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Improvement of the stability of a modified bituminous binders within EVA.

Introduction

The bitumen is heavy hydrocarbons by-products, black residues of crude oil obtained by direct distillation, serving in the covering manufacture in hot conditions.

Whatever, the used manufacture mode is, the objectives remain the same, to get a residue having particular properties which make its application essential in road works for the sake to guarantee the adhesion properties and mechanical ones satisfactory in.

The traffic and climates conditions to which one deals, but as well as the thermoplastic materials, it endures of a significant disadvantage: It is very sensitive to temperature, it tends to becoming hard and breakable, brittle in cold weather while

In hot weather, it softens and flow. Moreover, the significant increase in the request of road coatings, because of the very strong traffic density, the increasing axial loads and the forces of braking and acceleration require an adaptation of binders in the sense of mechanical characteristics' improvement. Indeed, the numerous experiments Cumulated in matter of road works made it possible to notice that: [1].

Bitumen doesn't allow to the traditional covering type, even by choosing a hard grade (35/50), to ensure in all cases a satisfactory resistance to rutting under a very heavy traffic.

A harder grade, if it brings to covering a better behaviour to rutting increases, contrary to thermal cracking risks at low temperatures.

Certain mixtures covered with open granularity will not have the desired Mechanical resistance, because of the cohesion and adhesiveness obtained with bitumen; this is why, solutions to these disadvantages are sought in the modification by polymers' adjunction.

The polymeric bitumen

The modified bitumen generally results from the pure bitumen combination with the polymers. It consists of adding to the bitumen a chemical agent which modifies its chemical structure, the physical and mechanical properties. The double objective required is:

--To decrease thermal susceptibility: fragility at low temperatures, satisfactory consistency at high temperatures.

--To increase the resistance to deformation and rupture's threshold.

The two polymers' families the most used bitumens' modification is the copolymers of Ethylene and Acetate of Vinyl Acetate (EVA) and Butadiene copolymers and Styrene (SBS).

In road domain, they are the essentially used [1] [3].as well as called functional copolymers.

They are static copolymers obtained by radicalizing copolymerization [2] (successive hooking of two monomers), under high pression of Ethylene the co-monomer of Acetate Vinyl (OCOC[H.sub.3]), which their essential properties are the resistance to flexion crack; and their thermal stability of copolymers EVA used in bitumen and acetic acid's modification.

Can be made to exhibit a dipole structure, which is the separation of two point charges of equal magnitude and opposite sign by a small distance.

The modification of bitumen' properties by polymer addition are often explained by polymer's inflation by bitumen's oils. [2] For the sake that a polymer is usable in bitumen modification, it must be at least ineffable, possibly soluble in the hydrocarbons fractions of the low molecular masses of the binder [5], this implies that a polymer-bitumen binder can be regarded as a two-phases system [4]:

--A polymeric phase inflated by oils.

--A bituminized phase richer in resins and asphalting (this doesn't intervene in salvation) than bitumen.

Experimental trials

Our study is based on Marshall test, to make it, we focus on the modification's influence on compactness (and the residual vacuums), stability and flowing while making a comparison between the modified bituminous concretes and that of pure bitumen 35/50.

Thereafter, the mixtures having given the Marshall performances will be submitted to indirect determination test of the rigidity module to traction.

Experimental methods

Testing program applied to the binders and bitumen concrete

The physical properties of bitumen that are most important to hot-mix paving are penetration "PI" at 25[degrees]C (ASTM D5-73) and softening point "SP" [degrees]C (ASTM D36-76). Penetration test performed at 25 [degrees]C with a 100 g load for 5 s according to ASTM D5 specifications. Softening point test is based on the ring and ball method according the ASTM D36.

Characterisation methods applied to the bitumen concrete

Bitumen concrete mixes should be designed to meet the necessary criteria based on medium traffic design according to the specifications of Algerian highways. These criteria require carrying out some tests such Marshall Stability, flow test. In this investigation, the hot mixtures asphalts were characterised through Marshall test. The dimensions of the cylindrical specimens are 101.6 mm diameter by 63.5 mm height. The specimens were compacted by applying 50 blows on each side of the specimen at 150[degrees]C in accordance with ASTM D 1559 then stored at ambient temperature for one day. Before performing Marshall test, the compactness is measured after which, standard specimens were immersed in water at 60 [degrees]C for 30 min and then loaded to failure at a constant rate of compression of 51 mm/min.

The Marshall stability value (in kN) corresponds to the maximum force recorded during test while the flow (in mm) is the deformation noted at the maximum force.

Materials characterization

Aggregates

Aggregates are the granular material used in asphalt concrete mixtures which make up between 90% and 95% of the mixture weight and provide most of the load bearing characteristics of the mix. Therefore, the quality as well as the physical properties of the aggregates is critical to the pavement performance. Moreover, the gradation of aggregates must be carefully considered since it provides the voids in the mixture required to accommodate the asphalt film thickness and allow thermal expansion of the bitumen within the mix.

Crushed aggregates are used in this work. They are obtained from quarries located of Algeria. The sand comes from Keddara located in Bouira whereas the gravels are from Cap Djinet in Boumerdes. Calcareous fillers are incorporated to the mixture to satisfy gradation requirements. Fig.1 shows the grading curves of the aggregates and sand and table 1 summarises their main characteristics. The chemical analysis shows that the sand is calcareous while the gravel contains an important proportion of silica and oxides.

[FIGURE 1 OMITTED]

The chemical characteristics, the wear well and the shock, the surface cleanliness as well as aggregates' specific weights are given in tables 1 and 2.

The 0/3 fraction has a small fine elements' percentage; by its calcareous nature it should have a good adhesiveness with the binder. However, its small content in fine elements must be corrected by the addition of fines for the bituminous covering's formulation.

Bitumen

It is well know that the properties of the bitumen depend on the main parameters, the crude source and the manufacturing process. In this investigation, the main source of cued oils is unknown while the bitumen is product by Naftec Company. The physical properties of the bitumen used in this investigation obtained through typical standard laboratory tests are listed in table 3. it appears that AC 35/50 has a low softening point. This means that, in summer, the used AC will soften and will suffer from rutting and permanent deformation since, in Algeria; the temperatures reach easily 50[degrees]C.

Polymer EVA

An ethylene vinyl acetate (EVA) copolymer is used as a modifier agent. EVA is a semi- crystalline copolymer containing 18% vinyl acetate with a melt index of 2.7 g/10 min and a specific gravity equal to 0.939 g/cm3. it is provided by SAEL Company (Algerian Company of elastomers).

Preparation of polymer modified bitumen PMBs

Polymer modified bitumen is prepared using a vertical low shear. Temperature control during mixing is achieved by means of thermocouple attached to the mixer. The machine was operated at a speed of 300 rpm. The utilisation strategy must be coupled with environmental energy considerations to use available materials most efficiently. Since the bitumen polymer mixing temperature and bitumen-polymer mixing time play a crucial role in determining the performances of polymer modified asphalt concrete. In this work, 500 g of Ac was heated to fluid condition into a 1000 ml spherical flask. Upon reaching 175[degrees]C reweighed polymer amount was added to the bituminous material. Three contents of EVA copolymer by weight of unmodified bitumen are selected ranged from low polymer modification at 3% to higher degrees of modification at 5% and 7%. Mixing was then continued at 180 [degrees]C for 4 hours.

Results and discussion

Polymer modified bitumen

In figure 3 and 4, we give the results of the penetrability tests and softening point of the modified bitumen at 3%, 5% and 7% of EVA.

[FIGURE 2 OMITTED]

We can note that 3% of EVA can effectively, decrease greatly the penetrability. The contrary is obtained with 7% of EVA: An increase in penetrability.

This variation of penetrability between the bitumen let us state, that with the addition of EVA, it passes from a very hard state at 3% to less hard at 7% of EVA. The greatest variation is obtained at 3%. That is all the remarkable as it is the content which makes it possible to decrease the bitumen's penetrability (41, 62%). The effect of 5% of EVA relative to the other contents can neglected since, its very little penetrability increase.

If these results emphasize the unquestionable influence of EVA on penetrability, they are in other hand not in a whole agreement with those other studies carried on the subject [1].Thus, certain assumptions can be advanced:

--The first problem encountered in the modification within EVA is compatibility. It is said that a polymer is compatible with bitumen, in abroad meaning, from the moment that the mixture heterogeneity can't be highlighted by a simple visual examination (no precipitation of asphalting). The three modified binders' compatibility in the study was visually controlled. However, this doesn't imply anything for optical microscopy.

--The pure bitumen was heated on several occasions for the study's needs (during transport, in the laboratory and during the manufacture of modified binders), what causes an evaporation of bitumen's oils. That is not without effecting the obtained properties of bitumen-EVA, the modification takes place by oils' absorption by the polymer.

[FIGURE 3 OMITTED]

The softening temperature increases with the content of EVA. The greatest point is obtained by 5%; and with 7% of EVA the softening temperature decreased compared with those obtained with other contents. Yet, the softening temperature goes down compared to that of the basic bitumen which is notorious and significant,. It proves then, that the polymer makes the pure bitumen more viscous.

After having analyzed the different components of the bituminous covering, the formulation study which begins with the definition of the mixture composition would allows us of taking in account a formula for the continuation of our study.

Compactness and residual vacuums of bitumen concrete

After correcting the lack of fine elements by addition of lime, the selected gradation for the control mix is 40% of sand 0/3, 18% of gravel 3/8, 40% of gravel 8/15 and 2% of filler.

The results on compactness' variation and the residual vacuums are presented as follows in the figure 4 and 5:

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

We can notice that, although the modified bituminous concrete has greater compactness than that containing pure bitumen, the EVA doesn't seem to have a great deal on the covering compactness.

Indeed, the variation of compactness (and the residual vacuums) between the covering containing pure bitumen and those modified, is highly reliable and not gradual. It's very improbable that the EVA could be the cause of this variation. The latter would be due to compactness and measurements methods' conditions.

Stability of bitumen concrete

The mixtures' stability is represented in the figure 6.

[FIGURE 6 OMITTED]

The stability of the bituminous concrete increased for the three contents of EVA. However, a modification at 5% gives the greater stability followed by that of 3%.

These figures show that the EVA influences considerably the Marshall stability of the covering. Indeed, the modification within EVA brought a profit of stability.

Flow of bitumen concrete

The results on flowing variation are presented in the figure 7.

[FIGURE 7 OMITTED]

The flowing increases progressively with the content in EVA, what gives, as one increases the polymer's content, more and more bituminous concrete tend to flow. However, the obtained flowing at 7% is the most important, doesn't satisfy the requirements. With 3% and 5%, the flowing increase is lower than 1mm, and its covering remain in the acceptable flowing standards.

Lastly, these figures show that the polymer confers to the covering a greater elasticity although, flowing tends to increase with EVA, the stability profit is such as the covering with the EVA will have a better resistance under heavy traffic. However, a modification at 3% gives more satisfactory overall performances.

--Penetrability at 25[degrees]C of the binder weaker (-41, 62%).

--Increase in the point of softening (38, 44%).

--Stability profit (16, 47%).

--Acceptable flow.

The figure 8 shows the variation of stability and softening point.

[FIGURE 8 OMITTED]

We note that the variation of bituminous mixtures 'stability follows that of the softening point. This can be explained in Marshall Test's conditions. Indeed, the covering is at the temperature of 60[degrees]C, which is in fact an extreme condition taking in consideration the softening point of bitumen and classical bituminous concretes. Thus, this condition hasn't a changing effect on the coverings with EVA. All softening temperatures of bitumen-EVA, being are about 70[degrees]C. What follows, in the same temperatures conditions. We note a better resistance to temperature and a greater stability for the covering with EVA. In other words, it would have been more comparative to carry out the test on the mixtures with EVA at 70[degrees]C.

Conclusion

The bitumen' study made it possible to see the influence of polymer on penetrability at 25[degrees]C and the softening point ball and circle. This reveals that a modification at 3% of EVA had a better results, since it makes it possible to obtain a polymer-bitumen at the same time more hard and more viscous that of the pure one 35/50.

We state that, the modification by EVA have no deal on compactness and the residual vacuums of the bituminous concrete, but it allows a significant stability profit and these increase is connected to the rise of temperature ball and circle.

However, we couldn't establish a correlation between bitumen' penetrability at 25[degrees]C and the mixtures' stability in addition to the flowing increase, what led to the following assumptions:

--The use of EVA should be made with bitumen rich in oils (soft) in order to optimize the polymer-bitumen performances

--The Marshall test, principal in the formulation studies, occurs at 60[degrees]C and would allow a better evaluation of the performances profit if it takes in account the softening temperature of bitumen-EVA which actually is about 70[degrees]C.

References

[1] Kamel, H.. Study of action of modified bitumen on the coverings' behaviour. Doctor Thesis USTHB 2003.

[2] Airey, GD, Rheological evaluation of ethylene vinyl acetate polymer modified bitumen, J. Constr Build. Mater. 2002. 16:473-87.

[3] Lu X, Isacson U. Modification of road bitumen with thermoplastic polymers, J. Polymer test. 2001. 20:77-86.

[4] Kraus, G & Rollman, K.W. Morphology and mechanical behaviour of bitumen modified with S.B.; Block polymers, Int. Rubber Conference Nordberg, Germany, Sep1980.

[5] Brule, B & Brion, Y. Bitumen-polymers Association, The relation between component, structures and properties, Linking Report of the bridges and fitted laboratories. 2001. 145, 09-10.

S. Saoula University M'hamed Bouguera, ALGERIA

K. Ait Mokhtar * University Houari Boumedienne, ALGERIA

H. Haddadi University Houari Boumedienne, ALGERIA

E. Ghorbel University Cergy Pontoise, FRANCE
Table 1: Results of chemical analysis of aggregates

Properties               Sand 0/3   Gravels 3/8   Gravels 8/15

(Si[O.sub.2]+Silicate)   4.37       70.12         70.18
[Fe.sub.2][O.sub.3] +    1.32       20.40         23.10
  [Al.sub.2][O.sub.3]
(CaS[O.sub.4],           Traces     Traces        Traces
  2[H.sub.2]O)
NaCl                     0.17       0.12          0.17
(CaC[O.sub.3])           94.01      6.83          5.12
C[O.sub.2]               41.36      3.00          2.25
Boiler feed water        0.27       1.85          2.52

Table 2 : Results of physical properties of aggregates.

Test                                 0/3       3/8     8/15

Los Angeles abrasion test (%)          /     13.96    14.09

Micro deval test (%)                   /     17.65    13.56
Humid                                         5.80     2.18
dry

Specific gravity (g/[cm.sup.3])     2.67      2.69     2.72

Table 3: Characteristics of based bitumen used in this investigation

AC                                  35/50
Penetration (25[degrees],1/10mm)    37
Softening point SP                  53.2
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Author:Saoula, S.; Mokhtar, Ait K.; Haddadi, H.; Ghorbel, E.
Publication:International Journal of Applied Engineering Research
Article Type:Report
Date:Apr 1, 2008
Words:2759
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