Semi-flexible multi-layer paving

Abstract

 A semi-flexible, multi-layer paving (pavement) for the construction of carriageways with high load stresses is described, comprising a bottom foundation layer (1) composed of a mixture of cement with crushed stone aggregates, and a base layer (2), positioned on top of the layer (1), composed of a mixture of cement with crushed stone aggregates with a high cement content, in which above the layer (2) are positioned a tack coat of overstabilized bituminous emulsion and a continuous surface layer (3) composed of a composite mixture deriving from a matrix formed by an open-graded bituminous mix, the voids being filled with a highly-fluid cement mortar and in which the stone aggregates of the layer (1) and of the layer (2) are recycled cementitious materials deriving from crushing of recycled concrete.

Description

[0001] The present invention refers to a multi-layer paving (pavement) of the semi-flexible type with a high bearing capacity for use in particular in the airport sector, in which the superficial layer is formed by a an open-graded bituminous mix the voids of which are filled with highly-fluid cement mortar.

[0002] In particular, the present invention relates to a pavement as defined above further comprising two underlying layers, a foundation layer and a base layer, of different thicknesses, formed of cement-bound recycled crushed stone material, said paving having a stiffness that increases from the deepest layers to the superficial layer.

[0003] Airport pavings are generally made with concrete slabs (rigid pavements) or bituminous mix (semi-rigid pavements). In both types of paving a rigid foundation is provided.

[0004] In rigid pavings, the surface layer, composed of concrete slabs possibly cooperating by means of steel connecting rods and in some cases provided with wire mesh for strain distribution, is placed on a base layer consisting of cement-bound virgin quarried stone material, which in turn is placed on the compacted ground (subgrade). This type of pavements has a high bearing capacity but presents various disadvantages: long times for construction and for opening to traffic at the end of the works, complex maintenance, almost complete impossibility of installing services without resorting to demolition of the slabs.

[0005] Semi-rigid pavings are characterized by overlaying of layers of bituminous conglomerate resting on a rigid foundation of cement-bound granular material. For these semi-rigid pavings, three bitumen-bound layers having volumetric and mechanical characteristics that increase towards the surface of the paving are normally used. The use of bitumen binders presents great advantages both in application, due to the simplicity and speed of laying, and in maintenance, deriving from the ease of removal and restoration of small portions of paving, as well as the ease of installing services during the useful life of the work. However, this type of solution presents various criticalities related to the heat sensitivity of bitumen, with the formation of various types of disruption such as rutting, due to the loss of stiffness of the bitumen at high operating temperatures, and cracking of thermal origin due to tensile failure of the bituminous mixture in conditions of high thermal gradient at low temperatures.

[0006] In the airport sector the need is constantly felt to have available pavings affording simple, rapid maintenance, without decreasing the bearing capacity and life cycle of the paving and allowing the airport to operate safely. This latter requirement has become greater in recent years because the global demand for airborne mobility of people and goods has undergone strong growth, subjecting airport pavings to increasing load stresses and accelerating their deterioration.

[0007] Furthermore, the use of considerable amounts of virgin stone material leads to further disadvantages, such as, for example, a long and costly movement of material from its quarrying site to the paving construction site. This movement leads to a considerable increase in lorry traffic with consequent problems of pollution and considerable transport costs given that said sites are generally far apart. This is particularly onerous in the construction of extensive pavings as in the case of airport pavings. Furthermore, continuous quarrying of new materials leads to a deterioration and impoverishment of the environment.

[0008] In the state of the art, aggregates deriving from recycled stone materials of various sizes are used in the construction of paving for road infrastructures. However, the use of such materials is limited to a single layer of paving, normally a sub-base or sub-foundation layer, in any case sufficiently far away (more than 15 cm) from the running surface of the tires, whereas the other layers are composed of superior stone materials. Generally, if the thickness of said paving is 100, the use of recycled material is limited to about 35-40%, whereas the predominant part of the thickness is represented by new inert materials. This is due to the fact that the grain size of the aggregates upon which the structural properties of the paving depend is more easily regulated when the stone materials are new rather than recycled.

[0009] Object of the present invention is to eliminate the drawbacks of the prior art by providing an airport paving (pavement) having bearing capacity, stiffness and durability absolutely comparable to those of the rigid and semi-rigid airport pavings of the prior art.

[0010] A further object of the present invention is that of providing an airport paving that can be produced using alternative ecological materials capable of giving the same performance as quarried materials without requiring excessive movement. These objects are achieved in accordance with the invention with the characteristics of the paving listed in appended independent claim 1.

[0011] Advantageous embodiments of the invention are apparent from the dependent claims.
The paving of the present invention comprises three different overlapped layers one on top of the other in which a first cementitious foundation layer and an intermediate cementitious base layer are composed of recycled stony material bound with cement, in different quantities, whilst the superficial (surface) layer is formed by a bituminous mix made of crushed stone aggregates whose voids are filled with highly-fluid cement mortar. The paving of the present invention is semi-flexible and has a high load-bearing capacity. In addition the stiffness of this paving increases from the deepest layers to the surface layer. The paving of the present invention is therefore ideal for the construction of carriageways with high load stresses, particularly for airports.

[0012] Further characteristics of the invention will be made clearer by the detailed description that follows referring to a purely exemplifying and therefore non limiting embodiment thereof, shown in the appended drawing, which illustrates a vertical section of the paving according to the invention, the layers of which have specific thicknesses.

[0013] The multi-layer paving comprises:

• a bottom foundation layer 1 comprising a mixture of cement with crushed stony aggregates,
• a base layer 2 consisting of a mixture of cement with crushed stony aggregates (cement-bound material) with a high cement content, positioned on top of the layer 1;
• a continuous surface layer 3, positioned on top of the base layer 2, composed of a composite mixture deriving from a matrix formed by bituminous mix (asphalt concrete) having a suitable void content and from filling of the voids with a highly-fluid cement mortar.
• a tack coat (adherence coat) between the layers 2 and 3 composed of an overstabilized bitumen emulsion,

in which the stone aggregates of the layer 1 and of the layer 2 are recycled cementitious materials deriving from granulation of recycled concrete.

[0014] In the layer 3 the amount of bituminous mix varies from 70% to 80% by volume and the amount of cement mortar varies from 20% to 30% by volume.

[0015] The components that form the layer 3 are characterized as described below.

[0016] The open-graded bituminous mix of the layer 3, also named open grade asphalt concrete, essentially comprises virgin crushed stony aggregates having discontinuous grain size and bitumen, the bitumen content being between 3% and 5% by weight with respect to the aggregates, preferably 3-4%.

[0017] The bitumen of the layer 3 can be of the standard type or bitumen modified with polymers. The latter type of bitumen allows the open-graded paving, not yet filled, to reach suitable levels of strength, in order to obtain a layer capable of not deteriorating under transient loads. This characteristic of the open-graded bituminous mix is particularly interesting for road and airport applications for which it is necessary to temporarily remove the works and open to traffic between the stages of laying the open-graded bituminous mix and injecting the cement mortar.

[0018] Examples of the bitumen that can be used to obtain the present bituminous mix are as follows:

Parameter	Bitumen as is		Modified bitumen
type	50/70	80/100	50/70-65	50/70-60
Penetration at 25°C[dmm]	50-70	80-100	50-70	50-70
[EN 1426/2002]
Softening temperature
[EN 1427/2002] [°C]	46-56	40-44	> 65	> 60
Fraas breaking point [°C]
[EN 12593]	< -10	< -8	< -15	< -12
Elastic recovery at 25°C [%]
[EN 13398]	> 80	> 80	> 75	> 50

[0019] The crushed stony aggregates of the bituminous mix of the layer 3 derive from crushing of virgin stone material, such as, for example, calcareous rocks, freed of any impurities, and are suitably selected for size. The stone aggregates preferably show the following characteristics:

	Standard	Value
Los Angeles Coefficient	AASHTO T96	< 25%
Angularity	ASTM D5821	100/100
Flattening coefficient	BS 812	< 20%
Apparent bulk density	AASHTO T85	> 2.6 g/cm3

[0020] The variety in grain size of the stone aggregates of the bituminous mix of the layer 3 allows the formation, in the laid bituminous mix, of a network of communicating voids between the aggregates able to receive the mortar in the injection stage.

[0021] The bituminous mix of the layer 3 has a void content between 20% and 30%, more preferably around 30%. In particular the mix has the following characteristics:

	Standard	Value
Bulk density [g/cm3]	ASTM D2726-88	1.8 ö 2.0
Void content [%]	CNR BU 39/73	20 ö 30

[0022] As far as the mechanical properties are concerned, the bituminous mix preferably has the following characteristics:

	Standard		Value
Stability [kN]	CNR BU 30/73		> 4.00
Flow [mm]	"	2 ö 3
Stiffness [kN/mm]	"		> 1.30
IDT [MPa]	CNR BU 97/84		> 0.50

where IDT is the indirect tensile strength measured under almost static loading conditions (v = 51 mm/min) and at a temperature of 20°C and the stiffness is Marshall stiffness at 60°C. The above mentioned characteristics of the bituminous mix were checked by preparing cylindrical specimens by the Marshall compacting method (CNR BU 30/73), applying 75 blows per side.

[0023] The highly-fluid cement mortar, also known commercially by the name Darimix ®, is composed of a mixture of selected cements, able to ensure the necessary requisites of fluidity and workability for the time needed for laying as well as the consistency of the physico-chemical characteristics, and highly-fluidifying, expanding, accelerating, additives, de-areation, set retardant, antisegregation, and antiagglomeration additives, with specific amounts of water so as to give the mixture such a fluidity as to fill the voids without difficulty. The amount of water to be added is generally less than 40% by weight with respect to the cement mixture containing additives, preferably between 30-35%. The cement mortar preferably has a fluidity measured in a cone with an 8 mm nozzle (UNI EN 445) between 20 e 50 sec, preferably 25 sec ± 3 sec.

[0024] Other characteristics of the mortar are as follows:

bulk density	1.98 kg/dm3 ± 0.02
maintenance of workability during mixing	60 min

[0025] The cement mortar thus obtained has a high mechanical strength and substantial containment of shrinkage which is also compensated for by the presence of expanding additives. The mortar thus obtained shows the following characteristics:

	Standard	Value
Compressive strength at 24h	UNI EN 445	≥ 20 [MPa]
Compressive strength at 7 days	UNI EN 445	≥ 55 [MPa]
Compressive strength at 28 days	UNI EN 445	≥ 70 [MPa]

[0026] The layer 3 deriving from filling of the open-graded bituminous mix with the cement mortar shows characteristics which are intermediate between a bituminous conglomerate and a cement mixture of the traditional type and shows the following technical characteristics (after setting):

	Standard	Value
Bulk density [g/cm3]	ASTM 2726/88	≥ 2.30
Elastic module at 20°C [MPa]	EN UNI 12697-26	≥ 7000
IDT at 20°C [MPa]	CNR BU 97/84	≥ 1.60

where IDT is the indirect tensile strength measured under almost static loading conditions (v = 51 mm/min) and at a temperature of 20°C.

[0027] The layer 3 generally has an elastic modulus approximately equal to that of the underlying base layer 2. Furthermore, the thickness of the layer 3 is less than the thicknesses of the layer 2 and the layer 1 since its function is essentially to distribute loads to the underlying layers.

[0028] The thicknesses of said layer 3 are preferably between 4 and 8 cm, or can even be greater, for example up to 12 cm.

[0029] Said layer 3 also has the task of compensating for high local loads, even shearing stress, due to direct interaction with aircraft/vehicles in transit or standing.

[0030] The complete interaction between the surface layer 3 laid with bitumen-cement technology and the base layer 2 is ensured by means of the use of an overstabilized slow breaking cationic bitumen emulsion as the tack coat, in which the residual bitumen, the binder by means of which transmission of stresses takes place, is modified with SBS (styrene-butadiene-styrene) polymers able to offer a high chemical and adhesive affinity with hydraulic (cement) and bituminous binders.

[0031] Thanks to the particular composition of the above described layer 3, it is able to absorb the stresses through the mutual collaboration between stone aggregates and cement mortar, thus showing substantially different mechanisms of structural interaction with respect to those that are established inside a layer of asphalt (bituminous conglomerate), in which the distribution of strains takes place through transfer of stresses to the points of contact between the grains.

[0032] Therefore, the response to stresses of the layer 3 is not of the discrete type, but of a continuous type with a consequent greater evenness in the distribution of stresses, reaching high levels of bearing capacity. Furthermore this continuous distribution of stresses means that the quality requirement for the inert materials used in the underlying layers is lower.

[0033] The base layer 2 is an intermediate layer consisting of a mixture of cement, in high quantities of between 4% and 7% with respect to the total weight of the mixture, and water in an amount between 6% and 10% by weight with respect to the weight of the mixture, with recycled cementitious stony aggregates deriving from granulated recycled concrete.

[0034] The recycled aggregates used are have a grain size of 0/40 mm deriving from crushing of debris from demolition of cement concrete structures and subsequent grain size selection.

[0035] The cement/aggregates mixture of the layer 2 is prepared in a concrete mixing plant, applied and laid by means of a mobile vibratory finishing machine and a compacting roller, after which a de-stressing procedure is performed during the stage of setting of the cement. This de-stressing procedure is carried out with metal roller compactor in static mode, 24 hours after the end of the works.

[0036] The de-stressing operation allows the formation of micro-cracks able to absorb excessive stresses deriving from the typical effects of shrinkage of cement when the latter is used in predominant quantities.

[0037] Therefore, the layer 2 presents as a sufficiently rigid material (elastic modulus approximately equal to that of the layer 1), due to the high amount of cement (between 4% and 7%) such as to provide the structural performance, but is not fragile thanks to the de-stressing procedure. The elastic modulus of the mixture of cement and aggregates (cement-bound material) of which the layer 2 is formed is between 6000 MPa and 9000 MPa, measured according to UNI EN 13286-43:2006 on samples reconstructed in the laboratory, after curing in a moist cabinet for a period of 7 days.

[0038] The thickness of the layer 2 varies according to the final application of the paving and is in any case generally between 20 and 30 cm.

[0039] The foundation layer 1 comprises a mixture of cement, in amounts between 3% and 5% by weight with respect to the total weight of the mixture, water in amounts between 7% and 10% with respect to the total weight of the mixture, and stony aggregates deriving exclusively from granulated recycled cement concrete. Said layer 1 is formed by mixing cement and water with recycled aggregates preferably with a grain size of 0/70 mm, deriving from crushing of concrete slabs at the end of their life cycle.

[0040] Laying of the layer 1 takes place by an in situ stabilization method. This method, known to a person skilled in the art, takes place by setting of the recycled aggregates, adding the necessary water and cement contents, mixing in a suitable mobile/on-site cement mixer, levelling with grader and subsequent compaction.

[0041] The thickness of the foundation layer 1 varies according to the final application of the paving and in any case is generally between 30 and 50 cm.

[0042] As far as the mechanical performance is concerned, the foundation layer 1 has an elastic modulus between 3000 and 5000 MPa, measured according to UNI EN 13286-43:2006 on samples reconstructed in the laboratory, after curing in a moist cabinet for a period of 7 days.

[0043] The stiffnesses of the layers 1, 2, 3 are generally such that the elastic modulus of the surface layer 3 is substantially equal to the elastic modulus of the intermediate layer 2 and such that the stiffness of the paving increases from the layer 1 to the layer 3 and the stiffness of the foundation layer 1 is equal to 50% of that of the base layer 2.

[0044] The thicknesses of the layers 1, 2, 3 which make up the paving of the present invention are proportioned so as to have a distribution of stresses across the thickness of the paving according to the end use of the paving and the type of structural performance required of said paving.

[0045] In the embodiment of Figure 1 the paving has the following thicknesses:

- layer 1	40 cm
- layer 2	30 cm
- layer 3	6 cm

[0046] In the paving of the present invention the layer 1 is such as to have a stiffness equal to about 50% with respect to that of the layer 2. Furthermore the total thickness of the layers 1 and 2 containing recycled materials represents essentially at least 90% of the total thickness of the paving.

[0047] An advantage of the paving of the invention is represented by the high content of recycled materials: as stated, in fact, if the total thickness of the present paving is 100, they represent at least 90% of the total thickness. This leads to a greater ease in finding the materials, a decrease in procurement costs for the contractors, and no exploitation of precious natural resources deriving from borrow pits. In addition, the paving thus provided is able to achieve high performances typical of airport paving.

[0048] Therefore, with the paving of the present invention it is possible to combine the environmental advantages deriving from the use of recycled material in the lower foundation and base layers with the high performance of a superficial layer formed using the technology combining bitumen and cement described above.

[0049] A further advantage of the paving of the present invention is represented by the considerable load-bearing capacity of the paving structure, which is substantially able to receive the same loads as rigid and semi-rigid paving of the prior art. In this manner the advantages typical of a continuous surface deriving from the usual working of layers of bituminous conglomerate are combined with the strength of concrete paving. Furthermore, the surface layer shows a modest sensitivity to temperature and load changes if compared with typical bituminous conglomerate pavings; this is due to the continuous matrix made of the stone aggregates contained in the bituminous mixture and the cement contained into the mortar.

[0050] The paving according to the present invention can be formed according to various methods known to the art. An example of the process is that comprising:

• preparation of a sub-base 4 for the paving comprising excavation of the site for the paving and compaction of the ground of said site;
• laying of the layer 1 by means of setting the recycled aggregates on the sub-base, adding water and cement in situ, and subsequent compaction;
• laying of the layer 2 comprising producing the cement mixture (cement-bound material) defined above in a fixed or mobile concrete mixing plant, applying the cement mixture on top of the layer 1 by means of a mobile vibratory finishing machine and subsequent compaction;
• rolling the layer 2 using metal rollers in static mode, 24 hours after the end of works in order to perform de-stressing;
• applying the overstabilized bituminous emulsion on the layer 2;
• applying the bituminous mix on the layer 2 obtained in the previous step and subsequent injection of the cement mortar.

[0051] As has been said, the paving of the present invention finds its use in the airport sector thanks to its ease of construction, absence of surface joins, high strength and load-bearing capacity.

[0052] Thanks precisely to these properties, said paving can also be applied in other sectors, both for ex novo construction of pavings that are particularly challenging from a static and dynamic point of view, and for routine and supplementary maintenance operations but also for reinforcement of roadways in a high state of disrepair, and in the construction of all those infrastructures whose end use involves high load stresses, including static stresses. Among the sectors most interested are industries (e.g. warehouses and depots, supply centres, sales areas, goods terminals, industrial floors, goods storage yards, including those for pallets and containers), ports and dry ports (e.g. goods handling and storage areas, warehouses and depots), roads subject to heavy traffic or chemical spillage (e.g. bus lanes, public transport stopping and terminus areas, urban roads with heavy traffic, heavy vehicle climbing lanes, road intersections, lay-bys, tunnels, motorway toll gates, services and petrol stations, refuse collection and storage yards), airports/heliports (e.g. runways and take-off/landing runway ends, taxiways, interchanges, aprons, De-icing bays, refuelling pads, and renovation of existing slabs).

[0053] Numerous changes and modifications of detail within the reach of a person skilled in the art can be made to the present embodiments of the invention without thereby departing from the scope of the invention as set forth in the appended claims.

Claims

1. Semi-flexible multi-layer paving for the construction of carriage ways with high load stresses, in particular for airports, comprising:

- a bottom foundation layer (1) comprising a mixture of cement with crushed stone aggregates,

- a base layer (2), positioned on top of the layer (1) composed of a mixture of cement with crushed stone aggregates with a high cement content,

- a continuous surface layer (3), positioned on top of the layer (2), composed of a composite mixture deriving from a matrix formed by an open-graded bituminous mix having voids filled by a highly-fluid cement mortar,

- a tack coat between the layers (2) and (3) composed of overstabilized bituminous emulsion,
the crushed stone aggregates of the layer (1) and of the layer (2) being recycled cementitious materials deriving from granulation of recycled concrete.

2. Paving according to claim 1, wherein the open-graded bituminous mix of the layer (3) essentially comprises virgin crushed stone aggregates having discontinuous grain size and bitumen, as such or modified with polymers, the bitumen content being between 3.0% and 5% by weight with respect to the aggregates, preferably 3-4%.

3. Paving according to any one of the preceding claims, wherein the open-graded bituminous mix is present in amounts of 70-80% by volume and the cement mortar is in amounts of 20-30% by volume.

4. Paving according to any one of the preceding claims, wherein the bituminous mix of the layer (3) has a void content between 20% and 30%, preferably about 30%.

5. Paving according to any one of the preceding claims, wherein the highly-fluid cement mortar comprises a mixture of cements, additives and water, the latter being in amounts less than 40% by weight with respect to the cement mixture containing additives, preferably in amounts between 30-35%.

6. Paving according to any one of the preceding claims, wherein the layer (2) has elastic modulus approximately equal to the elastic modulus of the layer (3).

7. Paving according to any one of the preceding claims, wherein the foundation layer

(1) has an elastic modulus approximately equal to 50% of the elastic modulus of the base layer (2).

8. Paving according to any one of the preceding claims, wherein the recycled cementitious crushed stone aggregates deriving from recycled granulated concrete of the base layer (2) have a grain size of 0/40 mm; the recycled cementitious crushed stone aggregates deriving from recycled granulated concrete of the layer (1) have a grain size of 0/70 mm.

9. Paving according to any one of the preceding claims, wherein the total thickness of the layers (1) and (2) containing recycled materials is at least 90% of the total thickness of the paving; preferably

- the thickness of the layer (3) is between 4 and 8 cm, or greater even up to 12 cm,

- the thickness of the base layer (2) is between 20 and 30 cm,

- the thickness of the foundation layer (1) is between 30 and 50 cm; more preferably the layers have the following thicknesses:

- foundation layer (1) 40 cm

- base layer (2) 30 cm

- surface layer (3) 6 cm

10. Process for preparing the paving as defined in any one of the preceding claims, comprising:

- preparation of a subbase (4) for the paving comprising excavation of the site for the paving and compaction of the ground of said site;

- laying of the layer (1) by means of setting the recycled aggregates on the subbase, adding water and cement in situ, and subsequent levelling and compaction;

- laying of the layer (2) comprising producing the cement mixture defined above in a fixed or mobile concrete mixing plant, applying the cement mixture on top of the layer (1) by means of a mobile vibratory finishing machine and subsequent compaction;

- rolling the layer (2) using metal rollers in static mode, 24 hours after the end of works in order to perform destressing;

- applying the overstabilized bituminous emulsion on the layer (2);

- laying and compaction of the bituminous mix by means of vibratory finishing machine and metal roller on the layer (2) obtained in the previous step and subsequent injection of the cement mortar.

Drawing