Fibrous additives to bituminous mixes and the method of obtaining the additive and use in bituminous mixes

The subject of the invention is a fibrous additive to bituminous mixes used for road pavements, the method of obtaining the additive and its use in bituminous mixes.

Increased road traffic intensity and heavy trucks aggressiveness due to increased axle loads demand road pavements that meet higher requirements and better performance. These increasing requirements can be met by using better materials and technologies. Usually, road pavement is made of various types of bituminous mixes, which are composites of mineral material, usually crushed aggregate, sand, calcareous filler and of petroleum bitumen.

Numerous publications describe different methods for improving performance of bituminous mixes which can improve pavement bearing capacity, resistance to rutting, low-temperature cracking, and resistance to atmospheric conditions. These methods consist in improving the basic properties of materials used for manufacturing of mixes, especially improving properties of road bitumen by modifying it, e.g. with polymer or various stabilizing and modifying additives.

The description of Polish patent application no. P.357612 specifies a bituminous mix with non-continuous grading consisting of bitumen or polymer-modified bitumen, mineral aggregate, granulated rubber (up to 10 mm grading) and polymer fibers. Such a mix provides resistance to cracking and is especially recommended for surface course overlaying damaged road pavements of cracked surface.

The description of the Polish patent application P.358370 specifies various known fibers that constitute additives to bituminous mixes and discusses the results

of their use and also the unfavorable characteristics of known fibers. These fibers include cellulose fibers, fibers of plant origin in general, mineral wool fibers, slag wool or glass wool and their mixes. Adding fibers to mixes results in improved resistance to cracking and crack propagation, at least to the extent to which these fibers can create micro-reinforcements inside bituminous mix. Disadvantageous properties of fibers of plant origin, i.e. cellulose fibers, include their degradation under influence of water and loss of cohesion between aggregate and binder in the mix, which results in loss of mechanical strength of such mix during use of pavement. Glass wool and slag wool fibers are not deteriorated by water, but their use has a negative impact on environment, because they can propagate in the air and have an irritating effect on humans; they are also hard to distribute in bituminous mixes and often remain on layer's surface.

The fibrous reinforcement material for bituminous mixes for road pavements, which constitutes the subject of the Polish patent application P.358370, is obtained from glass fibers with a maximum diameter of 5 microns and the length of 6 mm and more. This application also describes the manufacturing process of this reinforcement material, consisting in selecting the yarn from which the fibers are obtained, the grinding operation, during which the yarn is broken up into fibers, and also the use of the material obtained this way as a reinforcement material for bituminous mixes and mix for road pavements that contain the above-mentioned reinforcement material made from glass fibers.

The SMA mix, manufactured with an addition of a stabilizing agent, is a known and commonly used mix for making the wearing course of road pavement, irrespectively to the road class and traffic category, and also in some cases for making the binder course. This mix is used for ultra-thin wearing courses for road pavements, airport pavements and pavements on bridges. The requirements concerning mechanical and surface properties depend on the traffic category and type of construction (facility) where the mix is to be used. In general, SMA mix contains mineral aggregate of specific grading, bitumen or polymer-modified bitumen in amount from 6,0 to 8,0 % m/m, 0,2 to 0,9 % m/m adhesive agent in related to the bitumen and stabilizing agent in amount from 0.2 to 1.5 % m/m. Usually cellulose fiber is used as stabilizer. The fiber is added to stone material during mixing of components. Adhesive agent is added (if required) in appropriate quantity, depending on content of bitumen. Cellulose fibers is added to mix as stabilizing agent to prevent

components (aggregate and bitumen) segregation during transportation. It does not have however any other positive influence on pavement performance and its mechanical properties. The only characteristics used to determine suitability of stabilizing agent in SMA mix is binder drain-down test. This characteristics is usually determined using the Schellenberg method. The permissible drain-down was experimentally established at not more than 0.3 % (m/m).

It has been discovered that fibrous material of very good technical properties, such as resistance to water, elasticity and strength, can be recovered during the recycling of rubber products reinforced with a textile cord, such as automobile tires or conveyor belts. According to the invention, this material can be used as a fibrous stabilizing, reinforcing and modifying additive to bituminous mixes for construction of road pavements.

According to this invention, the fibrous additive is obtained through the process of grinding rubber wastes reinforced with textile cord, mainly automobile tires. The fibrous additive consists of a carded textile cord, which can contain some rubber residues in the form of rubber granulate of grading up to 8 mm maximum, preferably 2 mm maximum, at quantity of less than 40 % in relation to weight of fibrous material.

Preferably, textile cord consists of a mix of synthetic polymer fibers, and/or polyester fibers, and/or viscose fibers, and/or polyamide fibers, and/or para-aramide fibers with melting temperature of at least 220 ⁰ C, whose length does not exceed 30 mm, preferably does not exceed 10 mm.

In an optimum case, fibrous additive contains rubber granulate of following grading: size (rubber dust) below 0.085 up to 30%, size from 0.085 up to 2.0 up to 15%, size from over 2.0 up to 8.0 up to 5% m/m in relation to the fibrous material.

The process of obtaining the fibrous additive consists in mechanical breaking up of cut rubber products reinforced with textile cords, preferably worn automobile tires, and separating the textile cord from the rubber granulate. In a favorable make the grinding is carried out gradually in granulators of different sizes: 10-20 mm, 4-10 mm and 3-8 mm, and the separated textile cord is removed to a container, from which the fibrous material is taken and packed into packages or bags.

According to the invention, the fibrous additive, produced according to the described method, is used as stabilizing material and/or modifying and reinforcing material in bituminous mixes for road pavements.

According to the invention, the bituminous mix for road pavements consists of 4.0 to 8.0 % m/m of road bitumen, 92 to 96 % m/m of mineral aggregate, 0.2 to 0.9 % of adhesive agent in relation to the bitumen, 0.3 to 4.0 % m/m of stabilizing agent in relation to the bituminous mix.

According to the invention, the production process of the mix consists in heating the aggregate to the coating temperature, i.e. between 150 and 200 ⁰ C, adding the fibrous additive to the aggregate continuously or at regular intervals and mixing, then adding the bituminous binder and mixing the compound at proper mixing temperature.

The fibrous additive at ambient temperature does not require pre-heating and the process does not require additional, prolonged time for mixing with aggregate.

The bituminous mix with fibrous additive can be used for SMA mixes mainly, but also for other mixes such as asphalt concrete or gap-graded mixes. According to the invention, this mix can be used for laying wearing course of new and rehabilitated pavements, and for lower courses of road pavement. Depending on the mix gradation and intended use in asphalt pavement, thickness of the layer can range from 1.5 cm to 14 cm.

Optimum content of rubber granulate in the fibrous additive is determined depending on type and grade of bitumen, type of mineral mix and intended use of this mix. This is the advantageous characteristic of the invention because it is not necessary to completely separate the rubber granulate and dust from the carded cord fibers. It reduces cost of production of fibrous additive.

The length of the fibers in the fibrous additive is very important, because the function performed by the fibers in the mix depends on it. Short fibers, up to 2 mm length, usually perform the stabilizer function, because they prevent the drain-down of bitumen from the aggregate in the mix, whereas longer fibers, over 2 mm long, create the micro-reinforcement in the bituminous mix through their ability to develop bridging connections between aggregate grains.

The selection of grading in the fibrous additive results from the fact that the smallest size of rubber granulate, up to 0.085 (rubber dust), constitutes an asphalt binder modifier, whereas the contents of larger sized granulate, over 0.085 mm, has been limited so that it does increase the contents of bituminous mix filler, thus the number of air voids in the mix does not increase. This, in turn, results in an improved rutting resistance of the bituminous mix.

Benefits resulting from application of the invention mainly consist in obtaining an additive for bituminous mixes for road pavements, which can also act as the mix stabilizer that prevents segregation of components, a modifier that improves the performance characteristics of the mix due to very good mechanical properties of the textile fibrous material, such as tensile strength, resistance to water and atmospheric conditions. The rubber granulate and dust in the fibrous additive have a positive influence on bitumen properties by modifying it, which additionally improves the resistance of the mix to atmospheric conditions. According to the invention, these advantageous properties of the fibrous additive result from the fact that it is obtained from a textile cord that constitutes reinforcement of rubber products, including tires. This cord has a decisive influence on the strength of these products, and it is made of one or more types of highest-quality fibers, such as para-aramide, polyester, polyamide and viscose fibers. For example, the para-aramide fiber is one of the best fibers from the viewpoint of tensile strength at low specific weight. It also has excellent thermal strength, dimensional stability and low ultimate elongation. It does not corrode and is resistant to majority of chemicals, too. This fiber is also nonflammable, non-conductive and wear-resistant. The polyamide fiber is manufactured from a synthetic polymer with high tensile strength, high modulus of elasticity, hardness and abrasion resistance.

Polyester fibers also have high tensile strength, bending strength, abrasion resistance, fastness to light, resistance to chemicals (weak acids and alkali) and biological resistance and low water absorption.

Viscose fibers, designated on tires as Rayon, are actually cellulose fibers with significantly changed and improved properties.

Fibers from carded textile cord separated during the grinding of rubber products wasted, such as automobile tires, do not lose their physical properties and are perfectly suitable as an additive to bituminous mixes for road pavements.

An additional, and very important characteristic of the invention is the use of textile waste produced during the recycling of rubber products reinforced with textiles, especially automobile tires, conveyor belts, etc., in an environment-friendly way. Until now this textile waste has been utilized by incineration in furnaces in the power industry, which causes air pollution emission due to carbon oxides, nitrogen oxides, sulfur oxides, etc. released during the burning process.

The invention is explained in a more detailed way in the provided examples and on drawings, Figure 1 Recovery process chart of fibrous additive to bituminous mixes, and Figure 2 Binder drain-down according to Schellenberg method.

The fibrous additive is obtained from recycled automobile tires using known methods, such as:

- mechanical method at ambient temperature using mills and special granulators,

- cryogenic method, by freezing cut tires with liquid nitrogen and then breaking them up using beater mills,

- using the ,,wet" method,

- breaking them up with water at high pressure.

This example describes to most widespread method of mechanical breaking up at ambient temperature, shown in Figure 1. This method consists in preliminary cutting the tires using so-called rippers 1. Then the obtained shreds are ground into different sizes using special mills and rolling mills, i.e. different sized granulators. The granulators 2, 4 and 5 are connected to screens that sort the rubber granulate according to sizes: granulator 2 - 10-20 mm, granulator 4 - 4-10 mm, granulator 5 - 3-8 mm. The steel cord is removed with electromagnets 3 to storages 8 and 9. The textile cord fibers are separated from the rubber granulate after each granulation stage and transported through conduit H to pneumatic separators 6, where the rubber granulate is separated from the fibrous cord. The textile cord, refined and carded during the breaking up process, is stored in silos 7. The product from silos 7, after additional optional screening of rubber granulate is packed at position IfJ into packages or fusible bags as the fibrous additive to bituminous mixes.

Table 1 shows an example composition of fibrous additive suitable for bituminous mixes. Table 1

Fibrous additive, with the composition specified in Table 1, has been used for the SMA bituminous mix as a stabilizer preventing the segregation of components, instead of the known cellulose additive used so far. Table 2 specifies the composition of 4 types of SMA8 bituminous mixes: Type A is a mix of SMA8 (reference) without any stabilizer. Type B is a mix of SMA8 with 0.3 % m/m of cellulose fibers. Type C is a mix of SMA8 with 0.6 % m/m fibrous stabilizing, reinforcing and modifying additive. Type D is a mix of SMA8 with 0.3 % m/m fibrous stabilizing, reinforcing and modifying additive, without rubber granulate.

Table 2. Compositions of SMA8 bituminous mixes.

The mixes were prepared in the following way:

Mix A: Bitumen, mineral powder and adhesive agent were added to aggregate heated to 18O ⁰ C. The composition was mixed at 18O ⁰ C until it became homogeneous.

Mix B: Produced similarly to Mix A, additionally a stabilizing agent in the form of cellulose fibers has been added in the quantity of 0,3 % m/m in relation to MMA.

Mix C: In this case, the fibrous additive whose composition is specified in Table 1 , in the quantity of 0.6% in relation to MMA, has been used as the stabilizer to produce the bituminous mix.

Mix D: In this case the fibrous additive whose the composition is specified in Table 1 , in the quantity of 0.3% (after screening the rubber granulate) in relation to

MMA, has been used as the stabilizer to manufacture the mix according to the following hot process:

A weighed amount of aggregate after drying in a rotary furnace is fed to the asphalt plant. During mixing the fibrous additive at ambient temperatures and calcareous filler are added in batches. After adding the entire specified amount of fibrous additive to the aggregate and mixing it with the aggregate at coating temperature of 150 - 200 ⁰ C, the binder is added, and the entire composition is mixed at the same temperature until it becomes homogeneous.

The produced SMA mix can be used to make the road pavement wearing course the thickness from 1.5 to 14 cm.

Table 2 and Figure 2 specify the composition of mixes A, B, C and D. Table 3. Comparison of test results of bituminous mixes.

Fatigue tests of SMA8 mixes (specified in Table 2) proved that Mix C containing fibrous additive can withstand a higher number of load cycles at the same deformation than Mix A, which is made without any stabilizing additive. This shows that the fibrous additive, apart from the stabilizing properties, has reinforcing properties, which in turn results in extended durability of the road pavement. Mix B with commonly known and used cellulose fibers displays the worst fatigue characteristics.

Table 4 shows the example of asphalt concrete of typical composition without any additive or of composition according to the invention - using fibrous additive.

Table 4. Composition BA12 asphalt concrete-type bituminous mix.

The asphalt concrete produced using the mix containing fibrous additive, whose composition is specified in Table 4, exhibits improved fatigue resistance and low temperature resistance in comparison with the mix without additive. Asphalt concrete can be used for all pavement layers of different thickness and intended purpose.

The composition of the fibrous additive has a significant influence on the performance characteristics of bituminous mixes. In case of SMA mixes, the contents of rubber granulate in the fibrous additive has a significant influence on the adequate drain-down of the mix, as shown on the graph (figure 2), where Line I shows the drain-down of SMA mix with road bitumen and fibrous material, which contains approximately 40% of rubber granulate, Line Il shows the same mix but with fibrous additive without rubber granulate, Line III shows the same mix as Line I but with modified bitumen, Line IV shows the same mix as Line III but with fibrous without rubber granulate, and Line V shows a mix with bitumen and cellulose fiber stabilizer.

The graphs show that, in order to obtain the required drain-down at approximately 0.3 % m/m, different quantities of the stabilizer must be added, as shown in Table 5.

Table 5. Examples of compositions using fibrous additive for bituminous mixes.

Table 5 shows that only when using bitumen and fibrous additive with rubber granulate it is necessary to increase the amount of this additive in bituminous mixes in order to obtain the required limit of drain-down.

Pavement made from a mix with the fibrous additive, according to the invention, has higher fatigue resistance. The presence of a high amount of synthetic fibers in the mix, which are not biodegradable and do not absorb the binder like the cellulose fibers, improves the pavement resistance to low temperature cracking. In SMA and other gap-graded bituminous mixes, the fibrous additive constitutes a stabilizer that prevents the asphalt binder drain-down from the aggregate. The stabilizing action of the fibrous additive, according to the invention, acts like micro- reinforcement that makes the binder stay on the aggregate, whereas while using cellulose stabilizers the drain-down prevention mechanism results from the high absorbability of cellulose fibers, which in turn leads to reduced fatigue resistance and resistance to low temperature cracking of mixes with such fibers.

Claims

1. The fibrous additive to bituminous mixes used for road pavements whose characteristic feature is that it consists of a carded textile cord from rubber products, preferably automobile tire, that can contain up to 40 % m/m of ground rubber.

2. The fibrous additive, according to Claim 1 , characterized by a carded cord fiber not longer than 30 mm, preferably from 2 to 10 mm.

3. The fibrous additive, according to Claim 1 , characterized by the contents of synthetic polymer fibers, and/or polyester fibers, and/or para-aramide fibers, and/or polyamide fibers, and/or viscose fibers.

4. The fibrous additive, according to Claim 1 , characterized by the fact that the rubber residue has the form of granulate the size of up to 8.0 mm.

5. The fibrous additive, according to Claim 1 or Claim 3, characterized by the following contents of individual sizes of rubber residue in relation to the fibrous material: below 0.085 up to 30% m/m, from 0.085 to 2.0 up to 15% m/m, from 2.0 to 8.0 up to 5% m/m.

6. The method of obtaining the fibrous additive to bituminous mixes characterized by the fact that during the grinding of used rubber products containing a textile cord, preferably automobile tires, the textile cord in the form of carded fibers is separated from the rubber, which can contain up to 40 % m/m of ground rubber residues.

7. The method according to Claim 6, characterized by the fact that the separation of carded fibers from rubber is conducted through grinding rubber products into rubber granulate, preferably by mechanical method at ambient temperature.

8. The method according to Claim 7, characterized by the fact that the carded textile fibers are separated from the rubber granulate through consecutive stage of grinding of cut rubber products in granulators into 10-20 mm (2), 4-10 mm (4) and 3-8 mm (5) sizes.