Abstract:
Pavement structure  
     for use in applications where special requirements to load bearing capacity and durability must be fulfilled, for example in warehouses, on docks, airports, distribution centres, retail areas, goods terminals, industrial floors, production halls and other places where heavy loads and excessive wear can be expected, said material consists of an asphalt part and a slurry part, said asphalt having a porosity of 20-40%, and said slurry substantially completely fills the voids in the asphalt, the slurry being a composite material comprising a binder, additives and water, and optionally cement, microsilica, flyash or other pozzolanic materials as well as optionally sand or other fine aggregates, wherein the pavement material ( 1 ) comprises at least one reinforcement layer ( 3, 4, 21 ). Furthermore, a method for the construction of a pavement, as well as use of a material is disclosed.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a pavement structure comprising a porous asphalt part and a composite material part, said composite material part comprising a binder, wherein said composite material substantially completely fills the voids in the asphalt.  
         BACKGROUND OF THE INVENTION  
         [0002]    Pavement structures of this type are used in applications where special requirements to load bearing capacity and durability must be fulfilled, for example in warehouses, on docks, airports, distribution centres, retail areas, goods terminals, industrial floors, production halls, shopping centres, roads and other places where heavy loads and excessive durability is needed.  
           [0003]    Pavements of this type provides some important advantages as compared to ordinary pavements. When pavements or roads traditionally are constructed in the applications mentioned above, either constructions consisting of only concrete or only asphalt are applied.  
           [0004]    Concrete pavements can give a very strong construction which gives a high load bearing capacity, and high durability. On the other hand the concrete pavement is compelled to have flexible joints, as all other concrete constructions, for the construction to be able to move, due to changes in temperature, humidity which again causes shrinkage in cement based materials. Often it is not desirable to have joints in a pavement, as these joints constitute an area where the concrete can be damaged due to high loads on the edge, which can cause the concrete to crack and break. Furthermore, joints must be maintained which results in extra maintenance costs for the user, resulting in time where the pavement cannot be used.  
           [0005]    Asphalt pavements are very flexible and do not need joints. The load bearing capacity of a traditional asphalt layer is relatively low, which makes it unsuitable for use in the applications mentioned above when the sub-base is not sufficiently rigid.  
           [0006]    The combination of asphalt and concrete material enjoys the good characteristics of both materials, if they are correctly combined. By combining the two materials the pavement has a relatively high load bearing capacity, good flexibility, high durability and does not need joints. Furthermore is the material substantially frost-resistant and very resistant to harmful chemicals and pollutants.  
           [0007]    Materials of this type has long been used in the industry with good results. Examples of such materials are sold under the registered names Salviacim, Betophalt by Alfred Kunz AG, Microfond by Dyckerhoff Baustoffsysteme or Densiphalt® by Densit A/S. These materials are all combinations of asphalt and concrete or concrete-like materials, where the asphalt is placed first with a porosity of 20%-40%. Thereafter the concrete is placed in the shape of a slurry which will, to a certain extent, fill out the voids in the asphalt matrix and subsequently harden. The combined material thereafter has a very low porosity and a, in comparison to regular asphalt, high compressive strength, which together gives the material the special characteristics mentioned above.  
           [0008]    Pavements of this type furthermore has very good resistance to frost and pollutants in the environment in which they are placed, due to their very compact structure and low permeabillity.  
           [0009]    The abovementioned pavements all have very good durability characteristics and usually have a longer service life than either concrete or asphalt. If however the sub-base settles or subsides the pavement will lose part of its support and consequently will crack. The normal procedure in order to restore the pavement is to cut out the cracked section, renovate the sub-base and place a new layer of for example Densiphalt®. The replacement/renovation often takes place long after the initial cracking occurred, as these pavements are often subjected to heavy and/or intense use, and airports, ports, ports, etc. seldom have time where the area is not needed and this type of maintenance can be carried out. The user therefore has an extended time wherein the pavement is not performing as it is designed to do. Especially in airports can this cause severe problems. These kinds of pavements are often placed where there is the heaviest traffic, for example in the so called channelised traffic paths, leading the aircrafts from and to the aprons for loading and unloading passengers, cargo and/or fuel.  
         BRIEF DISCLOSURE OF THE INVENTION  
         [0010]    The present invention therefore addresses the problem of how to turn a wearing course with high durability characteristics, i.e. the pavement into a load bearing course.  
           [0011]    The above mentioned problem is present in the situations described above and can also be seen as a means to extend the service life and minimize the downtime of pavement structures, where the pavement becomes unusable due to cracking caused by settlement of the base.  
           [0012]    This problem is solved by the present invention, by placing reinforcement in the pavement layer.  
           [0013]    Historically it has not been thought feasible to reinforce asphalt layers. Due to the viscosity of the bitumen it has not been possible to reinforce asphalt with traditional steel reinforcements, but only with certain textile reinforcements such as geotextiles. The asphalt structure is not strong enough to transmit forces to the reinforcement. Asphalt consists of a mixture of gravel and bitumen, wherein the bitumen acts as a flexible binder, keeping the asphalt matrix together. When the reinforced asphalt structure is subjected to bending forces the bending moment is transmitted to the reinforcement as shear forces in the zone around the reinforcement and then from the matrix around the reinforcement into tension in the reinforcement whereby the tension characteristics are utilised. Due to the soft and flexible nature of bitumen, the bitumen slips its grip around the reinforcement, and cannot hold the reinforcement, whereby no shear forces can be transmitted to the reinforcement. Consequently, the reinforcement has only a very limited effect on the asphalt layer.  
           [0014]    Reinforcing concrete constructions with steel reinforcement is well known in the art, and will not be further discussed here. Reference is made to the many textbooks on the subject.  
           [0015]    The pavements consisting of mainly asphalt with a void filling of a slurry comprising a binder, has not previously been reinforced. This is probably due to the known problems with reinforcing asphalt as described above.  
         DETAILED DESCRIPTION OF THE INVENTION  
         [0016]    In this application slurry is used to characterise a material comprising a binder, and different additives. The binder can be chosen among different cement materials as mentioned below as well as among other hydraulic binders, for example gypsum or lime, or polymer based binders, for example different suitable epoxy products or cement based polymer-modified binders. The slurry can in further embodiments also contain microsilica, flyash or other pozzolanic fillers as well as sand, other fine aggregates or polymer-based fillers.  
           [0017]    The slurry can be produced from a cement or other hydraulic binder. The binder particles will preferably be fine particles having a size in the range of from about 0.5 micro-metre to about 100 micro-metres. In a particularly preferred embodiment, the fine particles such as cement particles are combined with ultrafine particles having a size in the range of from about 5 nano-metres to about 0.5 micro-metre. Typically, the average particle size of the ultrafine particles will be at least one order of magnitude smaller than the average size of the fine particles, thereby allowing the ultrafine particles to become substantially uniformly distributed in the voids between densely packed fine particles to result in an extremely hard and dense matrix that provides optimal resistance against ingress of aggressive and harmful chemicals. In these embodiments, the fine particles will typically comprise at least one cement selected from the group consisting of Portland cement, low-alkali cement, sulphate-resistant cement, refractory cement, aluminate cement, slag cement and pozzolanic cement, and the ultrafine particles will typically comprise particles selected from the group consisting of silica fume and oxides such as iron oxide and titanium dioxide.  
           [0018]    A slurry based on cement and silica fume is especially preferred. In this case, and when using a combination of fine particles and ultrafine particles in general as discussed above, the slurry will typically be prepared from a mixture comprising ultrafine particles in an amount of about 5-50% by volume based on the total volume of the fine particles and ultrafine particles in the mixture. More typically, the amount of ultrafine particles will be about 10-40% by volume, such as about 15-30% by volume, based on the total volume of the fine particles and ultrafine particles.  
           [0019]    In such mixtures, the amount of water is preferably kept to the minimum required in order to wet the particles and provide a mixture with the required workability. Water is therefore normally added to the mixture in a volume ratio between water and fine+ultrafine particles of about 0.25-1.5, typically about 0.41.2, such as about 0.55-1.0.  
           [0020]    When using a rather small amount of water as indicated above in a cement-based mixture, the mixture will typically be prepared using a suitable effective amount of a surface-active dispersing agent (also known as a water-reducing agent or plasticizer), preferably a dispersing agent of the type known in the art as “concrete superplasticizers”. Examples of suitable concrete superplasticizers are naphthalene-based, melamine-based, vinyl-based, acrylic-based and carboxylic acid-based products, as well as mixtures thereof and derivatives such as vinyl copolymers.  
           [0021]    The concrete superplasticizer or other dispersing agent is typically incorporated into the mixture (either the dry mixture before water has been added or a wet mixture to which some or all of the water has already been added) in an amount of about 0.01-5% (dry weight based on the total weight of the fine and ultrafine particles), typically about 0.05-4%, more typically about 0.1-3%, such as about 0.2-2%. It will be understood that the amount of superplasticizer to be used in each individual case will depend in part on the nature of the superplasticizer. For example, when using one of the new generation of highly effective vinyl-, acrylic- or carboxylic acid-based superplasticizers, the required dispersing effect can be obtained with a significantly smaller amount of superplasticizer than when using e.g. a naphthalene sulphonic acid/formaldehyde or melamine sulphonic acid/formaldehyde condensation product Thus, the concrete superplasticizer should be used in an “effective” amount, i.e. effective for the given superplasticizer and in the given particle system to obtain the desired dispersing effect in the mixture using only the intended amount of water. The slurry must initially be very fluid in order to be able to penetrate and fill the voids in the asphalt matrix. In order for other materials in the art to attain a sufficient void filing in the asphalt matrix, it has been necessary to vibrate the slurry into the asphalt matrix. With the present invention using a material as described above this is not necessary, as this slurry substantially fills all voids without vibrating, due to the materials excellent flowability. When not having to vibrate the pavement at least two advantages are obtained. Firstly, when vibrating a relatively fresh porous asphalt it is easy to destroy the initial pore/void structure, and thereby difficult to control the porosity. The porosity is very important for the final structure as the porosity determines how much slurry can be accommodated in the matrix. Secondly, by not having to vibrate, a work operation is saved which limits the time when the pavement cannot be used and furthermore makes the procedure of installing the pavement more economic.  
           [0022]    An example of a slurry material as described above, is the slurry part of the material sold under the name Densiphalt®. This material has shown a better ability to penetrate and fill the voids in an asphalt structure, than other materials of this type. It is this fact together with the strength characteristics of the composite material, which makes it possible to effectively utilise the tension characteristics of a reinforcement arranged in the matrix.  
           [0023]    The slurry will harden and thereby attain at least two, for the application disclosed in this patent, very important characteristics, namely compressive strength, and a dense and compact micro-structure.  
           [0024]    Within the scope of this invention the hardend slurry is called composite material.  
           [0025]    The compressive strength of the composite material depends on many factors, for example the mix composition of the composite material, water and air content. From an economic viewpoint it is interesting to design the composite material for a particular use. The compressive strength can be varied by changing the mix composition, as described above. The compressive strength of the composite material, as for example in Densiphalt® is preferably around 110 MPa, for example in the range 100 MPa to 120 MPa, for other applications it might be lower, for example in the range 85 MPa to 120 MPa or 75 Mpa to 100 MPa or still lower 65 MPa to 90 MPa, or lower. Also for other applications the compressive strength may be in a higher range for example 110 MPa to 130 MPa or 120 MPa to 145 MPa or still higher, for example 135 MPa to 145 MPa or still higher, for example 135 MPa to 160 MPa or above.  
           [0026]    The present invention has proven that it is possible to strengthen these materials with a reinforcement, and utilise the tension characteristics of a reinforcement. By using a slurry with extreme characteristics as the slurry used in Densiphalte®, tests have shown that the pavement becomes more resistant to bending. Test results indicate that the pavement structure can be calculated in a fashion similar to conventional reinforced concrete.  
           [0027]    The structure consequently has attained all the good durability characteristics of the original materials. Densiphalt® has an extremly good ability to transmit the above-mentioned shear forces into tension in the reinforcement. This is due to the very dense and fine micro-structure of the composite material. The pavement structure has obtained strength characteristics almost comparable to conventional reinforced concrete. Hereby is achieved that the pavement has a much improved load distributing ability, whereby the pavement will be less sensitive to settlement in the base layers, as well as cracking due to this settling. Furthermore because the composite material is able to transmit forces to the reinforcement, the pavement structure as a whole will crack in a pattern similar to reinforced concrete. When reinforced concrete is loaded and cracks, this happens as very fine cracks evenly distributed along the side of the object in tension.  
           [0028]    The type of pavements, of which the present invention is an improvement, are traditionally made in layers having a thickness of between 40 and 60 mm. When layers of this thickness are reinforced according to the invention, the layer as a whole only gains a limited resistance against bending. The load distributing and crack distributing characteristics however improves significantly, so that a pavement according to the invention, gains some very advantageous characteristics in comparison to un-reinforced pavements known in the art. As the layer thickness increases, for example up to 100 mm or more preferably up to 150 mm or still more preferably up to 200 mm or thicker, the bending resistance also increases as well as the other advantageous characteristics, such as crack and load distribution.  
           [0029]    Therefore by varying the layer thickness and compressive strength of the composite material, a pavement can be designed specifically to a specific use. The different parameters, i.e. composition of composite material, type of reinforcement and layer thickness, as well as porosity of the asphalt, can be chosen within the ranges of the patent, in order to design a pavement for different applications.  
           [0030]    In some applications of the pavement structure or material according to the invention, special requirements to the appearance and surface characteristics of the layer must be fulfilled. This is especially important in shopping centres, airports and other areas where heavy and/or intense pedestrian use can be expected. It is also within the scope of the invention to use technical bitumen in the asphalt matrix. Technical bitumen is a polymer-based material with similar characteristics to bitumen. The material is colourless, but can be dyed to any colour. The same is true about some of the epoxy-based binders which can be used as binder material in the slurry material. By utilising these characteristics the pavement can be given an appearance according to the architect&#39;s requirements, and still maintain the characteristics of the pavement structure. Furthermore is bitumen slightly sticky, which is not desirable in pedestrian areas. This stickiness is avoided by using technical bitumen.  
           [0031]    The pavement according to the invention consequently achieves some important advantages in comparison to known materials: 1) due to the improved load bearing capability, the pavement will be able to handle heavier loads as the load will be better distributed through the pavement layer and 2) with excessive loading the underside, i.e. the side of the pavement opposite the surface where the load is placed, will crack in the shape of many small cracks evenly distributed along the tension zone of the underside of the structure, and 3) because of the ductility of the material helped by the reinforcement the fatigue strength of the pavement is vastly improved, which again results in a much improved service life, and consequently less maintenance and down time for the pavement.  
           [0032]    When constructing a pavement according to the invention the sub-base is prepared as known in the art. Thereafter a first asphalt layer is placed on the sub-base. On the first asphalt layer a first reinforcement layer is arranged, and then another asphalt layer is placed. In constructions where only one reinforcement layer is needed the second asphalt layer will be the final layer. Where two or more layers of reinforcement are needed, another asphalt layer will be placed, and another reinforcement layer and so on alternating the layers until the desired number of reinforcement layers are arranged in the asphalt matrix, and the uppermost finishing asphalt layer can be placed. The asphalt matrix is placed with a porosity of between 20% and 40%, more preferably 22-35% and still more preferably between 25% and 30%. After the asphalt matrix has cooled, the voids in the asphalt matrix is substantially completely filled with the slurry (composite material). In a further process step the finished surface can be given a treatment, for example a curing membrane, in order to avoid excessive drying of the pavement during hardening of the composite material. Also the pavement can be given a treatment to roughen the surface, in order to improve skid resistance. This treatment can be carried out by for example a steel ball shot blasting process, during which the uppermost thin skin of composite material is removed, in order to expose a rougher structure of composite material and gravel.  
           [0033]    The invention also comprises the use of a reinforcement in a matrix comprising a composite material part and an asphalt material part, said composite material part comprises a binder and different additives, said composite material having a compressive strength in the range from below 60 MPa to more than 160 MPa, preferably from 75 MPa to 145 MPa, still more preferably from 85 MPa to 130 MPa, yet still more preferably from 100 MPa to 120 MPa, but even more preferably around 110 MPa, said composite material substantially completely filling voids in an asphalt-matrix with a porosity between 20% and 40%, more preferably between 22% and 35% and still more preferably between 25% and 30%.  
           [0034]    In further advantageous embodiments the reinforcement can be steel rods or wires, carbon wires, stainless steel rods or wires, modified polymers, glassfibres or glassfibre spun wires or strands. The reinforcement can be used as rods, strands, wires, nets, mats or mesh. Also in this application three dimensional reinforcement structures can be used. For example when the reinforcement is arranged in an intermediate asphalt layer. This is especially useful when the reinforcement is in the shape of fibres. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0035]    In the following detailed description of advantageous embodiments of the invention reference will be made to the accompanying figures, wherein  
         [0036]    [0036]FIGS. 1 a ,  2   a  shows a known pavement in a uncracked and cracked state;  
         [0037]    [0037]FIGS. 1 b ,  2   b  shows a pavement according to the invention uncracked and cracked;  
         [0038]    [0038]FIG. 3 illustrates test specimens;  
         [0039]    [0039]FIG. 4 illustrates laboratory test rig;  
         [0040]    [0040]FIGS. 5 a  and  5   b  illustrate the relationship between load and deflection;  
         [0041]    [0041]FIG. 6 is a table showing the asphalt recipe;  
         [0042]    [0042]FIG. 7 is a table showing the mix proportions for Densiphalt® mortar;  
         [0043]    [0043]FIG. 8 shows crack patterns on test specimens; and  
         [0044]    [0044]FIG. 9 is a modelled cross section of pavement. 
     
    
       [0045]    In FIG. 1 a  is schematically shown a traditional pavement  1  of the type comprising an asphalt matrix with a porosity in the range 20-40%, and a composite material ( 15 ) completely filling out the voids in the asphalt matrix ( 16 ). A schematic cross section of the material is shown in FIG. 9. The pavement is placed on a sub-base  2 , which is generally known in the art. Hereby is created a pavement  1  combining the flexibility of asphalt with the durability and strength characteristics of concrete, resulting in a joint-free pavement.  
         [0046]    In FIG. 1 b  is schematically illustrated an inventive pavement  1  according to the invention, with in this example two reinforcement layers  3 , 4 , one layer  4  placed in the lower part and one layer  3  placed in the upper part of the pavement layer  1 . In FIG. 2 a  is illustrated how the pavement without reinforcement will crack with few large cracks  5 , because of a subsiding base  2  creating voids  7  between the pavement  1  and the sub-base  2 , and high loads on the surface of the pavement  1 .  
         [0047]    In FIG. 2 b  is illustrated the comparable situation, but with a pavement according to the invention. As will be seen from the laboratory results, outlined below, the bearing capacity of the pavement according to the invention is more than three times that of the unreinforced known pavements of this type. The crack patterns will therefore only develop due to higher loads. Because of the composite materials intimate contact with the reinforcement, crack patterns comparable to conventional concrete will show.  
         [0048]    In conventional concrete exposed to bending, fine evenly distributed cracks will appear in the concrete on the side subjected to tension. Concrete can only take a small fraction of its compressive strength as tension, for example a concrete with a characteristic compressive strength of 35 MPa can be subjected to about 1,9 N/mm 2  in tension without cracking. When cracking in the concrete occurs, the tension forces will be transmitted to the reinforcement. Steel&#39;s reinforcement can take very high tensile stresses before failure, and therefore a reinforced concrete construction utilises the concrete characteristics in compression and the steels characteristics in tension. This co-action is based on the assumption that the concrete is able to grip or hold the steel reinforcement sufficiently to transmit the forces to the steel reinforcement.  
         [0049]    The situation illustrated in FIG. 2 b  is as described above caused by a significantly higher load and the creation of voids  7  between the pavement  1  and the sub-base  2 . The fine cracks  6  will be evenly distributed along the side of the pavement exposed to tension stresses.  
         [0050]    In laboratory experiments tests have shown that a reinforced Densiphalt® pavement achieved impressive characteristics in comparison to a non-reinforced pavement of this type. The tests were carried out on specimens  100  as illustrated in FIG. 3. All specimens had the same outer dimensions, but varied in the placement and type of reinforcement  21 . All reinforcements  21  however were welded mesh structures. Each specimen was made from a type  20  open grade asphalt (OGA) produced in a standard asphalt plant and stored in containers. Before being placed in the specimen moulds, the asphalt was re-heated to 150° C. Details of the asphalt mix are listed in the table illustrated in FIG. 6. The bottom layer of the asphalt was then placed in the mould and manually compacted. Thereafter a reinforcement mesh was placed and the mould was filled with asphalt, and manually compacted. Where there are placed two reinforcement layers in the specimen, a second portion of asphalt was placed on top of the first reinforcement layer, which second layer was manually compacted, and the second reinforcement layer was then arranged on this layer, whereafter the top layer of asphalt was placed and manually compacted.  
         [0051]    After the asphalt had cooled the asphalt matrix was filled with Densiphalte® mortar mixed as shown in the table in FIG. 7. The specimens were then cured for 1 week at 20° C.  
         [0052]    The specimens  100  were placed in a test rig  20  as illustrated in FIG. 4, and two equal loads  22  were applied equidistant from the centre of the specimen  100 . A displacement transducer  23  was arranged underneath and in contact with the specimen  100 . As the loads  22  are increased, the specimen will bend and depress the transducer  23 . The relationship between load and deflection, as measured by the transducer is illustrated in FIGS. 5 a  and  5   b.    
         [0053]    All specimens were loaded to failure, and their crack patterns were recorded, see FIG. 8. Cracks  25  are illustrated as lines not representing the actual crack width. From FIG. 8 is it clear to see that an unreinforced specimen (specimen 1) has failure at a low load level, and furthermore failure occurs due to the creation of one large crack. The other specimens (specimen 2-7) all exhibits the formation of a crack pattern, comparable to crack patterns known from conventional reinforced concrete. Furthermore, these specimens could all be loaded to much higher ultimate loads (see FIG. 5 b ) and endure larger deflections before failure.  
         [0054]    From the above mentioned laboratory tests is it evident that the reinforced Densiphalt® pavement system has improved characteristics as compared to similar systems without reinforcement Furthermore the tests show that with the slurry/composite material filling out the voids in the asphalt matrix is it possible to utilise the tension characteristics of a cast in reinforcement.  
         [0055]    In the test series mentioned above traditional welded steel mesh reinforcement was used, but also steel or carbon wires, modified polymers, glassfibres or glassfibre spun wires and similar types of materials can be used as reinforcement.  
         [0056]    In the pavement-system sold as Densiphalt® the slurry/composite material has an average compressive strength of around 110 MPa, but also slurries with lesser or higher strengths can be used. This system is not dependent on the compressive strength of the composite material alone, but on attaining a fine micro-structure which will have an intimate contact with the reinforcement, as well as having a strong skeleton-structure in the asphalt matrix. The material further has an exceptional ability to fill all the voids in the asphalt-skeleton. These aspects together makes it possible to transmit the forces arising from loads on the surface of the pavement. These forces occur as shear in the pavement layer and moments due to bending in the pavement structure. The moments, as described above, must be transformed into tension stresses in the reinforcement in order to achieve the load distribution, resulting in a multitude of fine cracks, as illustrated in FIG. 8. Even if the unreinforced pavement could absorb the shear forces, the tension in the underside from the bending moment would cause one large crack, as illustrated in FIG. 8, specimen 1, whereby the pavement would be destroyed. With the present invention, the cracks  25  are distributed along the underside exposed to tension indicating very good load distributing as well as a strong connection between the asphalt-composite mix and the embedded reinforcement, whereby the whole pavement structure becomes much stronger. The laboratory results show up to more than 3 times ultimate loads with larger deflections, see FIG. 5 b.