitumen volatile compound decreased with service time by different rates depending on the properties of each compound. These results indicate that the heptanes fraction of the binder will lose about 62% of its volatile compounds in 1 year and about 98% in 4 years. 1.0 INTRODUCTION Hot arid regions are characterized by their very high service pavement temperatures and substantial daily temperature fluctuations, sometimes fluctuating between an average maximum of 75C to an average minimum of 5C of pavement surface temperature (1,2). Hardening of bitumen binders in these regions takes place due to the action of different reactions that occur in service and result in loss of its desirable properties. These reactions include: oxidation, volatilization, and polymerization (3). Several mix design methods can be found in the technical literature, all of which focusing on producing high performance dense bituminous mix. Nevertheless, most of these methods do not seem to directly address the unique conditions of the performance of dense bituminous mixes in hot arid regions. Cracking and disintegration of road pavements still appear in different sections of the recently constructed roads as shown in Figure 1.

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The problem comes from the fact that, the substantial daily temperature fluctuations increase the bituminous mix stiffness which make the pavement surface vulnerable to thermal cracking. In such a situation, only lower viscosity binder or reducing the hardening rate of the bitumen binder will be beneficial in reducing the potential of thermal cracking. It is generally accepted that bitumen hardening is a good relative measure of asphalt durability. Many researchers(4,5,6,7) have reported the poor quality of bituminous pavements due to premature aging problems in hot and dry regions in different parts of the world.

2.0 PAVEMENT TEMPERATURE Temperature has a significant effect on the performance and durability of pavement structures. Temperature extremes and their cyclic variations are among the environmental forces that play major roles in the continual deterioration of pavements, and hence have an important bearing on their design and serviceability. At high temperatures, bitumen becomes soft and the stability of the mix diminishes. Repeated load applications in conjunction with high tyre pressures cause excessive permanent strain and wheel path rutting in unstable mixes. High temperature also contributes to the long-term ageing of asphalt concrete by increasing the in-service hardening of asphalt binders due to the evaporation of the volatile components and progressive oxidation, which in turn increases the viscosity of the bitumen and causes the consequent stiffening of the mix leading to cracking failures and disintegration. Generally, pavement distress caused by temperature effects may be summarised as follows:

1. Thermal cracking; caused by thermal stresses generated in the pavement material by contraction due to negative temperature gradient.

Figure 1 Location of the investigated area

Area Investigated

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2. Thermal fatigue cracking; resulting from the accumulation of damage caused by daily temperature cycling.

3. Bitumen hardening; hardening of bitumen binder due to loss of volatile components at high temperatures and progressive oxidation.

4. Bitumen bleeding; accumulation of liquid bitumen on the pavement surface. It takes place when the viscosity of bitumen decreases due to high pavement temperature and the low porosity bituminous mix compresses under traffic load.

5. Rutting; caused by the softening of the asphalt due to high temperature at the site.

3.0 CALCULATION OF PAVEMENT TEMPERATURE IN HOT ARID CLIMATE The general method for calculating pavement temperatures developed by Imbarek and Smith (7) was used to predict temperature regimes in a typical bituminous pavement at two sites in the region of the Sahara desert located in the Southern part of Libya for which the required data for air temperature and solar radiation was available. This region was a typical hot arid climate. The selected sites are Sabha (latitude 27:01 N and longitude 14:26 E) and Kufra (latitude 24:13 N and longitude 23:18 E) as shown in Figure 1. The typical cross section of the pavement under consideration comprises of: dense asphaltic concrete surface layer, open graded asphaltic concrete base course and compacted aggregate sub-base. Table 1 summarises the maximum and minimum calculated pavement temperatures. Figure 2 shows the temperature variations at different pavement depths for the Sabha site during one of the hottest days in the year.

Table 1 Maximum and Minimum Pavement Temperature for Sabha and Kufra

Layer depth Sabha Kufra

31.12.1991 22.06.1988 21.01.1967 07.06.1961

6 mm max. 41.77 86.96 43.37 88.07 min. 7.93 40.20 8.39 39.03

100 mm max. 30.47 72.92 31.89 72.90 min. 13.00 47.71 13.75 46.09

200 mm max. 25.53 66.47 26.74 65.92 min. 15.95 52.00 16.77 50.56

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One of the unique problems experienced in the hot arid environment is the surface cracking along the white and yellow road markings. Each surface colour has its own temperature gradient with different rates of change and peak values. The rates of warming up and cooling down are different, being more steep in the case of the black surface because of the high rate of absorption and releasing of thermal energy. As shown in Figure 3, the drop or rise of

30

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60

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90

0 3 6 9 12 15 18 21 24Time (hr)

Tem

pera

ture

( C

)

Sabha, 22.06.1988Layer depth 6 mm 100 mm 200 mm

Figur 2 Temperature Variation for Sebha, 22nd 1988

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0 100 200 300 400 500 600 700 800 900 1000 1100 1200

Pavement width (mm)

Tem

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White strip

1 pm

9 pm

8 am

Figure 3 Temperature Gradient - Black Road Surface With a White Strip of Road Marking; Kuffra During Hot Season

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temperature between the two surface colours takes place at very close points (within 25 mm or less). Under this heating regime, the pavement material is subjected to the following effects: the bituminous material under the black surface is subjected to more rapid

thermal ageing compared with that under the white surface colour, hence making the material under the black surface stiffer, due to the relatively high temperatures.

the bituminous material along the line of separation between different surface colours is subjected to two different magnitudes of thermal stress due to difference in temperature; rates of cooling; stiffness and coefficients of thermal contraction between the two surfaces.

the rate of change of temperature transversely is so rapid that transverse shrinkage stresses may occur.

In this kind of environment (hot arid climate), there exist a big contrast between road surface colour (dark colour) and road marking colour (white or yellow) which results in the heating regime explained above that lead to differential hardening and stiffening in the pavement material. As a result of the different rates of hardening, the material under the light surface colour will work as a soft joint between to stiffer bituminous blocks. Due to the different contraction and expansion rates between these bituminous blocks under the effect of temperature changes, cracking and separation will be inevitable. 4.0 HARDENING OF BITUMEN In this study, special consideration is given to long-term hardening of bitumen binders in hot arid climates. Binder hardening in this climate is most influenced by thermal oxidation and volatilization which are caused by high temperatures and high rate of exposure to sun radiation. Earlier research (8) showed that the thermal oxidation is approximately doubled for every 10C rise in temperature. Thus, the rate of hardening in service depends, to a large extent, on the pavement temperature system. Hardening of the surface is generally greater due to the higher temperature and ultraviolet radiation (9). Other investigators showed that the top inch of the pavement was found to have a viscosity 50% greater than at depths of inch (10). Hardening is influenced by the percent of air voids content and mixture permeability. Pavements with lateral cracking and high air voids had a greater degree of hardening and oxidation than uncracked pavements with low voids (6). Two different approaches were used in this study to investigate hardening of bitumen, namely: monitoring the loss of volatile compounds and measuring the apparent viscosity of the bitumen binder. To study the loss of volatile compounds in bitumen binders, a chemical analysis was carried out. The analysis was designed to monitor the loss of volatile with both the duration and degree of exposure. Samples were obtained from a number of highways in Southern Libya, a typical hot arid climate. The samples ranged in age from zero to eight years. Each sample was sawn into slices of 15 mm thick through its depth to represent the change of exposure to environmental effects.

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The procedure used has been illustrated in a previous study (1). The bitumen sample was extracted from the sample matrix using normal heptanes solvent. The percentage of bitumen that was dissolved in normal heptanes represented the total volatile compounds in the sample. In addition, there are some non-volatile components with high molecular weight that dissolved in the normal heptanes. The extraction result is shown in Table 2. The percentages of bitumen were plotted against time (years) and the results are shown in Figure 4. The percentage of bitumen decreased sharply from 0 to 1 years, and then slowly from 1 to 8 years. These rates of decline indicate rapid loss of the volatile compounds from

Table 2 Results of Binder Extraction

Sample Age

(Year)

Sample weight (gram)

Sample residue (gram)

Sample Extract (gram)

Extract (%)

0 49.93 46.98 3.05 6.11 1 49.96 47.00 2.30 4.6 4 50.01 47.89 2.13 4.25 8 50.02 48.09 1.94 3.87

the top surface layer during the first year by the processes of evaporation and oxidation. The loss will continue in the following years but at a slower rate. It is expected that the loss in the lower layers will be less than the top layer during the first year but will continue at different rates in the following years. The chromatograms of the heptanes fraction for 0 and 8 years are shown in Figure 5. These results indicate that the heptanes fraction of the binder will lose about 62% of its volatile compounds in 1 year and about 98% in 4 years. The apparent viscosity of the bitumen was determined using a Stanhope Seta sliding plate viscometer according to the procedure detailed in the Australian Road Research Report ARR No 59. The mean of three determinations is reported as the apparent viscosity (log Pa.s units) at a temperature of 45C and shear rate of 5x10-3 s-1. A few specimens were tried at a temperature of 25C. One specimen only, the top surface specimen of 8 years old, could not be tested at the conditions specified above even by reducing the shear rate to 2x10-3 s-1 and using the largest weight. Extrapolating the results available, the approximate apparent viscosity was 7.27 (log Pa.s units). Figures 6(a) and 6(b) show the change of binder viscosity with age and depth respectively. The results showed that the viscosity of the binder increases rapidly with service age. The viscosity-age change follows a certain rate that reflects the combined influence of both the environment and the degree of exposure (depth below surface) of the bituminous material on the process of hardening. In other words, hardening of the bitumen binder in this kind of environment (hot arid climate) takes place mainly by two process; volatilization and oxidation. These hardening processes are increased by solar radiation, pavement temperature

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and the presence of oxygen. At the beginning of service life (age = 0 years), no hardening has taken place and the gradient of viscosity profile through the pavement depth is minimal. In the following years, hardening takes place due to the effect of environmental factors such as solar radiation, temperature and circulation of oxygen. As bituminous mixtures get older, most of the volatile compounds in the top layers evaporated and the rate of viscosity increase becomes less compared to that of lower layers where more volatiles still exist and the rate of hardening is faster. Thus the viscosity-depth profile becomes flatter again.

Figure 1 Percentage of extract against age

01234567

0 1 2 3 4 5 6 7 8Time (years)

Extra

ct (%

)

Figure 4 Percentage of extract against age

C25C24C23

C22C218 years

0 year

Figure 2 Chromatograms for the heptane fraction of 0 and 8 years Figure 5 Chromatograms for the heptane fraction of 0 and 8 years

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4

5

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7

8

0 2 4 6 8Age, N (years)

Vb (

log

Pa.s

)

d = 25.5 mm

d = 43.5 mm

d = 7.5 mm

Figure 6(a) Change of Viscosity With Age; Different Depth at 45 C and Shear Rate 5x10-3s-

1.

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8

0 10 20 30 40Depth, d (mm)

Vb (

log

Pa.s

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8 years

4 years

2 years

1 year

0 year

Figure 6(b) Change of Viscosity With Depth; Different Age at 45 C and Shear Rate 5x10-3s-1.

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5.0 CONCLUSIONS 1. The substantial daily temperature fluctuations increase the bituminous mix

stiffness, making the pavement surface vulnerable to thermal cracking. In such a situation only lower viscosity binders or reducing the hardening rate of the bitumen binder will be beneficial in reducing the potential of thermal cracking.

2. One of the unique problems experienced in the hot arid environment is the surface cracking along the white and yellow road markings Each surface colour has its own temperature gradient with different rates of change and peak values. The rates of warming up and cooling down are different, being steeper in the case of the black surface.

3. The bituminous material along the line of separation between different surface colours is subjected to two different magnitudes of thermal stress due to difference in: temperature; rates of cooling; stiffness and coefficients of thermal contraction between the two surfaces.

4. The chromatographic analysis showed that, the heptanes fraction of the bitumen binder in the asphalt wearing course of the pavement surface may have lost about 62% of its volatile compounds during the first year and up to 98% in 4 years.

5. Binder viscosity increased rapidly with increase in service age. The increase follows a logarithmic function controlled by an ageing factor which is attributed to the climatic conditions and the rheological characteristic of the bitumen.

6. Measurement of binder viscosity showed a significant increase of viscosity of the bitumen in hot arid climates (Sahara desert) compared with those in other parts of the world of different climates.

REFERENCES [1] M.O.Imbarek, and M.K.ALI, "Hardening and Aging of Bituminous Materials in

Hot and Arid Regions", Proceedings of the European Symposium on Performance of Bituminous and Hydraulic Materials in Pavements, Nottingham, United Kingdom, 11-12 April 2002

[2] Ali, M. K., and Imbarek;, M. O., Premature Surface Cracking of Bituminous mixes in Sahra 1st National Conference on the Building Materials and Structures, Sebha, Libya, 14 -16 October, 2002.

[3] Richardson J T G (1994). Bituminous Mixtures in Road Construction. Edited by Hunter R N, Pub. Thomas Telford, London.

[4] Omer M S, Gnabah T H and El-mudi M O (1994). Mechanism of distress of asphalt Pavements in arid desert areas. 2nd Maghrib Road Conference, 8-9 Nov., Biskara, Algeria.

[5] Smith H R and Rolt J (1990). The Durability of Bituminous Overlays and Wearing Courses in Tropical Environments. Proc. 3rd Inter. Conf. on Bearing Capacity of Roads and Airfields, 3-5 July, Trondheim, Norway, pp. 85-98.

[6] Gietz R H and Lamb D R (1968). Age-Hardening of Asphalt Cement and Its Relationship to Lateral Cracking of Asphaltic Concrete. Proc. Assoc. of Asphalt Paving Technologists (AAPT), Vol. 37, pp. 141-158.

[7] Imbarek M O, Yudono B, Roberts D J and Smith J W (1996). The Loss of Volatile Compounds of Asphalt Binder in Hot Arid Climates. Proc. 2nd Nat. Conf. on Asphaltic Mixtures and Pavements, Thessaloniki, Greece, 2-4 April.

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[8] Dickinson E J (1980). The hardening of Middle East petroleum asphalts in pavement surfacing. Proc. Assoc. of Asphalt Paving Technologists (AAPT), Vol. 49, pp. 30-63.

[9] Kumar A and Goetz W H (1977). Asphalt hardening as affected by film thickness, voids and permeability in asphaltic mixtures. Proc. Assoc. of Asphalt Paving Technologists (AAPT), Vol. 46, pp. 571-605.

[10] Coons R F and Wright P H (1968). An Investigation of the Hardening of Asphalt Recovered from Pavements of Various Ages. Proc. Assoc. of Asphalt Paving Technologists (AAPT), Vol. 37, pp. 510-528.